Figure: The 915 MHz Crystal Oscillator (A3029XO-915). The circuit consists of a 457.5 MHz crystal oscillator, a frequency doubler, and an amplifier.
The Implantable Sensor with Lamp (A3030C) is equipped with a 50-mm data transmit antenna and a 50-mm command reception antenna. Both antennas are designed for 915 MHz when implanted in an animal, but as with SCT antennas, they work well enough in air as well. Now that we have the same frequency in use for data and command communication, the command antenna picks up the outgoing 915 MHz power from the transmit antenna, resulting in self-reception by the 915-MHz crystal radio, as shown below.
Figure: Self-Reception at 915 MHz. The 50-mm data and command antennas are parallel and separated by 2 mm. Top trace is the crystal radio output recorded with ×1 probe, 20 mV/div. Bottom trace is the 5-MHz data transmission clock recorded with ×10 probe, 1 V/div. Time is 2 μs/div. The data transmission lasts for 7.7 μs.
A self-reception pulse like this during command reception is likely to corrupt the incoming command. So the A3030C disables its data transmission during command reception and processing. Because commands begin with a minimum 5-ms pulse of 915-MHz power, the self-reception cannot be confused for command initiation, and once initiation has taken place, the self-reception is suppressed until the command is complete. Given that the 915-MHz command transmission will in any case overwhelm the much weaker data transmission, the suppression of data transmission during command transmission does not cause any additional loss of data.
We place a Loop Antenna (A3015C) at the center of a Faraday Enclosure (FE2B). We connect a white LED to our A3030C. We transmit a command to flash the LED once, and repeat this command at around 20 Hz. We move the A3030C around in the enclosure for 60 seconds and observe a total of 2 s of loss broken up into around six different intervals. We appear to have 97% reception of commands within the enclosure.
We work for several hours with the circuit, stimulating it, turning on and off the data transmission, and measuring the noise induced in the EEG amplifier by the lamp current pulses. We find reception within the faraday enclosure to be reliable. With the enclosure lid off, we can receive commands within roughly 100 cm of the command transmit antenna.
We have taken the well-establisehd SCT antennas and faraday enclosure and used them in reverse for ISL command reception. The A3015C loop antenna now transmits 1 W of 915-MHz power, and the 50-mm ISL command antenna receives this power. Meanwhile, the 50-mm ISL data antenna transmits 200 μW of 915-MHz power, and another A3015C loop antenna receives this power.
In the long run, we will be able to combine the two ISL antennas into one, and combine the two A3015C antennas into one, with the help of radio-frequency switches. The ISL can transmit "command received" messages through its data antenna, which means it will eventually be possible for us to know when a command has not been received. Just as we have multiple data reception antennas in the SCT system, we could have multiple command transmission antennas in the ISL system. If a command is not received from one command antenna, we can re-broadcast the same command with another command antenna.
Because the SCT 915-MHz communication works well for implanted SCTs, and because we can provide multiple command transmit antennas in the future, we believe we can rely upon 915-MHz command reception in implanted ISLs.
