Navigation:: Home >> Circuits and design >> this page

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

Share this page


Want more like this? Register for our newsletter








Energy Efficiency of Paramount Importance to 5G Rollout Mauro Boldi | Environmental Engineering Technical Committee, ETSI
Energy Efficiency of Paramount Importance to 5G Rollout
Whilst many will focus on the RF and data capabilities of the new 5G mobile communications system, energy efficiency is one of the primary design aims. Not only will operators be able to save on OPEX, but with cellular telecommunications continuing to grow, energy efficient 5G systems will enable green operation.
Training
Online - RF Connector Types & Connector Care
Know your connector types and avoid expensive damage

More training courses

Whitepapers
VoLTE Testing Explained
Download this free eBook to find out how testing addresses the challenges of bringing your VoLTE networks, VoLTE-enabled mobile devices and new services to market quickly and efficiently.

More whitepapers










Radio-Electronics.com is operated and owned by Adrio Communications Ltd and edited by Ian Poole. All information is © Adrio Communications Ltd and may not be copied except for individual personal use. This includes copying material in whatever form into website pages. While every effort is made to ensure the accuracy of the information on Radio-Electronics.com, no liability is accepted for any consequences of using it. This site uses cookies. By using this site, these terms including the use of cookies are accepted. More explanation can be found in our Privacy Policy