[sdiy] ASM-1 VCO troubles another time

[Tim Ressel]

I had some trouble with my VCO (similar to your I
believe). Replacing the oldish CA3140 (3130 in my
case) with a newer opamp (OPA602) cleaned up alot of
stuff that looked like the FET but was actually the
opamp.

--TR


--- "Oren B. Leavitt" <oleavitt at ix.netcom.com> wrote:
> Hello,
> 
> The 2N3819 JFET may not be discharging the
> integrator capacitor fast enough
> due to slow switching time and the higher rdson.
> You'll want to use a good
> 'high speed switching' or 'chopper' JFET for this -
> such as the 2N4391, or
> NTE466, NTE467, etc.
> Touching the 18pf cap, which sets the 'on' time for
> the LM311, gives you a
> little bit longer pulse time from the comparator and
> therefore the JFET has a
> chance to fully reset the integrator.
> Your oscilloscope input resistace/capacitance will
> also skew the accuracy your
> readings if you touch the probe directly to the
> CA3140 or LM311 outputs. You
> may want to do your measurements thru a buffer amp.
> As a 'band-aid' fix, you can use a larger capacitor
> for C3, say 33 or 47pf,
> which will give you more discharge time, but will
> also increase the
> high-frequency tracking error..causing the VCO to go
> noticibly flat at about
> 3kHz or so and higher.
> 
> Another posibility is that the comparator (LM311)
> may be self-oscilliting as a
> result of feedback.
> Are any traces from the inputs of the 311 near or
> next to the trace coming
> from the 311's output on your PCB? The LM311
> datasheet recommends keeping PCB
> traces that run to the 311's inputs and output well
> away from one another,
> preferably with a ground or power supply trace
> between them, to prevent this
> parasitic oscillation. This proplem happens fairly
> easily with the 311.
> 
> 
> Oren
> 
> Fortner Florian wrote:
> 
> > Hi list,
> >
> > Excuse me for annoying everbody again with ASM-1
> VCO problems, but I don't
> > know how to go on.
> >
> > I've built the VCO on a selfmade PCB, and the
> Output of the Integrator
> > (CA3140) gives a strange high-frequency sawtooth
> (between 70 and 160 kHz
> > with a magnitude of ~1V and an offset of 4.5V),
> the Voltage on the gate of
> > the FET (I used a 2N3819 which has a slightly
> higher rdson) is a kind of
> > "rounded" rectangle, not spikes. The cool thing
> is, that when I touch the
> > 18p Cap (C3) with my finger the freq. drops to the
> desired 8-10kHz and the
> > FET voltage becomes the spike-waveform. Any ideas
> what this could be? I
> > already double checked the board and defluxed it.
> >
> > regards,
> >
> > flo
> 
> --
> Oren Leavitt
> oleavitt at ix.netcom.com
> 
> 


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