DSP Blockset

Bandpass and Bandstop Discrete-Time FIR Filters.   The normalized cutoff frequencies for a bandpass filter are illustrated in the next figure. The normalized lower cutoff frequency, fn1, is 0.25, and the normalized upper cutoff frequency, fn2, is 0.75. For an input signal sampled at 1 kHz, these would be equivalent to cutoff frequencies of 150 Hz and 375 Hz, respectively.

The equivalent MATLAB code for this filter design is

b = fir1(41,[0.25 0.75])

and the filter can be designed in the Digital FIR Filter Design block by making the following selections in the dialog box:

• Filter type = Bandpass
• Order = 41
• Lower cutoff frequency = 0.25
• Upper cutoff frequency = 0.75

The lower and upper cutoff frequencies for a bandstop filter (Bandstop selected from the Filter type menu) are defined in the same way: -6 dB down from the passband level. The equivalent MATLAB code for this filter design is

b = fir1(41,[0.25 0.75],'stop')

Whenever the last (right-most) band is a passband, the filter order must be even (so that the filter is Type I). MATLAB therefore automatically increases the filter order to 42 for the stopband design.

Lowpass and Highpass Discrete-Time FIR Filters

Multiband Discrete-Time FIR Filters