Animal Location Tracker (A3038)

© 2020 Kevan Hashemi, Open Source Instruments Inc.



The Animal Location Tracker (A3038) provides a horizontal array of fifteen detector coils that receive signals from our subcutaneous transmitters and implantable stimulators. Above the coil array is a platform upon which we place a cage containing one or more animals with implanted transmitters. The A3038 decodes and records the signals transmitted by each implanted device. In addition, the A3038 measures the power received by each of its detector coils from each implanted device, and so estimates the position of each device. The A3038 is designed to measure the activity of animals in cages, and to make it possible to identify animals in video by correlating their movements with the movements of their implanted transmitters. Each transmitter has its own unique channel number, so each animal has its own unique location and velocity.

Our implanted transmitters emit 7-μs bursts of electromagnetic radiation in the range 902-928 MHz. Each burst contains a digital message. Each detector coil provides a measurement of radio-frequency power in this same frequency range. When one of the detector coils reports that a message transmission is in progress, all fifteen detector coils record the power they are receiving. The A3038 saves the transmitted message, along with fifteen eight-bit power values, in its memory, available for download by the Recorder Instrument in the LWDAQ Software, or download and storage to disk by the Neuroarchiver Tool. The Neuroarchiver contains a Tracker button that opens the Neurotracker Panel, which displays the measured position of transmitters on the tracker platform.

Figure: Neutotracker Panel Provided by the Neuroarchiver. Default parameters to control the location calculation are shown along the top.

The absolute accuracy of the A3038 in measuring position of a transmitter in a beaker of water or implanted in an animal with respect to the coordinate system defined by its coil centers is ±2 cm 90% of the time, and ±10 cm the rest of the time. Because there is no way to know whether the absolute position is accurate or not, we find that the absolute position measurement provided by the A3038 is not useful. Its measurement of the direction and magnitude of movement, however, provides a reliable measurement of animal activity, and a reliable way to identify animals in video. The correlation between the A3038 measurement of movement and video blob-tracking permits us to be 100% certain which blob corresponds to which animal, even when there are a dozen animals in the field of view.

Version X (cm) Y (cm) Coil Pitch (cm) Coil Type Num Coils Comment
A3038A 52 26 13 33 nH SMT 15 100.0% accurate disambiguation for height 0-5 cm.
Table: Versions of the A3038.

The A3038 communcates with our data acquisition computer through an ethernet connection that also provides power for its detectors, amplifiers, and data buffer. We use a standard Power Over Ethernet (PoE) switch to provide communication and power delivery for multiple A3038s installed in faraday enclosures. We connect our data acquisition computer to the same switch, and so download the signals and power measurements from multiple A3038s.


S3038X_1: Prototype power detector and demodulator. Gerber Files for A3038X printed circuit board.
A303801X_Top: Top view of A303801X printed circuit baord.
A303801X_Bottom: Bottom view of A303801X printed circuit baord.
LT5534: Radio-frequency power detector.
ADC081S101: Eight-bit serial ADC.
BGA2803: Low-power 23-dB DC-2GHz amplifier.
2014VS: Vertical, surface-mount inductor.


Current Consumption




[14-JUL-20] We apply a −6 dBm sweep 840-980 MHz to a Loop Antenna (A3015C). We generate the sweep with a Modulating Transmitter (A3014MT), and we split the sweep and mix with 910 MHz to produce an IF ±70 MHz, which we run through a 21-MHz low-pass filter before viewing on the scope. We hole the loop antenna above Coil 10 on V0384 and obsever the following response on PW, which is U1004-3.

Figure: Sweep Response of Power Measurement. Yellow: PW on U1004-3, 500 mV/div. Green: Ramp voltage that controls A3014MT. Blue: IF reference, center is 910 MHz, left and right edges of bulge are 890 MHz and 930 MHz respectively.

We repeat on all fifteen coils and find the same response on each one, ±100 mV variation in 890-930 MHz, which is roughly ±3 dB. We remove the A3051C loop antenna and replace it with a 3-dB attenuator and a 50-mm bent wire antenna. We observe the same ±3 dB variation on power through the pass band. But in rare orientations of the antenna, all surrounding objects remaining stationary, received power drops suddenly, and variation is ±6 dB.

[29-JUL-20] We are considering using detector diode such as the SMS7630 in our coil amplifier to provide power limiting and power detection.

Figure: Behavior of the SMS7630-061 Schottky Diode. Left: Current versus Forward Voltate. Right: Rectified Voltage versus Input Power.

In SkyWorks application note APN1014, we see the detector ciruict they used to obtain the above rectified voltage versus input power. They deliver power from a 50-Ω source, but do not load the source with 50 Ω. Instead, they place a diode and balast capacitor in series with the 50-Ω source impedance. The voltage across the diode will be roughly double what we would see if we loaded the source with 50 Ω. Their "Incident Power (dBm)" is the power that reflects off the detector circuit, which would be equal to the power delivered to a 50 Ω load. The "video resistance" they refer to in the detector plot is the resistor loading the balast capacitor.

We assemble a power detector made out of an SMS7630 diode and a 100 pF balast capacitor attached to a 50-Ω transmission line carrying the output of an A3029B amplifier. We measure the voltage across the balast capacitor, which we call the rectified voltage. We vary the transmission line power from −30 dBm to +28 dBm. Below −30 dBm our rectified voltage is swamped by noise. At +28 dBm, our amplifier is saturating. When we remove the diode and capacitor, our amplifier saturates at +30 dBm. With +6 dBm we add 1 kΩ in series with the diode and see only 90 mV, compared to 360 mV with no resistor.

Figure: Rectified Voltage versus Input Power for SMS7630 Power Detector. Incident power is terminated by a resistor. In parallel with the resisstor is the diode and a capacitor.

We solder 51 Ω from the center pin of a BNC socket to ground. In parallel we place a SMS7630 diode and a 1.0 nF balast capacitor. We supply power down a 1-m coaxial cable from our synthisizer, and vary power from −30 dBm to +10 dBm. We measure the voltage rectified voltage versus power and plot.

[30-JUL-20] We solder a 33 kΩ resistor across a BNC socket. In parallel we place a SMS7630 diode and a 1.0 nF balast capacitor. We connect the detector directly to the output of our synthisizer. Without the 33 kΩ, we see no sustained rectified voltage, because the incoming power is capacitively coupled.

[07-AUG-20] We have the schematic of a prototype detector coil circuit, which provides both power measurement and demodulation of SCT messages, S3038X_1. Top view of circuit board here. This circuit will provide the D input for an A3007D so as to provide SCT signal reception, and will also produce an output P from the power meter. Power supply will come from a two-pin molex plug.