How to set the order in iircomb filter?

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I am trying to get rid of the fundamental (1.1 MHz) and 6 harmonics (2.2 MHz, 3.3 MHz...6.6 MHz) from my signal sampled at 250 Msamples/s. IHow should I set the order of the 'iircomb' filter since it is supposed to be a positive integer?

Accepted Answer

Chunru on 9 Sep 2021
The order of the IIR notch filter (using iircomb) is determined by the number of notches (equal to filter order plus 1). The sampling frequency should be integer multiples of the fundamental frequency. The sharpneess of the notch will determin the bandwidth. You may need to resample your data to a new sampling frequency as shown below. It seems that you also have a quite higher order IIR filter.
If your just want to get away of 6 harmonics, an alternative is to cascade 6 "iirnotch" filters. doc iirnotch for details.
fo = 1.1e6;
fs = ceil(250e6/fo)*fo
fs = 250800000
q = 40; % Q facctor of the notch
bw = (fo/(fs/2))/q;
[b,a] = iircomb(fs/fo,bw,'notch'); % Note type flag 'notch'
Use the Filter Visualization Tool (fvtool) to generate the plot showing the filter notches.
freqz(b,a, 8192*4, fs)It

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