Radio №11, 1983
Sometimes T-shaped RC-bridge circuits are used for suppression of 60 Hz mains hum. But this circuits have poor selectivity, so, in the result, the useful signal components getting weaker. To improve performances of RC-bridge circuits we can use op-amps for notch filter design.
The circuit diagram of the notch filter is shown in the figure above. This filter has very high input impedance of megohms. The quality factor Q of the filter can be adjusted with the potentiometer R4 (Qmin = 0.3, Qmax = 30). It is very suitable to choose a compromise between useful and unwanted components of a signal.
Frequency response of the active notch filter id shown in the figure below.
If R1 = R 2= 2R3 and C1 = C2= C3/2 then the frequency of the notch filter can be calculated using this formula:
Fo = 1 / (2 * π * R1 * C1.)
With the components values used in this circuit diagram, the frequency will be equal to:
Fo = 1/(2*3.141*10*(10^6)*270*(10^-12)) ≈ 60 Hz
Funk technik, 1982, 5