High speed ASM-1 VCO question

[Synthaholic AKA The Shark]

On Fri, 2 May 1997 11:02:47 -0700 (PDT), you wrote:

>   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
>

Your point about the use of a high quality VCO this way is well taken,
however, what difference would it make whether saw or triangle is
used?  Isn't VC-PWM produced by comparing some kind of linear ramp
(good saw or triangle waveforms both have linear ramps...) to a
control voltage?  It would seem to me that both methods are viable
approaches, both producing a predicable duty cycle output.  The fact
that the saw, in this example, is somewhat dirty, is more important I
think.  From that stand point only, a *different* kind of oscillator
might be better.  The DV/DT of the ramp on a triangle might be less
prone to cause ringing, but it may be possible to produce just as
clean a sawtooth.  IMHO.

To be sure, the art in art is not in it's sameness, but rather in the
diversity of thinking from artist to artist.  Wouldn't the world be
somewhat more mundane if everyone did things the 'usual' or 'accepted'
way?  Nothing ventured, nothing gained... Again, IMHO.
- Scott Gravenhorst (Synthaholic)

Programming: The Ultimate Computer Game.  Unfortunately, you never win.
Hell: Windows 95, 16 bit apps and Banyan Vines

A Quote: "I didn't do it."     -- Bart Simpson