ISL Operating Range at 146 MHz

We compare reception of repeated commands by the A3024A and the A3030A. We fasten an example of each to a piece of cardboard.



Figure: Two ISLs Strapped to Cardboard for Command Reception Test.

The A3024A simply turns on its lamp when it detects radio-frequency (RF) power above a threshold. We set the threshold as low as we can. The A3030A turns the incoming RF signal into a sequence of byte instructions with a checksum at the end. When we send repeated commands to flash the A3030A lamp, the A3024A lamp illuminates while the command is being transmitted, and the A3030A lamp illuminates afterwards, provided the command has been received without corruption.

We use our 146 MHz Command Transmitter (A3029A) with boost power to produce the RF commands. We measure maximum range of reception, for a favorable orientation, and maximum range for reception in 95% of orientations. We also test reception with the two devices in water, as shown below.



Figure: Two ISLs In Water for Command Reception Test.

We measure maximum range for reception in water. Now we add 5 g of salt to the 500 ml of water and stir until dissolved. We measure maximum range again. We present our observations below.

Test	A3030A No1.2	A3024A No7
Robust Reception Range on Cardboard	70 cm	100 cm
Maximum Range on Cardboard	250 cm	600 cm
Maximum Range Held By Lamp and Dangling	300 cm	900 cm
Maximum Range in Water	70 cm	450 cm
Maximum Range in Saltwater	70 cm	450 cm

Table: Comparison of Reception for Encapsulated A3024A and Encapsulated A3030A.

It occurs to us that encapsulation or an exhausted battery might affect No1.2's reception, so we try an un-encapsulated A3030B with a fully-charged battery and measure maximum range in air. We get 250 cm, the same as for No1.2. We walk around with the lamp held in our fingers, and the device dangling below, and repeat our measurement of maximum range.

The A3030A must monitor and interpret the exact timing and content of the digital RF signal it receives on its two-turn loop antenna. Compared to the A3024A, which simply turns on its light in response to any received RF power, the A3030A has substantially shorter operating range (70% far in air for robust reception). In air, the A3030A's maximum operating range is one third that of the A3024A, but in water it is only one sixth that of the A3024A. In 1% saltwater, operating range is unchanged.

We place the A3030A in a jar of fresh water. It rests on the floor. We move it around 50 cm from the antenna and obtain robust reception. But there are orientations that are hard for us to obtain with the transmitter loose in the water, so we suspect that robust reception in all orientations would be closer to 30 cm. At ION, we observe robust operating range for an implanted device is only 20 cm.

915-MHz Command Frequency

The reliable operating range of command transmission in ISL Stage 5 appears to be 3 m with the ISL held in open air, 50 cm with the ISL in a jar of water, and only 20 cm when implanted in a rat. Within one of our radio-frequency isolation chambers, command reception at 146 MHz is reliable only up to 20 cm, and in one of our Faraday enclosures, reception can be unreliable even at 10 cm. Meanwhile, data transmission at 915 MHz from implanted A3030Bs is reliable in isolation chambers and Faraday enclosures, but not reliable out in the open. Another problem with the 146 MHz command reception is the 30-mm diameter, two-turn loop antenna it requires on the implanted ISL device. This antenna is tolerated by rats, but would be intolerable to mice. A higher command frequency will mean a smaller antenna.

The Command Transmitter (A3030B) and ISL (A3030C) of ISL Stage 6 will use a command frequency of 915 MHz. We already have data transmission working well at 915 MHz. In theory, if we can transmit 915 MHz with our Loop Antenna (A3015C) and receive with our 50-mm bent wire antenna, we can do the same in reverse. We begin by adapting the crystal radio of the A3030B to operate at 915 MHz. The traces below show the tuning we achieved with C7 = 0.5 pF, C8+C9 = 1.8 pF, and L1 = 2.7 nH (see schematic).



Figure: Crystal Diode Output (VR) vs. Frequency. The bottom trace is 5 mV/div obtained with a ×1 probe on VR. The top trace shows 890-930 MHz, with the zero-crossing at 910 MHz.

We apply 20 mW of 910 MHz to an A3015C antenna. We hold and rotate our A3030C at range 30 cm. We obtain ≥20 mV demodulated signal strength, which is sufficient for robust command reception. With 1000 mW of 910 MHz, assuming an inverse square law of power distribution, we expect to get robust reception at range 2 m in air.

We adapt the radio frequency amplifier of the A3029A to amplify a 915 MHz input and generate as much power as it can at its command antenna output connector. We arrive at the following amplifier circuit, in which the amplifier gives us 10 V peak-to-peak, which the matching network boosts to 24 V peak-to-peak. We end up with 800 mW available for our loop antenna. Our calculations suggest that this is the maximum possible power we can deliver in a stable manner with our chosen amplifier chip and our 5-V amplifier power supply.



Figure: Matching Networks Around 915 MHz Power Amplifier.

With these two outcomes, it appears that we will indeed be able to adapt the Stage 5 circuits to 915 MHz for Stage 6. The A3030B will have two 50-mm antennas: one for data transmission and one for command reception. These two antennas can be combined in a future design, but not by adapting our existing circuit. The A3029B will have its own precision 915-MHz oscillator to provide input to the power amplifier. We plan to load this oscillator onto the auxiliary connector footprint, thus leaving the metal enclosure intact.