915 MHz Command Reception

We test the reception of ISL commands with our new 915 MHz Command Frequency. Our Command Transmitter (A3029B) is equipped with a 915.000 MHz 915 MHz Oscillator. This auxiliary circuit can stand alone, as shown below, or be soldered onto the A3030B circuit board as its source of 915 MHz.


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.

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.

Lamp Current Noise

In November 2014, before we shipped the ISL5s, we put one in water with a head fixture and obverved up to 10 mV of switching noise on the EEG monitor when we flashed the lamp. The amplitude of the noise did not correlate well with proximity of the lamp and the EEG leads. After half an hour of work, we ran out of time to examine the problem. Since then, Rob Wykes has implanted No2.5 and observes the following noise on the EEG signal during what appears to be 18-Hz, 10-ms optical pulses of 0-100% power.


Figure: Lamp Flashing Noise on EEG Input. ISL No2.5 is implanted in a rat. One-second intervals, full 27-mV voltage range.

Detail of the 20% power noise, shown below, shows a rising and falling edge on the pulse start and stop, and another 130-Hz interference that we see also when the light power is 0%.



Figure: Lamp Flashing Noise on EEG Input for 20% Power, Detail. ISL No2.5 is implanted in a rat.

We believe both these types of noise are electrical and magnetic, rather than photovoltaic. The boost regulator runs even at 0% power, so its switching noise is most likely the source of the persistent 130-Hz. The large spikes when the lamp switches on and off may be due to proximity of the four leads, or be electrical through the body fluids. We do not see such noise when the devices are unencapsulated and in air.

ISL Stage 5 Delivery

We ship to ION the devices shown below. There are five Implantable Sensors with Lamp (A3030B). These are encapsulated in acrylic, transparent epoxy, and clear silicone. They include command receiver, lamp power supply, EEG amplifier, and data transmitter.


Figure: ISL Stage 5 Deliverables. We have 5 Head Fixtures A3024HFC-B and 5 ISL A3030B. The ISL No2.1 is non-functioning. We see the two-turn command antenna bound up with the one-turn data antenna.

The lamp wires are 100-mm long. The orange lead is L+ and the purple lead is L−. Each pair of lamp wires presents resistance 65 Ω to the flow of lamp current, so that the current through a blue lamp will be close to 35 mA. The EEG leads are 150 mm long. The blue one is X+ and the red one is X−.


Figure: Implantable Sensor with Lamp, A3030B.

In addition to the five ISLs, we are sending five Head Fixtures (A3024HFC-B) with blue LEDs. These are No3.4, 3.5, 5.1, 5.2, and 5.2. They produce 13, 13, 13, 15, and 14 mW blue light respectively at their fiber tips when supplied with 40 mA forward current, as marked on their petri dishes. Our A3030B lamp current is 35 mA, so the light emitted by the fiber tips when connected to the A3030B will be around 12 mW.


Figure: ISL Controller Tool.

Accompanying these circuits, we have the ISL Controller Tool, Version 2.2, which fixes some device-addressing problems present in Version 2.1. Download the script and place it in the LWDAQ Tools/More folder to replace any previous version of the tool you may have installed.

Four of the five A3030Bs are fully-functional: No2.2-2.5. But No2.1 failed after water wash and encapsulation, along with our spare circuit, No2.6. In the future, we have to figure out how to load the battery and wash the solder joints to make the circuit clean, without damaging the lithium-ion battery, which is vulnerable to liquid water, and without causing the circuit, which has power attached, to enter a parasitic state that drains the battery quickly.

Before shipping, we used No2.2 to measure the effect of lamp flashing on the EEG amplifier. We will report on our observations in the next post.

EEG Monitor Incorporated

We have our first Implantable Sensor with Lamp (A3030B) with functioning EEG amplifier and transmitter. The circuit includes the same Command Receiver and Lamp Power Supply as the existing A3030A, but we have now loaded the amplifier, ADC, radio-frequency oscillator, transmit antenna, and electrode leads. In order to get the digitization and transmission working, we translated our existing data transmitter firmware from its original ABEL to the more modern VHDL required by the A3030B's CPLD. The graph below shows the frequency response of the amplifier as observed in the Recorder Instrument after digitization at 512 SPS, transmission at 902-928 MHz, and reception in by loop antenna in a faraday enclosure.



Figure: Amplitude versus Frequency for 11-mV pp input on X. We see the bump in gain at around 130 Hz, which is a characteristic of our three-pole Chebyshev low-pass filter.
We turn on and off the A3030B's 915-MHz radio-frequency data transmission with 146-MHz radio-frequency command transmissions. The A3030B is either digitizing and transmitting its analog input, or it is doing nothing with its analog input. We update the ISL Controller Tool to permit us to turn on and off data transmission with the press of a button.


Figure: The Updated ISL Controller Tool. We now have buttons Xon and Xoff to turn on and off data transmission. We have labels along the top that indicate the status of each of fourteen possible ISLs.
Of particular concern to us was interference between the data transmission and command reception. With data transmission turned on, there was the possibility of the 915-MHz power on the transmit antenna disrupting or stimulating the 146-MHz command receiver. But no disruption appears to occur. Command reception is equally effective with simultaneous data transmission.

We are also concerned about the effect of command transmission upon the A3030B's analog input. In an earlier experiment, we observed 20-mV pulses at the analog input in response to command transmission at range 70 cm. The A3030B includes a 40-kHz low-pass filter (R5, R6, and C here) at the analog input, which we expect to attenuate 146-MHz by a factor of 3500. In initial experiments, we are unable to observe any effect upon the analog input by command transmission. We will conduct more thorough measurements in the coming weeks. For now, with 100 μV of mains hum on our un-encapsulated circuit with external battery, we see no effect, so it appears that we have reduced its magnitude to less than 100 μV.

Only one A3030B exists today, which we made entirely by hand. We have ordered another thirty to be assembled by machine and shipped to us by the end of October. Of these, we will use 5, 5, and 10 to fulfill ISL Development Stages 5, 6, and 7 respectively.

ISL Stage 4C Delivery

We ship to ION the devices shown below. There are five Implantable Sensors with Lamp (A3030A). These are encapsulated in acrylic, clear epoxy, and clear silicone. They do not include EEG monitors, but they can be commanded individually, and instructed to perform extended optical stimuli with a single command. All five have 100-mm long stretched helical leads and through these they deliver 40 mA to an implantable lamp.



Figure: ISL Stage 4C Deliverables. We have (1) Rubber Duck antenna, (2) telescoping half-wave antenna, (3) Command Transmitter, (4) Head Fixtures A3024HDC-B, (5) Implantable Lamps Circuits A3030A.

There are five Head Fixtures (A3024HFC-B). All produce blue light. The power output from the taper tip at 40 mA forward current is written on the top of their petri dishes. Head fixture No3.1 produces 18 mW, which it obtains from a total lamp output of 34 mW. Its coupling efficiency is 53%, which is a little over our theoretical maximum, and 3.4 mW higher than our previous best head fixture. Another head fixture provides 15 mW. For more details see here.

The Command Transmitter (A3029A) is programmed to transmit serial commands to the ISLs. We plug LWDAQ power into its RJ-45 socket, and 24-V boost power into its power jack. We do not include a boost power supply in our ISL4C shipment. The boost power supply from the ISL4B shipment is still at ION, and will do the job. Note: we sometimes have to reset the LWDAQ Driver before the green power light illuminates on the A3030A.

We operate the Command Transmitter (A3029A) with our new ISL Controller tool, which you can download here. Place the TclTk text file in your LWDAQ Tools folder, or run it with the Run Tool option, and a window like the one shown below will pop up.


Figure: ISL Controller Tool in LWDAQ Software. The A3030A does not provide data transmission (Xon and Xoff).

The A3030A does not support randomized pulse generation. But it does support lamp dimming to 0%, 20%, 40%, 60%, 80%, and 100% of maximum intensity. The ISL Controller allows you to select the lamp intensity with a menu button. We can send the stimulus command to all A3030As or to an individual A3030A. We select channel 1-14 for individual devices, according to the serial number label on its battery.

We ship two antennas. One is a telescoping half-wave 146-MHz antenna. With this antenna we obtain reliable reception at ranges 0-1 m. The other is a short, flexible antenna (called a "rubber duck" antenna). With this one, we obtain reliable reception at ranges 0-50 cm.

We hope that ION's implantation of these devices will answer the following questions. Does the encapsulation resist condensation and corrosion? We have clear encapsulation to allow us to examine the devices after explantation. At what range is command reception reliable? What lamp power is required to provide stimulation, and what pulse length and frequency is optimal for stimulation?

We will visit ION on 16-OCT-14 to measure the operating range. In the meantime, we will start work on the A3030B, the same circuit as the A3030A, but with the EEG amplifier and data transmitter loaded.