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Design >> Analog Design >> Switched Capacitor Channel Select Filter
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Message started by aaron_do on Mar 30^th, 2009, 1:27am

Title: Switched Capacitor Channel Select Filter
Post by aaron_do on Mar 30^th, 2009, 1:27am

Hi all,

just wondering, is there any major reason why we wouldn't use a switched capacitor filter for a channel-select filter for a direct-conversion receiver? Suppose the filter needs to work with a signal as low as -80 dBm.

thanks,
Aaron

Title: Re: Switched Capacitor Channel Select Filter
Post by rf-design on Mar 30^th, 2009, 1:56am

The reason is that there are alternatives with better efficiency.

For inband noise/power dominated designs the cap size is equal in continuous-time and switch cap filters.

But the sampling frequency should be a factor higher than the maximum rejection frequency. Otherwise rejection is limited by alias. Second to have an accurate frequency response the caps should charged within a fraction of the sampling period. The need much more current again. If you compare the required bias current of continuous versus discrete filter opamps there is a big factor.

That rules out switch cap!

Title: Re: Switched Capacitor Channel Select Filter
Post by aaron_do on Mar 30^th, 2009, 7:45am

Thank you very much.