[sdiy] OTA performance (was SSM chip reissue)

[Tom Wiltshire]

On 27 Apr 2017, at 16:03, Mattias Rickardsson <mr at analogue.org> wrote:

> On 27 April 2017 at 16:57, Richie Burnett <rburnett at richieburnett.co.uk> wrote:
>>> But if the gains match well at 0dB, pretty well at -20dB, and not so well
>>> by the time you're down at -80dB, then you'd expect resonance to drop off at
>>> lower frequencies.
>> 
>> Or it could do the opposite, depending which of the two integrator gains is
>> lowest/highest!  Like in that paper you referenced about varying SVF gain by
>> manipulating the integrator gains.
> 
> True!
> This is intriguing. Does this indicate that the loss of resonance
> towards the bottom end is actually caused by another effect than the
> gain mismatch? Possibly capacitor leakage (e.g., through zeners) or
> something else?


There are three potential causes we've identified:

1) Phase lag
2) Gain mismatch being worse at greater attenuation
3) Zener current robbing the integrators.

The trouble is *all* of these effects operate in the same direction, making the filter less likely to resonate at lower frequencies. 

How much of each is causing the effect we're seeing is still yet to be determined! It's quite possible that it's a mixture of all three and that sorting out any one might give an improvement but won't fix the problem on its own.

Any ideas for experiments we could try to find out which is the worst offender?

Tom