[sdiy] OTA performance (was SSM chip reissue)

[Mattias Rickardsson]

Hej Tom,  :-)

the consequences, if any, of capacitance in zeners would probably show
up in high frequencies and not only low ones, no?

/mr

On 28 April 2017 at 09:47, Tom Bugs <admin at bugbrand.co.uk> wrote:
> 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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>
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