Magnus Danielson cfmd at
Mon Nov 1 23:40:25 CET 1999

From: Dr Steve Grainger < at>
Subject: ASM-1 VCO
Date: Mon, 01 Nov 1999 11:18:05 +0000

> Dear All,
> 	In amoment of madness had a go at the ASM-1 VCO at the weekend.
> 	It works fine except for the amplitude of the sawtooth which instead
> 	of being 0 to +5V is up at +2.5V to +5V (peak to peak) ie. it has
> 	a large DC offset.
> 	I suspect this might be caused by the ON resistance of the JFET
> 	(Rds (ON)) for which I am using a 2N3819, causing the integrator
> 	capacitor to not discharge completely.
> 	Any comments on this problem - given that this is the UK and 
> 	JFETs seem as rare as hen's teeth !

One of the things that comes to mind is the time for which the FET is being
open. If this time is too short, then will it not reset the cap properly and
it will manifest itself as a DC offset. Increasing the capacitor over the
LM311 will increase this time. That was basically what I had to do to get
mine working.

Another things that migth happend is that the resistor in series with the
capacitor is actually magnitudes too large. You can easilly see this by
varying the CV, if the DC level follows along more or less linearly with
intended frequency then this is the error.

There is much more to this than FETs alone.


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