Figure: RF Spectrometer (A3008B) with Loop Antenna.
The RF Spectrometer (A3008) measures RF power in 3-MHz windows between 850 MHz and 1100 MHz. If you connect the A3008's RF input to an antenna, the A3008 will measure the power spectrum of RF signals arriving at the antenna with 1-MHz resolution and sensitivity 10 pW (−80 dBm). You can use the A3008 to measure average power or peak power. If you would like some help with radio-frequency jargon, see our presentation of Terminology.
The A3008 is a LWDAQ device. The LWDAQ software's Radio-Frequency Power Meter Instrument (RFPM) obtains RF power measurements from the A3008 and the Spectrometer Tool gathers these power measurements together into a graph.
The A3008 allows us to measure the ambient radio frequency power in the 875 MHz to 1050 MHz range, so that we can choose a 20-MHz wide band free of interference in which to operate our Subcutaneous Transmitter (A3009). We can also use the A3008 to measure the power received by an antenna from such transmitters, even though the transmitters are active for only 7 μs out of every 2000 μs. The A3008, combined with the RFPM instrument, allow us to measure peak power received rather than average power, and this peak power will detect the bursts of power from our transmitters.
The A3008 uses an eight-bit DAC to set the TUNE input of a MAX2624 VCO. For DAC values 0 to 255, the VCO output frequency increases from roughly 850 MHz to 1050 MHz. The A3008's RF input is amplified and passes into a mixer, where it is downshifted by the VCO frequency. The resulting IF is low-pass filtered with cut-off frequency 1.5 MHz. The power present at the output of the low-pass filter is proportional to the power present at the A3008's RF input in a 3-MHz wide frequency window centered upon the VCO frequency.
All version of the A3008 suffer from a +0.4 MHz/°C shift in measured frequency with ambient temperature. This shift occurs as the VCO frequency changes by −0.4 MHz/°C. Firmware P3008A01.abl for the A3008A suffers from calibration drift as you take continuous spectra. The VCO warms up as you use it, but cools down when you don't. Firmware version P3008A02.abl for the A3008A solves this problem by leaving the VCO on all the time, so it reaches thermal equillibrium. But you must let the spectrometer warm up for a few minutes to allow it to reach this equillibrium. The A3008B provides a READY light that flashes after power-up until the warm-up period has expired.
The A3008 is a simple circuit. It uses no sophisticated components, other than the programmable logic chip that does the job of communicating with our data acquisition system. The A3008A and A3008B differ in several respects, as described below. The two versions have enough circuit differences that they have separate schematics.
S3008A_1: The LVDS receiver for connection to the LWDAQ, the programmable logic chip, and 32.768 kH reference oscillator, U6.
S3008A_2: The antenna aplifier, mixer, low-pass filter, IF amplfiers, gain selector, local oscillator, local oscillator control, calibration oscillator, and calibration frequency SAW filters.
S3008B_1: The LVDS receiver for connection to the LWDAQ, the programmable logic chip, and 32.768 kH reference oscillator, indicator LEDs, and U6.
S3008B_2: The antenna aplifier, mixer, low-pass filter, IF amplfiers, gain selector, local oscillator, local oscillator control, calibration splitter and calibration socket.
P3008A: Firmware program for the logic chip for A3008A.
P3008B: Firmware program for the logic chip for A3008B.
The A3008 amplifies its RF input by 18 dB and feeds it into a frequency mixer for downshifting. The mixer's LO signal comes from a MAX2624, whose output frequency is set by its TUNE input, as shown in the figure below. The IF output from the mixer passes through a 1.6 MHz low-pass filter. The filtered IF signal is the downshifted version of the RF frequencies within 1.6 MHz of the LO frequency.
Example: If the LO frequency is 950 MHz, then any RF signal whose frequency is in the range 950±1.6 MHz will emerge from the mixer as an IF signal in with frequency in the range ±1.6 MHz, and so pass through the IF filter.
The A3008 measures RF power in a 3.2-MHz band centered upon its LO frequency. We set the LO frequency using U11, and eight-bit DAC. The DAC can vary the LO chip's TUNE input from 0 V to 2.8 V in steps of 11 mV. The RFPM Instrument allows you to specify the DAC value with its daq_dac_value parameter. When daq_dac_value is 0, the TUNE input will be 0 V. When daq_dac_value is 255, TUNE will be 2.8 V.
Because each step is 11 mV, and the slope of frequency with tuning voltage is 75 MHz/V, we expect the frequency to increase by ≈ 0.8 MHz per count.
The filtered IF signal is called IF1 in the schematic. The A3008 amplifies IF1 by ×11 to create IF2, and then amplifies IF2 by 11 to create IF3. The A3008 allows you to select any one of these three IF signals, and also a 0-V reference. The RFPM records all four of these signals using the LWDAQ driver's 8-bit ADC. Each ADC sample takes a minimum of 500 ns. If you set daq_delay_ticks to 0 in the RFPM, the ADC samples will be spaced by 500 ns, so the IF signals get sampled at 2 MSPS. We decrease the sampling frequency by adding 125 ns "delay ticks" to the sampling steps. The daq_delay_ticks parameter is the number of 125-ns delays you would like added to each 500-ns ADC sample. The daq_num_samples parameter tells the RFPM how many samples to take of each IF signal. We obtained most of the graphs below with 20 delay ticks and 10,000 samples. The sample period is 3 μs, the sample frequency is 333 kHz, the time per display division is 3 ms, and the total observed interval is 30 ms.
The RFPM plots all IF signals that do not exceed the dynamic range of its display (this makes sure we don't obscure a useful trace with a large, saturating IF3 signal). It returns the peak-to-peak voltage it obtains from each of its four IF channels. The Spectrometer Tool translates these peak-to-peak voltages into RF power in decibels, and plots the power in its graph window. The spectrometer provides its own help with its Help button.
The A3008A and A3008B differ in the following respects.
Plug the A3008 into the LWDAQ. On the A3008B, the green LED should start flashing. If it doesn't, unplug the A3008B and plug it in again. The green LED is the READY indicator. It flashes for a few minutes, after which time the A3008B has warmed up, and it shines continuously. The A3008B does not measure the temperature of it's on-board VCO (voltage-controlled oscillator). All it does to generate the warm-up period is measure a time interval from power-up. Even a short interruption of power will start the READY light flashing again.
If you use the A3008 before it's warm-up period of roughly four minutes has passed, its frequency calibration may be off by up to 4 MHz. Other than this frequency error, the A3008 will work perfectly well. For a plot showing how the A3008B's on-board oscillator frequency varies during warm-up, see below.
Open the RFPM instrument in your LWDAQ software. Set it up to acquire from your A3008. You will need to set the daq_driver_addr and the daq_driver_socket. Press Acquire. You should see waveforms on the screen, and with each acquisition, four numbers appearing in the text window, like this, or this. Whenever you press Acquire, you should see the yellow ACTIVE light on the A3008B flash.
Open the Spectrometer Tool from the Tools menu. Press Run. It will begin to plot a spectrum for you. Starting with Spectrometer Version 11, the plot is in units of dBm (decibels above 1 mW). Earlier versions plot the power in units of dB relative to an arbitrary and uncalibrated power level.
Once you see a spectrum appearing, stop the Spectrometer with the Stop button, press Clear to clear the graph, and cover the A3008 with a foil shield. Set the step parameter to -1 and press Run. Let the spectrometer plot its own noise. When it has finished plotting the entire graph from left to right, press Stop and select a new active graph. Remove the foil shield, set step to -1, and start again. this will be a measurement of your ambient radio-frequency power.
With the Save and Load buttons, you can save and load spectra as text files. The raw Spectrometer data we link to in the sections below is data we saved to disk in this way. You can download it, look at it with a text viewer, and read it into your own Spectrometer to see the graphs.
The A3008A provides a crude and ultimately unhelpful means of calibrating its own frequency response. The RFPM activates this self-calibration when daq_calibrate is 1. There is a second VCO (U16) on the A3008, which we call the calibration oscillator (CO). We use the CO to excite three SAW filters. The outputs of these three filters are weakly coupled into the A3008's RF input. We sweep the CO frequency from 875 MHz to 1050 MHz over the course of 30 ms, and simultaneously record on of 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. But that was after we performed experiments with each circuit using each filter in isolation.
The autocalibration 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 with some simple external-calibration circuits. We removed the calibration option from the LWDAQ software's RFPM instrument.
The A3008B does not provide the self-calibration circuits. Instead, it provides a socket from which you can extract a fraction of the LO power, and so calibrate the LO with respect to DAC output. The new circuits are shown on page two of the schematic. For this calibration, you would need a precision local oscillator and a mixer, as we use here. We use the A3008B's RF input to make precise calibration measurments in the following sections.
You can perform a one-point calibration of the A3008A or A30008B by turning on a reference oscillator, such as the 910 MHz SAW Oscillator (A3016SO). Take a spectrum, and you will see a peak at the SAW Oscillator frequency. From there, you can estimate nearby frequency response with the nominal 0.8 MHz/count slope of the A3008 frequency response.
[MAY-07] We took the LO output from our A3008B and fed it into the RF input of a mixer. We used our 868-MHz SAW Oscillator (A3014SO) as a source of LO. We measured the period of the IF on our oscilloscope. We set plugged power into the A3008B, set its frequency-control DAC to 10 counts immediately, and started measuring the A3008B's VCO frequency. We obtained the plot below.
We concluded from this plot that a four-minute warm-up period would be adequate for 1-MHz calibration accuracy. There is little point in looking for more than 1-MHz accuracy, because the temperature coefficient of the MAX2624 frequency is −0.4MHz/°C, so a 5°C change in room temperature will cause a 2 MHz error.
Using the apparatus we described above, we measured the VCO frequency with DAC setting across the full A3008B operating range. We obtained the graph below.
We confirmed with separate measurements that the VCO frequency was 900 MHz at DAC=35, 915 MHz at DAC=52, and 930 MHz at DAC=70. We wrote these numbers on the circuit board, thus giving us the 900-MHz ISM-band calibration.
With count=1, the VCO frequency is within 200 kHz of our 868-MHz LO. The IF we are looking at on the oscilloscope is 200 kHz. We know from other measurements that the phase noise on our 868-MHz SAW Oscillator is unobservable on a 200 kHz IF, but we see ±200 ns jitter on the 200 kHz period we obtain with the A3008B's VCO output. We also observe drif of ±1 μs in the period. The VCO frequency is unstable on the level of 100 kHz. Because our spectrometer window is 3 MHz wide, this 100 kHz instability is insignificant.
Our 910-MHz SAW Oscillator (A3014SO) has power output 13 dBm. We fed its input through attenuators to the RF input of our A3008B. We started with a 30-dB attenuator. We opened the Spectrometer Tool and obtained a spectrum of the 910-MHz signal. We found the peak lay at DAC value 48. We set step_increment to 0. Each time we pressed Acquire, we obtained a new measurement of the 910-MHz power arriving at the RF input. We now used attenuators to supply between −20 dBm (10 μW) and −110 dBm (10 fW) to the RF input. We plotted the Spectrometer Too's measured power against actual input power.
The slope of the linear part of the graph is 0.98 dB/dBm, which is close enough to 1.0. Starting with Spectrometer Version 11, the display will be in dBm, with the plot running from power_min to power_max in units of dBm. To convert the RFPM voltage measurement into dBm, we set the pp_1mW parameter to the peak-to-peak amlitude that corresponds to a 1-mW (0 dBm) input. For the A3008A and B, we use pp_1mW equal to 1600 V. The plot above was obtained with pp_1mW equal to 0.008. (The intercept of the plot is −106 dBm and 1600 is 106 dB greater than 0.008.)
All the following spectra we obtaines with the Spectrometer Tool, which saves and loads graphs. All the graphs recorded berfore June 2007 use the Spectrometer's raw, uncalibrated power scale, and are all measurements of the peak power received during the measurement time interval and within the A3008's 3.2-MHz frequency window. After that, the graphs are in dBm, which is dB relative to 1 mW, and the measured power can be peak or average power observed during the measurement time. We will specify peak or average in our description of the spectrum.
Click here to see the numerical data acquired by the Spectrometer for dual A3006 SCTs at range 4 m. Each point in the graph consists of a LO fequency step (0-255), a measurement of RF power made at that frequency step (in decibels), and a graph letter, which assigns the data to one of the several graphs the Spectrometer can plot.
SCT Spectrum: Radio frequency power spectra recorded with the A3008 in the Brandeis University Physics Department basement. The graph shows peack RF power received during a 30-ms recording period for frequencies between 875 MHz and 1050 MHz. The vertical axis gives the received power on a logarithmic scale, with each division being a factor of ten (10 dB). The horizontal axis gives frequency on a scale that is almost linear. The red spectrum we obtained with the antenna covered with foil. The green spectrum is with the foil removed, but no SCT active. The blue spectrum we obtained with an A3006 SCT 2.5 m from the A3008 antenna. You can see the 875-MHz zero-bit transmission power on the left side of the plot, and the 950-MHz one-bit transmission power towards the center. The SCT modulates its frequency between 875 MHz and 950 MHz. The yellow line shows power passing through the A3008's on-board 915±12 MHz calibration SAW filter. The dark blue line shows power passing through its 957.5±12.5 MHz calibration filter. The vertical lines are frequency markers we have placed in accordance with the yellow and dark blue spectra. Each line is defined by three numbers: frequency in megahertz, position on the horizontal axis, and color.
SCT Near and Far: Here 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 selectted 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.
Second Circuit: We made a second A3008 and used it to record these spectra. Note how the step-value corresponding to the SCT peack has changed.
Dual SCT: The spectrum obtained from two operating SCTs at range 4 m in the basement lab. You will find the raw data for this graph here.
Outside: You will find the raw data for this graph here. We recorded the green spectrum outside the Brandeis University Physics Department 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 tempreature. 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.
Receiver Emissions: You will find the raw data for this graph here. The two peaks at approximately 900 MHz and 1000 MHz in the red spectrum are emissions from two A3005 Receivers. We unplugged the first receiver, which is one we have enclosed in an aluminum box, and took the green spectrum. The 1000 GHz peak disappeared. We unplugged the second receiver, and plugged the first one in again. The second receiver does not have a box. It is the one we received back from London recently, and which we modified and adjusted extensively in London during my January visit. With this receiver unplugged, we see the 900 MHz peak disappear. Its LO frequency was adjusted to 900 MHz instead of 1000 MHz. The receiver will still work with LO 900 MHz, but it will give poor performance at close range, which is what we observed in London. The receiver is more effective when the LO is 950+50 MHz instead of 950−50 MHz. You will also notice that the 1000 MHz peak due to the LO of the first receiver appears to move up in frequency when we plug it in again. This is because the circuit in its aluminum box had time to cool down between spectra. The VCO was cooler during the second spectra, because the aluminum box had not yet warmed up with the heat of the RF amplifiers (the VCO itself generates very little head). When the VCO is cooler, its output frequency rises for the same TUNE input, which explains the rise in the frequency of the peak.
We obtained the above 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. To our delight, we see the curious 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. You will find the raw data here.
Precisely-Calibrated Spectra: These are taken [MAY-07] with the new A3008B after we calibrated its ISM-band frequencies to 1-MHz accuracy, as described above. The raw data is here. Trace A (red) is with the RF input terminated with 50 Ω. Trace B (green) 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. Trace C (blue) 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 Trace D (orange) we insert a 20-dB attenuator in the antenna line. For Trace E (yellow), we turn off the 910 MHz and turn on a Subcutaneous Transmitter (A3013A) and place it 50-cm from the Spectrometer antenna. For Trace F (purple), we move this same transmitter to a range of 5 m.
Below is a detail from the low-frequency end of our outside spectrum.
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.
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 intereference, 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 |
And finally, the interference in our basement laboratory, ignoring that coming from the SCT dual receivers themselves.
| 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, becase we have observed the reception at range 6 m to be stable with a dual receiver in our laboratory.
It is clear that here on Brandeis University campus, operating in the 1000-1050 MHz would be perfect. A SAW filter with 30 MHz bandwidth and center frequency 1030 MHz is available from Golledge. Below is a fragment of a chart giving the United States radio frequency allocations.
As you can see, 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 measureable 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.
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.
Figure: Spectrum and RFPM Display for Interference in Geneva. Green trace is with no SCTs plugged in, red trace with one plugged in at range 1 m. The blue lines mark 900, 910, 920, and 930 MHz.
The above figure shows interference power peaking about 10 dB higher than our signal power at 1 m, even in a favorableorientation. 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.
