Mon May 19 11:02:40 EDT 2014

Switched capacitor variable resistor?

Is it possible to make a variable resistor using a switched capacitor
design where the frequency is varied[1]?

For practical use in audio apps the low end needs to be > 20kHz.  Say
we can vary 20kHz to 20MHz, that's 1000x which spans the useful
frequency range.

Of course the idea is not new[2].

Now the question is: can frequency be changed accurately enough?  Is
there a way to construct base frequency + frequency difference using
multiplication?  At low frequencies, uC resolution is high enough.  At
high freqs it's not.

The thing is that the EIDAC circuit can probably be used for that.
Wide frequency range, with errors proportional to frequency, and a
digital control loop that could in principle reach zero average error.

[1] http://en.wikipedia.org/wiki/Switched_capacitor#The_switched_capacitor_resistor
[2] http://en.wikipedia.org/wiki/Switched_capacitor#The_Parasitic_Sensitive_Integrator