[sdiy] OTA performance (was SSM chip reissue)

[Tom Bugs]

One thing I wondered about Zeners a couple of years ago was whether 
different types gave different results - I wasn't entirely sure whether 
I was barking or looking at the wrong parameters though!

I *think* that BZX84 type were worse than BZV55 for example. One 
parameter in the datasheet I'd wondered on was the capacitance measured 
at 1MHz.
But I now can't remember if this changed the oscillating amplitude at 
all freqs or just at the low freqs which is what I was trying to figure 
(as per current discussion)

Barking up the wrong tree?



On 27/04/2017 18:25, Tom Wiltshire wrote:
> 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
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