A notch filter is a special type of Band Stop filter used by electrical and electronics engineers. In this article, We will discuss in detail the complete concepts about Notch Filter. But before that, we should discuss some basics of the Bandstop filter to understand the complete concepts of the Notch filter in an easy way.
What is a Band Stop filter?
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| Block diagram of band-stop filter |
The circuit diagram of an active Bandstop filter is shown as:
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| Active Band stop filter |
Unlike high-pass and low-pass filters, band-stop, and band-pass filters have two cutoff frequencies. It will pass above or below a particular range of frequencies whose cutoff frequencies are defined by the components used in the circuit. Any frequency in between these cut-off frequencies is attenuated. So it has two passbands and one-stop band. The characteristics of the band-pass filter are shown below:
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| Characteristics of band-pass filter |
Here FL is the cut-off frequency of the low-pass filter and FH is the cut-off frequency of the high-pass filter. So the center frequency of the band-stop filter will be:
Notch Filter Theory
A Notch filter is nothing but a narrow band-stop filter. If the stop-band of the band-stop filter is very narrow and highly attenuated over a few hertz, then that special type of band-stop filter is known as a Notch filter. Since the stopband of the Notch filter is narrow up to a few Hz the Notch filter is also known as a Narrowband stop filter in some cases. It is a highly selective (High Q value) form of band-stop filter that can be used to block a single or very small band of frequencies rather than the whole bandwidth of different frequencies. It acts as a gain for one frequency component and an attenuator for all other frequencies.
Notch Filter Frequency Response
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| Notch Filter Frequency Response |
Practically perfection is very tough to achieve. However, by using the operational amplifier circuit, a high level of attenuation and narrow notch can be achieved.
Notch Filter Circuit
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| basic Twin-T Notch filter circuit |
The upper T-configuration of resistor 2R and capacitor 2C form the low pass filter section, whereas the lower T-configuration of resistor R and capacitor C 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 Notch frequency. So notch frequency is formulated as:
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| single Op-Amp Twin-T Notch filter circuit |
Notch Filter Design
For designing a two Op-Amp narrow band RC notch filter with a center notch frequency of 1kHz and 3-dB bandwidth of 100 Hz. Consider the 0.1uF capacitor and calculate all the values of the required components as explained in the below steps.
- Step 1: Calculate the value of R for the given capacitance of 0.1uF.
- Step 2: Calculate the value of Q.
- 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:
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| Notch filter design |
Notch Filter Applications
- In communication electronics, the signal is distorted due to some harmonics (Noise) which makes the original signal interfere with the noise signal which leads to an error 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 equalizers, synthesizers, and PA systems.
- In telephone technology, Notch filters are also used as the telephone line noise (Harmonics) reducer and DSL performance of internet service.
- Also, the notch filters are widely used in 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 make it the best equipment. These are also used in acoustic applications like Mandolin, Bass instrument amplifiers, etc.
- It is also used in Optical communication technologies. The best example is Raman Spectroscopy. At the end of optical fiber, there may be some interfering frequencies of light that make distortions in the light beam of optical fiber. Then these distortions are eliminated by the 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 is commonly used in our daily lives.
- These filters are also used in medical field applications, i.e., in biomedical instruments like ECG for removing lines.
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