Fast analog sw. as VC resistor

[Christopher_List at Sonymusic.Com]

Hmmm, seems like my mail's going through, so I'll give this a try...

I did a linearity test on this last night and got very good results. Mark,
I don't know what we're doing differently, but here's what I did;

1. Took an ASM-1 and replaced the .0022uF cap with a .001uF. Put a resistor
in parallel with the 560K - the resistor's value was chosen to level shift
the output into the 0 - 10v range, rather than -5/+5. I think it was just
another 560K. This output a decent sawtooth up to about 150-175kHz - a
little curvy at the higher frequencies, but not that bad...

2. Set up an LM311 on a bread board as follows;
     ASM-1 Sawtooth Output to 100Ohm Resistor
     Other end of resistor to pin 2 (+) of the LM311
     LM311 Pin 1:   Ground
     LM311 Pin 3:   PWM refence (from a buffered pot outputing 0-13.5v)
     LM311 Pin 4:   Ground
     LM311 Pin 5:   No Connection
     LM311 Pin 6:   No Connection
     LM311 Pin 7:   OUTPUT : 4.7K resistor to +V, 560K resistor to Pin 2
     LM311 Pin 8:   +V

3. Set up a CD4053 as follows;
     Pin 1:    Ground, 100K resistor to pin 15
     Pin 2:    +10v,     100K resistor to pin 15
     Pin 6,7,8: Ground
     Pin 10:   Comparator output
     Pin 15:   Output -> 1.8K resistor / .1uF cap for filtering
     Pin 16:   +V

I got excellent linearity (my scope is old, but it's got a handy storage
feature so I could etch-a-scetch the PWM voltage (x) vs. the output volage
(y) and see the exact path). The only problem with it was that it was
slightly verticle at the top and bottom end - like it was kind of "turning
on" and "turning off". This is not so good for an ADSR knob. I'd like to
try it with a good quality triangle wave and see if I get the same
results...

I also don't know if this acts like a "true" resistor as far as current is
concerned. Also the 1.8K resistor for filtering on the output gets in the
way - but a single .1uF cap between output and ground didn't seem to do the
filtering for me - I'm curious that it worked for you. I'll play more
tonight, maybe I missed something...

- CList