Fast analog sw. as VC resistor

Mark Smart smart at nn.com
Thu Jan 9 01:31:13 CET 1997


> 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...

Now you're hacking up your synthesizer! You're nuts, man.

> 
> 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).

Really? I'm starting to think that my 4053 was partially toasted while I
was twiddling. At the end it was very definitely toasted and got
really hot...I assumed I had zapped it with static. I do these experiments
during off hours at my workplace, which is an electronic repair lab with
carpeting and really dry air!! I wasn't wearing the wrist strap I usually
use while I was doing the last experiment. I have fried a lot of stuff in
this room...we're going to get tile flooring before long.

> 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

The 0.1 cap worked when I just hooked the vc pot up as a voltage divider
and looked at the output on the scope. The cap made it look flat...that was
the extent of the test! Maybe the 0.1 is inadequate for time-varying
signals.

Let me know what you come up with next, you madman.


************************************************
*     Mark Smart                               *
*     Network Technician                       *
*     University Communications Inc. (UCI)     *
*     smart at medusa.nn.com                      *
************************************************



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