volt controlled synthetic capacitor?

[Rene Schmitz]

At 00:05 16.12.1997 +1100, you wrote:
>The annoying thing about the ICL8038 is that it doesn't
>quite cover the full audio range in one voltage sweep
>with a fixed capacitor/resistor.
>But if the cap or resistor was in some way to change,
>at the same time as the sweep voltage, and in the right
>direction, then perhaps it would.
>Is there a simple/easy/obvious way to do this?
>All I want is a one chip sin oscillator that goes from say 0.1 Hz to
>20KHz with say 0 to 5 volt control voltage....
>
I have had the same problem with the NE566, but I won't use them anymore.

If the "resistor" pin were a current input (I'm not sure but it could 
be like in the good old XR2206), 
then just use a voltage controlled current source at that pin.
It could be a two transistor current reflector and a resistor for simplicity.
(The pin is kept at some voltage and the IC senses the current 
thru the pin, thats how it works in the 2206)


This was my first weird idea:
 What you could do is using a "capacitance multiplier" and a 
 varicap diode (varaktor). That way one could get a voltage controlled 
 capacitance with a range of approx. 1:10. 
 A problem could be that C is *insversely*
 proportional to V (actually C ~ 1/V ^(3/2)). 
 (I have not fully analysed this, maybe not linear enough)
 The capacitance multiplier is described in Nationals Appnote 31, page 17
bottom.


>please dont tell me not to use the ICL8038, I know that already,
>but I have a whole shitload of them andI cant afford Maxim anything....
>
>BTW, is there a cit\rcuit that 'synthesises' a capacitor the way a
>gyrator synthesises an inductor?

Well a gyrator simulates an inductor via a capacitance,
and a gyrator can be used to simulate a cap via an inductor, so this won't 
get us home, unless we had a voltage controlled inductor.

>paul perry melbourne australia going to bed and hoping to find wit
>and wisdom from this list tomorrow ;)
>
>
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