High speed ASM-1 VCO question

[gstopp at fibermux.com]

     Don has a good point here; it didn't register with me that this is may 
     be a fixed-frequency application. If the reference waveform for the 
     pulse width modulator is to be a fixed frequency at around 100 KHz, 
     you're probably better off using a triangle wave rather than a 
     sawtooth wave. You'll definitely have less overshoot and ringing 
     problems.
     
     You can probably rig up a nice clean triangle with a TL082 
     integrator/schmitt trigger, however I suspect that things will start 
     to get funky if you go much higher than 100KHz. For a clean waveshape 
     at higher frequencies, it may be worthwhile to invest in an EXAR 2206 
     or something like that.
     
     To be honest I haven't done exactly this application, so I'm just 
     making some stabs here. Experimentation is the best way so have at it!
     
     - Gene
     gstopp at fibermux.com


______________________________ Reply Separator _________________________________
Subject: Re: High speed ASM-1 VCO question
Author:  don at till.com at ccrelayout
Date:    5/2/97 10:36 AM


   Date: Thu, 01 May 1997 14:52:49 -0700 
   From: John Speth <johns at oei.com>
     
   I am experimenting with the ASM-1 VCO trying to get it run cleanly at about 
   100KHz.  I was wondering if you anybody could shed some light on some of 
   design choices that were made in the circuit.  I understand how it works 
   but need info on the more practical aspects.
     
   My goal is to make a CVable duty cycle modulator for a time-multiplexed 
   panner using analog switches.  I figure I would use the ASM-1 core sawtooth 
   generator (a standard Electronotes VCO isn't it?) to provide a ramp and an 
   LM311 to make a CVable duty cycle convertor.  Problems I have seen is 
   excessive ringing (about 10% of signal) in the sawtooth at the end of 
   retrace and an LM311 problem (see below).
     
Umm, I'm confused.  Are you saying that you're planning on using an 
especially-optimized-for-accurate-tracking VCO for a fixed frequency 
source?  Why are you using a sawtooth wave for PWM instead of the 
usual triangle wave?  What are the goals of the circuit? 
     
  -- Don