[sdiy] OTA performance (was SSM chip reissue)

[Mattias Rickardsson]

Speaking of differences I guess that parts within the same batch can
differ too - I've sometimes found that an occasional lack of resonance
in LF can be fixed by either replacing the zeners (with the same type
of zeners from the same maker) or replacing the 2164 (again with the
same type from the same maker), but not always.

/mr


On 28 April 2017 at 11:45, Tom Wiltshire <tom at electricdruid.net> wrote:
> Different types might have different leakage currents too though, no?
>
> I  don't suppose that's a specification they put in the datasheet, although perhaps it correlates with something that is there if you know about zener construction.
>
> (other) Tom
>
> On 28 Apr 2017, at 08:54, Mattias Rickardsson <mr at analogue.org> wrote:
>
>> 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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