High speed ASM-1 VCO question

[gstopp at fibermux.com]

     A quick look in some old EN issues has revealed the follwing:
     
     The 680 ohm resistor is a "Franco compensation" resistor, which is 
     used to allow for comparator switching time at higher frequencies so 
     that the 1V/oct tracking doesn't tend towards the flat side relative 
     to the current from the exponential converter. I'm not sure if it 
     needs to change as the cap size changes.
     
     The 15K resistor and the 18pf cap around the LM311 are the hysteresis 
     components that let the FET stay conducting as the main integrator cap 
     discharges. Here's the deal - as the sawtooth rises above the 
     comparator threshold (set by the 10K/20K divider), the comparator 
     fires and causes the FET to conduct. The sawtooth will start to 
     discharge, but as soon as it discharges just a little bit the 
     comparator threshold is "un-crossed" and the FET will stop conducting, 
     theoretically. This means that (in theory) the sawtooth should hover 
     around +5 forever with a teeny little ripple on it because the 
     comparator never stays on long enough to discharge the main integrator 
     cap all the way to ground. This is what the 18pf/15K RC is for - they 
     set a time long enough for the FET to totally discharge the cap (400 
     ns according to the book). Now 400 ns is a drop in the bucket compared 
     to the 10 us period of the 100KHz sawtooth so it's okay.
     
     As for the rest of your concerns, specifically the overshoot/ringing 
     problems, those types of things are quite tricky sometimes and almost 
     always have to do with circuit layout. One thing you might want to do 
     is make sure that they are really there in the first place! What I 
     mean is that sometimes the scope probe can cause these things to show 
     up on the scope, either due to the wrong type of probe being used or 
     the particular probe grounding at the time. In our lab environment 
     here this happens sometimes, as we realize we need another probe and 
     just grab something from the nearest bench, and it's the wrong type. 
     (Of course this may not be your problem at all, I'm just trying to 
     suggest all kinds of things!)
     
     And you say you are using a push-in type protoboard.... yeah, that 
     could be it... especially for anything above, say, a hundred hertz! OK 
     I'm exaggerating a little, but those things are terribly capacitive at 
     anything above audio.
     
     Ringing and overshoot at high frequencies can be caused by 
     transmission line effects due to impedance mis-matches between the 
     transmitter and the receiver of a signal (this is one of the reasons 
     why LAN coax has terminators). Sometimes in logic circuits it is 
     reduced by using a series resistance, such as 47 ohms or 100 ohms, 
     between output and input to carry the signal.
     
     I guess I'd suggest setting the component values back to original, 
     except for the main integrator cap, and changing one thing at a time 
     to see what the effect is. If things just don't seem to get better, 
     bite the bullet and solder up the circuit on a vectorboard. Don't 
     forget to de-flux like crazy.
     
     Hope this has helped,
     
     - Gene
     gstopp at fibermux.com


______________________________ Reply Separator _________________________________
Subject: High speed ASM-1 VCO question
Author:  John Speth <johns at oei.com> at ccrelayout
Date:    5/1/97 2:48 PM

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?