Power frequency noise double T filter circuit

The figure below shows a power frequency noise filter circuit . This circuit is a double-T filter that can be used to filter out the mixed noise of the 50Hz (or 60Hz) power supply frequency when amplifying weak signals (such as sensors, etc.). If only RC components are used to form this type of filter, its Q value is generally low and has broadband attenuation characteristics. Using an op amp and adding positive feedback can improve the Q value. Suppose the Q value at this time is Q', then Q'=Q/(1-K), in the formula, K=RA/(RA+RB). Changing the coefficient K can increase the Q value. The resonant frequency of the circuit is fo=1/2πRC, and the positive feedback components are R/2 and 2C. Considering the dispersion of the characteristics of the capacitor, the resonant frequency should generally be set lower, and then the potentiometers PR1 and PR2 are used to adjust the frequency to 50Hz (or 60Hz). When adjusting, input a 50Hz (or 60Hz) sinusoidal voltage, and adjust PR to minimize the output voltage passing through the filter. When using this filter, if the input power supply noise is not a sine wave, high-frequency components will pass through the filter. High-order harmonics in the AC power supply are generally odd-order harmonics such as 3, 5, and 7. These harmonics can be filtered out by connecting a low-pass filter.

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