Notch filter- Theory, circuit design and Application

The most common notch filter design is the twin-T notch filter network. In its basic form, the twin-T also called a parallel-T configuration. It consists of two RC branches in the form of two tee sections connected in parallel.

The basic Twin-T Notch filter circuit is shown in the below figure as:

The upper T-configuration of resistors and capacitor form the low pass filter section, whereas the lower T-configuration of resistor form the high-pass filter section of the design.

The frequency at which this basic twin-T notch filter design offers maximum attenuation is called the “notch frequency”. So notch frequency is formulated as.

For obtaining the high level of attenuation and narrow notch, an operational amplifier is used to design single Op-Amp Twin-T notch filter circuit. The single Op-Amp Twin-T notch filter circuit is shown in the below figure as

Notch Filter Design

For designing a two op-amp narrow-band RC notch filter with a center notch frequency of 1kHz and a -3dB bandwidth of 100 Hz. Consider 0.1uF capacitors and calculate all the values of required components as explained in below steps.

Step 3: Calculate the value of feedback fraction K.

Step 4: Calculate the value of R3 and R4.

Step 5: Finally Calculate the value of notch depth in dB as:

Hence after using these values in the circuit we can design the Notch filter as

Notch Filter Applications

In different technologies, Notch Filters are used in different ways.

• In communication electronics, the signal is distorted due to some harmonics (noise) which makes the original signal to interfere with noise signal which leads to errors in the output. Thus, Notch filters are used to eliminate these unwanted frequencies of harmonics.
• These filters are used by musicians in high quality audio applications such as graphic equalizer, synthesizer and PA systems.
• In telephone technology, Notch filters are used as the telephone line noise (harmonics) reducers and DSL internet services. It will help to eliminate the interference on the line which will reduce the DSL performance of internet service.
• Also, the notch filters are widely used in the electric guitar amplifiers. Actually, the electric guitar produces a ‘hum’ at 60 Hz frequency. Then this filter is used to reduce that hum by rejecting 60 Hz in order to amplify the signal produced by the guitar amplifier and makes it the best equipment. These are also used in some of the acoustic applications like mandolin, bass instrument amplifiers, etc.
• It is also used in Optical communication technologies. The best example is in Raman spectroscopy. At the end of the optical fiber, there may be some interfering frequencies of light which make the distortions in the light beam of optical fiber. Then these distortions are eliminated by narrow-band stop filters.
• In image and signal processing, these filters are highly preferred to eliminate noise.
• It is also used to reduce the static on the radio, which are commonly used in our daily life.
• These filters are also used in medical field applications,i.e., in biomedical instruments like EGC for removing line.

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Originally published at https://www.electrical-technology.com.