[sdiy] VCO sync question

[Michael Ruberto]

Check the pulse output for DC offset. In most VCO designs the pulse is 
generated from an op amp functioning as a comparator and invariably there is 
no offset null trim. I've run into this sort of thing before. If this turns 
out to be the case the solution is simple, AC couple the pulse through a cap 
and the offset will be blocked.
In VCOs where an actual comparator chip is used I usually find the output of 
the comparator is not buffered and gets loaded down easily when connected to 
other modules. Check for no signal or a tiny signal from your pulse output 
when connected to an input. If this is the prob, try using a diode in series 
with the pulse output or using an op amp to buffer the signal. Use an op amp 
with a fast slew rate like an LF353, or an AD712. If you have a positive 
only pulse you can also use a CA3130 which is single supply and has like 
30V/uS slew and includes the + and - rails in it's output. It also has 
connections for offset nulling. It is a CMOS device though so take 
precautions on it's output.

M. A. Ruberto


>From: megaohm <megaohm1 at gmail.com>
>Reply-To: megaohm1 at gmail.com
>To: synth-diy <synth-diy at dropmix.xs4all.nl>
>Subject: [sdiy] VCO sync question
>Date: Mon, 5 Sep 2005 01:31:20 -0500
>
>Hmmm... I'm scratching my head on this one.
>  The VCO I just built (EN/CGS/Blacet/etc. style) is working great. 
>But...the
>sync input behavior is mysterious to me. It syncs fine to a master rising
>ramp (no surprise) but it won't sync to a master pulse or square...unless I
>invert them first. Since the square is either +5v or -5v, why does 
>inverting
>it make any difference?
>  peng