Notch filter is a kind of resonant circuit, or an automatic switch inductor, which can be used in antenna engineering. It can automatically extend or shorten the length of antenna according to the frequency of signal. The filter in radio receiver is used to eliminate some useless signals to reduce the interference to useful signals.

Notch filter is used in the circuit to filter the signal of unnecessary frequency. For example, notch filter is added at the edge of band-pass filter’s pass band. Usually, a parallel resonant circuit is connected in series or a series circuit is connected in parallel. Their resonant frequency is the frequency to be filtered, which is very common in the medium frequency part of TV electromechanical circuit.

## Basic circuit diagram of notch filter Double T-Notch filter circuit

In the figure, A2 is used as an amplifier, and its output is used as the output of the whole circuit. A1 is connected in the form of voltage follower. Because the double-T network can achieve better attenuation characteristics only when it is far away from the center frequency, the Q value of the filter is not high. The purpose of adding voltage follower is to improve the Q value. In this circuit, the Q value can be increased to more than 50. Adjust the resistance values of R, R and two resistors to control the filter characteristics of notch filter, including the band width of band stop filter and the Q value.

## Specific calculation and selection of parameters

In this design, it is required to eliminate the 50 Hz power frequency signal contained in the test signal with the frequency above 1 kHz, so the center frequency f0 = 50 Hz, RC = 1 / 314, if r = 31.85 ohm, then C = 100 NF.

Taking FH > = 251hz, FL < = s49hz, we can get BW = 4 (1-k) F0 < = 2, Q > 25

That is, k = R2 / R1 + R2 > 0.99, R2 > 99r1, R2 = 100k Ω.

In order to prevent center frequency drift, silver plated mica capacitor or carbonate capacitor and metal film resistor should be used. The common attenuation is 40-50db. In order to get 60dB attenuation, the error of resistance and capacitance must be less than 0.1% and 0.1% respectively.

## Circuit composition Multisim Simulation circuit of double T notch filter

## Component parameters in simulation diagram 