Subcutaneous Transmitter System

Contents

Introduction

Our Subcutaneous Transmitter System consists of small, battery-powered sensors implanted beneath the skin of laboratory animals, antennas to pick up radio-frequency transmissions from the sensors, faraday enclosures to isolate the sensors from ambient radio-frequency interference, and electronics to receive, decode, and transfer unmodified samples over the internet. The photograph below shows one of our first fully-functional, fully-encapsulated sensors, the Subcutaneous Transmitter (A3013A-E).

Figure: Encapsulated Subcutaneous Transmitter, A3013A-E. The leads are helical stainless steel insulated with silicone. The antenna is stranded stainless steel insulated with silicone. The transmitter volume is 2.4 ml, not counting the leads and antenna. Operating life is over eight weeks.

The A3013A provides one differential input channel with impedance 10 MΩ and 2 pF, a dynamic range at the input of 9 mV, and noise of 12 μV rms. The A3013 limits the bandwidth of the input with a single-pole high-pass filter at 0.2 Hz and a three-pole low-pass filter at 160 Hz. It amplifies the signal by a factor of three hundred and digitizes the amplified and filtered signal with with sixteen-bit precision. It transmits 512 samples per second through its radio-frequency antenna. Its operating life with a fresh battery is usually nine and a half weeks. The battery-powered differential input, combined with the isolation provided by faraday enclosures, reduce mains hum at the input to below 1 μV in a laboratory setting (that is to say: no arc-welding going on near the sensors). According to our calculations, radio-frequency communication between the sensor and the antenna will be robust within faraday enclosures up to ten meters wide. The faraday enclosures we have built so far are less than one meter wide. Reception within them is almost perfect with transmitters implanted in live animals.

The Subcutaneous Transmitter (A3019) is a newer transmitter. The A3019A has a shorter battery life of four weeks, but its volume is less than 1.0 ml, not counting the leads and antenna. The A3019 and its various versions will replace the A3013 in 2010. The chief advantages of the A3019 are far longer shelf life, reduced size, and larger dynamic range.

If you want instructions for setting up your own subcutaneous transmitter system, try our set-up section. The following diagram gives you an overview of the system's various components.

Figure: Example Subcutaneous Transmitter System. Shown are three faraday enclosures. One is large, perhaps 2 m square, and contains four rats living together, each with their own implanted transmitter. Two other enclosures are smaller, and contain rats and mice isolated in their own cages. There is one Loop Antenna (A3015A) in each faraday enclosure. The antenna signals are carried by coaxial cables to a Four-Way Antenna Combiner (AC4A). One input to the combiner is unused in this example. The Data Receiver (A3018) amplifies, demodulates, and decodes the antenna signals. It gets its power from the LWDAQ Driver. The LWDAQ Driver with Ethernet Interface (A2037E) downloads the transmitter messages from the Data Receiver and serves them over the internet. A data acquisition computer reads out the transmitter messages with the help of our LWDAQ Software and Neuroarchiver Tool. Also connected to the LWDAQ Driver in this example is a Camera (A2056), which takes photographs of the four communal mice when certain EEG signals are received.

In our conclusion, we give examples of subcutaneous transmitter systems and their performance. You will find the prices of subcutaneous transmitter system components in our price list. In the body of this report, we describe the performance and operation of our Subcutaneous Transmitter system. The descriptions are mingled with the history of our development, which we find fascinating, but you may find irrelevant. For that we apologise. We have done our best to extract as much of the history as possible and place it in our history section.

If you can't understand our radio-frequency terminology consult our terminology page, where we define the terms RF, IF, LO, mixwer, attenuator, dBm, and dB. We explain the significance of 50 Ω in our circuits, and we provide a table that relates power in 50-Ω circuits like our to signal amplitude.

Much of our discussion concerns itself with robust radio-frequency reception. We define reception robustness as the fraction of the time during which we receive 80% or more transmitted messages while the transmitter rotates and moves randomly at a particular range. Our sample rate and filtering is designed to tolerate the lost of 20% of the samples without loss of signal quality. We define robust reception as reception with robustness 95% or higher. In other words: signal quality is degraded by loss of samples for less than 5% of the time. A transmitter's operating range is the maximum range from the receiving antenna at which reception is robust. We use faraday enclosures to guarantee robust reception at ranges up to 300 cm in all laboratories. A transmitter's operating life is the maximum time for which the transmitter can provide robust reception in the absence of ambient interference. Its collision tolerance is the maximum number of transmitters of its type that can share the same receiver and still obtain robust reception in the absence of ambient interference.

History

In our Technical Proposal, we layed out our plan to develop a transmitter small enough to be implanted in the body of a rat, fast enough to transmit four hundred data samples per second, powerful enough to be detected at a range of three meters, and efficient enough to operate for three months on a lithium battery. We divided our development into four stages.

1. Technical Proposal
2. Dummy Transmission Circuits
3. Data Transmission and Reception Circuits
4. Prototype Circuits
5. Appendix: Encapsulation
6. Appendix: Flexible Wires
7. Appendix: Faraday Enclosures

The electronics of Stage Four were ready in March of 2007. It was then that we confronted the problem of robust, water-proof encapsulation for the transmitters. At the time we wrote our Technical Proposal, we believed that two coates of silicone dispersion would be adequate to protect the transmitter from water. Given the simplicity of the procedure, there would be no point in coating the transmitters ourselves. We proposed to send transmitters to our users without batteries, wires, or coating. Our user could solder their preferred wires to the transmitters, install the battery, and apply the coating.

Water-proof encapsulation is not straightforward. Capillary action makes water a relentless invador of any opening or crack. During encapsulation, air bubbles trapped beneath components emerge into the curing encapsulation. In the low pressure of an aircraft cargo hold, the bubble beneath the battery pushes outwards, and will burst an encapsulation made of silicone. Our work on encapsulation is an appendix to the work we committed to in our Technical Proposal. After nine months of effort, we arrived at an encapsulation process that uses both epoxy and silicone, which we called Process X. We made our first transmitters with Process X in December 2007. We describe our work on encapsulation and encapsulation methods in our Encapsulation report. We later improved Process X and arrived at Process B. We made our first transmitters with Process B in June 2009.

Once our transmitters were water-proof in Process X, they endured for long enough in live animals for the wires to break from repetetive stress. Wires broke at the neck of a rat, at their solder joints, and where they emerged from epoxy encapsulation. Our work on flexible wires is an appendix to the work we committed to in our Technical Proposal, and was funded in part by ION's purchase of ten prototype transmitters. We describe our work on wire fatigue in our Flexible Wires report. In Spring 2009 we began trials using a variety of steel wires, including steel springs, as we describe in the Trials section of Flexible Wires. We eventually arrived at a stranded wire for the antenna and helical steel wires for the input leads. We insulate both with silicone, and this insulation forms part of the outer cover of the transmitter body.

Interference with our subcutaneous transmitter signals from outside sources proved to be a problem in ION's London laboratory. We solved the problem of interference with faraday enclosures. Our work on faraday enclosures is an appendix to the work we committed to in our Technical Proposal, and was funded in part by ION's purchase of ten more prototype transmitters. We describe the development of a practical faraday enclosure, and document its success, in faraday enclosures. Faraday enclosures are now an integral part of the subcutaneous transmitter system. Not only do they provide immunity to outside interference, but they allow many transmitters to operate simultaneously in the same space without interfering with one another.

Please note that the Recorder and Neuroarchiver in LWDAQ 6.5+ are compatible with the Data Receiver (A3018), but incompatible with the Data Receiver (A3010).

Electronics

Our Subcutaneous Transmitter (A3013) provides two analog inputs, one or both of which can be enabled. One input is intended for high gain and high bandwidth. The second is intended for low gain and low bandwidth. We imagine the former monitoring neural activity, and the other monitoring heart rate.

Our Data Receiver (A3018) comes in a 9" × 7 " × 3" metal box. The box connects to our data acquisition system (LWDAQ) through an RJ-45 connector. It receives power through the same connector. A BNC socket provides connection for a coaxial cable to the antenna. There are four lights on the box. One of these, a green one, turns on when the Data Receiver receives messages from a transmitter.

Our Antenna (A3015) attaches to the end of a coaxial cable through a BNC socket. We recommend a 96" cable (240 cm). The antenna stands up on its own, and can take one of three forms: loop, dipole, or whip, but the loop antenna performs best in the subcutaneous transmitter system.

Figure: Stage Four Electronics. The Data Receiver is the aluminum box with its lid off. Alice is holding a transmitter (prototype version with external battery and programming connector). The loop antenna on a cable sits at one end of the table. A dipole antenna (disconnected) is next to the loop. Two Modulating Transmitters are in the background. The black box is the LWDAQ Driver, connected to our computer.

The encapsulated Subcutaneous Transmitter (A3013) is 18 mm × 20 mm × 8 mm, as shown here.

We recommend that all transmitters operate inside faraday enclosures. The faraday enclosure shown here is large enough for two of ION's rat cages, including water bottles. Each faraday enclosure has its own internal antenna. In theory, we can obtain robust reception while monitoring twelve transmitters with a single Data Receiver. We could combine the signals from six antennas and feed the combination into a Data Receiver. In practice, we recommend combining only four antenna signals, using our Four-Way Antenna Combiner (AC4A).

We designed our receiver for ease of use, construction, experimentation, and modification. Inside the box are three circuits: the Demodulating Receiver (A3017), SAW Oscillator (A3016SO), and Data Recorder (A3007C). The receiver's antenna input, which is a BNC socket in the side of the box, connects to the RF input of the Demodulating Receiver. The output of the SAW Oscillator connects to the LO input of the Demodulating Receiver. The output of the Demodulating Receiver connects to the input of the Data Recorder. The receiver connects to the data acquisition system through a single RJ-45 socket. Power for all three of the internal circuits arrives through this socket.

We can disconnect our Antenna from the Demodulating Receiver and instead connect some other RF source with known power and frequency. We can vary the power we deliver with the help of fixed attenuators in series with the cables. We made use of our Modulating Transmitter (A3014MT) as a sweeping source of RF to measure our RF amplifier frequency response. We used our 910 MHz SAW Oscillator (A3014SO) as a stable source of RF power that we varied from +10 dBm down to -80 dBm. We used the same 910 MHz osxillator as a source of RF power for a transmitting antenna, and so tested radio transmission at a known frequency. We switched antennas quickly, from loop to dipole to whip, using our BNC connectors. We used our 868±1 MHz SAW Oscillator (A3014SO) as a frequency reference to calibrate all our other oscillators. Any time we doubted a result, we could switch all the circuits with duplicates, to see if the result was sound.

We spent some time developing and checking several ways of measuring RF power, as we explain elsewhere in a section called Power Measurement. The procedure we settled upon in the end was to downshift the RF signal with our ZAD-11 mixer, and measure the IF power. We obtain the original RF power by adding the ZAD-11's insertion loss, and our cable losses, to the IF power. This gives us a measurement accurate to ±1 dB.

When we measure RF signal frequency, we compare frequencies to our reference 868±1 MHz SAW Oscillator (A3014SO) with the help of a mixer. All our frequency measurements are calibrated with respect to this 868 MHz reference. We mix the RF signal whose frequency we want to measure with the 868 MHz, and measure the IF frequency using our oscilloscope. The precision of our IF frequency measurement is 0.5 MHz or better.

And so it is that we can measure the power of RF signals to ±1 dB (10%) and the frequency of RF signals to ±1 MHz (0.1%). In our previous work, our power measurements were no better than ±3 dB, and our frequency measurements were all referred to the center of our SAW filter passbands, which turned out to be off by up to 5 MHz from their nominal centers. Our increased accuracy, combined with the convenience of our BNC cables, fixed attenuators, and mixer, allow us to measure the performance of our RF circuits in more detail than before.

Antenna combiners and faraday enclosures are a later addition to the subcutaneous transmitter system. The faraday cages isolate transmitters and antennas from ambient radio-frequency interference, greatly increasing our operating range. The antenna combiners allow the signals from multiple faraday cages to be combined and applied to a single data receiver.

Set-Up

Start by printing out our system diagram (the one in the introduction). Check that you have all the parts required. We list those parts below. We'll start with a singl-enclosure system, and later we'll add an antenna combiner for multiple-enclosures.

1. Power Supply (plugs into the AC wall socket)
2. Ethernet Cable (standard ethernet jumper cable)
3. LWDAQ Driver (such as the A2037E, a black box)
4. LWDAQ Cable (you can use a short ethernet cable instead)
5. Data Receiver (such as the A3018, a silver box)
6. Coaxial Cable (with BNC plugs, usually 2.4 m)
7. Faraday Enclosure (such as the FE2B for two animal cages)
8. Short Coaxial Cable (with BNC plugs, 30 cm)
9. Loop Antenna (such as the A3015A)
10. Transmitter (such as the A3013A)
11. A Magnet (we like cow magnets, which are big and strong)

The power supply provides 24-V for the LWDAQ Driver. When we ship the power supplies to our non-US customers, we don't provide the power cord that runs from your native wall socket to the power adaptor's industry-standard AC power plug. We leave it to you to dig out an old computer or monitor power cord for use with the power adaptor. Your power cord will be compatible with your own native wall socket.

There is a DC power jack on the LWDAQ Driver. Plug the power adaptor in here. Some of the indicator lights on the front side of the driver should turn on, to indicate the power supplies inside the driver are working.

Plug the ethernet cable into the ethernet socket on the LWDAQ Driver. On the A2037E, the ethernet socket is the one on the back, next to the power jack. The driver has other sockets of the same type as the ethernet socket, but these are not ethernet sockets. They are LWDAQ Sockets. You can plug your ethernet cable into them if you like, but it won't do you much good. On the other hand, you won't break anything either.

Plug the other end of your ethernet cable into your laptop or some other computer upon which you have administrator access. Download the LWDAQ Software suitable for your computer's operating system from the LWDAQ software download page. Go to the LWDAQ Manual and follow the software installation instructions for your operating system. Once you have installed the software follow the instructions in the Configurator section of the same manual in order to establish communication between your computer and the LWDAQ Driver. This configuration may take you an hour or two because it requires that you set up your computer to operate on a local network that contains only the computer and the driver. We have done this a bunch of times with our MacOS computers, so it only takes us ten seconds. But the first time can be frustrating.

Once you have established communication between your computer and the driver, you may wish to configure the driver with an IP address in your local area network, so that you can plug it into one of your lab's ethernet sockets and communicate with the driver from anywhere on the Internet. Once again, follow the instructions in the Configurator section of the manual. You may find this step takes a while to accomplish because you will have to work with your network administrator to obtain an Internet address, and they may not be happy about allowing you to put a strange device like the LWDAQ Driver on their network.

Let us assume, therefore, that you have communication between your computer and the LWDAQ Driver, and that you have the LWDAQ software installed and running on your computer. You can test this connection at any time with the Diagnostic Instrument in the LWDAQ Software. Try turning on and off the LWDAQ power supplies and obtaining plots of the power supply voltages. Consult the Diagnostic Instrument section of the LWDAQ Manual for detailed instructions.

Connect your Data Receiver to the LWDAQ Driver with a LWDAQ Cable. You can use a short ethernet jumper cable as a LWDAQ Cable, and that's what we send our customers when we ship them a data receiver. Pick one of the LWDAQ sockets on the driver for the connection. Any one will do. Let's suppose you pick socket number 1. You should see a power light come on the Data Receiver. If not, press the hardware reset button on your LWDAQ Driver to bring up the power supplies again. The hardware reset button is next to the indicator lights.

Connect the longer coaxial cable to the Data Receiver. Whenever you connect a BNC plug to a BNC socket, make sure that you turn the locking flange on the plug until it locks. If you don't lock the connector, your system will still work, but it will work poorly now and then, which makes for a difficult problem to diagnose.

Plug the other end of your long coaxial cable into the bulk-head BNC feed-through on the base of your faraday enclosure. Remove the lid of the faraday enclosure. If the lid holds a microwave absorbing foam pad, it will be heavy. Set the lid aside and connect the short BNC cable to the inner side of the feed-through. Connect the antenna to the open end of the short cable.

Take your transmitter and place it next to the antenna in the cage. Don't bother to put the lid on the cage. Check the Receive indicator on your Data Receiver. If it's off, then bring the magnet next to the transmitter and move it away again. The Receive light should come on. If not, try again. Pick up the transmitter in your hand and move it around near the antenna. Repeat a few times with the magnet. If it still doesn't come on, you have a problem somewhere. Try another transmitter, check your antenna connections, and write to us.

If you get the Receive light, open the Recorder Instrument in the LWDAQ Software. Scan through the Recorder Instrument manual and look at the signal you are receiving from the transmitter. Short the leads of the transmitter together and check that the noise matches its specification. The A3013A, for example, should have around 90 counts of noise with the inputs shorted, and the A3019A should have around 30 counts. Check your transmitter manual for more information.

Data Acquisition

The Data Recorder (A3018) plugs into a Long-Wire Data Acquisition (LWDAQ) system. It acts as a LWDAQ device, although it violates the current-consumption limits of the LWDAQ.

We use the Data Recorder with the LWDAQ Driver with Ethernet Interface (A2037E). The A2037E connects to the Internet, your Local Area Network, or directly to your computer via an RJ-45 Ethernet socket. You communicate with the A2037E, and therefore the Data Recorder, via TCP/IP.

On the computer you use for data acquisition, you run the LWDAQ software, which you can download from here.

Software

Download the latest version of the LWDAQ software from here. To help you with installation and use of the LWDAQ software, consult the User Manual. With the Subcutaneous Transmitters and Data Receiver, you will use the Recorder Instrument and Neuroarchiver Tool.

The Recorder Instrument is a set of routines that run inside the LWDAQ program. You can use the routines by opening the Recorder Instrument window from the Instrument menu. You can call the routines from the LWDAQ's console. The Recorder Instrument software downloads blocks of binary data from the Data Recorder hardware and divides them into blocks of fixed time-duration. It displays transmitter signals as it receives them, each transmitter trace in a different color, and prints a summary of the received signals to the screen.

The Neuroarchiver Tool uses the Recorder Instrument to download signals from the Data Receiver. The Neuroarchiver downloads, filters, displays, and stores to disk selected signals from the Data Recoder hardware, and does so using the Recorder Instrument as an intermediary. The Neuroarchiver calculate and display the fourier transform of the incoming signals. It stores data and transforms to disk.

Signal Path

The Subcutaneous Transmitter (A3013) provides two analog input channels by means of four flexible, insulated copper wires. One or both channels will be active, depending upon how the device has been programmed. Each channel is high-pass filtered with an RC network that rolls off at around 0.1 Hz. The primary input channel is low-pass filtered with a three-pole active filter with roll-of anywhere from 10 Hz to 320 Hz. The auxilliary channel is low-pass filtered with an RC network at anytwhere from 1 Hz to 320 Hz.

Each version of the A3013 provides a different combination of sampling rate, dynamic range, and channel activation. The A3013A, for example, has two wires for one active channel with dynamic range 9 mV, bandwidth 160 Hz, and sampling rate 512 SPS. The A3013A can detect a 5-μV sinusoid of any frequency between 5 Hz and 160 Hz, and measure its amplitude with 1-μV accuracy.

The A3013 converts its amplified and filtered inputs into sixteen-bit numbers and transmits them in radio-frequency messages.

The A3013 transmits messages through its RF antenna. The A3013A transmits in the 902-928 MHz ISM band. It transmits at two frequencies to represent two logic levels. The messages propagate through space as radio waves. (See How Antennas Work in our Technical Proposal for an explanation of antennas and radio waves.)

The transmitter's analog circuits operate continuously to amplify and filter its analog signals. But the logic and RF circuits are inactive almost all the time. In the case of our A3013A, the logic and RF circuits wake up for 7 μs every 1953 μs, which is 0.36% of the time. During this 7-μs burst of activity, the transmitter converts its single analog input into a sixteen-bit number, and transmits this number, together with some synchronizing bits, the transmitter's four-bit ID, and a checksum. We call this transmission a message. The A3013A's message rate is 512 per second. Within each message, the transmitter sends bits at the bit rate. The A3013 bit rate is 5 MBPS, or 200 ns per bit, just like its predecessor, the A3009, and its transmissions are fully-compatible with the Stage Three electronics. For more details of the message encoding, see below.

The radio messages are received by our Antenna (A3015). The messages are joined in the antenna by RF signals from other sources of similar frequency. The messages and the interference propagate along a coaxial cable to the Data Recorder (A3018).

The radio signal enters the metal enclosure of the Data Recorder and connects to the RF input of the Demodulating Receiver (A3017) inside (see schematic). The RF signal, messages and interference, are amplified by 20 dB. They pass into a bandpass filter that rejects signals outside the 902−928 MHz ISM band. What remains are the messages and ISM-band interference. These are amplified by another 20 dB and enter a mixer. The mixer downshifts the 902-928 MHz ISM-band RF signal to a 38-64 MHz IF signal.

The IF signal passes through three limiting amplifiers. Each provide 22 dB of gain, and limit their output. The output of the third stage is called IFL in the schematic. When we disconnect the antenna, the signal on IFL is random, but not quite saturated. When we connect the antenna, the RF interference in our laboratory causes IFL to saturate almost all the time. Because IFL is saturating, its amplitude is constant. Only its frequency changes.

The fixed-amplitude IFL passes into a tuned circuit called the discriminator. The discriminator attenuates the lower IF frequencies, so that its output amplitude depends upon the frequency of IFL.

The discriminator output passes into a full-wave demodulator. The demodulator turns the AC signal into a signal proportional to the AC amplitude. The demodulator output is D in the schematic. The alternating radio frequencies of the transmitter messages appear in D as a square wave.

Figure: SCT Message as Seen at Demodulator Ouput (D, Upper Trace) and at SCT Test Point (Lower Trace).

The D signal passes along a short BNC cable to another circuit inside the Data Receiver box, the Data Recorder (A3007C). There it is mildly band-pass filtered by two RC networks to form the signal S in the schematic. The transmitter messages appear in S as a square wave centered about the 0-V potential.

Figure: SCT Message as Seen at Comparator Input (S, Upper Trace) and at SCT Test Point (Lower Trace). The central horizontal graduation on the screen is 0V.

A comparator transforms S into a logic HI when S is above 0 V, and logic LO when it is below 0 V. The comparator output is called C. The logic levels of the original transmitter message now re-appear as logic levels on C. Outside the messages, C is a random or pseudo-random sequence of logic levels. It is random when the dominant source of RF power at the antenna input is electronic noise, and pseudo-random when the dominant source is interference in the ISM band.

Figure: SCT Message as Seen at Comparator Output (C, Upper Trace) and at SCT Test Point (Lower Trace). Note random values of C before the transmitter builds up output power.

The C logic signal passes into the Data Recorders's logic chip, where it is sychronized with the message clock. Our message clock is 40 MHz. The synchronized version of C is called SC in the logic chip's firmware. The SC signal changes only on the rising edges of the message clock, so changes pulses on C that take place between these edges do not appear in SC.

The Data Recorder constantly monitors the stream of bits on SC, looking for messages embedded within the stream. These messages could arrive at any time. Messages can also appear in the random SC bit sequence by chance. These random messages we call bad messages, and we discuss them in detail below.

The Data Recorder stores any messages it detects in its 512 KByte first-in first-out buffer as thirty-two bit records, as we describe below. The last eight bits are a timestamp, which counts cycles of the Data Recorder's 32.768 kHz clock.

The Data Recorder also stores clock messages 128 times per second. The clock messages takes the form of a transmitter message from a transmitter with ID zero. We reserve ID zero for clock messages, and will never be use it as the ID of an actual transmitter. The sixteen-bit data of a clock message is a sixteen-bit counter that increments by one with every clock message stored. The final eight bits are a timestamp, which is always zero, because the clock messages are stored whenever the Data Recorder's 32.768 kHz cycle counter wraps around to zero. You can think of the clock messages as being the product of a virtual transmitter with ID zero, transmitting 128 messages per second, and whose data is a counter that increments from one message to the next.

The Data Recorder connects to the LWDAQ. The LWDAQ in turn connects to an Ethernet. Your data acquisition computer runs the LWDAQ software and communicates with the LWDAQ by TCPIP.

The LWDAQ software on your data acquisition computer uses its Recorder Instrument to download blocks of messages from the Data Recorder's first-in first-out buffer. It plots these messages on the screen, including the clock messages. The Recorder provides you with blocks of data that cover a time interval you specify. As it acquires data, the recorder makes no effort to eliminate bad messages or insert substitute messages. Its data blocks are therefore of variable memory-size, but fixed time-duration.

The Neuroarchiver Tool calls the Recorder to supply fixed time-duration data blocks. You tell the Neuroarchiver which channels you want to display and record. Each channel corresponds to a transmitter identifier. Our Stage Four transmitters can have identifiers between 1 and 15 (0 is reserved for clock messages). The Neuroarchiver reconstructs each channel from the data block by removing any bad messages it can identify, and inserting substitute messages where necessary. It displays the reconstructed channels, and calculates their fourier transforms.

The Neuroarchiver will store the raw Recorder data blocks to disk, or it can store reconstructed channels to disk in a variety of formats. It will store fourier transforms to disk also. And so the transmitter signals are available on disk for you to analyze as you see fit.

Bit Rate

The bit rate is the rate at which the transmitter sends bits during one of its message transmissions. The A3013A, for example, transmits a total of 40 bit-values in 8 μs. The bit rate is 5 MBPS (megabits per second). We describe the function of these bits in the Message Encoding section here.

With the exception of a few temporary, slower versions we made for the following tests, all Stage Four and Stage Three circuits run at 5 MBPS. Here we compare the performance of the Stage Four system at 5 MBPS and 2.5 MBPS and show that the gain in operating range we make by dropping the bit rate is slight, while the loss in operating life is significant.

We set up a Demodulating Receiver (A3017) with a SAW Oscillaotr (A3016SO) and Data Recorder (A3007C). We connected a Dipole Antenna (A3015B) on a 240-cm (96") coaxial cable (RG58C/U). We recorded messages from a Subcutaneous Transmitter (A3013). We configure the transmitter and Data Recorder for message transmission at 5 MBPS (200 ns per bit), then for 2.5 MBPS (400 ns per bit). The switch between bit rates requires the following changes.

1. The transmitter logic must be re-programmed.
2. The Data Recorder logic must be re-programmed.
3. Capacitor C4 must be 1 nF for 5 MBPS and 2 nF for 2.5 MBPS (see schematic).

We measure a variety of performance parameters for each bit rate. We present our measurements in the following table.

The 5 MBPS and 2.5 MBPS transmissions perform best with a modulation depth of ±4 MHz. The 90% power bandwidth of the 5 MBPS messages is 18 MHz, which leaves us with at least 8 MHz of extra room in the 902-928 MHz band. The power bandwidth of the 2.5 MBPS messages is 13 MHz. In theory, the narrower the bandwidth, the less likely a hole in the reception caused by interference (such as the this) will lie within the transmission bandwidth, and therefore the less likely that the transmission will be corrupted by interference.

The biggest benefit of the lower bit rate is a 50% increase in operating range. The biggest cost of decreasing the bit rate are the increase in active current. The current rises from 75 μA to 108 μA. The operating life drops from nine weeks to six weeks. Another cost of decreasing the bit rate is the increased transmission time, which means a greater probability of collisions between multiple transmitters sharing the same receiver. The probability of collision between any two transmitters on any given message transmission is 0.7% at 5 MBPS and 1.4 % at 2.5 MBPS. With ten transmitters, each would lose 7% of its samples at 5 MBPS and 14% at 2.5 MBPS. Our system can handle a 30% loss of messages. At 2.5 MBPS we would use up most of this missing message budget in the normal operation of ten transmitters.

We decided to stay with a 5 MBPS bit rate. Our collaborators at ION stated that battery life and bandwidth were more important than operating range, because in either case the operating range appears to be greater than the size of a standard rat cage.

Message Encoding

Our Stage Four circuits use the same message encoding as the Stage Three transmitters, which we describe in the Message Encoding section here section of an earlier report. Each message carries a four-bit channel number and sixteen bits of data. Channels 0 and 15 are reserved, leaving 1 through 14 for transmitters.

Note: Channel number 0 we reserve for the timestamp messages stored by the receiver. Channel 15 we reserve for future use in slow data transfer. A channel 15 message contains a four-bit id, a four-bit field address and eight bits of data. A transmitter that measures EEG and temperature could transmit 511 messages per second on channel 1 and one message every second on channel 15. The 512'th EEG message would simply go missing in the EEG signal, and be replaced during signal reconstruction in software. The sixteen bits of the No15 message would begin with four bits indicating that this a message is from transmitter No1. The next four bits identify the top byte of a sixteen-bit temperature measurement. The next No15 message would send the bottom byte. The next No15 message could transmit the number of hours the transmitter has been active, and so on, with up to fourteen different byte of data being transmitted at one byte per second. We reserve field addresses 0 and 15 for future use.

We considered using a cyclic redundancy check at the end of the message, instead of the inverse of the transmitter ID for our error-checking bits. The cyclic redundancy check would allow us to detect corruption of data bits. The inverse-id bits are sensitive only to corruption of the ID or the bits themselves. But we decided the benefit of improved error checking did not outweigh the disadantage of incompatible transmitters.

The Data Recorder (A3018) stores these messages as four-byte records. The first byte is the transmitter ID. The next two bytes are the sixteen-bit data. The last byte is a timestamp. The timestamp counts periods of the Data Recorder's 32.768 kHz clock oscillator.

Blocked Messages

The Data Recorder can fail to identify the message in its incoming bit stream (see Signal Path), even though they are received clearly by the Demodulating Receiver. The Data Recorder might be occupied with something that looks like a message, and when the real message arrives, all it knows is that the previous message was invalid. The recorder goes back to looking for a new message, but is too late to identify the real message. We measure the rate at which the Data Recorder loses messages by putting a single transmitter next to the antenna and watching for missed messages. Our signal is strong and there are no other transmitters to collide with it. We assume that any missing messages have been lost in the Data Recorder. It turns out that the number of synchronizing bits that the Data Recorder requires at the beginning of a message is what dictates the missing messages rate.

Only after it has received its required number of synchronizing bits does the Data Recorder proceed to message recording, and it will lose an incoming message only if it is already recording a false message. By insisting upon five synchronizing bits, we ensure that the Data Recorder loses less than one in a five hundred messages. As it turned out, we settled upon 11 synchronizing bits because this improved the performance of the system when the signal was weak.

Missing Messages

Each transmitter, when active, transmits messages in a continuous stream. The A3013A transmits 512 sixteen-bit data samples per second. A message that the Data Recorder fails to receive is a missing message.

The Neuroarchiver Tool handles missing messages by creating a substitute message with equal data sample value to the previous message received from the same transmitter. It does this only for the channels you are recording.

Despite the Neuroarchiver's message substitution, missing messages degrade the effective bandwidth and quality of our received signals. But they do not cause catastrophic problems, nor do they confuse our data acquisition system, which can rely upon a guaranteed stream of messages from each transmitter it monitors.

The primary cause of missing messages is lack of signal strength at the antenna when compared to ambient interference power, as we discuss in Ambient Interference. The primary cause of loss of signal strength is cancellation of the RF signal by its own reflections arriving at the antenna, as we discuss in Operating Range. The primary source of ambient interference is cordless phones operating in the 902−928 MHz band, and mobile phones operating in the frequency bands immediately above and below the 902−928 MHz band. Even if such mobile phones transmit only 1% of their power outside their designated frequency bands, they will still combine to produce substantial interference in neighboring bands. Another source of interference is other subcutaneous transmitters, in which case we refer to the interference as a collision.

Mobile phone interference in the ION laboratory in London, which is on the eighth floor of a building, was so severe that reception from an implanted transmitter at range 50 cm would sometimes drop as low as 20%, and was rarely above 80%. With the help of one of our faraday enclosures, reception jumped up to an average of 99%, with a minimum of 98% during any four-second period.

Bad Messages

Noise and interference can generate messages on their own. We call these bad messages. Bad signals can raise false alarms, spoil the scale of self-adjusting displays, and wreak havoc with your fourier spectrums. We have made every effort to avoid them.

Bad messages have a valid transmitter ID number, but their data is invalid. The Recorder Instrument rejects them when they occur with ID numbers you have not asked it to record.

The bad message rate in our laboratory is less than one per ten minutes when there are no transmitters active. In London, the bad message rate was roughly one per second, sometimes coming in bursts of ten in a second. With transmitters active, we find that some messages get corrupted, as we describe below, and these message become bad messages.

We spend the rest of this section showing how bad message can, in theory, arise from noise and interference, and how our message encoding makes such bad messages very unlikely.

As we describe in Signal Path, the Data Recorder monitors a logic signal called SC, looking for messages. These messages could arrive at any time. If the stream of bits is random, there is a chance it will generate a message by chance. If the stream of bits has some pattern to it because of radio interference, the chance of a message appearing in it at random might be much higher.

When we disconnect the Data Recorder's antenna, and replace it with a 50-Ω terminator, we stop radio signals from reaching our receiver. All that remains at the input to our RF amplifier is white noise, whose power in our radio-frequency passband is around −87 dBm. The receiver amplifies this noise until it is large enough to generate an active pattern of zeros and ones on SC. This alternation is random, because the white noise is random. The output of the Demodulating Receiver (A3017) is an analog signal, S. This signal is sharply bandwidth-limited by the Demodulating Receiver's radio-frequency passband, and mildly high-pass filtered when it enters th Data Recorder. The Data Recorder uses a comparator to generate the logic level SC from S. When we examine SC on an oscilloscope with the antenna disconnected, the bandwidth of the bit stream is around 10 MHz, which we determine with the help of various low-pass filters.

Our messages represent individual bits values as edges in SC, not as levels of SC. An edge is a transition from logic LO to HI or from HI to LO. A rising edge is a one, and a falling edge is a zero. A sequence of synchronizing one-bits at the beginning of the message tells the Data Recorder when to look for data-carrying edges, because it knows the rising edges in the synchronizing sequence are the data edges in these initial one-bits. Every subsequent data-carrying edge allows the Data Recorder to adjust its expectations for when the next data-carrying edge will arrive. With a 5 MBPS bit rate, the data-carrying edges are separated by 200 ns. Each bit takes roughly 8 periods of our 40-MHz message clock. We say "roughly" because the bit rate is not exact. The transmitter's message clock is generated by a ring oscillator, and is accurate to only ±10%.

If the Data Recorder detects an edge within one or two clock periods of another, it rejects the entire message and goes back to looking for synchronizing bits. It rejects synchronising bits using the same contraint. The Data Recorder tests the level of SC for premature edges twice during each bit period. In order for a random sequence of bits to create a valid message, it must satisfy both tests for all bits of the message. There are around 25 bits in the message, so the random stream must take on the correct value 50 times. Furthermore, the message must have the correct checksum at the end, which is the same as saying that it must take the correct value for another five bits.

The likelyhood of a random sequence on SC matching our message protocol is roughly 0.5³⁰. To the first approximation, there are 10⁵ opportunities for the match to occur every second, because each message occupies roughly 10 μs. We expect our bad message rate from noise and random interference to be of order on per hour. And indeed this is what we observe: after running for ten minutes we saw no bad messages with the antenna disconnected.

Corrupted Messages

A corrupted message is one that has been interfered with, but which still passes through our error-checking. A corrupted message has the same effect upon data acquisition as would a bad message with the same ID.

We can reduce the likelyhood of message corruption by checking for errors in the content of the message, as opposed to only by comparing the final four bits to the first four bits. A four-bit cyclic redundancy check at the end of all transmitted messages would give us better rejection of corrupted messages. Instead of ending up as bad messages, most corrupted messages would end up as missing messages. But the cyclic redundancy check would make our Stage Four transmitters incompatible with those of Stage Three, so we decided to tolerate a higher than necessary corrupted message rate for the sake of backward-compatibility.

Collisions

When two transmitters send a message at the same time, we say that they collide. In almost all collisions, the result is the loss of both messages. If the first message is much more powerful than the second, the second will be lost, and therefore act as a missing messages. The first message will be received correctly. But if the first message is weaker than the second, the second will interfere successfully with the first, while it is unlikely that the second will be received properly itself. When the two signals are equal in power, they will certainly interfere with one another.

Collisions occur because two transmitters are transmitting at the same time. If the transmission period were exactly regular, and two clocks drifted into exactly synchronicity, collisions could occur systematically at every transmission instant. But the transmission period is not regular. The average period is exact, but the moment of each individual transmission is displaced by the transmitter by a small, random, amount of time. In the A3013A, the transmission period is 64 cycles of its 32.768 kHz clock, or 1952 μs. The A3013A delays its moment of transmission by 0 to 15 clock cycles, so that the actual moment of transmission can be delayed by up to 456 μs.

We call the displacement of the transmission instant transmission scatter. We describe how the data acquisition system handles transmission scatter in our Recorder Manual. The transmitter uses the lower four bits of its sixteen-bit ADC conversion as the source of a random number. We describe in more detail how the hardware implements the scatter in the Transmission section of our A3013 Manual. The figure below shows transmitter scatter on the oscilloscope screen. As you can see, the scatter is not perfectly random.

Figure: Transmission Scatter. The transmitter's sample frequency is 512 Hz. The oscilloscope triggers on one message and then displays the next. The delay between the two messages is the sum of two instances of transmission scatter.

Faraday enclosures provide isolation of transmitters from ambient interference, and also from transmitters that do not share the same receiver. If we have eight transmitters in four separate faraday enclosures, each faraday enclosure will have its own antenna. We combine the signals from the six antennas and feed the combination into our Data Receiver (A3018) using an Antenna Combiner (AC4A). Transmitters in these six enclosures will interfere with one another whenever their transmissions overlap.

The A3013A transmission period is 1952 μs and the transmission itself lasts for only 7 μs. Each transmitter transmits for 0.35% of the time. The chance of one 7 μs transmission overlapping another is 0.7%. The average loss due to collisions when we have n transmitters sharing the same receiver is (n-1) × 0.7%. If we have If we have twelve transmitters sharing a receiver, we will lose roughly 8% of messages to collisions when averaged over a long time period. We say the average collision loss is 8%.

But the collision loss varies with time, as shown in the following graph. Its average value may be only 0.7% for two transmitters, but it's peak value can be ten times higher.

Figure: Cyclic Changes in Message Reception from Two A3013A Transmitters. Transmitter No4 has been running for 6 weeks in a jar of water. Transmitter No7 is new and in air. Average reception from No4 is 98.9% and from No7 is 99.5%.

In the above graph, we see cyclic variation in reception caused by a slight difference in the transmitter clocks. Over the course of eighteen minutes, the cycles grow in amplitude from 1% to around 8% and shrink again. Another twenty minutes goes by with no evidence of collisions, and the collision cycles begin again.

The period of message transmission for the A3013A is roughly 2 ms (64 cycles of the transmitter's 32.768 kHz oscillator). An entire collision sequence, such as the thirty-six minute sequence captured in the figure above, takes place as the two clocks drift with respect to one another by 2 ms. The OV1564 clock we use on the A3013A is accurate to ±10 ppm, so we expect differences between the clocks of order 5 ppm to be common. In the example above, it takes 2200 s for the two clocks to drift apart by 2 ms. The two clocks differ by 0.9 ppm. This combination of clocks is unusual. More often we see collision cycles of period several hundred seconds, like this one taken from transmitters in live animals at ION in London.

The individual cycles in the above sequence have period thirty seconds, as we can see in the following figure.

Figure: Cyclic Changes in Message Reception from Two A3013A Transmitters. Detail from figure above showing 500 s to 600 s.

Each cycle corresponds to a drift of 30.5 μs between the two clocks (one period 32.768 kHz). Because our clocks are only 0.9 ppm apart, we get a 34-s cycle. We see the structure of the cycle clearly. There is a 18-s period with no collisions in each cycle, and a 16-s period where the collisions take place.

The first period of collisions in the entire collision sequence begins when the earliest transmission window of one transmitter coincides with the last transmission window of the other. In our case, the two 7 μs windows overlap for 16 s (that's twice 7 μs divided by 0.9 ppm). Collisions in this first period should be rare, because only one window overlaps. The chance of a collision should be 1/16 × 1/16 = 0.4%.

As we see in the oscilloscope trace above, the four-bit values the transmitters use for random numbers are not uniformly-distributed across their sixteen possible values. Furthermore, one transmitter might be so much stronger than another that the interference is only one-way. In our example sequence, we see the first cycle has a depth 6% for No4 and 0% for No11. The peak cycles have depth 8% for both transmittes.

The second cycle of collisions occur 34 s after the first, when the clocks have drifted 30.5 μs to the next window overlap. Now the earliest window of one transmitter coincides with the second-to-last last window of the other. But now we have the second-earliest window coinciding with the last window as well. Our chance of collision is, in theory, 0.8%. The overlap lasts for 16 s, and 18 s later comes the next overlap. The chance of a collision is 1.2%. On the sixteenth cycle, all sixteen windows overlap, and the collision probability is 6%. The cycles shown in detail in the figure above are the largest in the collision sequence. Both transmitters experience a 6% drop in reception.

Sixteen cycles after the peak, none of the windows overlap. The clocks drift another 1 ms apart over the next 1200 s and there are no collisions. Now the entire sequence starts again.

If we place n transmitters in the same faraday enclosure, it is inevitable that their sixteen transmission windows will coincide at some point in time. At that time, the collision rate between transmitters will be at its greatest, and reception will be at a minimum. If we consider a message occuring in one of the windows, the probability of no other message occuring in the same window is 0.94⁽ⁿ⁻¹⁾. If we assume near-perfect reception in the absence of collisions, which appears to be the case in faraday enclosures, this probability is the minimum reception rate for the n transmitters.

A transmitter's collision tolerance is the maximum number of transmitters of its type that can share a receiver and still give us robust reception. For robust reception we need to receive more than 80% of messages for 95% of the time (which is to say: we need robustness of 95% or higher). It's not obvious from the minimum and average reception values whether reception will be robust. To obtain a good estimate of robustness, we simulated the collision cycles of n transmitters. Our simulation program, collisions_1.pas, simulates sets of n transmitters working together with randomly-distributed clock periods over a period of two thousand seconds. The figure below shows reception from the first four transmitters in a set of forteen.

Figure: Simulated Collisions for Four Out of forteen Transmitters. For detail, see here. For simulated collisions of four out of four transmitters, see here.

When the simulation begins, all clocks are synchronous, and we obtain the minimum reception. After that, the interactions between the clocks become complex. For a 100-s detail, see here. For each simulation of n transmitters, we obtained n values for minimum reception, average reception, and robustness. These values were consistent from one transmitter to the next to within a few percent, so the values we give in the table below are the average values taken over the n transmitters.

Robustness is greater than 95% all the way up to n = 12, so the collision tolerance of the A3013A is 12. This is why we talk about 12 transmitters sharing a single receiver in other sections. Robustness for n ≤ 5 is 100%.

In our simulation, we assume that any collision between two transmitters will result in the loss of both messages. This is not true in practice. If the power received from one transmitter is 12 dB greater (16 times greater) than the power received from another, the more powerful signal will dominate. If reception of the more powerful signal is taking place, the weaker signal will be ignored. If the weaker signal is being received, there is a good chance that the receiver will have time to abandon the weaker message and receive the stronger message. In our recording of two stationary transmitters, shown here, we see that fewer messages are lost from No7 than No4. Transmitter No7 is in air with an 80 mm antenna. Transmitter No4 is in water with a 50-mm antenna. We assume the signal from No7 is stronger than that from No4.

Operating Range

We define operating range as the greatest range at which we obtain robust reception. An A3013A transmits 512 messages per second, so the operating range is the greatest range at which we receive 410 or more messages per second in 95% of orientations and positions at the operating range. In practice, we operate the transmitters inside a faraday enclosure, so their operating range must be adequate to cover the range from the antenna in the center of the enclosure to the farthest corner of the enclosure volume.

When we hold a transmitter between two fingers in our basement laboratory, and move it around randomly at range 100 cm for a minute (see movie), we receive 98% of the messages it transmits. The operating range of the A3013 is greater than 100 cm in our basement laboratory. In our our office in the center of Waltham, operating range is closer to 70 cm. Operating range in the ION laboratory in London appears to be around 30 cm. The decreasing operating range is the result of ambient interference, which dominates transmitter messages whenever the transmitter signal is attenuated by reflections and unfavorable orientation.

Regardless of the operating range of transmitters in a laboratory, we recommend the use of enclosures with conducting walls to block out ambient interference. Such enclosures are called faraday cages. We call them faraday enclosures so we don't get them confused with animal cages, which are used to contain animals. We discuss ambient interference below, and we describe the performance and construction of faraday enclosures in our Earaday Enclosures report.

Because faraday enclosures give us at least a 30-db (one thousand-fold) reduction in ambient interference, they increase the operating range of our transmitters by a factor of 30 (square root of one thousand). Even if the operating range is only 10 cm without a faraday enclosure, it will be 300 cm within a faraday enclosure. We expect robust reception from transmitters operating within an enclosure 500 cm × 500 cm × 200 cm, provided the antenna is in the center of the floor of the enclosure. Our FE2A enclosure is far smaller than this.

Robust reception within a faraday enclosure appears to be guaranteed. Nevertheless, we always benefiit from increased signal strength compared to ambient interference, and we have worked hard to increase the operating range of our transmitters in the absence of faraday enclosures. We devote the remainder of this section to a discussion of what phenomena limit the operating range of our transmitters. The discussion explains why we use a poorly-tuned loop antenna to receive our transmitter signals, and why we use a poorly-tuned bent antenna to transmit them.

When we implant a transmitter in an animal, the 50-mm antenna of an A3013A-E will resonate more efficiently, and so transmit more power (see Antenna Length). We may lose some or all of that additional power by absorption in the animal's body (see Transmitter in Rat Corpse).

We compared the operating range of the Data Receiver (A3010B), which is the Stage Three receiver upgraded with new firmware and an antenna socket, and the Data Receiver (A3018A), which is the original Stage Four receiver. (The original Stage Three receiver, the A3010A, suffered from a firmware bug that reduced its performance even farther with weak signals.) We used the same Subcutaneous Transmitter (A3013A) for both receivers. We used the same Loop Antenna (A3015A) for both receivers. We unplugged it from one receiver and plugging it into the other.

1. At range one meter in air, with the transmitter held between two fingers, and rotating in all directions at random for ten seconds, the A3010B (Stage Three Upgraded) lost ≈ 6% of messagse. The A3018A (Stage Four) lost <1%.
2. We placed the transmitter 7 m away in a randomly-chosen location. The A3010B (Stage Three Upgraded) lost 75% of messages. The A3018A (Stage Four) lost 2%.
3. We were able to receive messages from up to 14 m away with the A3018A (Stage Four), but no more than 7 m away with the A3010B (Stage Three Upgraded).

It appears that our Stage Four development has doubled the effective range of the transmitters. We claim that we are nearing the physical limits of message detection, as we shall now explain.

The noise in our receiver is the thermal and amplifier noise at the antenna input. The interference is RF power in the receiver's pass-band arriving at the antenna from transmitters other than those the receiver is intended to receive. The effective noise power at the input of our demodulating receiver is −90 dBm (see Noise and Interference). But interference power, even in our basement laboratory, is over a hundred times more powerful than this noise, at −68 dBm (see Noise and Interference).

Our signal must have power 12 dB greater than the interference in order to avoid corruption (see Foreign Interference). When we transmit power across 1 m of space from a quarter-wave antenna to a loop antenna, we lose at least 32 dB compared to connecting the power directly to the receiver circuit with a cable (see Reception). Even with our omini-directional antennas, an unfavorable relative orientation of the transmitting and receiving antennas causes a 17 dB drop in received power (see Omni-Directional Antennas). We must also deal with reflections, or multi-path interference (see Multi-Path Interference and Radiated Power). Reflections of the main signal interfere at the receiver. We can easily get a 10-dB loss due to destructive interference. In other words: we can lose 90% of our power easily

Our transmitters produce roughly −4 dBm (see Transmission and Modulating Transmitter). Transmission across 1 m of air with can drop our received power to −53 dBm. This −53 dBm is still more than 12 dB above our interference power of −68 dBm.

But we must contend with multi-path interference as well, which give rise to reception dead spots. With the antenna in an unfavorable orientation, in which we receive only −53 dB from line-of-sight transmission, we have power radiating more effectively in other directions. This power can reflect off nearby conducting surfaces and arrive at the receiving antenna with as much strength as the line-of-sight signal. The reflection adds to the line-of-sight signal. It might reinforce the line-of-sight signal, or cancel it, depending upon their relative phase.

The phase difference between a reflected and line-of-sight wave is a strong function of frequency. If the reflected path length is three meters long, this is a hundred wavelengths. A 1% change in frequency will cause a 2π change in phase. A 0.25% change in frequency will cause a π/2 change in phase. The figure below shows two holes in the response of our our Demodulating Receiver (A3017). The antennas are 1 m apart, on either side of our body.

Figure: Two Holes in Frequency Response Caused by Reflections. The linear trace is proportional to frequency. Each vertical division is 5 MHz, starting with 895 MHz on the left. The top trace is the demodulator output.

We see that each hole is about 1 MHz wide. A 0.1% change in frequency causes the cancellation to stop. We take such sharp holes in the frequency response to be evidence of multi-path interference. The width of the holes is consistent with a reflected path length of several meters. Longer path lengths would give holes that were even more sharp.

Aside: The two holes are separated by 24 MHz, and you will notice yet another hole, on the left side, which is 24 MHz below the middle hole. As the frequency changes by 24 MHz, the phase difference between the line-of-sight and refelected waves changes by 2π. The number of wavelengths in the path difference changes by one. Because the frequency changes by 2.5% to bring about this one-wavelength change, the path difference must be 40 wavelenghts, or 12 m. This suggest to us that the wave is bouncing off one of the metal shelves five or six meters from our transmitting antenna. If that's the case, then the reflected signal will be about 25 dB weaker than the line-of-sight signal, except for the fact that our body is in the line of sight. Human tissue attenuates 900 MHz by approximately 1 dB/cm, so we expect a loss of around 25 dB through a human torso. Both signals are each strong enough for reception, but they cancel one another at particular frequencies.

With 10 dB loss due to cancellation by reflections occurring at the same time as 17 dB loss due to poor antenna orientation and 32 dB loss due to transmission across 1 m of air, our signal drops to −63 dBm, which is only 5 dB above our −68 dBm interference. Reception will fail.

With a favorable orientation of the transmitter, however, we can expect −36 dBm at 1 m, and − 56 dBm at 10 m. Even at 10 m, we can receive a signal that is 12 dB above our interference. In fact, we find we can get reliable reception at up to 15 m in our lab by orienting the transmitter properly.

Ambient Interference

Radio frequency power from sources other than our subcutaneous transmitters is what we call ambient interference. If it's large enough, ambient interference can dominate the transmitter signals and cause missing messages. In early 2009, we determined that our operating range in the ION animal laboratory in London was only 25 cm, after observing a 50-cm operating range for most of 2008 (details in this e-mail). A range of 25 cm is not adequate to cover even a single animal cage, let alone four at once. We assumed that ambient interference was to blame for the the short operating range, and began to look at ways to block ambient interference from reaching our receiving antenna.

We can block out ambient interference with a faraday enclosures, which is a box with electrically conducting walls. There can be holes in the conducting walls, but the holes must be less than 1% of the wavelength of the radiation if we want to block 99% of the power. In our case, the wavelength in air is around 300 mm, so holes less than 3 mm will be fine.

One way to block ambient interference is to turn your entire laboratory into a faraday enclosure. You can paint the walls, ceiling, and floors with shielding paint. You could cover the windows with aluminum mosquito mesh, or make curtains out of shielding fabric.

Another way to block ambient interference is to move your animal laboratory into a basement with a concrete ceiling.

We believe the most cost-effective and convenient way to block ambient interference is to build a faraday enclosure that encloses the animal cages and the receiving antenna. We describe our experiments with home-made faraday enclosures in our Faraday Enclosures report. This work is on-going today (08-APR-09).

Analog Inputs

Our experiments with the Subcutaneous Transmitter (A3009) showed us we needed to increase the gain of the analog input from ×30 to ×300 if we were to see normal EEG activity in a mouse brain. We also detected a couple of sources of noise in our analog-to-digital conversion process. Apart from the increase in amplification, and the addition of an auxilliary input, the Subcutaneous Transmitter (A3013) is much the same as its predecessor.

The A3013, with its greater gain, has roughly 12 μV of cross-talk and thermal noise on its input. It can detect a 5-μV 160-Hz sinusoid with no difficulty, once you take the fourier transform of its output in the Neuroarchiver.

See the Analog Inputs section of our A3013 Manual for a detailed discussion of the analog inputs. See our Mains Hum report for more on the origins of mains hum. Our transmitters pick up no measurable mains hum when implanted in animals. The electrodes measure the difference in potential between two parts of the brain. Mains hum tends to operate upon the entire animal body. Furthermore, with the introduction of Faraday Enclosures to isolate transmitters from radio-frequency interference, we find that the transmitters are isolated from mains hum as well.

Encapsulation

We encapsulate transmitters in epoxy and coat them in silicone. The result is shown here. For an account of our work on rugged encapsulation that is resistant to water, fatigue, and vacuum, see our Encapsulation report.

Flexible Wires

We use stainless steel coiled wire with silicone insulation for the analog leads, and a stranded steel cable with silicone insulation for the antenna. We describe our selection of wires suitable for long-term implanting in lab animals in our Flexible_Wires report.

Operating Life

Battery life of the A3013A-E is roughly nine and a half weeks running continuously, starting with a fresh battery. The graph below shows four transmitters running down their batteries.

Figure: VBAT versus Time for Four Transmitters. We label the graphs by transmitter channel number.

For details of how the operating life is affected by battery size and transmission rate, see the Current Consumption and Battery Life sections of our A3013 Manual. For a table of the operating and shelf lives of the various A3013 versions, wee the Versions section of the same manual.

Demonstrations

In the Implanted Performance section of our A3013 Manual, we give an account of the evolution of the transmitter's performance in live animals. The account proceeds in chronological order, so the most recent results are last.

Faraday Enclosures

With the introduction of faraday enclosures, robust reception from implanted transmitters became assured. We describe the development of our faraday enclosures in our Faraday Enclosures report. Our faraday enclosures reduce ambient interference by at least 30 dB, and most likely 40 dB. The result is an increase in our operating range of 30-fold to 100-fold. Not only is interference kept out of the enclosure, but the transmitter signals are kept in the enclosure. Transmitters in separate enclosures do not interfere with one another unless they share the same receiver. Nor will they interfere with other users of the same radio-frequency band.

Conclusion

The A3013A-E is encapsulated in epoxy and silicone. Its analog leads are steel springs insulated with silicone. Its antenna is a stranded steel wire also insulated with silicone. The encapsulation and wires will resist the fatigue and corrosive fluids within an animal's body for at least eight weeks. It will resist the vacuum, cold, and heat it is likely to experience during shipping and storage. When active, the A3013A transmits 512 samples per second for at least eight weeks when it starts with a fresh battery. Each sample is sixteen bits, with a dynamic range of 9 mV, thermal noise of 12 μV, and bandwidth limited to 160 Hz by a three-pole low-pass filter on the transmitter. Its near-ideal differential input picks up less than 1 μV of mains hum when implanted in a live animal. With a volume of only 2.1 ml, the A3013A is suitable for implantation in rats and adult mice.

The A3013's operating range is limited by the strength of local radio-frequency interference. If we make no effort to isolate the transmitter from ambient interference, we find that we obtain robust reception at ranges up to 10 cm in a hotel in Geneva, 30 cm on the eighth floor in London, and 200 cm in a basement in Boston. When we place the transmitter and receiving antenna in a faraday enclosure, we increase the operating range by at least a factor of thirty. A faraday enclosure suitable for two independent rats is only 70 cm square. We obtain near-perfect reception continuously from an implanted transmitter within such a enclosure.

There are other advantages to the use of faraday enclosures in the subcutaneous transmitter system. They eliminate mains hum and electrostatic fields, leading to a dramatic improvement in the quality of the EEG signal measured by the transmitter. The enclosures serve to isolate transmitters from one another, so that we can have multiple transmitter collections in the same laboratory. The faraday enclosures allow us to expand the subcutaneous transmitter system indefinitely in the same space, subject only to the requirements of physical space for the faraday and animal cages.

Example: We could have four systems of eight transmitters in the same laboratory. Each system has its own receiver and four faraday enclosures. Each enclosure contains two transmitters in two animals, each living in its own animal cage. Each faraday enclosure has its own antenna receiving signals from two transmitters. The four antenna signals of each system are combined to feed the input of the system's receiver. The eight transmitters in each system will interfere with one another when their transmissions happen to coincide, but the transmitters of separate systems are isolated from one another by the faraday enclosures. The transmitters within each system must have unique channel numbers (from one to fourteen), but transmitters in separate systems can have the same channel number.

Here is a parts list with prices for each component required by an eight-transmitter system with animals in separate cages, and two cages in each of four faraday enclosures. Our price does not include the price of the animal cages or the animals themselves.

There is no reason why the animals must live in separate cages. Many animals could live together in a large faraday enclosure, and we could monitor all signals with a single antenna.

Example: We implant eight A3013A transmitters in eight animals. We place all eight animals together in a 60 cm × 50 cm × 20 cm cage (if we can find such a thing, we can't see anything that size advertised on the web), and place the cage within an FE2B faraday enclosure. We push the 60-cm side of the cage up against one of the aluminum side walls. We place the antenna flush up against the opposide 60-cm side of the cage. We don't want the antenna flush up against the wall of the enclosure. This way, the antenna is 20 cm from the nearest wall. We plut our water bottle on the same side of the cage as the antenna. We now monitor the eight rats living together with a single antenna and a single data receiver.

Here is a parts list with prices for each component required by an eight-transmitter system with animals in a single cage.

We can provide a telemetric system for eight isolated rats for $9300, and a system for communal rats for only $8000. The transmitters will operate for at least eight weeks. When they run down their batteries, they must be discarded. There is no way to replace their batteries. A new set of eight transmitters costs another $4800, unless you purchase in hundreds and schedule delivery every two months, in which case each set of eight would cost $3200.

Our LWDAQ Software is free and distributed under the GNU Public License. It runs on Linux, MacOS, and Windows. The software downloads the continuous stream of transmitter data from the LWDAQ Driver, which in turn reads the data out of the Data Receiver. Eight A3013A transmitters (160 Hz bandwidth, 512 sixteen-bit SPS) produces roughly 16 kBytes/s of data, which must be transferred over the local ethernet to the data acquisition computer. Because all our designs are public under the GNU Public License, you will have no problems interpreting, translating, and analyzing the data obtained from the transmitter system.