[sdiy] ASM-1 VCO troubles another time

[Oren B. Leavitt]

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