The notch or band-reject filter is named for the characteristic shape of its frequency-response curve in figure. Unwanted frequencies are attenuated in the stopband B. The desire frequencies are transmitted in the passband that lies on either side of the notch. Notch filter usually have a passband gain of unity or 0dB. The equation for Q, B, fl, fh and fr are identical to those of its associated bandpass filter.
The best way to implement a band reject filter is to sum together the outputs of a low pass and high pass filter:
- The figure shows that although both filters have identical 3 dB points, there is much more rejection of unwanted signals in the stop band with the low pass summed with the high pass than there is with the notch filter - with the single exception of the center frequency.
- The performance increase that comes with summing low pass and high pass filter outputs comes at the expense of an additional opamp - the opamp that performs the summing function.
- Higher order low pass and high pass filters will improve the performance of the band reject filter.
- The farther apart the passbands are, the better the performance of the band reject filter.