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I have designed a fir filter with my desired frequency response at a specific sampling rate and filter order. If I were to redesign my filter for a new higher sampling rate, should I change the filter order? I am having trouble understanding how the same filter order influences the desired frequency response at different sampling rates.

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  • $\begingroup$ To be more clear, I assume you have an analog signal $x_a(t)$ which is to be sampled at a rate $F_s$ samples per second and you will process those samples $x[n]$ in discrete-time with your designed FIR filter $h[n]$ and you wonder what to do with the FIR filter, if the sampling rate $F_s$ is changed, such as doubled ? $\endgroup$ – Fat32 Apr 27 '16 at 23:06
  • $\begingroup$ Right that is what I am asking but specifically for the filter order when the sampling rate is changed. The typical way I create a Fir filter is by windowing the corresponding iir filter impulse response. $\endgroup$ – Malz Apr 27 '16 at 23:27
  • $\begingroup$ You already selected the answer but you have a more serious problem than the filter order here. Your Effective Frequency response on the signal will change due to sampling rate change (or stated the other way: the input signal's Frequency spectrum on the Discrete time domain will shrink) and you must redesign your filter (not only changing the order) to have the same Frequency Response with the previous one. $\endgroup$ – Fat32 Apr 27 '16 at 23:38
  • $\begingroup$ I stated that I am redesigning my filter for the new sampling rate. Assuming that it was redesigned but the order was kept the same is where my concerns start. $\endgroup$ – Malz Apr 28 '16 at 0:03
  • $\begingroup$ ok I just missed it somehow.! $\endgroup$ – Fat32 Apr 28 '16 at 0:27
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Keeping the FIR filter the same order and doubling the sample rate will effectively halve the accuracy of frequency resolution which will be noticable at low frequencies (this is based on the assumption you are using something like FFT to create your FIR, otherwise the order of the FIR and it's resolution is a bit more complicated).

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