A polyphase filter is formed by decomposing a filter into N subfilters, each of which operates at a lower sampling rate, Fs/N. The N possible polyphase filters have (approximately) the same magnitude response, but each has a different delay.

The Select Polyphase Filter dialog allows you to select a single polyphase subfilter from a larger filter, replacing the coefficients of the original filter. (Note, however, that the filter design parameters are unchanged.) You specify the total number of polyphases, N, and the phase to select, numbered 0 through N-1. These parameters must be chosen in accordance with the following rules:

• The number of total phases, N, must be greater than zero, less than the number of taps of the filter, and evenly divisible into the number of taps.
• The selected phase must be in the range of 0 through N-1. (Phase number zero has the least delay and phase number N-1 has the most delay.)

The subfilter itself has a gain which is 1/N times the gain of the original filter, so the Scale gain option causes the gain of the subfilter to be automatically scaled by N, resulting in a subfilter whose passband gain is the same as the original.

Each subfilter operates at 1/N of the sampling frequency of the original filter, so the Scale sampling frequency option causes the sampling frequency to be automatically scaled by a factor of 1/N.

Tips:

• The stopband lower frequency of the original filter must be less than half Fs/N otherwise, the polyphase filters will not be valid.
• The stopband attenuation of the individual subfilters often is less than that of the original filter. Therefore, you might have to design the original filter to have extra attenuation in order that the subfilter you select will have the stopband attenuation you require.

Example:

Suppose you want to design a filter with the following specifications:

• Sampling frequency: 100 Hz
• Passband upper frequency: 10 Hz
• Stopband lower frequency: 20 Hz
• Passband ripple: 1 dB
• Stopband attenuation: 80 dB

However, you want to design the filter to have a fractional-sample delay.

First, design the filter specified above. It turns out that 28 taps is sufficient to meet this specification.

Next, multiply the sampling frequency and the number of taps by ten, resulting in a sampling frequency of 1000 Hz and 280 taps. Then design the filter, and use the Select Polyphase Filter feature to select polyphase filter number 0 from 10 total polyphases. Compared to the original filter, this filter has approximately the same magnitude response in the passband, has about 10 dB less stopband attenuation, and has the desired fractional-sample delay.