Fast analog sw. as VC resistor
Paolo Predonzani
predo at dist.dist.unige.it
Tue Jan 7 13:36:27 CET 1997
>
> That's a pretty good idea. I saw a couple of references to the switched
> capacitor style filter in my Electronotes. The reason it didn't seem to fit
> what I want to do is that I was looking for a way to control a LOT of
> voltage-controlled three-terminal pots with only one oscillator. To
> generate a complementary resistance (the other half of the pot) with the
> switched-capacitor filter would require another oscillator whose frequency
> was proportional to 1/F of the first oscillator, right? There would have to
> be two voltage-controlled oscillators and 1/x circuitry for every function
> to be adjusted.
>
Yes. Ok, I didn't understand that you want 3 terminal pots. Were do you
use them in the envelope generator?
I spent some time in the weekend trying to figure out what you have in mind.
I include a uuencoded GIF of the result. There are 2 solutions.
The first uses 3 VCR and 1 multiplexer; the second uses 1 VCR and 2 multiplexers.
The design is not complete, as you can see. The control logic requires at
least one more input from the attack threshold comparator (not shown in the
picture).
I would go for the first solution (3 VCR and a multiplexer) because the design
is safer. Moreover if I use switched capacitors as VCR I don't need the
multiplexer because the VCR can be turned off (resistance = infinity) by
gating its input clock.
> It's almost totally linear as long as R is a lot bigger than the on-resistance
> of the analog switch. I tested this circuit out last night with a couple of
> 100K resistors and a 4066, which has Ron of about 130 ohms. It worked great once
> I got a good pwm'ed clock for it. The envelope generator circuits and filter
> resonance which I need this for are all either 1M or 100K pots, so it should
> work fine for them. The basic idea is that the overall resistance is the
> time average of the resistances in the two states. Like if it spends 20% of
> the time being on and 80% being off, then the resistance is 0.2(130//100K) +
> 0.8(100K). The first term is small compared to the second term, so the
> resistance is close to R*(1-(fraction of time switch is on)), and the
> resistance of the other half of the pot is close to R*(fraction of time switch
> is on). If this still wasn't linear enough, two or more analog muxes could be
> hooked up in parallel to reduce the on-resistance still further.
>
This is true if you use it as a voltage divider with no load at the output.
I know you use it as a pot but be careful if you use it as a variable resistor
in an RC network.
Your version of a VCR is a resistor with a switch in parallel. Right?
Now, if we assume that the switch is ideal (Ron = 0) then the capacitor is
charged immediately when the switch is closed, no matter how wide the control
pulse is.
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`
end
--
+-------------------+----------------------------------+
| Paolo Predonzani | email: predo at dist.dist.unige.it |
+-------------------+----------------------------------+
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