[sdiy] sync circuit woes

[David G. Dixon]

>Then why is the 22p in a positive feedback loop?  (It's there to hold the 
>discharge pulse on long enough to totally drain the cap.) 

Oops.  I stand corrected.  Still, is the value of this cap all that
critical?  Also, would it be necessary at all if an opamp had been used as a
comparator here instead of the LM311?

David G. Dixon
Professor
Department of Materials Engineering
University of British Columbia
309-6350 Stores Road
Vancouver, B.C.  V6T 1Z4
Canada
 
Tel 1-604-822-3679
Fax 1-604-822-3619
 
"PERFECTA FINGAMUS SERVIAT NATURA"

The information in this email and in any attachments is confidential and
intended solely for the attention and use of the named addressee(s).  It
must not be disclosed to any person without the writer's authority.  If you
are not the intended recipient, or a person responsible for delivering it to
the intended recipient, you are not authorized to and must not disclose,
copy, distribute, or retain this message or any part of it.

-----Original Message-----
From: Ian Fritz [mailto:ijfritz at comcast.net] 
Sent: Thursday, November 20, 2008 11:18 AM
To: David G. Dixon; 'Dan Snazelle'; 'sdiy'
Subject: RE: [sdiy] sync circuit woes

At 09:55 AM 11/20/2008, David G. Dixon wrote:
>Dan,
>
>I didn't see any 18 pF caps in the schematic.  However, the only timing cap
>there is the 2200 pF in the sawtooth oscillator.  This cap sets the pitch
of
>the VCO.  All the other caps in the opamp feedback loops (the 22 pF and the
>100 pF) are there to prevent spurious oscillations in those circuits.

Then why is the 22p in a positive feedback loop?  (It's there to hold the 
discharge pulse on long enough to totally drain the cap.)