Implantable Stimulator-Transponder (A3036)

© 2019-2020 Kevan Hashemi, Open Source Instruments Inc.

Contents

Description
Versions
Design
Set-Up
Battery Capacity
Optical Power
Fiber Tapering
Modifications
Development
Summer-19Fall-19Winter-20Spring-20

Description

Warning: The IST cannot activate when its stimulus leads are in electrical contact. Make sure the resistance between the leads is always greater than 100 Ω.

[07-DEC-19] The Implantable Stimulator-Transponder (A3036) is a wireless electrical stimulator that receives commands and responds with acknowledgements, battery measurements, and synchronizing signals using a single antenna. The displacement volume of the A3036A is only 0.90 ml. The electrical stimulus is delivered through two silicone-insulated helical wires terminated with miniature pins. A single radio-frequency command initiates an arbitrarily long stimulus. When combined with the A3036IL series of implantable lamps, the A3036 provides optogenetic stimulus. When combined with a bipolar depth electrode, the A3036 provides direct electrical stimulus. Between implants, we can recharge the A3036's battery through its stimulus leads with the help of a Battery Charger (A3033A).


Figure: Implantable Stimulator-Transponder (A3036A). Displacement volume 0.9 ml. Extreme dimensions of the body are 21 mm × 10 mm × 5 mm. The red lead is L+ the blue lead is L−. The electrical stimulus is delivered to the pins at the tips of the leads. Recharging takes place through the same pins. The 30-mm loop antenna work poorly in air, but is efficient when implanted in an animal body. The single antenna provides both command reception and acknowledgement transmission.

In its standby state, the IST is ready to receive commands, but is doing nothing else. Its standby current consumption is only 7 μA, so the A3036A, with its 19 mA-hr battery, can remain in its standby state for 2700 hrs before its battery is exhausted. Commands arrive from a transmitter such as the 915-MHz Command Transmitter (A3029C), which we control with the same software we developed for the Implantable Sensor with Lamp (ISL). This software operates with our LWDAQ data acquisition hardware. The IST transmits its acknowledgements and metadata at 915 MHz also, for detection by a data receiver such as the Octal Data Receiver A3027E. The IST transmissions use the same protocol as our Subcutaneous Transmitter System (SCT). The IST command transmissions take tens of milliseconds, and during command transmission, SCT samples will be lost.

All ISTs that drive implantable lamps are equipped with a lithium-polymer battery. No other miniature battery can provide 20 mA for the lamp. The IST applies its battery voltage to the lamp through its lamp leads. The resistance of the lamp leads limits the LED current, and we make this resistance part of the design of the device. The average voltage of lithium-polymer battery during its lifetime is 3.7 V. The forward voltage of the blue EZ500 LEDs at 20 mA is 2.9 V. The forward voltage of the green EZ500 and both the blue and green TR2227 LEDs is 3.1 V at the same current. Suppose we choose the resistance of the leads to set the lamp current at 20 mA for battery voltage 3.7 V. The 19-mAhr battery of the A3036A will provide 57 minutes of continuous light output. If our stimuli consist of 10-ms pulses at 10 Hz, the A3036A can deliver a total of 570 minutes of stimulus before it exhausts its battery.


Figure: The Implantable LED (ILED, A3036IL-A) For Surface Illumination. The LED dies is mounted in a 3-mm square package, sealed with clear epoxy. Two stimulator leads are plugged into sockets at the back. A steel tube provides a means of holding the ILED during implantation. All electrical contacts on the front side, which touches the animal bod, are covered with epox or solder mask.

The A3036IL implantable lamps use a variety of LEDs, such as the blue and green TR2227, the blue and green EZ500, and the EZ290. All A3036ILs are equipped with a hypodermic tube on the opposite side of the LED to allow the lamp to be located and held securely during imlantation. The tube is thinned near its base, so it may be cut more easily after the lamp has been secured with dental cement. The optical fibers we use with our implantable lamps are polished at the base and tapered at the tip. We glue the base to the surface of the LED and the fiber captures roughly half the light emitted by the LED and carries it to the tapered tip, wher the light is emitted in all directions. These are our Fiber-Coupled LEDs (FC-LEDs).


Figure: The Implantable LED (ILED, A3036IL-A) Back-Side. Two sockets accept the IST stimulation pins. There are markings + and − next to the scokets. The pin of the red IST lead plugs into the socket marked +. The steel mounting tube is soldered to the ILED, and thinned just outside the solder joint so it may be cut more easily during implantation.

With no optical fiber glued to the LED, the LED is instead covered with clear epoxy so that it may be placed in contact with the tissue to be illuminated. The surface of the epoxy may be flat or domed, but this makes little difference to the power delivered to the tissue surface. These epoxy-topped lamps we call Implantable LEDs (ILEDs).


Figure: The Fiber-Coupled LED (FC-LED, A3036IL-A8) For Depth Illumination. In a well-made FC-LED, 40% of the light emitted by the LED reaches the fiber tip. The steel mounting tube is thinned close to the LED so that it may be cut more easily after the FC-LED is fixed in place with cement.

One application of the IST is to implanted it together with an SCT so that stimuli can be generated in response to EEG activity in real time, with the help of the Event Classifier running on the data acquisition computer. Earlier ISL device combined stimulator and sensor in the same device, powered by the same battery, and suffered from lamp artifact. When we separate the stimulator and the sensor into two circuits each powered by their own battery, we find that we eliminate lamp artifact in optogenetic applications. The IST's synchronizing transmission, when enabled, is recorded along with SCT signals, and allows us to obtain perfect superposition of the stimulus pulses and biometric signals.


Figure: Connections to an Optical Stimulator and Local Field Potential Sensor. We have L+ and L− from an Implantable Stimulator-Transponder (ISS) connected to a fiber-coupled LED, and X+ and X− from a subcutanesou transmitter (SCT) connected to a depth electrode and a skull screw.

We first proposed the development of the IST in our IST Technical Proposal. Development of the IST began on 06-AUG-19 with a purchase order from UCL. Further development is funded by an SBIR grant from the NIH.

Versions

The table below lists the existing versions of the Implantable Stimulator-Transponder (A3036).

Version Battery Volume
(ml)
Lead Length (mm) Lead Resistance (Ω) Comments
A3036A LiPo PP031012AB 19 mA-hr 0.9 45 56
A3036B LiPo PP031012AB 19 mA-hr 0.9 45 56 Decodes channel numbers 17-222
Table: Versions of the Implantable Stimulator-Transponder (A3036).

The gold-plated pins on the end of the IST's lamp leads mate with a pair of sockets on the A3036IL implantable lamps. The table below gives the available and planned versions of these lamps.

Version LED Light Guide Wavelength
(nm)
Optical Power
(mW at 20 mA)
Status
A3036IL-A C469EZ500 Epoxy Dome 460 (Blue) 20 Available Now
A3036IL-B C527EZ500 Epoxy Dome 527 (Green) 10 Available March 2020
A3036IL-A8 C469EZ500 450 μm Dia Fiber, 8 mm long 460 (Blue) 10 Available Now
A3036IL-B8 C527EZ500 450 μm Dia Fiber, 8 mm long 527 (Green) 5 Available March 2020
A3036IL-C C460TR2227 Epoxy Dome 460 (Blue) 20 Available March 2020
A3036IL-D C527TR2227 Epoxy Dome 527 (Green) 10 Available March 2020
A3036IL-C6 C460TR2227 270 μm Dia Fiber, 6 mm long 460 (Blue) 10 Available March 2020
A3036IL-D6 C527TR2227 270 μm Dia Fiber, 6 mm long 527 (Green) 5 Available March 2020
A3036IL-E6 C460EZ290 300 μm Dia Fiber, 7 mm long 460 (Blue) 8 Available April 2020
A3036IL-F6 C527EZ290 300 μm Dia Fiber, 7 mm long 527 (Green) 4 Available April 2020
Table: Versions of the Implantable Lamp (A3036IL). We quote expected optical power at the tip of the fiber or the surface of the LED for an LED current 20 mA. Optical power is approximately linear with current.

Design

The IST is managed by a field-programmable gate array (FPGA) in a 2.5-mm square package, the XO2-1200. This device provides both volatile and non-volatile memory as well as thousands of programmable logic gates. It is capable of implementing arbitrarily-complex stimuli in response to a single command. The A3036A uses the same firmware as the Implantable Sensor with Lamp (A3030E), in which a single stimulus consists of a set number of pulses, each of fixed length, generated at regular intervales, or at random intervals with a known average value.

S3030A_1: IST Version A Schematic.
S3030B_1: IST Version B Schematic.
Code: Logic Programs and Test Scripts.
LCMXO2-1200ZE: The programmable logic chip data sheet.
WLCSP-25: The 2.5 mm square BGA logic chip package.
A303601A.zip: PCB for A3036A, Gerber files and drawing.
A303601A_Panel.zip: Panel A303601A, Gerber files for assembly.
A303601A_Top: Rendering of top side of A303601B circuit board.
A303601A_Bottom: Rendering of bottom side of A303601B circuit board.
A3036A.ods: Bill of Materials and Pick and Place for A3036A.
A303601B.zip: PCB for A3036B, Gerber files and drawing.
A303601B_Panel.zip: Panel A303601B, Gerber files for assembly.
A303601B_Top: Rendering of top side of A303601B circuit board.
A303601B_Bottom: Rendering of bottom side of A303601B circuit board.
A3036B.ods: Bill of Materials and Pick and Place for A3036B.
A303602A_Top: Rendering of top side of TR2227 Head.
A303602A_Bottom: Rendering of bottom side of TR2227 Head.
A303602B_Top: Rendering of top side of EZ500 Head.
A303602B_Bottom: Rendering of bottom side of EZ500 Head.
A303602C_Top: Rendering of top side of TR2227 Head.
A303602C_Bottom: Rendering of bottom side of TR2227 Head.
A303602D_Top: Rendering of top side of Wire-Bonded LED Head.
A303602D_Bottom: Rendering of bottom side of Wire-Bonded LED Head.
EZ500: Manufacturer's data sheet for EZ500 blue and green LEDs.
EZ290: Manufacturer's data sheet for EZ290 blue and green LEDs.
TR2227: Manufacturer's data sheet for TR2227 blue and green LEDs.

Set-Up

We provide hardware configuration and connection instructions in the Set-Up section of our Implantable Stimulator System guide, and software control instructions in the Software section of the same guide.

Battery Capacity

Each version of the IST has a nominal battery capacity. The A3036A battery is nominally 19 mA-hr. We discharged and charged the battery of device A215.10 five times, and charged once more with an A3033A. Once charged, it reported battery voltage 4.3 V when inactive. We attached a white LED, turned on lamp power continuously and measured 20.0 mA current with the battery reporting 4.1 V. The lead resistance is 56 Ω. The LED forward voltage drop is 2.98 V. We turned on a 50% stimulus ofd 2-ms pulses at 250 Hz and recorded battery voltage versus time.


Figure: Discharge of an A3033A Battery Delivering 50% Stimulus to a White LED. Total charge delivered is 17 mA-hr.

Part-way through, we measure 11 mA current with the lamp turned on continuously, and the battery is reporting 3.6 V, which is also consistent with LED forward voltage 3.98 V. Thus we are able to calculate the LED current for intermediate battery voltages, and accumulate to obtain the total charge delivered by the battery versus time.

Optical Power

[11-APR-20] We measure the optical power output of each of our implantable lamps at an LED current of 10 mA, this being the approximate current when driven by a 3.7-V lithium-polymer battery and 50-Ω stimulus leads. The power output at 10 mA is our calibration of the implantable lamp. The output power is propostional to current, so we get twice as much power at 20 mA and half as much at 5 mA.

Our Implantable Lamps (A3036I) use an LED die wire-bonded to a printed circuit board as the source of light. These LEDs come in two colors: 460-nm blue and 527-nm green. The LED dies come in several sizes: the TR2227 is 220 μm × 270 μm, the EZ290 is 290 μm square, and the EZ500 is 480 μm square. We either cap the LED die with clear epoxy to make a surface illuminator, such as the A3036IL-A, or we glue the base of an optical fiber onto the LED and use the tapered tip of the fiber as a depth illuminator, such as the A3036IL-A8. The "A8" means a C460EZ500 LED die as light source, with an 8-mm long, 450-μm diameter, conical-tipped fiber as the depth illuminator.


Figure: Optical Power versus LED Current for Various LEDs.

The blue LEDs produce roughly 1 mW per 1 mA forward current, and the green LEDs produce roughly 0.5 mW per 1 mA forward current.


Figure: Optical Power versus LED Current for Various Fiber-Coupled LEDs.

We measure optical power using an SD445 photodiode and an ammeter. We convert photocurrent into optical power using 0.18 mA/mW for blue light and 0.25 mA/mW for green light. We have two power measurement stands that permit us to make consistent measurements of power for any combination of LED and fiber. The drawing below shows the arrangement of LED or FC-LED and photodiode in each stand. For more information on the calibration and appearance of the test stands, see our development notes.


Figure: LED Power Measurement Stand (Left) and Fiber-Coupled LED Power Measurement Stand (Right).

When we cap an LED with epoxy, we make sure the surface is convex. We avoid loss by total internal reflection within the epoxy, and we direct the light more in the forward direction, which favors illumination of a surface directly in front of the ILED. But the epoxy dome is never uniform, so measuring the power emitted by the ILED after application of the epoxy dome is difficult. We assume the total power transmitted by the ILED is equal to the total power emitted by the LED die before we apply the dome.

We measure the power emitted by fiber-coupled LEDs directly with our FC-LED measurement stand. The fraction of LED power reaching the fiber tip depends upon the area of the fiber and the LED, and upon the numerical aperture of the fiber itself. We make our light guides out of optical fiber with numerical aperture 0.86 (core index 1.72, cladding index 1.49). All light within ±60° of the fiber axis will be captured by the fiber and transported to the tip. For a typical LED, this ±60° accounts for 75% of the light emitted. A 450-μm diameter light guide, perfectly polished at the base, and perfectly positioned on a 480-μm square EZ500 LED will couple 51% of the emitted light to the fiber tip. The A3036IL-A8 uses a 450-μm fiber on a C460EZ500, and we obtain coupling efficiency of around 40%.

The IST applies its battery voltage to the LED through its stimulator leads. The A3036A stimulator leads, for example, are 45 mm long and each have resistance 28 Ω, making a total of 56 Ω. To determine the LED current we need to know the approximate forward voltage drop of the LED at 10 mA, and we need to estimate the battery voltage.


Figure: Current and Power versus Voltage for an A3036IL-A8 Implantable Lamp. Blue C460EZ500 LED coupled to 450-μm, 8-mm fiber, power measured at the tapered tip of fiber.

The battery voltage drops from 3.8 V to 3.6 V during the operating life of the device. If an A3036IL-A8 has forward voltage 2.77 V at current 15 mA, we expect this current when the battery voltage is 3.6 V. At 3.8 V, the current will be 19 mA. To determine optical power, we consult the plots for our particular implantable lamp. At 19 mA, the A3036IL-A8 fiber tip emits 6.9 mW, and at 15 mA it emits 5.5 mW.

Fiber Tapering

Our fiber tapering machine uses two motor controllers, two micrometer stages, a vertical mounting stand, and a heating coil. We mount a 150-mm length of fiber, polished flat at the lower end, to two mounting plates, one attached to each micrometer stage. As we mount the fiber, we pass it through the heating coil. During the tapering process, the heating coil is red-hot and softens the glass within its diameter. The two stages move up, bringing the target taper location into the coil. The lower stage stops while the upper stage continues upwards. The heat-softened fiber stretches into a thin thread above the coil. The lower stage moves downwards to break the fiber and create the taper on the lower section of fiber. The short, lower section is the one we keep. The upper section we discard.


Figure: Tapermaker Tool on MacOS. These settings generate a 1-mm long, uniform taper on the end of a 270-μm diameter TD5 fiber.

We control the tapering machine with the Tapermaker Tool, available in LWDAQ 9.1.8+. The tapering procedure is as follows.

  1. Press Reset to configure the motor controllers and establish the home position. The motors move to their limit switches and then to the home position, which is the starting position for tapering.
  2. Adjust the settings if you need to. The default values work well for 450-μm and 270-μm TD5 fiber.
  3. Install a fiber in the stretcher by passing it through the heating coil and securing it with magnets.
  4. Turn on heating coil.
  5. Press Taper to start tapering process.
  6. When the motors stop, turn off heating coil.
  7. Remove the tapered fiber and the scrap fiber from the stretcher.
  8. Press Home to return to home position, and you are now ready for next taper.

At any time you can press Stop to stop the motors. You will occasionally need to stop the motors when you forget to return to the home position before you press Taper. The stretcher will collide with the heating coil. The Off button turns off power to the motors, so you can rotate the shafts by hand. After you rotate the motors by hand, you must press Reset to re-establish the home position in the controller's memory.

The approach distance is how far the two stages move upwards together to bring the target point into the coil. The approach speed is how fast they move. The approach differential is ratio of the top stage speed to the lower stage speed during the approach. This differential should be slightly greater than one so that imprefections in the synchronous movement of the stages do not cause the fiber to buckle. With the stages separating very slightly, the fiber remains straight. The top stretch distance and speed define the movement of the stage once the target poin is in the heating coil. The bottom stretch delay is how long the lower stage waits before it starts to move down. The bottom stretch distance and speed define its movement downwards. More details of the tapering process are given in the comments of the Tapermaker.tcl file, which you will find in the Tools of the LWDAQ distribution.

Modifications

[18-NOV-19] The A3036AV1 needs the following modifications.

  1. Connect D2-1 to U5-2 with wire link in order to correct a PCB error and complete the 0V net.
  2. Replace D1 SMS7621079LF with SMS7630079LF. The Pin-1 marker on the new diode is the anode, while on the previous diode it was the cathode.
  3. Add 0.5 pF in parallel with C6 to bring matching network resonant frequency close to 910 MHz.

[12-FEB-20] For the A3036BV1 we make the following enhancements to the circuit.

  1. Fix connection to 0V on ground plane at battery.
  2. Switch D1 to SMS7630040LF in P0402 package.
  3. Switch D2 to CDBQC0130L in P0402 package.
  4. Change C6 to 1.5 pF.

Development

Summer-19

[23-AUG-19] Schematic complete. Arrange components on 10-mm × 10 mm circuit board, narrowed at the top to fit into the terminal end of our 19-mAhr LiPo battery. The A3036 crystal radio uses the SMS7630079LF detector diode in a 1.7-mm long SC-79 package. The threshold comparator is the same MCP6541 but in the smaller SC-70-5 package. The stimulus and battery-check switches are two N-channel enhancement mode mosfets provided by a DMG1024UV in a 1.7 mm × 1.0 mm SOT-563 package. The OR gate is is the same SN74AUP1G32, but in the much smaller UDFN-6 1.0 mm × 1.5 mm package, which we use on our A3028GV1 circuit boards. This package fails to load properly in roughly 1% of assemblies and is impractical to replace by hand, but it is small. The logic chip is the same LCMXO2-1200ZE, but in a 2.5 mm square WLCSP-25 package with balls on a 0.4-mm pitch. We are not yet sure how to make a footprint for so fine a ball pitch.

[04-SEP-19] The WLCSP-25 package has twenty-five balls on a 0.4-mm (15.7-mil) pitch. With 10-mil diameter pads the clearance between pads is only 6 mil, which is barely enough to run a 2-mil track, let alone the 5-mil tracks of our usual fabrication process. We consult with Epectec for a solution to the layout problem. They propose that we use 12-mil pads with 10-mil soldermask opening and a 4-mil laser-drilled microvia from the top copper (L1) to the first middle copper layer (L2).


Figure: BGA Footprint on A303601A Printed Circuit Board. Shown are the top copper (L1), top soldermask, and the drill holes.

The second layer of copper (L2) has 10-mil pads to receive the 4-mil microvias, as shown below.


Figure: The Middle Copper (L2) Beneath the BGA. Shown are the middle copper tracks with 10-mil pads to receive the microvias, and the 4-mil microvia holes themselves.

There are no pads for the 4-mil holes on the remaining four copper layers: ground plane (L3), power plane (L4), middle copper (L5) and bottom copper (L6). But our drill file specifies 4-mil holes, and we see these rendered in the plot below.


Figure: The L3-L6 Copper Layers. There are no pads for the 4-mil holes, because the 4-mil holes are indented to pass only between the L1 and L2 layers. These holes are, nevertheless, shown in this plot, because our drill file cannot specify partial drill holes.

We could have routed tracks directly beneath the microvia holes, but our layout software is incapable of understanding drill holes that pass between only two layers. So we kept the bottom four layers clear of copper around the drill holes. In order to generate the gerber files for the bottom four layers, we deleted the 4-mil vias temporarily.

[06-SEP-19] Completed draft version of P3036A01 firmware, including battery voltage measurement by timing how long it takes to charge up C4. When we request a battery measurement, the firmware will assert BT, which closes the battery test switch U3-6 to U3-1. Capacitor C4 starts to charge through R1 and R2. These resistors present a voltage source 80.5% of VB through a resistance of 6.43 kΩ. Capacitor C4 is 1.0 μF. The charging time constant is 6.4 ms. The BTV signal connects to an input on U8, a 3.0-V logic input with logic threshold around 1.5 V. When VB = 3.6 V, voltage BTV will take around 3 ms to reach 1.5 V, or 100 cycles of our on-board 32.768 kHz clock. To the first approximation, time taken is inversely proportional to the battery voltage, so we expect 86 periods when VB = 4.2 V and 106 periods when VB = 3.4 V. Submit A303601A Rev 1 printed circuit board for fabriction on a ten-day turn.

Fall-19

[30-SEP-19] We have the A303601A printed circuit boards, quantity 100, in panels of 10 each. We solder a battery to a blank circuit board, and a charging connector. We succeed on the fourth battery, having figure out how to fold the battery tabs, solder them to the pads on the circuit board, and press the board into place. One of our 0-V connections is missing due to a bug in the way our PCB layout software implements relief connections to the ground plane. We find we can correct this error with a short wire link.


Figure: A 19-mAhr LiPo Battery Loaded On A303601A PCB.

Once the programming extension is clipped off, the battery and circuit will be contained within a 10 mm × 20 mm × 3 mm cuboid. Allowing for 0.5 mm of epoxy and silicone over all surfaces, final volume will be ≤0.92 ml. We are hoping for 0.80 ml. We charge the battery.

[03-OCT-19] We ship components and circuit boards to our assembly house. We have ordered 20 circuits made. We should receive first article in two weeks.

[04-OCT-19] We receive 1254 of C527TR2227 (green ≥10 mW @ 20 mA) and 1777 of C460TR2227 (blue ≥30 mW @ 20 mA). They come on plastic sheets. They are so thin they are transparent.


Figure: Twelve C527TR2227 On Plastic Sheet. Each is 270 μm × 220 μm.

We ship all the green ones to an assembly company we have contracted to load the dies onto the A303602A printed circuit board, which we submit for fabrication on a 25-day turn.

[11-OCT-19] We have our first three assembled A3036AV1 circuits, No1-No3. We connect D2-1 to U5-2 in order to correct a PCB error and complete the 0V net. When we connect power, current consumption is 7 μA. We see a 3.3-V 32.768-kHz square wave on both sides of R7. The A3036AV1 has no resistor between OND and its two destinations U6-2 and P3-3. If we tie P3-3 to P3-4, we connect a LO output of U8 to a Hi voltage. We remove U6 on No1 and use its pads to connect 3VB to 3VA. We set VB to 3.3 V. On our programming cable, we connect ispEN to VCC. When we plug the programmer into P1, the cable drives PEN HI so as to enable the JTAG interface. We program U8 repeatedly from our laptop. Once we program U8, we see a 2.8-V square wave on U7-2 and a 1.9-V square wave on RCK. No matter how we configure TP1 and TP2, we cannot change their state. TP1 remains Hi and TP2 remains Lo. We remove U6 on No2 and repeat the above steps, arriving at the same result.


Figure: Assembly of A3036A. A blue EZ500 LED is faintly illuminated to make the picture. Clear glue covers the base of a fiber light guide. The LED sits in a 3-mm square package, bonded to the interior pads.

[14-OCT-19] We use the Lattice Diamond Programmer V3.9, instead of the older ispVM 18.0, to program No1. Now RCK is a 3.0-V square wave, and we can program the behavior of TP1 and TP2 as we wish. We add output KEEPER on unused pin B1 and set equal to STBY in the firmware so as to stop the compiler eliminating the power control unit. We had deleted this output when we adapted the A3030 firmware for the A3036. We solder a white LED with 100-Ω resistor to the L+ and L− outputs of No3, and a steel antenna to A. The No3 circuit still retains U5. We place the antenna over a Loop Antenna (A3015C) driven with 10 dBm of 910 MHz. With the logic chip erased, XEN is Lo, the RF power causes RP to go Hi, and chip power turns on, after which the erased chip asserts OND, ONL, and XEN, keeping its own power turned on, turning on the lamp, and disconnecting the antenna from the tuner. We connect the programming cable. We remove from our source of 910 MHz. We program the device successfully. Stanby current is 7.4 μA.


Figure: A3036AV1 Prototype Circuit with 3-mm White LED and Steel Antenna.

We look at the RF power signal on D1-1, which we call VR. We have the A3036AV1 over a source of 910 MHz. When VR reaches 10 mV, RP is asserted and both 1V2 and 3VB turn on. With a Command Transmitter (A3029C) we are able to flash the white LED. When the device receives a STOP command, it flashes its lamp. When 1V2 turns on, OND, ONL, and XEN all rise to about 1.5 V for 1 ms. We enable aknowledgements and the No3 circuit transmits an RF signal. The signal is not received by our data receiver. We have not yet calibrated the RF canter frequency or its modulation clock.

During stimulation, the current in No3 when the lamp is off is 220 μA. We solder antennas and lamps to No1 and No2. These are permanently on because of our removal of U6. Their quiescent current consumption is 150 and 250 μA respectively. This is what we call the "active" current of the device, and we expect it to be around 60 μA. Looking at the Lattice Diamond power calculator, this 200 μA quiescent current is consistent with a failure to turn off the band gap references. We try to calibrate the RF center frequency and the transmit clock, but we find that the ring oscillator is not working. It is being eliminated from the design. We believe we can fix these problems by attention to the firmware, so we will proceed with assembly of another 17 circuits.

We add routing priority for the ring oscillator bits, and the fck divisor bits, see P3036A.lpf. Now the compiler preserves all bits and we see the ring and divisor in the device view. We add frequency estimates for the four clocks. We note that we have SDM_PORT set to the default value DISABLE, which means that outputs DONE, INITN, and PROGRAMN are available for general-purpose I/O. We are using one of these, INITN, for our output XEN.

[16-OCT-19] We are able to persuade the compiler to retain our ring oscillator only if we disable the ring oscillator for some fraction of the time. So we turn it on and off with RCK and route it to TP1 directly on No1. The ring oscillator runs 50% of the time. Current consumption is 5.2 mA. The ring oscillator output has period 8.5 ns, frequency 118 MHz. Instead of running the ring oscillator directly to TP1 we run TCK to TP1 and RCK to TP2. We see TCK with period 201 ns. Current consumption 750 μA. Of this, 150 μA is present without the ring oscillator, so the oscillator consumes 600 μA for 50% of the time, the equivalent of 1200 μA for 118 MHz continuously, or 10 μA/MHz, which compares well to the 11 μA/MHz we observed with the A3030 oscillator.

[05-NOV-19] We calibrate the ring oscillator on No2, one of the circuits that is always on. We have not calibrated its RF center frequency. The transmit clock period (TCK period) is 190 ns, which is on the edge of being too low. But we receive acknowledgements from the device. We study the battery monitor, which consists of U3-2, R1, R2, and C4. When we assert BT, U3-2 turns on, charging C4 (1 μF) through R1 (8 kΩ), with R2 (33 kΩ) present to limit the voltage we attain at BTV, and to discharge C4 between measurements. Eventually BTV reaches the threshold of the Schmitt trigger input on U8-D5, which we have set to have "small" hysteresis. We measure the charging time in RCK periods, which are 30.5 μs. The charging time constant is 6.4 ms, or 200 periods. The A3036A returns the charge time in the data of an auxilliary message on channel 15. We vary VB and transmit battery commands to No2 and plot the charge time versus voltage.


Figure: Capacitor Charge Time versus Battery Voltage. The charge time is the number of 32.768 kHz periods it takes to charge C2 to the logic threshold of roughly 1.5 V. The device does not transmit for VB ≤ 2.5 V.

We prepare ISL Controller Tool 6.1, which supports the A3036A battery measurement. We are able to measure the battery voltage with better than ±0.5 V precision around the critical region of 3.6-3.8 V.

[08-NOV-19] We take No2 and remove the wire between 3VA and 3VB. We see 3VB at 0.2 V and 1V2 at 0.0 V. Current consumption from VB is 8.7 μA. We connect 3VB to VB with 1 kΩ using the pins of P1 and P3. We apply 3.3 V to VB. We see 2.3 mV across the 1 kΩ, implying that U8's I/O circuits, U9's shutdown current, and U2's enable input current are together 2.3 μA. We find we can program the board with the 1 kΩ VB to 3VB series resistor in place. When we turn on the lamp, TP2 is asserted and we see 360 mV across our 1 kΩ resistor, which is the current flowing through the 1 kΩ and R13 (8 kΩ). With the lamp off, the total current consumption from VB is 213 μA. The Lattice Diamond power calculators says the core current consumption of U8 when the band-gaps are turned off is 60 μA, which was what our A3030 circuits consumed with almost identical firmware.

We install the Lattice Diamond 3.11.0. We were previously using 2.2.0. In both cases, we select the LSE synthesis tool. We re-create the P3036 firmware for LD3.11 and compile. Total current consumption 220 μA, command reception and acknowledgement working. With fck_divisor = 11 we get TCK period 188 ns, implying ring oscillator 117 MHz. With fck_divisor = 12 we get 220 ns, implying ring frequency 109 MHz. We have some work to do stabilizing the ring oscillator with respect to fck_divisor.

We are using Lattice Diamond Programmer 3.9.0. We discover the TCK Divisor setting. Programming is unreliable with the divisor set to 1. With the divisor set to 2, scanning and programming works every time, but takes twice as long: 123 seconds. Current consumption of No2 remains 224 μA. In order to free up U8 pins A4, B4, A5, and C5 for general-purpose I/O, we have disabled the JTACK interface by default. We now enable the JTAG interface and program again. We remove R1 because BT (TDO) is HI. Current consumption 218 μA.

We turn on the RF oscillator continuously, at the LO frequency. With frequency_low = 7, the center frequency is 909.5 MHz, and with frequency_low = 8, center is at 915.5 MHz. We set frequency_step = 2 and frequency_low = 7 and obtain reception of acknowledgements.

We route the BTV logic signal to TP1. This signal is the voltage BTV at U3-1 passed through U8-D5's Schmitt trigger input. Let's call it BTVL. We look at BT, BTV, and BTVL during a battery test for No1. Battery voltage is 3.58 V. Acknowledgments are turned off, so the only response of the device is the battery message.


Figure:BTV (yellow, 500 mV/div), BT (blue, 2 V/div), and BTVL (red, 2 V/div) During Battery Test, No1. Charge time is 4.68 ms according to the cursors.

We are not receiving acknowledgements from No1. The TCK period is correct, but we have not checked the RF center frequency. We don't have its report of the charge time in RCK periods. But we expect from our earlier calibration that this will be around 120 periods, or 3.66 ms, which is not consistent with the above measurement. The above measurement was performed with this same circuit, but acknowledgments were enabled. We repeat for No2.


Figure:BTV (yellow, 500 mV/div), BT (blue, 2 V/div), and BTVL (red, 2 V/div) During Battery Test, No2. Charge time is 5.16 ms according to the cursors.

We do receive acknowledgements from No2, and it says 172 every time, or 5.25 ms, which is consistent with the the above observation.

[20-NOV-19] We check the response of the A3036AV1 to a −5 dBm sweep applied to an A3015C antenna, duplicating our earlier measurement of the A3030E antenna network. We see no response on VR in the A3036AV1. The A3036AV1 is loaded with D1 = SMS7621079LF, which turns out to be the wrong member of the family. The SMS7621 is designed to measure power from a 50-Ω source, and requires forward biase 300 mV for a current of 1 mA. The diode we are replacing is the now-obsolete HSMS285C, which requires only 150 mV for 100 μA and 250 mV for 1 mA forward current. Its video resistance is 8 kΩ at zero biase. Instead of the SMS7621, we should have used the SMS7630, a zero-biase detector diode with video resistance 5 kΩ, requiring 100 mV for 100 μA and 200 mV for 1 mA forward current. We order these. In the meantime, we supply 10 dBm of RF power to an A3015C loop antenna and place an A3036AV1 on top. Matching network resonance is at 1000 MHz. We add 0.5 pF to C6 and resonance drops to 920 MHz.

[25-NOV-19] We apply +10 dB to an A3015C. We place an un-tuned A3030E circuit board on the antenna and measure VR as we increase frequency from 900-1000 GHz. We see a maximum of 60 mV at 970 MHz. We repeat with A3036AV1 with SMS7621 diode and C6 = 1.5 pF. We see a maximum of 60 mV at 912 MHz. We apply −5 dBm sweep at 10 kHz and see 20 mV peak on the A3030E at around 975 MHz and <2 mV on A3036AV1 at around 915 MHz. Replace D1 with SMS7630, taking care to place the pin 1 marker on the other side of the footprint. We see 20 mV on A3036AV1 at around 915 MHz. The video resistance of the SMS7621 is so high that the diode cannot charge C7 = 100 pF fast enough to respond to the sweep with a 20-mV peak. That of the SMS7630 is 5 kΩ, giving a time constant of 0.5 μs with C7.

[26-NOV-19] We vary the value of the ring oscillator divisor and measure the transmit clock period. We want to see the period increasing with divisor, and we do from 7 to 14, but not at 15. We find that our ring oscillator is being optimized for speed, but not the fast clock divider. When we turn on optimization for the fast clock divider, the TCK period is unstable, so we are leaving the divider without optimization for now.


Figure: TCK Period versus FCK Divisor in P3036.

We connect a Modulating Transmitter (A3014MT) to a Loop Antenna (A3015C). We modulate the RF with a square wave so that it moves in and out of the center frequency of our A3036A with SMS7630 diode. Rise and fall times are around 1 μs.


Figure:Rise and Fall Time of VR with D1 = SMS7630. For the same response of the SMS7621 diode see here.

We select a circuit with D1 = SMS7621 and add 0.5 pF to C6. We repeat the above experiment with this new circuit and our rise and fall times are of order 4 μs. We sweep the A3014MT output frequency with a ramp. We use our SSG-6001 synthesizer and a ZAD-11 mixer to calibrate the sweep on our oscilloscope screen. We connect the sweep to an A3015C that is flat on our bench. We move the A3036A around on the loop until we maximize the peak in VR.


Figure:Response of VR to 10-kHz Sweep 840-940 MHz when D1 = SMS7630. For the same response of the SMS7621 diode see here. The left curor is 890 MHz, the center is 910 MHz, and the right cursor is 930 MHz.

With the SMS7630 diode, we obtain a peak of 40 mV. But with the SMS7621 diode the peak is no more than 10 mV. We add 0.5 pF to C6 and switch D1 to SMS7630 on all remaining A3036 circuits. Peak of sweep response for all ten circuits in the range 903-913 MHz.

We switch the BTV signal input hysterisis from SMALL to LARGE. We set battery_calib in the P3036A03 firmware to 58 and now the battery counter is 200 for VB = 3.70 V. We do the same for No3. In order to calibrate No3 we send command 0081 to the Command Transmitter (A3029C) to turn on RF continuously, which allows No3 to remain on during programming. We have battery calibration 61, which results in battery counter 199 at 3.70 V, 184 at 4.00 V, and 218 at 3.40 V. We use −60 for the scaling factor in V/cnt and our reference will be a count of 200 for voltage 3.7 V.

[27-NOV-19] Inactive quiescent current of No3 and No4 is 7.3 and and 7.3 μA. We program and calibrate No4. We now have 3 A3036A calibrated for battery monitoring, their battery calibration values are 56-61. We attempt to program and calibrate more circuits, but have difficulty programming, and once programmed, they do not respond to commands, not even with an increase in current consumption.

[28-NOV-19] We devise an easier way to connect U5-3 to 0V, by connecting D2-1 to U9-3. This leaves D2-2 free for a wire to look at VR. We start with No6, and succeed in programming and calibrating. We modify No7 and program, but U7 is not producing RCK. We modify No5 and program, but VR is oscillating, spending 90% of the time at 10 mV and rising for 10% of the time to 100 mV, every 70-100 μs, the period varying as we move our finger around the circuit. This behavior is consistent with a matching oscillation on XEN, whereby U4 is connecting the antenna to the tuner only 10% of the time. WEe put a probe on XEN, but the oscillations stop and XEN stays HI, turning on U9, and disconnecting the tuner. Current consumption is 12 mA.

[02-DEC-19] Of 10 A3036AV1 circuits we have corrected with modifications, six are calibrated and ready for battery and encapsulation.


Figure: Calibration Constants and Current Consumption of A3036A. Circuit A3036AV1 with modifications. Active current is with stimulus with rare pulses, Xmit current is while transmitting synchronizing signal, no stimulus.

The active current consumption remains roughly 70 μA higher than the 60 μA suggested by the Lattice Diamond power calculator for this design. The same functionality loaded into the A3030E circuit yields active current consumption of around 70 μA.

[03-DEC-19] We have five A3036A programmed with firmware P3036A03 ready for encapsulation, with batteries loaded, 45-mm lamp leads, and 30-mm stranded antennas.


Figure: Batch of A3036A Ready for Encapsulation.

The leads are our 0.7-mm diameter silicone insulated leads, red for L+ and blue for L−. On one device, we measure lead resistance 27 Ω and 29 Ω. We expect 6.0 Ω/cm = 54 Ω. Assuming battery voltage 3.7 V, LED forward voltage drop 2.9 V and lead resistance 54 Ω, we expect LED current (3.7 − 2.9)/50 = 16 mA.

[04-DEC-19] We have 14 of 460-nm EZ500 LEDs mounted to A303602B printed circuit boards. We run 30 mA through each and measure optical power output. We get 25.3±1 mW. This compares well with the average 26.9 mW we obtained with LEDs from the same batch earlier.

[06-DEC-19] We have five encapsulated A3036A. Devices A215.3, A215.4, A215.8, and A215.10 respond to commands, transmit the synchronizing signal, and deliver power to LEDs. Device A215.9 does not respond to commands. We place the four working devices with lamps in our Faraday enclosure with data antenna 15 cm to one side and command antenna 15 cm to the other. We connect boost power to the command transmitter. Reception of commands is unreliable. We place the four devices in a beaker of water. Their bodies rest upon the glass bottom. Reception is now reliable. We turn on the lamps 90% of the time with a continuous stimulus. It takes ten to twenty minutes for the batteries to drop to 3.5 V according to the ISL Controller's battery voltage measurements. We connect 5.0 V to the lamp pins of A215.10 and see 9 mA flowing in.


Figure: Current versus Voltage for Charging Diode. We connect 200 Ω from VB to V0 and apply a voltage across VB and L−. We measure current flowing into VB and we measure the voltage between 0V and L−. The former is the diode current, the latter is the diode forward voltage drop.

The re-charge diode is the diode in parallel with the drain of U3-3, see schematic. The recharge current enters through L+ end exits through L− In order to exit through L−, it travels along the 0V net through the re-charge diode. The current through the battery is the total incoming current minus the circuit's inactive quiescent current, which is around 7.3 μA. We want to charge the battery to 4.2 V and then reduce the current slowly to zero while maintaining a battery voltage of 4.2 V. 5.0 V connected to the lamp leads, the voltage across the battery will be 4.2 V when the current flowing through the diode and the 54-Ω lamp leads is 4.0 V. When the current drops to 1 mA, however, the battery voltage will be 4.5 V, which could damage the battery. If we stop the re-charge when the current is 3 mA, we stop it when the battery voltage reaches 4.3 V, which may be okay. But if we set the charging voltage to 4.7 V, the battery voltage will be 4.0 V at 3 mA, 4.2 V at 1 mA, and 4.3 V at 0.1 mA.

We have been charging A215.10 for twenty minutes with 5.0 V and the charge current has dropped to 7.5 mA. We drop the charge voltage to 4.7 V and the current drops to 3.4 mA. We connect all four ISTs to the same 4.7 V source and leave to charge.

[09-DEC-19] We have two A3036IL-A8 prepared. No1 we moved the fiber before the epoxy was cured. No2 we allowed to cure without interference. Today we measure their output power with a photodiode and obtain 2.9 mW for No1 and 4.0 mW for No2. Average LED output at 30 mA is 25 mW, so let us assume 12.5 mW at 15 mA, giving us 23% and 32% coupling efficiency. In earlier work our best head fixture provided 41%, our worst provided 22%, and our median was 38%. Low coupling efficiency was a strong function of the quality of the taper base. These two tapers were both chipped at the base, but only in the cladding, so we trusted that they would work adequately.

[10-DEC-19] We push three of A3036A into a 10-ml graduated tube containing 5 ml of water. The water level goes up to 7.7 ml, making the individual A3036A volume 0.9 ml. We are testing lamps and batteries, and we notice that the lamp flashes when we perform a battery measurement, on both No8 and No10. We have not checked No3 and No4.

[11-DEC-19] We have A3036IL-A8 No3, having snapped the fiber on No2, leaving us with only No1 24% efficient. With 15 mA current into No3 we see photocurrent 0.33 mA, or 4.1 mW for 33% efficiency. We glue the fiber onto No4 and leave to cure. We use our prototype A3033A to charge A3036A No3, No4, No8, and No10 until charge current is less than 1 mA.


Figure: Green TR2227 Mounted on Circuit Board. From the Palomar Technologies.

[12-DEC-19] A3036IL-A8 No4 emits 3.7 mW at 15 mA for 30% efficiency. And then the fiber fell out of the glue joint. We throw away the glue cartridge, which is three or four years old, even though our glue samles have all set well. We take out a new cartridge that is less than a year old. We set up a new LED No5 and measure power output before applying fiber: 11 mW for 15 mA. We glue a new fiber.

[13-DEC-19] A3036IL-A8 emits 4.0 mW at 15 mA for 36% efficiency. We place an IST A3036A attached to an A3036IL-A in 200 ml of water and move the beaker around on an ALT platform, which is 32 cm by 16 cm, all parts, especially the corners. We try the IST lying flat on the bottom of the beaker, suspended against the side wall, and suspended in the center. We open and close the enclosure door. We send stimulus commands and observe by eye if the IST received the command. Of 100 stimuli, the IST received 96%. The lost 4% were all in one far corner of the platform with the door closed.

We have the IST Prototype system ready to ship: 3 of IST A3036A (A215.3, A215.4, A215.8), 8 of ILED A3036IL-A, 2 of FC-LED A3036IL-A8, 1 of Command Transmitter A3029C, 1 BNC elbow, 1 Boost Power Supply for A3029C, 4 BNC feedthroughs, 4 short antenna cables, 1 of A3015C with cable, and 1 Battery Charger A3033A. We ship DHL to London.

We have A3036A A215.10. When inactive, battery measurement is 4.2 V. When active, 4.0. We start a 10-Hz, stimulus of 30-ms pulses. After 9 minutes, battery reports 3.9 V, after 17 minutes, battery reports 3.8 V. We stop stimulus, battery reports 4.1 V.

[16-DEC-19] Continue 30% stimulus with A215.10. We note that frequency of stimulation, as seen in Neuroarchiver spectrum of synchronizing signal, is 10.25 Hz, not the 10 Hz wer requested. After a total of 58 minutes since start of drain, battery reports 3.7 V. We stop stimulus, battery reports 3.8 V.

[17-DEC-19] Continue 30% stimulus with of A215.10. Before start, battery reports 4.0 V, after start, reports 3.7 V, but this rises to 3.8 V after a few minutes. After a total of 76 minutes since start of drain, 3.7 V. After a total of 126 minutes, 3.6 V. We stop, battery reports 3.8 V. Later, we continue for another 33 minutes until the battery voltage has dropped to 3.4 V, and then stop. Total time is now 159 minutes. We connect stimulus leads to 4.7 V.

[18-DEC-19] Disconnect A215.10 from 4.7 V charging voltage. It now reports 4.3 V battery when inactive, 4.1 V when transmitting its synchronizing signal, and 4.1 V when generating 30% stimulus. After 23 minutes, reporting 3.9 V. We now switch to a 1% stimulus: 1 ms pulses at 10 Hz and leave running. After 490 minuts of 1%, reporting 4.0 V.

[19-DEC-19] At 9:21 am lamp still flashing at 1%, battery reports 3.8 V. So far 22 min at 30% and 11 hrs at 1 %. Change to 5%, battery reports 3.8 V.

[20-DEC-19] At 9:13 am lamp not flashing. Cannot get a battery report or acknowledgement from the device. Connect to 4.7 V charger at 9:18 am, charge current 0.01 A. At 9:37 am charge current is 5.6 mA. Disconnect and obtain battery report 3.7 V. Reconnect to charger, 5.6 mA. At 10:33 am charge current is 4.6 mA. At 11:20 am battery reports 4.0 V and charge current is 4.0 mA. At this current, charging diode drops 0.55 V. Assuming leads are 56 Ω, we expect VB = 3.93 V. At 15:27 charge current 0.7 mA, reports 4.2 V. Start 50% stimulus, battery reports 4.0 V.

Winter-20

[23-DEC-19] Friday and today we get 129 minutes of 50% illumination from A215.10 at which point battery reports 3.4 V. We continue with 10% for 17 minute until battery reports 2.9 V. The lamp still flashes, but we perceive it to be dimmer. Connect to 4.7 V charger and see 11 mA. After 18 minutes 6.0 mA, 27 minutes 5.7 mA, 120 minutes 4.0 mA, 300 minutes 1.1 mA. Remove and connect LED. Battery report 4.2 V. Start 20% stimulation at 15:40.

[24-DEC-19] Our 20% stimulation ended some time before 10:30 am. We charge battery for half an hour, then run 250 Hz 1-ms pulses, after half an hour, battery reports 3.5 V. We attach to A3033A charger at 11:45 am. We have ten more A3036A circuits modified, cleaned, and dried. We are numbering these No4-No13 using the labels on circuit boards. Inactive current 7.47plusmn;0.1 μA for all but No9, which consumes 12 mA and we reject.

[26-DEC-19] We use the Tapermaker Tool to make tapered optical fibers using our new 270-μm diameter high-index glass fiber, with numerical aperture 0.86 and diameter tolerance ±30 μm. We have not bothered polishing the bases of the fibers. We are working on getting a symmertic, uniform taper at the tip.


Figure: Tapered 270-μm Diameter Fibers. The longest is 9.6 mm from tip to base, and the shortest is 6.5 mm.

We are confident we can make 6-mm long fibers with 1-mm tapers at the tip. We can control the length of the taper with the configuration of the tapering process.

[27-DEC-19] We have A215.10 recharged over-night, reporting 4.3 V when inactive. We start 10 ms pulses at 1 Hz for 1% stimulus at 12:30.

[30-DEC-19] Device A215.10 reports 3.5 V at 12:21. The active current consumption of A210.10 is 109 μA, combined with 1% of an average of 15 mA lamp current is 259 μA. So far the battery has provided 18.6 mA-hr.

[31-DEC-19] We enhance our ring oscillator implementation, separating it into its own VHDL file P3036A05_OSC. In P3036A05_Main we specify fck_calib to configure the ring oscillator. The ring oscillator output is FCK, which we route to TP1. We no longer route TCK to TP1 because we encounter timing and routing issues when we do so.


Figure: FCK Period versus FCK Calib. "A05" is for A215.19 with P3036A05 firmware.

We calibrate A215.17-19. We have enhanched the ISL Controller to accommodate higher channel numbers. For fck_calib = 17, we are making a ring oscillator containing nine gates, and dividing its frequency by six. The result is an FCK period of 99 ns. The single-gate ring oscillator would run at 545 MHz. A single gate delay is around 1 ns.

We try various alterations to the power controller firmware in the hope that we can reduce the standby current consumption to 60 μA, but our standby current consumption for 17-19 is 154, 143, and 116 μA with only slight variations regardless of firmware.

[03-JAN-19] We complete calibration of nine more A3036A taken from our second set of ten. There were no hardware issues with these devices except for 12 mA inactive current in one circuit. Calibration constants and current consumption shown below.


Figure: Calibration Constants and Current Consumption of A3036A Batch 2. Circuit A3036AV1 with modifications. Active current is with stimulus with rare pulses, Xmit current is while transmitting synchronizing signal, no stimulus. Table updated as we damage and repair circuits.

The average difference between the Xmit and Active current ius 93 μA with standard deviation 4 μA. The slope of current versus sample rate, dI/dS, is 0.091 μA/SPS, which is less than the 0.11 μA/SPS we observe in the A3028GV1 circuits. Our concern remains the average active current, which is 160 μA with standard deviation 30 μA when we expect 70 μA with standard deviation 3 μA.

[06-JAN-20] Take A215.25, place on RF power. Current 179 μA. Connect P1-4 to P1-7, making sure PEN is unasserted. Current 180 μA. VB = 3.7 V, 3VA = 3.0 V, 1V2 = 1.2 V. Immerse entire circuit in acetone. Place in vacuum chamber, evacuate, allow to boil for ten seconds, remove, blow dry. Current 169 μA. Start stimulus with a flash every three seconds. Cool U8 with freezer spray. Current drops to 25 μA and lamp is still flashing. We cool again, then attach a thermocouple and record current with temperature as the device warms up.


Figure: Active Current versus Temperature. We compare the A3036A equipped with LCMXO2-1200 in WLCSP-25 and A3030E equipped with same device in TQFP-100.

The Lattice Diamond Power Calculator tells us that the typical current consumption of the chip at 25°C is 67 μA, while the worste case is 238 μA. At 37°C the typical is 96 μA and the worste case is 353 μA. All our chips are within this range of typical to worste case.

[07-JAN-20] We shatter the corner of U8 on A215.24. We heat U8 from above with iron at 400°C. After twenty seconds, it comes loose. We load another WLCSP-25 package by hand, heating from above with iron at 400°C. After ten seconds, the package attaches. We heat for another ten seconds. We wash and dry and the new U8 programs first time, and operates with the same calibration constants except we must drop battery_calib from 72 to 57. Active current consumption 150 μA.

The U8 on A215.24 may have been damaged by heat when we loaded it at 400°C. But it was not damaged by handling. We held it with tweezers. We measure the variation in current consumption with temperature and add to the above plot. The U8 on A215.25 may have been damaged by machine placement. But it was not damage by temperature. The reflow temperature profile is here, and reaches only 247°C for four seconds.

Summary of failures for B88120, 20 of A3036AV1, 4 faulty circuits. 2 with short or open circuit under U6. 1 with U7 not producing RCK. 1 with open circuit under U8. We replace U8 on this last one and produce fully-functional A215.26. We measure active current versus temperature for an A3030E, E157.1, and plot.

[09-JAN-20] We have 5 of A3036IL-C blue TR2227 and 5 of A3036IL-D green TR2227 back from Palomar Technologies. We connect 5 V through 200 Ω expecting 10 mA to flow into each LED and see of order 15 mW of light from blue LEDs and 6 mW from green LEDs. Some emit no light, some emit a flickering dim light, some emit a steady dim light. We pick a blue one and measure current and power output versus applied voltage. At each measurement, we connect power, make our measurements quickly, and disconnect power so as to allow the LED to cool down.


Figure: Current (mA) and Optical Power (mW) versus Forward Voltage (V) for Prototype Blue TR2227 LED of A3036IL-C.

The plot below is taken from the TR2227 data sheet. The optical power output is, to the first approximation, proportional to the current. Our plot shows the optical power increasing with voltage as suggested by the data sheet. But the forward current does not agree with the data sheet. At 3.5 V, for example, the forward current in our prototype is 120 mA, while the data sheet suggests 30 mA. The data sheet suggests 45 mW of blue light for forward voltage 3.5 V, but we see only 12 mW.


Figure: Current (mA) versus Forward Voltage (V) for Blue TR2227 LED of A3036IL-C, Manufacturer's Data Sheet.

We try cleaning one board with hot water and a brush, but the LED comes off. We knock another off by accident. When we apply 7.0 V through 200 Ω, all eight remaining LEDs emit light, some dim, some bright. Forward current is 75 mA for the six bright ones and 90 mA for the two dim ones. We measure the resistance between the input terminals. One of the bright green LEDs has resistance 2 kΩ in both directions. One of the bright blue LEDs has resistance 300 kΩ in both directions. Among the rest, the resistance varies 10-100 kΩ, the same in both directions, with no correlation between brightness and resistance.

[10-JAN-20] We equip A215.26 with two 120-mm long leads, OD 0.8 mm, 6-mil wire, MDC26398 spring. Lead resistance 19.6 amnd 19.8 Ω. We connect to an A3036IL-A and power from benchtop supply through ammeter. We measure VB at the circuit board. We turn lamp on to full power continuously. We measure total current versus VB.


Figure: Total A3036A Current with Lamp at Full Power versus Battery Voltage. Each plot gives lead length, lead wire diameter, and LED version.

The inverse slope of the graph is 44.5 Ω. We repeat with A215.25 with two 50-mm long leads, OD 0.7 mm, 4-mil wire, MDC13867A. Lead resistance 28.2 and 27.0 Ω. The inverse slope of the graph is 63.9 Ω.

[14-JAN-20] We hear from Lattice Semiconductor in response to our question about combining their Mico8 microprocessor with our existing VHDL code in the LCXO2-1200. "You just need to have a top design connecting to the MICO8. Just add the .msb generated by LMS to your Diamond Design. Just follow the Development Guide of LMS (MICO8/32) and check if how you can connect the .msb file to your Diamond Design."

[21-JAN-20] We started an A3030E and A3036A flashing their lamps at 1 Hz with 10 ms pulses yesterday at 11 am. Today, the A3036A flash is occurring 100 ms before the A3030E flash, suggesting that the difference in their clocks is around 1 ppm.

We have No25 at room temperature. We connect 3.7 V and activate with continuous RF power on antenna beneath the circuit. Current consumption is 177 μA. Re-program with same code again, freshly compiled. Immediately after programming see 198 μA, falling to 178 μA. We enable the JTAG interface and disable mux_configuration_ports, recompile and re-program. Standby current now 375 μA includes current caused on by TDO = BT being HI, and BTV being neither HI nor LO. Restore code, 185 μA. Try with No1, 175 μA before. Disable JTAG and see 400 μA. Remove R1, R2, and R3 and R13. Now current is 151 μA. Disable JTAG again and get 158 μA after a couple of minutes to cool down.

[24-JAN-20] We create standby-only firmware STDBY_WLCSP25 for the A3036A and STDBY_TQFP100 for the A3030E so as to make it easier to measure the standby current consumption of LCMXO2 chips. The VHDL source code is the same for both, but the pin numbers assigned to RCK and STDBY are different, to suit the WLCSP-25 and TQFP-100 packages, and we generate a larger number of unused inputs in the for the TQFP-100. We enable JTAG port and program A3030E E157.1, get standby current 65.3 μA. With E157.2, 65.7 μA. We connect U10-82, which is JTAGEN, to P1-4 and disable JTAG. We program E157.2 and get 65.6 μA. Reprogram with JTAG enabled again, get 65.8 μA. We program A3036A No1 with STDBY code with JTAG disabled and get 154 μA.

We have another 40 of LCMXO2-1200ZE in WLCSP-25 from another distributor, date code 1810. We load one onto our A3036A No1. We program with our STDBY code, current is 138 μA.

[28-JAN-20] We removing parts from A3036A No1 in order to isolate the component that is consuming an extra 70 μA. We remove the lamp switch, battery test switch, and DAC resistors. We have already removed U6, and we connect 3VB to VB. We still see 140 μA total consumption. We remove U8. Consumption is 70 μA. So U8, in standby mode, was consuming only 70 μA, which is correct. We connect 3VB to 0V. We remove U2 and C3. No change. We remove U4, current drops to 6 μA. Consulting the PE4239 data sheet, we see that its quiescent current is 250 nA so long as CTRL1 is HI.


Figure: Table from PE4239 Data Sheet. Current consumption is low provided U4-4, the CTRL input, is HI.

When the A3036A is inactive, quiescent current is 7.5 μA, and CTRL of U4 is LO. So the current consumption of U4 with CTRL LO must be less than a micramp also. When CTRL is floating, however, the current consumption is unspecified. We take No26, activate with RF power, heat U4 with soldering iron. Quiescent current rises from 110 μA to 200 μA in three seconds. We do the same wth U8 and see an even more rapid increase. We remove U4 and U9 and the 110 μA current persists. We program with our STDBY code. Current persists. We remove U9, current drops to 9 μA. Load a new U9 and program with STDBY and see 140 μA.

[29-JAN-20] Rebuild No26 with new U4 and U9, re-program, re-attache thick-wire leads. Works fine. Active current 120 μA.

[30-JAN-20] We receive 3 green and 5 blue TR2227 mounted to printed circuit board, having passed 5 V through 1 kΩ 2-mA current and bright light test at Palomar Technologies. All eight emit bright light when we run current through them. We measure upward optical power with an SD445 photodiode. We observe 6.2 mW at 10 mA from one blue LED, and 0.67 mW from another at 1.0 mA. This compares well to 7.0 mW at 10 mA we observed with the blue EZ500. We damage two blue LEDs by connecting 5 V to them without a resistor. Another two suffer damage when we leave them with 10 mA running through them for thirty seconds. We damage a green LED with 10 mA as well. The remaining two green LEDs emit 0.44 mW at 1.0 mA, compared to 6.0 mW at 10 mA for the green EZ500.

[31-JAN-20] We left a green LED with 1 mA for eight hours and it had failed by the time we returned. We separate failures into two types. One type is open-circuit, where the LED appears to have broken from its epoxy bonds. This occurred in two LEDs that we over-heated with a direct application of 5 V with no resistor. The other failure is similar to the one for which we plotted current and light output versus voltage above. The LEDs are producing close to the nominal output power versus forward voltage. This suggests another component in parallel with the LED. But this component is not a resistor, because its current is 2 mA at 1 V, 40 mA at 2 V, and 80 mA at 28 V.


Figure: Damaged Green TR2227 Mounted on A3036C Circuit, Closeups. The LED is 270 μm long. We see a signs of a dark film on the bottom side of the LED in left and right images. Note that the 3-mil gab between the gold-plated pads is almost too wide for the pads on the LED.

We place a working LED in water to see if it will remain cool while illuminated. The LED durns off and acts as if damaged. We remove and blow dry, but the LED is now acting like the rest of the damaged LEDs. We examine another damaged LED with a 20× loop. We see a dark stain on the underside of the LED. We take the above photographs with our microscope, in which the stain is not clear, but suggested. We now suspect that metal migration is creating a parallel diode junction in the gap under the diode. This will happen after 8 hr at 1 mA in air, 30 s at 20 mA current in air, and 1 s at 10 mA in water.

We cover a fully-functional green LED with epoxy, place in our vaccuum chamber to remove bubbles, and drive epoxy beneat the LED. Later in the day, the epoxy is not yet cured, but it's tacky. We run 10 mA through the LED and see 1.4 mW of green light with 2.98 V forward voltage. We increase to 20 mA. Light is flickering. Current is changing too quickly for us to make self-consistent measurement of power, voltage, and current. After a few minutes we return to 10 mA and see 0.88 mW. The current is 0.3 mA at 1 V, 2 mA at 2 V, and 7.5 mA at 2.8 V. At this final current, power output is 0.64 mW, ten times less than the data sheet suggests.

[04-FEB-20] We have ordered the A303602C printed circuit board for TR2227 with 10-mil long mounting pads separated by 3 mil. We take one of our existing A3036IL that draws around 100 mA for a forward voltage of 3.0 V, producing roughly the correct amount of green light, and we push the TR2227 off its pads and over-turn it. We obtain this view of the epoxy and underside of the LED chip.


Figure: TR2227 Removed and Overturned Near Epoxy Mounting.

Now that we have removed the LED, the current is 0 mA for forward voltage 3.0 V. We have eight remaining A3036IL-C/D circuits that produce light. With forward voltage 3.0 V they shine brightly, but the current they consume varies greatly, from 70 mA to 150 mA.

We take A3036A No22, active current 213 μ and transmit current 300 μA at time of calibration. Active current 208 μA today. We replace U8 and re-program. Active current 143 μA, transmit current 227 μA. We apply 7.5 V as the battery voltage for the A3036A through 200 Ω. No harm comes to the circuit. We load batteries onto six A3036A with the help of water-soluble flux. We check immediately after soldering, and all six boards work fine. We triple wash in hot water after and three of them have ONL asserted, which we observe on P1-3. We check PEN on P1-4 and find it LO, but as soon as we connect a probe to PEN, ONL is unasserted. We re-program these three boards using their original calibration constants and they all work fine after. We assemble two more, the first works after washing, the second we re-program. We now have eight working boards with batteries loaded. To load the battery, we bend the battery tab back upon themselves twice, so as to bring to solderable surfaces face-up. One of the tabs, the one with the spot-welded extension, we can solder only on one side of the extension. We place a lump of solder on each tab end, add lumps to the circuit board battery pads, add flux, and lower the pads onto the tabs. We melt solder from the side of one joint then the other, alternating as we push the circuit baord down onto the tabs. The resulting circuit plus battery is 19 mm long.

[05-FEB-20] We have eight A3036A with batteries loaded yesterday A215.18-25. We measure battery voltage with a voltmeter on the programming extension and get 3.8=7-3.95 V. We turn on and off data transmission. We check the battery voltage measurement and get 3.9, 4.1, 3.9, 4.0, 3.7, 3.9, 4.0, 3.9 V respectively. The lowest reading must be a poor calibration. The existing value of batter_calib we used when re-programming this device after washing is 62, but we find that 52 gives an accurate battery measurement. Now A215.22 says 3.9V and our multimeter agrees.

[11-FEB-20] We have eight encapsulated A3036A circuits, numbers 18-25. All respond to commands, flash their lamps, transmit synchronizing signal, and report battery voltage2 3.8-3.9 V when inactive. We have 13 of A3036IL-A. We measure optical power for 10 mA current and see 4.4-6.0 mW, with the exception of one with an air bubble over the LED, which produces 4.2 mW. The average power of 5 mW is roughly half what we expect from our earlier measurements of the same LED with no epoxy cap. We have two EZ500 without epoxy cover, they emit 9.8 and 9.5 mW for 10 mA. We add one more drop of epoxy to all thirteen caps, so that the caps are domed. We now measure 6.7-7.8 mW at 10 mA.

[14-FEB-20] We receive 5 blue and 5 green mounted TR2227 LEDs from Palomar. Of these, all turn on brightly with 1 mA forward current for one second, except for one blue, which turns on brightly with 70 mA and then flickers and fails. We cover the nine working LEDs each with a drop of clear DP270 epoxy and place in our vacuum chamber at −25 psi for five minutes, then drop pressure over five minutes. We leave to cure in their gel pack.

[15-FEB-20] At 4 pm yesterday we left No10, 24, and 25 flashing an A3036IL for 100 ms at 1 Hz. Today No10 has stopped. We connect to charger. No24 and No25 flashing 100 ms apart. We measure this separation by looking at their synchronizing signals. Battery reports are 7.0 V and 3.4 V. We stop stimulus and battery reports are 2.9 V and 3.7 V. After half an hour of charging, No10 reports 3.9 V.

We number our nine new epoxy-encapsulated TR2227 LEDs. No1 is green. We run 2.0 mA through it and observe 0.76 mA light output. After ten minutes, forward voltage is 2.80 V for 2.03 mA and power output is 0.72 mW upwards. We flip the circuit board over and measure 0.16 mW coming through the printed circuit board. So the total power output is 0.88 mW. While we are handling the circuit board, the LED flickers and goes dim. We take out No6, which is blue. At 2.0 mA its forward voltage is 2.8 V and power upwards is 1.0 mW. After one minute, it flickers and goes dim. We take out No2, green, and run 10 mA through the LED. It shines brightly for ten seconds, then goes dim. We take No7, blue, and run 0.5 mA through it, seeing 2.5 mW. But it starts to flicker after twenty seconds.

[17-FEB-20] We consider how best to mount the TR2227. Right now, we are mounting the chip by its two bond pads with silver epoxy to a gold-plated pad, as shown below.


Figure: Drawing of Our Silver-Epoxy TR2227 Mounting Scheme.

We consult the TR2227 manual, which contains the following drawings of the device package. We assume these sketches are to scale.


Figure: TR2227 Package Drawings from Data Sheet.

According to the data sheet, we are mounting the die with the top side down. The data sheet states in its opening paragraph, "The design is optimally suited for industry standard sideview packages as it is die attachable with clear epoxy and has two top contacts, consistent with industry standard packaging." We see that the LED junction comes within 10 μm of the cathode pad. We sustpect that our silver epoxy is spreading from the cathode pad to the junction and creating another current path in parallel with the LED. We propose to try the following mounting arrangement of the TR2227 on our A303602C printed circuit board.


Figure: Drawing of Our Wire-Bonding TR2227 Mounting Scheme.

In the new mounting scheme, we attach the die to the substrate with clear epoxy and then wire bond to the nearby gold-plated pads. We are not sure what fraction of light emitted will end up propagating upwards past the bond pads, but we want to know if this scheme produces a reliable LED.

The A303602D printed circuit board provides a generic two-pad wire-bonding footprint for an LED die. The top side provides the footprint with masked vias leading to the bottom side. The top side will of course be the one that is facing down when implanted. The bottom side provides two pads for sockets, each anchored with its own via, as well as a 1.4-mm diameter clearing in the solder mask directly opposite the center of the LED. This clearance allows us to glue a steel tube directly to the FR4 substrate.


Figure: Rendering of the Top (Left) and Bottom (Right) Sides of the Wire-Bonded LED Head. The vias on the top side are masked. The boards will be made with ENIG finish to allow wire-bonding. The anode pad is the larger square, the cathode pad the smaller square.

This wire-bonding footprint is compatible with the TR2227, EZ290, and EZ500. The TR2227 will mount with clear epoxy to the center of the anode pad, and then be wire-bonded to the anode pad and cathode pad. The gold-plated pad beneath the LED will reflect downward-going light back upwards, increasing the efficiency of coupling into our fiber. The EZ290 and EZ500 are both square, 170-μm thick chips with bottom-side anode and top-side cathode. These will be glued with conducting epoxy to the center of the anode pad and wire-bonded from the top-side to the cathode pad. We receive the following drawing from Cree showing how they intend the TR2227 to be mounted in an epoxy package.


Figure: Drawing from Cree Showing How TR2227 Should be Mounted.

[19-FEB-20] Yesterday we recharged three A3036A, but something went wrong with one of them. Afterwards, its lamp power was stuck on, and it would not repond to commands. We left it in front of a video camera to exhaust its battery. After 100 minutes, the light is half as bright. After 120 minutes, the light is out. We connect to 4.7 V and see 11 mA flowing in. After five minutes, current is 8.5 mA. We disconnect and are able to turn on data transmission with a command. We return to charger. The other two devices we leave flashing at 1 Hz and sixteen hours later they are flashing 30 ms apart.

[21-FEB-20] We charged No25 and No25 overnight. We set them to flashing A3036IL-A for 300 ms at 1 Hz, as well as transmitting synchronizing signal. At first, their battery voltages are 4.0 V and 3.9 V respectively during stimulus. After 190 min their voltages are 3.3 V and 3.4 V respectively. Turn off all functions and batteries report 3.6 V and 3.7 V. We charge No10 for five hours. Battery reports 4.2 V. Start 300 ms, 1-Hz continuous stimulus. Battery reports 3.9 V. After 210 minutes, battery reports 3.4 V. Turn off stimulus, reports 3.6 V. After 210 minutes of charging, No24 and N25 report 3.9 V and 4.0 V.

[24-FEB-20] We have No24 and No25 recently charged. We connect to A3036IL-A and A3036IL-B and set to 300 ms pulses at 1 Hz average, but random spacing. The result is an engaging, never-repeating display. We begin at 09:30. One stops flashing at 12:24 and the other at 12:39.

[25-FEB-20] We turn three A3036A on at the same instant, set to random 10-ms flashes every second. They flash randomly, but synchronously. The random number sequence is the same for all three.

[28-FEB-20] Today something went wrong while re-charging one of our three A3036As, No24, where the lamp is stuck on, we are letting it run its battery down. This same problem arose on 18-FEB-20, but we did not record which stimulator is was that exhibited the problem. Both times, however, one of our staff was working next to the three recharging devices and moving them around.

[02-MAR-20] Charging No24. Connect No26 to power supply with ammeter in series. When we issue an Xon command, we look for current to jump up to about 150 μA for half a second to indicate that RP has been asserted, and for current to remain at 150 μA to indicate that data is being transmitted, and we look at reception from No26 to see if data transmission is effective. The minimum value of VB for which we can turn on data transmission is 2.6 V. Reception is 100%. Once data transmission is turned on, VB = 2.4 V is the minimu battery voltage that keeps it on. At VB = 2.3 V, transmission turns off. With the device inactive, the minimum voltage for which RP is asserted is 2.4 V. We insert 50 Ω in seriess with VB to simulate a battery's source impedance. The minimum VB to turn on data transmission is 3.4 V. Once turned on, it remains on all the way down to 2.3 V. The minimum voltage for which RP is asserted is 2.4 V.

[03-MAR-20] No24 recharged, connect lamp, it is stuck on. It does not respond to commands. Leave with lamp shining from 9:35 am to 2:30 pm, when it is dark. Connect to 4.7 V charger and see only 5 mA flowing in. This is the same behavior as No9, which failed after encapsulation. We put No24 and No9 in our physical sample bag.

We deliver 4.0 V through 50 Ω to A3036A No26 and observe VB when we issue an Xon command to activate the device. The power-up takes 2 ms and peak current is 42 mA. We cannot deliver such a current surge with an MnLi battery. We need a LiPo battery.


Figure: Power-Up Current from Inactive to Active. Yellow: VB 500 mV/div. Blue: 4V0 power supply 500 mV/div. We deliver the 4.0-V power supplky through 50 Ω to VB. Instantaneous current is displacement of VB below 4V0 at 10 mV/div.

We resolve to try the ADP5301 micropower buck converter. It can deliver 50 mA at 1.2 V, which is sufficient. When delivering 200 μA to the sensor circuit from a 3.7-V LiPo battery, the ADP5301 is 87% efficient, implying a battery current of 75 μA. The ADP5301 comes in a BGA-9 with 0.5-mm pitch, and the entire buck converter with inductor, capacitors, and control resistor occupies 3.6 mm × 3.0 mm.

[10-MAR-20] We receive form Micro Precition Technology the first 10 C460EZ500 on Wire-Bonded LED Head (A303602D) printed circuit board.


Figure: EZ500 LED Die Bonded to Anode and Cathode Pads of A303602D.

The cathode bond wire is close to the edge of the cathode pad so as to allow us to put more optical fiber on the LED surface. The anode is glued with conducting epoxy. We damage one of the cathode wire bonds during handling. We apply 10 mA to the remaining nine. Average power 10.1 mW with standard deviation 0.5 mW. According to the manufacturer calibration of these LEDs, we expect 155 mW total power output for 150 mA. The EZ500 data sheet suggests that should see something like 11 mW at 10 mA. Given that we are losing light by unwanted reflection off the edges of our photodiode, and by imperfect reflection off the conducting epoxy below the LED, we are well-satisfied with 10.5 mW at 10 mA. We also have an EZ500 in 3-mm package coupled to a 450-μm fiber polished entirely by hand, without using a puck. For this one, we get 2.8 mW at the fiber tip for 10 mA forward current.

[17-MAR-20] We have a dozen A3036IL-A assembled. The first eleven with the EZ500 in a 3-mm SMT package, the last one with the EZ500 mounted directly onto an A303602D wire-bonding board. We apply 10 mA forward current and measure output power at the taper tip using an angled photodiode.


Figure: Fiber Tip Optical Power. A 450-μm fiber on EZ500 LED, 10 mA forward current.

When the output power is less than 3.5 mW, as for 9-12, misaligment of the fiber base with respect to the LED surface is obvious when viewed with a loupe.

[19-MAR-20] We have our first set of ten blue TR2227 glued and wire-bonded to our A303602D wire-bonding board. Nine out of ten emit 3-4 mW of light for 10 mA forward current. One draws current but emits no light. These LEDs have been glued with conducting epoxy rather than clear, non-conducting epoxy. We suspect the conducting epoxy has wicked up the side of one of the LEDs and shorted the anode pad to the cathode junction. We expect 10 mW for 10 mA. We suspect the conducting epoxy is absorbing the downward light. We leave one LED with 20 mA forward current, emitting 6 mW.

[20-MAR-20] Today our blue TR2227 is still shining, now emitting 3 mW at same current. We receive 240 of C460EZ500 mounted to circuit boards. We apply 10 mA and measure output power for 25 LEDs, get an average of 9.4 mW. Manufacturer's calibration of these diodes says an average of 156 mW @ 150 mA.

Spring-20

[23-MAR-20] We have 250 C527EZZ500 wire-bonded to A303602D boards. We apply 10 mA and measure output power with photodiode. Average is 6.0 mW. Manufacturer's calibration of these diodes says an average of 71 mW @ 150 mA.

[30-MAR-20] We construct a power measurement stand for bare LEDs mounted on our A303602D wire bonding circuit board. We mount an SD445 photodiode, 10 mm square, in a vertical plane 10 mm from the LED die, with the die at the height of the lower edge of the photodiode, and centered upon it.


Figure: Lateral Photodiode Power Meter for Wire-Bonded LEDs. The bottom edge of the photodiode is at the same height as the top surface of the printed circuit board. The center of the LED die is 10 mm from the surface of the photodiode. The photodiode receives 7.1% of the light emitted by the LED.

We use a Monte-Carlo simulation, PCV1.tcl, of rays of light originating at LED die randomly distributed over a quarter-sphere to determine the fraction of of photons the photodiode would receive from a point source. We modify this calculation to determine the fraction of photons received from a planar source, where the intensity is proportional to the sin of the angle between the ray direction and the plane of the source (Lambertian distribution). The result of our simulation is shown below.


Figure: Power Fraction for Increasing Range of Lateral Photodiode. Fraction of hemisphere covered by photodiode (Blue), fraction of emitted rays striking photodiode (yellow), and ratio of photodiode area to hemispherical area (orange).

At small ranges, the fraction of photons striking the photodiode is equal to the fractional coverage of the photodiode, or 0.5. At large ranges, the fractional coverage of the photodiode approaches the ratio of the photodiode area to the hemispherical area. But the fraction of photons striking the photodiode decreases more quickly, because the photodiode sees a smaller emitting surface from its lateral position. At range 10 mm, the simulation predicts that 7.1% of photons emitted by the LED will strike the photodiode. The simulation itself produces a map of its simulated rays. The map below shows the rays generated for range 10 mm with ten thousand iterations, Lambertian distribution.


Figure: Hits (red) and Misses (green) For Photons Generated by PCV1 Simulation (y_range = 10 mm, z_max = 10 mm, z_min = 0 mm, x_min = −5 mm, x_max = +5 mm). We assume a Lambertian distribution of photons across the hemisphere above the LED, place the 10 mm square photodiode in a vertical plane 10 mm from the die, with lower edge of photodiode at same height as LED.

We place a C460EZ500 in our power meter. With 10 mA forward current, we see 148 μA photocurrent. Using 7.1% we get 148 μA ÷ 7.1% ÷ 0.18 mA/mW = 11.6 mW. We place the LED 1 mm from the surface of the photodiode, facing the center, and get 1.6 mA photocurrent, suggesting 8.9 mW is received by the photodiode. We consider the reflection of light for large angles of incidence. The SD445 data sheet says its field of view is ±60°. If we assume all light with incidence >60° will be reflected, the fraction lost will be 25%. In which case, our 8.9 mW suggests total power output of 11.8 mW, which is close to our measurement with 10-mm lateral photodiode.

We repeat with C460TR2227 and see 82 μA for 10 mA forward current, 6.2 mW. With LED up against photodiode, face-on, 720 μA, or 0.72 mA ÷ 0.75 mW/mW ÷ 0.18 mW/mA = 5.3 mW. The TR2227 radiation pattern deviates from Lambertian: at 68° from the optical axis, intensity is 70% of the value of the optical axis. The EZ500 radiation pattern is close to Lambertian: 33% at 68°, while cosine 68° is 0.37.

[01-APR-20] A green C527EZ500 in our measurement stand produces a photocurrent of 127 μA at 10 mA, suggesting power 0.127 mA ÷ 7.1% ÷ 0.25 mW/mA = 7.1 mW.

[02-APR-20] We construct a power meter for fiber-coupled LEDs, where we want to know the total optical power emitted by the fiber tip. Our objective is to provide a meter that does not damage the surface of the photodiode with the hard fiber tip, and provides a consistent measurement from one fiber to the next.


Figure: Lateral Photodiode Power Meter for Fiber-Coupled LEDs. The fiber tip is 1.3 mm from the reflecting surface of the silicon photodiode, and approximately centered on the photodiode area. Installed is an A3036IL-A8. Note the aluminum foil baffle along the bottom edge of the photodiode.

The aluminum-tape baffle at the base of the photodiode reflects light from the LED away from the photodiode. When we cover the base of the fiber with a separate piece of foil, we reduce the photodiode current by less than 5%. We use our PCV1.tcl ray simulator to estiamte the fraction of light emitted by the fiber tip that is received by the photodiode. We set Lambertian to disabled, z_max = +5.0 mm, z_in = −5.0 mm, and y_range = 1.3 mm.


Figure: Hits (red) and Misses (green) For Photons Generated by PCV1 Simulation (y_range = 1.3 mm, z_max = 5 mm, z_min = −5 mm, x_min = −5 mm, x_max = +5 mm). We assume a uniform distribution of photons across the sphere around the fiber tip.

The simulation suggests that 39% of the light will be received. This results in our measuring 3.7 mW for this fiber tip at 10 mA. We get 3.5 mW with our existing angled-photodiode measurement.

[15-APR-20] We receive 5 Blue EZ290 and 5 Green EZ290 from our wire-bonding company. We measure power output at 10 mA forward current in our lateral-photodiode measurement stand.


Figure: Optical Power Output of C460EZ290 (Blue) and C527EZ290 (Green).

In our original tests with the EZ290, we measured an average of 7.2 mW at 10 mA for the C460EZ290 and 4.0 mW at 10 mA for the C527EZ290. Those measurements we performed with a photodiode held over a 3-mm plastic package in which the LED was mounted.

We apply a −6 dBm sweep to an A3015C loop antenna. We load 30-mm wire antennas onto our three A3036BV1 circuits. We set the circuit down on the antenna and measure VR. We must apply power to the circuit to see antenna resonance. We solder a battery directly to the circuit, or we plug one into the programming extension. In the latter case, we must have the battery leads running away from the A3015C antenna or else we see additional peaks in the VR response. The directly-soldered battery gives us a reliable peak. All three circuits show resonance at 885±5 MHz.


Figure: VR versus Frequency with C6 = 1.5 pF. Green: VR 2 mV/div. Blue: Sweep signal downshifted by 915 MHz and low-pass filtered to ±21.4 MHz. Yellow: ramp applied to A3014MT voltage-controlled oscilator. Sweep power is approximately −6 dBm.

We try 40-mm and 20-mm antennas and resonant frequency remains 885±5 MHz. These A3036BV1 boards come loaded with C6 = 1.5±0.1 pF. We replace with 1.0±0.1 pF and resonance is at 940 MHz. We load 0.2 pF in parallel with 1.0 pF to make 1.2 pF and see the following sharp resonance at 910 MHz.


Figure: VR versus Frequency with C6 = 1.5 pF. Green: VR 5 mV/div. Blue: Sweep signal downshifted by 915 MHz and low-pass filtered to ±21.4 MHz.

Once tuned, we obtain robust reception of commands at range 1 m from our command antenna. We have two bare A3036AV1 circuits, and we find they, too, have resonant frequency 885 MHz for C6 = 1.5 pF when we connect the battery. We drop C6 to 1.2 pF and resonance for both is at 910 MHz. During our work, one A3036AV1 and one A3036BV1 required re-programming.