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Twin T notch filter

- design and circuit considerations for a resistor capacitor (RC) twin T notch filter

The twin T circuit is very useful as a notch filter. Here the twin T provides a large degree of rejection at a particular frequency. This notch filter can be useful in rejecting unwanted signals that are on a particular frequency. One example may be to filter out unwanted mains hum at 50 or 60 Hz that may be entering a circuit.

The response provided by the filter consists of a low level of attenuation away from the notch frequency. As signals move closer to the notch frequency, the level of attenuation rises, giving the typical notch filter response. In theory, at the notch frequency the level of attenuation provided by the twin T notch filter is infinite.

RC - resistor capacitor twin t notch filter

RC - Resistor Capacitor Twin T Notch Filter

The circuit for the twin T notch filter is shown above and can be seen to consist of three resistors and three capacitors. It operates by phase shifting the signals in the different legs and adding them at the output. At the notch frequency, the signals passing through each leg are 180 degrees out of phase and cancel out. In theory this provides a complete null of the signal. However in practice close tolerance components are required to achieve a good null.

In common with other RC circuits, the RC twin T notch filter circuit has what may be termed as a soft cut-off. The response of the notch circuit falls away slowly and affects a wide band of frequencies either side of the cut-off frequency. However very close to the cut-off frequency the response falls away very quickly, assuming that close tolerance components have been used.

Calculation of the value for the circuit is very straightforward.

fc     =     1 / (2 pi R C)

Where:
  fc = cut off frequency in Hertz
  pi = 3.142
  R and C are the values of the resistors and capacitors as in the circuit

By Ian Poole

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