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Multiplying Frequency

In article , Phil Hobbs wrote: [... me ...]

It sure sounds like a "nervous Nellie" fit on your part. You first have to define "work" before you can say if something works. Using the control voltage to vary the gain of OTAs and using the VCO output to run a switched capacitor filter both have the same problem WRT the filter being set for where the VCO is and not where it should be.

Eventually, the system will settle and the VCO will be where it should and the transfer function for small changes will be good.

I have made things like this "work" for "work" as defined in that case. The PLL has a nasty tendancy to hang at the low frequency end but if you've got the time, it will climb out of it.

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kensmith@rahul.net   forging knowledge
Reply to
Ken Smith
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Hmm. I guess we can differ as to whether that constitutes working or not...the difficulty is how you can adequately test such a circuit, given how nasty the parametric nonlinearity is, and how prone such systems are to oscillate in hard-to-guess ways. I sure wouldn't ship a product designed that way. Of course, with a MCU in it, one could do quite a lot of belt-and-suspenders type checks, and probably contain the worst misbehaviour.

Cheers,

Phil Hobbs

Reply to
Phil Hobbs

In article , Phil Hobbs wrote: [...]

For most switched capacitor filters, the variation in clock frequency can be modeled as a change in all the resistor values in real time. You don't see effects like would happen if the capacitor values were changed while charged.

If you want to model the situation, it is easier to assume that the VCO and the filters hold still and that the control voltage changes the input frequency. You end up with a funny "time" axis but if the VCO is kept well away from zero Hz, the predicted results can be trusted.

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kensmith@rahul.net   forging knowledge
Reply to
Ken Smith

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