ABSTRACT
Another way of making notch filters is the “one-minus-bandpass” principle, usually called ‘1-BP’. The input goes through a bandpass filter, typically the Multiple FeedBack type described earlier, and is then subtracted from the original signal. The accuracy of the cancellation and hence the notch depth is critically dependent on the mid-band gain of the bandpass filter. Figure 9.5b shows an example that gives a notch at 50 Hz with a Q of 2.85. The subtraction is performed by A2, as the output of the MFB filter is phase-inverted. The MFB filter is designed for unity passband gain, but the use of E24 values as shown means that the actual gain is 0.97, limiting the notch depth to −32 dB. The value of R6 can be tweaked to deepen the notch; the nearest E96 value is 10.2 kΩ which gives a depth of −45 dB. The final output is inconveniently phase-inverted in the passband.
