High speed ASM-1 VCO question

[John Speth]

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).

Referring to just the core integrator/reset part: My integration cap is
100pf reduced from 2200pf, discharge resistor is 47 ohm reduced from 680.
The rest stays the same.

Q1: What is the function of the 15K resistor (opamp output to comparator
input) and 18pf cap (bridging comparator output and input)?

Q2: Does anybody know of any LM311 specific problems where the output
switching noise can feed back into the comparator inputs?  (Might this be
one reason for the 15K/18pf parts in the reset circuit?).  My sawtooth to
PW comparator (also LM311) shows this annoying little ripple (maybe 100 mv)
feeding back into the sawtooth input of the 311 whenever the 311 output
goes high (meaning the output transistor of the 311 turns off - using a 1K
pullup).  This results in a little positive going half glitch right before
the pulse settles high.

I use those nice white breadboard testing thingys you can get in radio
shack with all the holes in them for testing new circuits.  So I expect a
fair amount of ringing and crosstalk at higher frequencies.  I would
imagine a tight layout when building the final product would greatly reduce
ringing but I wonder if the components around the reset comparator could be
tuned to reduce it also.  And that little feedback problem: ever heard of
that before?

Hopefully these details aren't too specific to make no sense at all.  Any
ideas?


John Speth (johns at oei.com)
Object Engineering, Inc.
Vancouver, WA