[sdiy] Resistor matching in Bergfotron complex VCO

[Tim Stinchcombe]

> At higher frequencies, it looks pretty triangular, but at low  
> frequencies it's more like a 20% triangle. (Also, when twisting  
> various knobs to get the lowest frequency, it seems to only get down  
> to 600 Hz, but I suspect that will be fixed once we get the triangle  
> more triangular.)

Since the lack of symmetry would tend to indicate some imbalance in the
current into/out of the main timing cap, are you using high input impedance
op amps like the TL072s shown on the Bergfotron schematics? If a
bipolar-input type, could the input bias current be causing grief? A quick
calculation suggests this idea might be a bit iffy, but it was just a
thought...

Tim
__________________________________________________________
Tim Stinchcombe 

Cheltenham, Glos, UK
email: tim102 at tstinchcombe.freeserve.co.uk