FM and tracking

Just a quick thought about filter frequency modulation and tracking.

Since I've removed tracking-tuning and do CV lookup instead to get good filter CV tracking, FM may be an issue. 1V of exponential FM may not be 1 octave up anymore. 

However, I assume that the tracking across octaves will be pretty much stable, at least within the middle ranges. If for example +1V increases the pitch by 0.8 octaves (instead of the correct 1 octave), -1V should reduce the pitch by 0.8 octaves. This means that tracking correction can be done in the FM VCA. Also, this means tracking VCA should have a range > 0-5V.

As for linear FM, that changes the exponential converter's reference voltage and that is unaffected by the tracking pot anyway, so no issue there.

AS3364 gain > 1

Since my juno filter might output less than unity gain, I wondered if it is possible to make up for this in the output VCA mixer, effectively making gain > 1

Normally, I use a 47k input resistor and a 47k output resistor, as per the AS3364 datasheet.

I tested this quickly and got a sligtly more than 8Vpp output (I didn't measure the input, it may have been as high as 10Vpp but could also be lower).

5v/cell, approx 8Vpp

I then replaced the input with a 22k resistor. The resulting output was 17-18Vpp.

Still 5v/cell, 17Vpp

Changing the input resistor would increase the input current by 47k/22k = 2.14. 

8 * 2.14 = 17.12, so this looks like it is doing exactly what we want.

The datasheet says that maximum signal input current is between +/-300 and +/-500uA.

With a 47k resistor and max input 11.5V, the input current is 244uA. Reducing to 22k, the max is 522uA. I'm not sure if going higher actually kills the chip, but it is something to think about. It could be that it will just start clipping.

If we choose to use a smaller resistor we should also make sure that the input amplitude does not go too high. For example, if we go for the lowest maximum signal input:

max V input = 300uA * 22kOhm = 6.6V, or 13.2Vpp.

Oh - and to make up for a change from 100k to 230k input to the juno filter, use a 47k / 2.3 = 20.4k resistor (or just two 10k resistors in series)