I want to design FIR and IIR Filters...

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Never heard of that cycle explanation??

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Consider it as my personal rule of thumb "where not to use FIR filters". I didn't say, that a filter with fc = 0.02 and 55 taps isn't feasible. I claim, that it's ability to achieve a specified frequency response will be very limited and thus an IIR filter will better achieve a typical low-pass characteristic.

If you doubt, that there is a relation between filter performance and relative filter length, you should compare the results of fc 0.01, 0.02 and 0.1 low-passes with L=55.

FIR filter design literature gives more exact expressions for the required filter length depending on stop band attenuation and pass and stopp band frequency. Typical relative lengths are considerable larger than L > 1/fc.

The FIR design chapter of U.Meyer-Baese digital signal processing with fpga gives a brief overview of popular design methods.