LM13700 based filter

Heiko van der Linden h.j.vanderlinden at el.utwente.nl
Mon Mar 6 12:16:57 CET 2000

Hi all,

I have made a LM13700 based filter this weekend and I had a few questions about it's functioning.

I have built the a filter that consists of a 2 pole high pass stage and a two pole low pass stage.
The schematics I have used for the filters comes from the national semi datasheets for the LM13700.
The ones I have used are the single high pass and single low pass filters and I've put two of each in series.
The filter works fine and I sweep the control voltage to the OTA's from +15 v to -15v (I am not really sure if I 
can sweep the CV up to 15v but it works ok.)
The thing is that the lowpass filter works fine but the high pass gives some cracks when I turn the CV almost to
-15V (the bias current is almost zero uA then). Has anybody got ideas how this comes.
And when I turn the CV even lower both filters stop working because the outputs of the OTA's run into one of the
supply rails. I have checked if this is because of a small voltage difference betweent the inputs of the OTA and
this indeed is the case. 
I wonder if this has something to do with the fact that I don't use 1% resistors.

And I was sondering what the expected tuning range is for such a filter. The datasheets say it's possible to tune over a
few decades and I have designed the filter to go from 20 to 20000 Hz. But it doesn't sound like it is reaching the 20000 Hz.
When I have a 808 snaredrum across the filter the snaredrum stays a little muddy.

If anybody can give me some hints I would be very happy.

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