Command Transmission Noise

We turn on two Subcutaneous Transmitters (A3019). One has 150-mm leads and the other has 45-mm leads. We place the tips of the leads in water, so that they are connected by an impedance of order 100 kΩ. This connection reduces the amount of mains hum the transmitters pick up, allowing us to see other sources of noise. We turn on our Command Transmitter (A3023CT), which emits pulses of 146-MHz radio-frequency power to stimulate Implantable Lamps (A3024). We place the command antenna 70 cm from our two subcutaneous transmitters, and instruct the command transmitter to emit 10-ms pulses at 10 Hz. We see the following response from the subcutaneous transmitters.


Figure: Command Transmission Noise. Two A3019 subcutaneous transmitters responding to pulses of 146-MHz command pulses. Vertical range is 27 mV, horizontal range is 500 ms. Pulses are 10 ms at 10 Hz. Pink trace: 150-mm leads. Orange trace: 45-mm leads.
If we remove the leads from an A3019, we see no response to the command transmission. We record with a two-channel Subcutaneous Transmitter (A3028D) and observe the same pulses on both channels. We suspect that the leads pick up 146-MHz power, which penetrates the transmitter circuit and is somewhere demodulated by a non-linear element in the amplifier. The demodulated signal is added to the input, resulting in pulses. We record the following trace from an A3019 without encapsulation and equipped with 150-mm wire leads.


Figure: More Command Transmission Noise. One un-encapsulated A3019 responding to pulses of 146-MHz command pulses. Vertical range is 27 mV, horizontal range is 500 ms. Pulses are 10 ms at 10 Hz.
We add a 100-kΩ resistor in series with each lead, and a 10-pF capacitor at the far side of the resistors. Thus the resistors form a high-pass filter with cut-off frequency 80 kHz. At 146 MHz, the filter attenuates by a factor of two thousand in amplitude. A 20-mV pulse of command transmission should be reduced to 1μV. With the filter installed, we recording the following trace from the subcutaneous transmitter with the same command pulses.


Figure: Eliminated Command Transmission Noise. One un-encapsulated A3019 equipped with 80-kHz low-pass filter showing no response to pulses of 146-MHz command pulses. Vertical range is 27 mV, horizontal range is 500 ms. Pulses are 10 ms at 10 Hz.
The 10-pF capacitor of the filter appears in parallel with the 10-MΩ input impedance of the subcutaneous transmitter's EEG amplifier, making a low-pass filter with cut-off frequency 1.6 kHz, which is above the 1.3-kHz bandwidth we would obtain from an A3019 running at 4096 SPS. The two 100-kΩ resistors form a divider with the 10-MΩ input impedance, which reduces the EEG signal amplitude by 2%, which is insignificant. We claim this filter will have no effect upon the detection of EEG, and yet eliminates the command transmission noise.