lhlbluesky_lhl wrote on Nov 2nd, 2012, 6:10am:in S/C bandpass filters, the center frequency f0 can be tuned by clock frequency fclk, and normally, fclk is much larger than f0, 50 times or bigger. however, in some cases, f0 is just the input signal frequency (ex:in ir receiver, f0 is the carrier frequency), f0=fsignal, then, in half period of input signal, fclk works 25 cycles, so, how to decide the transient output for S/C filter? if tuning the frequency of fclk to obtain desired f0, then in half period of input signal, fclk may be not integral cycles, can this cause a wrong transient result?
Besides, in S/C bandpass filters, how to simulate the AC response (center frequency f0, Q, etc), just as the continuous filter (in hspice)?
thanks.
* Time domain: Integral cycles yes/no within one half period are no problem at all because S/C filters are no digital filters. They belong to the class of "sampled data systems" - somewhere between analog and digital. That means: Amplitude informations are updated at discrete times only, but the signal is a time-continuous one due to sample-and-hold operations at the output of each stage.
* Frequency domain (ac response): There a special simulation packages (like SPECTRE), which are able to calculate the frequency response (including periodic repetitions within the spectrum). If you prefer classical simulation packages (PSpice, HSpice, LT-Spice) you must previously convert the S/C circuit into a time-continuous equivalent. As far as I know, for this purpose there are two basic principles described in the literature.
(I have used both methods already - and they work perfectly).