[sdiy] I've made a VCO... On accident...

[Jeff Farr]

Heh, I wish I could take a pic of my 'scope at the moment. It is, as I 
suspected, removing the 100k resistor from the feedback between the second 
integrator and the summer that causes this. By using the same taps from the 
filter arrangement I get a nice square, triangle, sine. All with perfect 
symmetry and even gain, no DC shift through their range, the sine in 
particular looks perfect. There seems to be an upper limit to the range 
short of 20k, perhaps as low as 10k (my scope hasn't been calibrated since 
the early 90's at best). The lower limit is LOW, nice for a LFO. There is 
some clipping in the triangle and sine output above a given frequency, 
trimming affects this, but doesn't cure it. In any case it seems I have a 
nice VCF/VC-LFO module now ;) I have to look into if I screwed up the 
original feedback path from the schematic, the circuit was working with all 
filter responses before discovering this, so I assume it was correct. 


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