[sdiy] integrator / capacitor leakage

[jhaible at debitel.net]

> Do you think the amplitude increase is similar to high-frequency Q 
> enhancement in VCF's?  This is improved significantly by using 
> high-performance opamps.  Also it can be compensated, for example by 
> putting small caps across the OTA input resistors.  Might be hard to get it 
> flat to 1% though.  :-)

I should have been more precise.

I haven't used OTAs. I've built it with THAT2180 VCAs, to get rid of the
offset issues of OTAs. Unfortunately, these VCAs only have one input 
(inverting, to get a noninverting VCA in combination with an inverting 
opamp I->V converter), so I had to add an extra opamp inverter stage.
Unlike your OTA solution, that extra stage will break the "symmetry" 
between the two integrator stages, which isn't good for a constant phase
difference.

Last night's experiment:

CA3280, diskrete FET buffers, voltage swing 1Vpp, boosted (outside the loop!)
to 20Vpp, AD633 multipliers, 10nF capacitors.
Works great (except some offset issues) at 7mHz (2.5sec period), and up
to 14kHz. The latter is with Iabc of several mA, CA3280 getting warm, no
good tracking expected. But these 7mHz aren't the lower limit - it's just the
frequency I've counted with a stopwatch, while checking the beam on the scope
performing a circle in xy mode. Actually, just checking the beam to approach
the +/-1V marks on the x and y axis. But as I said, THD isn't important in the
sub-audio range.
So, with _that_ low end performance, I can certainly use smaller caps to 
improove the high end.
More experiments as I find the time.

JH.

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