Random Pulses

Firmware A3030D01.vhd for ISL7 (A3030D) is debugged and running. It provides random lamp pulses for any interval and pulse length. For a view of the modulation noise generated by random pulses, see our previous post. The ISL7 stimulus length is the expected number of pulses. The interval length is the average pulse period in milliseconds. The pulse length as a fixed multiple 30.5 μs.

The A3030D divides each pulse interval into thirty-two sections. For regular pulses, the A3030D always initiates a pulse in the first section and only in the first section. For random pulses, the A3030D initiates a pulse in each section with probability 1/32. Thus the average period of pulse initiation is the same as the interval length. With pulses longer than one section length, the pulses overlap. Pulses that occur at the end of an interval can overlap pulses occurring at the beginning of the next interval.

With 2-s intervals and 100-ms pulses, we counted 100 pulses in 207 s. With 100-ms interval and 10-ms pulses, we used a recording of lamp modulation noise to count 559 pulses in 60 s. The figure below shows the distribution of the number of pulses we counted per one-second recording interval.


Figure: Distribution of Pulses per Second for Random 10-ms Pulses with 100-ms Interval. Average pulse rate is 9.3 pulses/s. For 32-s view of recording and spectrum, see here.
We appear to have a passable approximation to a Poisson distribution of pulse generation. When the pulse length is significant compared to the interval length, the average number of pulses drops because the pulses are overlapping. But we still get 9.3 pulses/s average for 10-ms pulses at 10 Hz.

NOTE: The ISL6 accepted the interval length in multiples of 30.5 μs rather than 1 ms, so the ISL Controller Tool 4.1 has a version selector that allows it to control both ISL6 and ISL7 devices.

Quiet Modulation

The figure below shows lamp modulation noise on the X input of an ISL6 (A3030C) for 20% brightness. This noise occurs even with no lamp attached and no X input leads.


Figure: Lamp Modulation Noise on X for A3030C with 20% Brightness, 50-ms Pulses at 10 Hz.
When we modulate the lamp, we apply a 1-MHz clock signal to the gate of Q1 so as to turn on and off the lamp current. During lamp modulation, the A3030C battery current increases by around 5 mA. We suspect that this 5 mA causes the 3V0 output of our TPS70930 to drop by 500 μV, which in turn causes X to step up by 2 mV.

In the A3030C, Q1 is the NDS355AN, with 200 pF gate capacitance. We load our A3030D with NTR4003N for Q1. The figure below shows A3030D modulation noise for random 10-ms pulses.


Figure: Lamp Modulation Noise on X for A3030D with 80% Brightness, 10-ms Random Pulses at 10 Hz. Vertical range is 140 μV.
The new transistor reduces modulation noise to 50 μVpp. The NTR4003N has resistance 1.5 Ω, which is much larger than the 0.2Ω of the NDS355AN, but still insignificant compared to our 60-Ω lamp lead resistance.

ISL7 Head Fixture

The photograph below shows our first ISL Head Fixture (A3024HFD-B). Version D of the head fixture has the guide cannula cemented parallel to the optical fiber.


Figure: ISL7 Head Fixture with Blue LED (A3024HFD-B).

This guide cannula may be used to hold the head fixture in place during implantation. Once the head fixture is covered by cement, the silica guide cannula tube may be crushed with scissors, or a thin pair of cutters, just below the thread. We now have a more compact head fixture, with no guide cannula thread protruding from it, and the hole in the animal's skull must accommodate only the optical fiber. With our previous head fixtures, the skull hole had to accommodate both the fiber and the guide cannula.

In the above example we have roughly 3 mm space between the circuit board and the guide cannula thread. We believe this is sufficient to allow cement over the circuit board and still have space enough to cut the guide tube. But we await comments from ION before deciding the final geometry.

UPDATE [24-JUN-15]: ION tells us they would rather have the older A3024HFC-B.