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Design >> Analog Design >> A question about gm-c filter tuning
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Message started by smellyegg on Mar 16^th, 2007, 1:50am

Title: A question about gm-c filter tuning
Post by smellyegg on Mar 16^th, 2007, 1:50am

Good afternoon. :)I am now working on a gm-c filter, whose cutoff frequency is 3.75MHz and the attenuation at 4.25MHz is about -40dB. When designing the automatic tuning circuit, I met a problem. A friend tells me that for the variations caused by process, voltage and temperature, capacitors in the circuit vary greatly while gm varies little, but he is not too sure about it. So my question is, is it necessary to consider the variations of gm when designing the automatic tuning? can I just tune the variations of capacitors while ignoring the variations of gm?

Thanks!

Title: Re: A question about gm-c filter tuning
Post by ACWWong on Mar 16^th, 2007, 5:42am

It is envitable that there will be some gm shift with process, voltage and temeprature irrespective of how you generate it (although some circuits are better than others).
For frequency tuning you must try to preserve the gm/C time constants over PVT variation, so must consider the variation in both gm & C. But often you only use either gm or C to correct/tune out the frequency error to within spec.
Also, depending on your filter specifications, you may need to tune filter pole Q/Gain, which may not be the same as any frequency tuning.
cheers
aw

Title: Re: A question about gm-c filter tuning
Post by smellyegg on Mar 18^th, 2007, 4:30am

thanks :)

Title: Re: A question about gm-c filter tuning
Post by SATurn on Mar 18^th, 2007, 6:57am

I think it highly depends on your Gm (transconductor) topology. The nominal variations on C is +/-10%, while it can be much higher for Gm value (depending on your Gm circuit).

Saturn

Title: Re: A question about gm-c filter tuning
Post by smellyegg on Mar 18^th, 2007, 6:44pm

thanks :)