[sdiy] integrator / capacitor leakage

[Ian Fritz]

At 04:09 PM 12/3/05, JH. wrote:

>If this concept fails (which I expect), I'll go for an oscillating state
>variable filter,
>like you have presented, with the zener diodes, or with the x**2 + y**2
>method from Tietze/Schenk.
>I have made some experiments with both on breadboard. Your inexpensive
>zener diode circuit is really as good in THD as the expensive squaring
>method,
>which came as a big (and pleasant) surprise. But I found that above a
>certain frequency
>(ca. 3kHz with the cap values and medium-fast opamps I had used), the
>amplitude
>increases a lot. I'd say this is from additional phase shift, and the rather
>smooth
>zener diode amplitude stabilisation isn't strong enough to really keep the
>amplitude
>constant over the whole frequeny range.

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.  :-)

   Ian