Figure: United States Radio Frequency Allocation
| Design |
| Modifications |
| Spectra |
| Development |
[05-OCT-25] Our circuit diagrams and firmware are distributed under the GNU Public License 3.0.
[06-OCT-25] The A300D requires the following modifications to become an A300E.
[05-OCT-25] The 902-928 MHz Industrial, Scientific, Medical (ISM) frequency band is recognized for unlicensed use in North America, South America, and the UK. In countries where the 902-928 MHz band is reserved for the exclusive use of license-holders, our faraday enclosures ensure that no significant RF power is radiated by our SCT system, thus making sure we do not violate local regulations. The regulations governing the 902-928 MHz band have evolved in the years since we first chose it for our SCT telemetry in 2005. The band is still recognised for unlicensed transmitters up to 1 W in North and South America. But in 2013, the FCC granted a private company a license to transmit up to 30 W in the USA. A 30-W transmitter less than 10 m from one of our recording systems will disrupt reception. But such proximity should be easy to avoid.
In Europe, the UK recognises the 902-928 MHz band for unlicensed transmitters up to 1 W, but they have granted mobile phone companies license to use the same band with base stations transmitting up to 3000 W effective radiated power. An SCT telemetry system directly in the path of such a base station transmission will be disrupted at ranges less than 100 m. Mobile base stations tend to be over twenty meters above the ground, mounted on towers or the sides of tall buildings. The upper floors of neighboring buildings receive the most power from such base stations. Near base stations, lower floors are better than upper floors, and basements are best.
In the paragraphs below we present spectra we and our customers obtained with A3008 spectrometers in various locations around the world. During our earliest measurements, we were still considering which frequency band to use, but we soon settled upon the US 900-MHz ISM band.
[01-MAR-06] Below is a detail from the low-frequency end of our outside spectrum on a hill in Waltham, MA, near Boston.
We have a peak in the blue curve at 900 MHz and 1000 MHz. We have orange vertical markers at 930 MHz, 950 MHz, and 970 MHz. To match the outside spectrum with the blue peaks and the orange vertical markers, we must shift it by 7 frequency steps to the right. We obtain the following table of interference power in various frequency bands. We quote power relative to the bottom of our spectral graphs.
| Band | Power Density | SCT Range |
|---|---|---|
| 875-902 MHz | +30 dB | 60 cm |
| 902-928 MHz (ISM) | +15 dB | 400 cm |
| 928-945 MHz | +30 dB | 60 cm |
| 945-960 MHz | +15 dB | 400 cm |
| 960-990 MHz | +10 dB | 600 cm |
| 990-1050 MHz | +15 dB | 400 cm |
The +30 dB power at 930 MHz explains why we could not operate A3006 transmitters outside at any range. The weakness of the A3006 is that it does not transmit power in the A3005 receiver pass band during zero bits, so that any ambient power above about +15 dB will saturate it during zero transmissions, and so give false zeros. With the new A3009 transmitters, and modified A3005 receivers, we will be transmitting zeros and ones within the receiver pass band, so we expect to be able to dominate the interference by bringing the transmitter close enough to the receiver.
Below we see the spectrum recorded by the A3008 for an active A3006 Subcutaneous Transmitter at ranges 60 cm and 600 cm in the basement lab. Note that the spectrum extends up to 1050 MHz. This may be a failure in the A3008's ability to reject frequencies outside its current selected band, or it may be that the A3006's spectrum really does extend up into the 1050 MHz band. We have no way at the moment to distinguish between the two possibilities. Note that the ratio of the peak power at 950 MHz is roughly 20 dB (×100), which is what we expect with an increase in range of ×10.
For reliable data recording by a dual receiver, we believe the SCT power must be at least +10 dB greater than the interference. This minimum power sets the maximum range at which the SCT can operate. With +10 dB of interference, the SCT power must be >20 dB, which is the case for ranges less than 6 m. With +30 dB of interference, the SCT power must be >40 dB, which it is at ranges less than 60 cm. When we step inside the doorway, the SCT ranges improve because of the shielding provided by the building.
| Band | Power Density | SCT Range |
|---|---|---|
| 875-902 MHz | +20 dB | 300 cm |
| 902-928 MHz (ISM) | +8 dB | 700 cm |
| 928-945 MHz | +20 dB | 300 cm |
| 945-960 MHz | +8 dB | 700 cm |
| 960-990 MHz | +8 dB | 700 cm |
| 990-1050 MHz | +8 dB | 700 cm |
The interference in our basement laboratory, ignoring that coming from the SCT dual receivers themselves, is as follows.
| Band | Power Density | SCT Range |
|---|---|---|
| 875-902 MHz | +10 dB | 600 cm |
| 902-928 MHz (ISM) | +8 dB | 700 cm |
| 928-945 MHz | +10 dB | 600 cm |
| 945-960 MHz | +8 dB | 700 cm |
| 960-990 MHz | +8 dB | 700 cm |
| 990-1050 MHz | +8 dB | 700 cm |
We provide the above table as a reference, because we have observed the reception at range 6 m to be stable with a dual receiver in our laboratory. It is clear that here in Waltham, operating in the 1000-1050 MHz would be perfect. But the band 960 MHz to 1215 MHz has been reserved for Aeronautical Navigation. Transmitting 300 μW in this band within the confines of a laboratory would have no effect upon aeronautical navigation. Within five meters of the laboratory's external walls, the Subcutaneous Transmitter's signal will be beneath thermal noise. We could ship transmitters in metal boxes, or wrapped in foil. If the transmitter turned on by impact or magnetic field, no measurable power would escape the wrapping.
We would prefer, however, to operate in the ISM band, which is free for anyone to use. The ISM band appears to be the 26-MHz range between the peaks seen in the outside spectrum. We cannot be exactly sure of the frequencies to within a MHz, but we are confident to within 5 MHz. Certainly, it's worth trying the ISM band here in the United States.
[02-MAR-06] A few weeks ago, we recorded the following spectra in and outside our laboratory at 8 pm. The outside temperature was -2 °C. We recorded the red spectrum just inside the doors, where it was 15 °C. We see the same spectrum as outside, but attenuated by 10 dB, and shifted up by five frequency steps, or roughly 3.5 MHz.
The blue trace we obtained ten minutes later in the basement laboratory, with no SCTs turned on, but the SCT dual receiver with their 1-GHz local oscillators turned on. The temperature in the laboratory was 19 C. We see signs of the same features at around step 60 as we saw outside, but shifted up by seven frequency steps, or roughly 5 MHz. The VCO tuning curve above shows a 25 MHz drop in VCO frequency with a 125 °C rise in temperature. That's −0.2 MHz/°C. With our 21 °C increase in temperature from outside to inside, we expect our LO frequency to decrease by roughly 4 MHz. We observe roughly 5 MHz. We took another spectrum with our SCT dual receivers unplugged. The two peaks at 1 GHz (step 163) and 900 MHz (step 34) disappeared.
[10-MAR-06] We obtained the following spectra using our Modulating Transmitter (A3001A). We drive its TUNE input with a square wave that switches from 1.1 V to 1.4 V at frequencies 200 kHz, 2 MHz, 5 MHz, and 10 MHz.
We see the Bessel function curves predicted by theory when the modulation bandwidth is comparable to the separation of the two frequencies. In our case, the two frequencies are separated by 30 MHz, and we start to see the Bessel function at a modulation frequency of 5 MHz.
[31-MAR-06] It appears that the ISM band is respected in the UK also, as shown here. They describe the ISM band as "Fixed, Amateur".
These are the spectra from Matthew Walker's laboratory and recording room, recorded by Matthew Walker himself. The recording room is where the rats with implants will live. The laboratory is outside his office. There is broad interference across the entire frequency band in the laboratory. When someone turns their mobile phone on, the spectrum rises by 10 dB, and becomes erratic. We suspect that we are seeing the mobile phone's spurious power emissions, which may be transient. The Spectrometer measures the maximum received power in its measurement interval, not the average power. When we look at the two laboratory spectra without local mobile phone interference, we see what looks like the sum of many mobile phones transmitting in the same broad way, in neighboring buildings. In the recording room, we have three spectra from three different locations. They show residual power in the Fixed Mobile 5.317A band, 942 MHz - 960 MHz. From the location of this band, we think we can say that the vertical black lines in our plot lie upon 20-MHz boundaries. If that's the case, then the peaks to the left is at 930 MHz, in another fixed mobile band. Below that we have several power peaks in the range 910 MHz to 925 MHz. The power in these peaks is approximately equal to the power we record from an A3006 SCT at range 6 m. At range 3 m, an A3009 SCT would dominate the interference. We should be able to operate in the ISM band in Matthew's recording room at ranges less than 3 m.
The above figure shows interference power peaking about 10 dB higher than our signal power at 1 m, even in a favorable orientation. The 902-928 MHz ISM band must be in use for some unlicensed devices more powerful than our own, or France does not respect the band. The operating range of the transmitter in this interference is only a 10 cm.
[31-MAY-07] These following spectra are taken with the new A3008B after we calibrated its ISM-band frequencies to 1-MHz accuracy, as described above.
The red trace is with the RF input terminated with 50 Ω. The green trace is with a Loop Antenna (A3015A) connected to RF with a 96" coaxial cable, but all of our own sources of ISM-band power off. We leave the Loop Antenna connected for the rest of the spectra. The blue trace is with our 910-MHz +12-dBm SAW Oscillator (A3014SO) turned on, with a 75-mm wire as an antenna, place 50-cm from the spectrometer antenna. You can see the large bump corresponding to the 910-MHz, and many other bumps. For the orange trace we insert a 20-dB attenuator in the antenna line. For the yellow trace, we turn off the 910 MHz and turn on a Subcutaneous Transmitter (A3013A) and place it 50-cm from the Spectrometer antenna. For the purple trace, we move this same transmitter to a range of 5 m.
[09-APR-09] Subsequent work on the spectrometer reveals that its noise floor is at −80 dBm. This floor is visible on the right side of the plot above: its is the green straight line. The interference power rises to a peak of −51 dBm, with another broad peak at −60 dBm. The spectrometer has a measurement bandwidth of 3 MHz, so we can calculate the total interference power in the 900−930 MHz band by dividing the band into ten 3-MHz sections and adding the power together. We arrive at roughly −48 dBm total interference in the band. In a separate experiment, we measured the interference power in our basement laboratory in Boston with our Demodulating Receiver (A3017), and found it to be −68 dBm. Our operating range in there is 100 cm. With 20 dB more interference, our operating range will be ten times less, or 10 cm.
[15-APR-09] Today reception was good in the ION animal laboratory. The interference spectrum, taken by Pishan with A3008B number 001, shows less power than the one Matthew obtained in 2006.
[22-JAN-10] We obtained the following spectra in the Children's Hospital Boston basement, in the Operating Room (OR) and the Animal Room (AR). We also show spectra obtained the day before outside in Waltham.
[25-JAN-10] We obtained the following spectra at the OSI office on Moody Street, Waltham.
[20-SEP-13] We receive the following spectrum from her animal laboratory in Edinburgh.
Above 927 MHz, interference power is -45 dBm in each 3-MHz window examined by the spectrometer. Iris is using an A3015B antenna, which contains a 3-dB attenuator, so actual power is around -42 dBm. In 927-930 MHz, where we are sensitive, there is -42 dBm of power. This is great enough to require a faraday enclosure for reliable reception with only one receiving antenna.
[11-NOV-13] We take our first spectrum at our new OSI office on Pratt Ave.
The peak at 926 MHz is −46 dBm. Our Moody Street office has a peak at 926 MHz also, but it is only −60 dBm. Reception in the Bratt Avenue office is not robust even at range 20 cm.
[06-MAR-14] We take a spectrum at MRC Harwell, reception building. We equip the A3008B with an A3015C damped loop antenna.
There appears to be no interference in the 900-930 MHz band. We obtain robust reception with one antenna up to 50 cm.
[25-MAR-16] We take the following spectra in the OSI basement office in Waltham, MA.
Isolation provided by the FE2F is around 26 dB. Interference outside the faraday enclosure is centered on 926 MHz and −48 dBm. Previous measurements show reception of up to −45 dBm in other office locations. Other than this peak, there is no measurable interference. The same three transmitters emit roughly 6 dB more power when immersed in 200 ml of water. From the above graphs, the temperature coefficient of the A3028E center frequency is −0.2 MHz/°C, which agrees with the data sheet.
[30-JAN-17] Spectra from Philipps University, Marburg, showing RF power inside and outside faraday enclosures with subcutaneous transmitters running within the enclosures. We see no sign of significant interference within the 900-930 MHz band.
[27-JUL-17] We receive the following spectra Edinburgh University, where they recently moved their SCT recording system from one room to another on the top floor of their CCNS building. Room A is the original room, from which we obtained spectra in 2013.
Peak interference power in the Corner Room is −41 dBm (80 nW) outside the Faraday enclosure and −61 dBm (800 pW) inside. In the edge room, meanwhile, we have peak interference in 900-930 MHz of −54 dBm (4 nW). The spectrometer measures the power in a 3-MHz band, while the interferene we see above spans 6 MHz at the top of the 900-930 MHz range our SCT receivers are sensitive to. Thus we estimate the total interference power is equivalent to a single frequency of 160 nW outside the enclosure in the Corner Room, 1.6 nW inside the enclosure in the Corner Room, and 8 nW outside in the Edge Room. Reception from implanted SCTs is reliable only when interference outside the enclosure is less than 40 nW. Thus we expect reception to be unreliable in the Corner Room, and indeed it is: for 30% of the time, reception is less than 80%.
| Number | Measurement |
|---|---|
| 0 | Corner room, outside enclosure, average power |
| 1 | Corner room, outside enclosure, average power |
| 2 | Corner room, inside enclosure, average power |
| 3 | Corner room, inside enclosure, average power |
| 5 | Corner room, inside enclosure, SCT power |
| 6 | Corner room, inside enclosure, average power |
| 8 | Edge room, inside enclosure, average power |
| 9 | Edge room, outside enclosure, average power |
| 10 | Corner room, outside enclosure, average power |
The original recording room was the Edge Room, with one outer wall on the top floor of the CCNS building. The new room is the Corner Room, directly opposite another taller building with three mobile phone base stations. According to a map of the local base stations, the nearest is 60 m away from the Corner Room, with only the outer building wall as an attenuator. The Edge Room is 80 m away with three attenuating walls. The interference power we see in all three traces above correspond to the 925-960 MHz downlink band of the 900 MHz GSM network.
[30-MAY-07] The A3008A provided a crude and ultimately unhelpful means of calibrating its own frequency response. The A3008A and A3008B differ in the following respects.
There is a second VCO (U16) on the A3008A, which we call the calibration oscillator (CO). We use the CO to excite three SAW filters. The outputs of three filters are weakly coupled into the A3008A's RF input. We sweep the CO frequency from 875 MHz to 1050 MHz over the course of 30 ms, and simultaneously record the four IF signals (0-V reference, ×1, ×11, and ×121). At the same time, we record the IF output. When the CO frequency passes through the LO frequency, we always get a pulse in the IF signal, like this, but the pulse gets larger when the LO frequency lies within the passband of one of the three SAW filters. The three SAW filters specified in the schematic have passbands 869±1 MHz, 915±12 MHz, and 947.5±12.5 MHz. The first one is not much use, because it is outside the range of the LO frequency. The second and third filters turn out to overlap in most cases, so they don't give us two discrete calibration bands. In our two prototype A3008s, we omitted the coupling for the first two filters, and just left the 947.5±12.5 MHz filter. The auto-calibration is not helpful because the pass-band of the SAW filters can be much wider than their minimum specification, an is not centered to better than ±5 MHz. Because of the failure of the self-calibration, we removed it from the A3008A circuit and replaced it with an output socket for the LO frequency. We removed the calibration option from the LWDAQ software's RFPM instrument.
[31-MAY-07] The A3008A provided a crude and ultimately unhelpful means of calibrating its own frequency response. The A3008A and A3008B differ in the following respects.
There is a second VCO (U16) on the A3008A, which we call the calibration oscillator (CO). We use the CO to excite three SAW filters. The outputs of three filters are weakly coupled into the A3008A's RF input. We sweep the CO frequency from 875 MHz to 1050 MHz over the course of 30 ms, and simultaneously record the four IF signals (0-V reference, ×1, ×11, and ×121). At the same time, we record the IF output. When the CO frequency passes through the LO frequency, we always get a pulse in the IF signal, like this, but the pulse gets larger when the LO frequency lies within the passband of one of the three SAW filters. The three SAW filters specified in the schematic have passbands 869±1 MHz, 915±12 MHz, and 947.5±12.5 MHz. The first one is not much use, because it is outside the range of the LO frequency. The second and third filters turn out to overlap in most cases, so they don't give us two discrete calibration bands. In our two prototype A3008s, we omitted the coupling for the first two filters, and just left the 947.5±12.5 MHz filter. The auto-calibration is not helpful because the pass-band of the SAW filters can be much wider than their minimum specification, an is not centered to better than ±5 MHz. Because of the failure of the self-calibration, we removed it from the A3008A circuit and replaced it with an output socket for the LO frequency. We removed the calibration option from the LWDAQ software's RFPM instrument.
[06-MAY-16] We leave four A3008Ds running for five days, and re-calibrate them. Spectrometer Q0158's frequency calibration remains 57, but power calibration increases from 3.2 dB to 4.0 dB, and LO output power increases from 2.8 dBm to 3.7 dBm. Spectrometer Q0159's frequency calibration remains 50, but power calibration rises from 2.2 dB to 2.6 dB and LO power drops from 3.2 dBm to 1.6 dBm. Spectrometer R0201's frequency calibration remains 58, but power calibration increases from −1.2 dB to +0.6 dB, and LO power drops from 6.3 dBm to 4.5 dBm. Spectrometer R0202's frequency calibration increases from 57 to 58, its power calibration increases from −1.2 dB to 0.6 dB, and its LO power output drops from +5.3 dBm to 4.3 dBm.
The power calibration of our spectrometer circuits we obtain with a reference oscillator applied to the input. But the main purpose of the A3008 is to measure the power of subcutaneous transmitters. We have a Subcutaneous Transmitter (A3028) with a BNC socket soldered onto its RF output. We pass through a 6-dB attenuator and mix with 869.4 MHz in a ZAD-11. We trigger our scope from TP1 on the transmitter circuit. The IF amplitude is 37 mVpp. We set our Q0158 spectrometer to 915 MHz LO and measure LO power 4.0 dBm. We pass through a 6-dB attenuator and mix with 869.4 MHz. The IF amplitude is 142 mVpp. Thus the power delivered by the transmitter to its BNC connector is −7.7 dBm. We pass the transmitter signal through a 30-dB attenuator and apply to the Q0158 input. We adjust the Spectrometer's sct_to_dBm value to 1.0 dB, and the SCT peak power in the Spectrometer is −37.7 dBm. And so we obtain a value for sct_to_dBm, which takes a peak power measurement and converts it to transmitter power at the center frequency of the transmit signal.
We leave four A3008Ds running for five days, and re-calibrate them. Spectrometer Q0158's frequency calibration remains 57, but power calibration increases from 3.2 dB to 4.0 dB, and LO output power increases from 2.8 dBm to 3.7 dBm. Spectrometer Q0159's frequency calibration remains 50, but power calibration rises from 2.2 dB to 2.6 dB and LO power drops from 3.2 dBm to 1.6 dBm. Spectrometer R0201's frequency calibration remains 58, but power calibration increases from −1.2 dB to +0.6 dB, and LO power drops from 6.3 dBm to 4.5 dBm. Spectrometer R0202's frequency calibration increases from 57 to 58, its power calibration increases from −1.2 dB to 0.6 dB, and its LO power output drops from +5.3 dBm to 4.3 dBm.
The power calibration of our spectrometer circuits we obtain with a reference oscillator applied to the input. But the main purpose of the A3008 is to measure the power of subcutaneous transmitters. We have a Subcutaneous Transmitter (A3028) with a BNC socket soldered onto its RF output. We pass through a 6-dB attenuator and mix with 869.4 MHz in a ZAD-11. We trigger our scope from TP1 on the transmitter circuit. The IF amplitude is 37 mVpp. We set our Q0158 spectrometer to 915 MHz LO and measure LO power 4.0 dBm. We pass through a 6-dB attenuator and mix with 869.4 MHz. The IF amplitude is 142 mVpp. Thus the power delivered by the transmitter to its BNC connector is −7.7 dBm. We pass the transmitter signal through a 30-dB attenuator and apply to the Q0158 input. We adjust the Spectrometer's sct_to_dBm value to 1.0 dB, and the SCT peak power in the Spectrometer is −37.7 dBm. And so we obtain a value for sct_to_dBm, which takes a peak power measurement and converts it to transmitter power at the center frequency of the transmit signal.
[08-MAR-16] We consider the possible use of power received from a subcutaneous transmitter as an indication of its location with respect to the receiving antenna. We measure received power versus position for a 50-mm bent transmit antenna in water and both 100-mm and 10-mm loop antennas in air.
[11-MAR-16] The 100-mm loop antenna can receive less power as the transmitter comes nearer. The tiny loop antenna receives more power at shorter ranges, but far less power in absolute terms for ranges 5 cm and up. We make a 15-mm long quarter-wave antenna. The permittivity of water is 80 times higher than that of air, so the resonant length of the quarter-wave antenna in water is roughly 9 times shorter than the 80-mm it is in air. So we expect resonance at 9 mm in water. We are starting with 15 mm.
In air, this antenna picks up −61 dBm from a nearby transmitter. We pour water into the beaker as shown above, and received power increases to −49 dBm. We clip the antenna back to 9 mm and we get −54 dBm. We increase to 20 mm and get −52 dBm. We cut back to 15 mm and get −48 dB. See here for measurements of the ideal length of a transmit antenna in water. We measure received power for the 15-mm quarter-wave antenna in water and in air.
The straight antenna in water acts like a much larger antenn: power can decrease as the transmitter comes near. We make a 15-mm loop antenna and put it on a right-angle BNC plug, so that it rests upon our wood table. This antenna picks up −27 dBm when the transmitter is right over the antenna at a range of a few millimeters. Power greater than −45 occurs if and only if the transmitter is within a few centimeters of the antenna. We raise the 15-mm diameter loop 20-mm off our table and lay a conducting sheet on the table to act as a ground plane. We move our transmitter along the same path as before, but 60-mm above the ground plane. Now we pick up less than −60 dBm everywhere, although we do get more power the closer the transmitter is to the antenna. We move the transmitter around on a stick and find that, almost all the time, power greater than −50 dBm, regardless of orientation and position, indicates the transmitter is within 5 cm of the antenna, and power less than −60 dBm almost always indicates that the transmitter is farther than 10 cm from the antenna.
[15-MAR-16] We make an unterminated 10-turn coil of solid wire 20 mm in diameter and support it 20 mm above a ground plane. Along a platform 60 mm above the ground plane, we move an A3028B mouse-sized transmitter with 30-mm antenna in a 30 ml of water with its antenna loop roughly 20° to the horizontal. We move a distance 30 cm along the platform for various orientations of the transmitter cup about the vertical, so simulate an animal turning around as it moves about a cage.
The coil has rotational symmetry about the vertical axis, but the transmitter does not, hence the variation in reception with transmitter orientation. But we do not expect these graphs to vary as we change the line of motion with respect to the coil. We are usually able to detect the presence of the transmitter 10 cm away. Power at range 0 cm is always greater than power at range 5 cm. An array of such coils on a 10-cm grid could provide location tracking with accuracy ±3 cm, and resolution ±1 cm.
[22-MAR-16] We wind a 3.5-turn 13-mm diameter loop of hookup wire and solder it to a right-angle BNC plug. According to an inductance calculator, this coil should have indutance roughly 160 nH. We combine the antenna with a 3-dB, 50-Ω attenuator, after noting that the power delivered by the antenna increases by 10 dB when we add the attenuator. We place the center of the coil 30 mm above a ground plane made of conducting graphite sheet.
We move the transmitter as before, 6 cm above the ground plane, which is 3 cm above the coil, from position (−15 cm, 0 cm) to (+15 cm, 0 cm) in four orientations. We measure received power every centimeter.
We place the transmitter at the same height, but in five different locations with respect to the fixed receiving coil antenna. At each position, we rotate by 360° in 22.5° steps and record received power.
We see that our receiving coil provides at least 5 dB more power for a transmitter directly above the coil than for a transmitter 5 cm away from the coil, for all orientations and directions.
Suppose we have an array of five receiving coils arranged at positions (0,0), (-5,0), (5,0), (0,5), (0,-5). We have a transmitter directly above the central receiving coil. We measure power received by all five coils. We subtract a threshold of −80 dBm from all our power measurements, this being the power we get when we turn off the transmitter, and so obtain the power above minium for each antenna. We take a weighted centroid of this logarithmic power measurement for each of our sixteen orientations, and so obtain a measurement of position that simulates a transmitter rotating above a receive coil with an array of five coils spaced on a 5-cm grid. The plots below show how the x and y location varies with rotation.
Coils far from the transmitter will have a large effect upon measured position, even if the power they detect is barely higher then our noise level. We introduce into our weighted centroid calculation a power threshold. If a coil power is greater than the threshold, we subtract the threshold to obtain the net power. If the power is less than the threshold, we set the net power to zero. We will use net power in our centroid calculation so as to eliminate the influence of far-away coils. Conversely, a coil directly under the transmitter does not contribute significantly to the overall position measurement, other than to reduce the contribution of the other coils. We remove the central coil measurements of our experiment so as to obtain a position measurement from four coils arranged on a 10-cm square, with the transmitter in the center of the square.
The standard deviation of position is now 15 mm, and we have a step-change in measured position of 50 mm when we go from rotation −22.5° to 0°.
[29-APR-16] We take our 910-MHz SAW Oscillator (A3014SO) and apply its output to an un-calibrated Spectrometer (A3008B) and average power with an early version of the Spectrometer Tool. We use attenuators to vary the power we deliver to the A3008 and check the delivered power with our vector voltmeter.
The slope of the linear part of the graph is 1.00 dB/dBm. The A3008 fails for constant-frequency inputs higher than −20 dB, because the VCO that generates the LO locks its frequency to the RF input and the IF is too low to pass through the IF amplifier's high-pass filters.
[05-OCT-25] We define version A3008E to provide a programmable frequency sweep output. The sweep output on our hand-held and bench-top commercial instruments is too slow for our dynamic tests of demodulation tuning. In the case of tuning an A3038DM-D detector module, we are using a screwdriver to adjust a capacitor and we want to see immediately on the oscilloscope the effect of our movements. We update A3008 documenation.
[06-OCT-25] We open and modify R0201, previously A3008D. We drop C2 to 1.0 μF, which allows the MAX6239 regulator to power up more easily, so it does not latch up. We replace R30 with two diodes in series, using an MA125, so that the DAC reference voltage is close to 1.44 V, and more stable with respect to the value of our +15-V power supply, so as to improve accuracy. We solder 1.0 kΩ to U11-7 and bring this DAC ouput signal through the resistor to a front-panel BNC connector. We reprogram with P3008B01. We define the following opcodes for LWDAQ commands, where the opcode is the four-bit value [DC4..DC1].
| opcode | value | function | |
|---|---|---|---|
| nop | 0 | no operation | |
| set_start | 1 | start frequency := [DC16..DC9] | |
| set_end | 2 | end frequency := [DC16..DC9] | |
| set_dwell | 3 | dwell value := [DC16..DC9] | |
| sel_gnd | 4 | select ground analog return | |
| sel_if1 | 5 | select +0 dB gain | |
| sel_if2 | 6 | select +22 dB gain | |
| sel_if3 | 7 | select +44 dB gain |
The firmware increments the frequency count every (dwell+1) μs starting from the start frequency and ending with the end frequency. A dwell value of zero will lead to no sweep or DAC value writing at all. The dwell and end frequencies reset to 255. The start frequency resets to zero. In this reset state, our dwell time is 256 μs and our sweep takes 65 ms. The kind of sweep we are looking to implement for receiver calibration is 880-950 MHz, which will be around 90 steps. With dwell value of 3, this sweep will complete in 360 μs, so it will be running at 2.7 kHz, which is fast enough.
The new RFPM Instrument operates the A3008E, but is incompatible with the A3008D and earlier versions. So far as we know, there is only one A3008 in the field, at Edinburgh University. We have three in-house. We will update them all and drop support for the A3008A-D in LWDAQ 10.7.5. Because the DAC write is faster, the A3008E is faster at taking spectra. When measuring power, the RFPM does not use a constant LO value, but intead makes use of a sweep of "dither+1" steps with dwell time "dwell+1" microseconds. Our initial values are dither=1 and dwell=3. With the dither in the LO, we eliminate the occasional zeros in power measurement that occur when our source is in phase with our LO. We also increase the width of the spectrometer window. So will increase the capacitors in the IF filter from 100 pF to 200 pF, which will drop the corner frequency of the four-pole Butterworth filter from 1.6 MHz to 0.8 MHz. The window will now be 1.6 MHz. With our dither of 1 MHz, the effective window will be 2.6 MHz, slightly smaller than our original 3.2 MHz. New second page of schematic is S3008C_2.gif. We produce but do not test P3008B02, which drops DAC update time from 1.85 to 0.925 μs and adds a flash of LED2 for each command reception.
[07-OCT-25] Our P3008B02 is working. We are able to provide dwell times 2 μs to 256 μs. We run the LO output from R0201 through −30 dBm to an A3038DM-D detector module input. We trigger on the falling edge of our sweep's TUNE voltage. We look at the power measurement voltage, P, on the detector module, as well as the demodulator output, D. To configure the sweep we have: sweep start 0x00, sweep end 0x70, and dwell 0x03. With dwell = 3 we have 4-μs dwell time per step, and in the demodulator output we see the eight-bit ADC steps clearly. Each step is roughly 0.8 MHz.
We copy today's version of the RFX.tcl Toolmaker script and adapt for calibration of our A3008E with the help of an RF Explorer spectrum analyzer, which we are controlling and reading out over USB. We create Calib.tcl, a Toolmaker script that steps through a range of DAC values using the RFPM Instrument at each step, and measures for each step the A3008E output frequency and power. We revert to an older LWDAQ and repeat for an A3008D. We plot both calibrations below.
The steps in the calibration are due to the resolution of the hand-held spectrometer. Our objective is to obtain a good calibration of the spectrometers at a reference frequency 915 MHz, combined with an accurate slope. For R0201 we have dac_ref 57.2 cnt and slope 0.824 MHz/cnt. For R0202 we have dac_ref 60.8 cnt and slope 0.772 MHz/cnt.
[08-OCT-25] We improve our calibration code. We increase C14-C17 to 200 pF. We apply first −60 dBm at 905, 910, 915, 920, and 925 MHz and then −30 dBm. We choose our power calibration to correct the −60-dB signals and find that our −30 dBm signals are measured as 3 dB too low. At no point during these measurements do we see any zeros in the power values. Our dither of one count is proving effective at stopping coherence between the signal and the LO.
[21-OCT-25] We upgrade R0202 from A3008D to A3008E. We check our hand-held RFX spectrometer against our SSG-6001-RC and then connect the RFX to our calibration program. We upgrade Q0159 as well. Now we have three completed A3008E and we plot their calibrations all on the same graph below.
