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Band Stop Filter – Types, How it Works, Applications and Advantage

Band Stop Filter also called as Band Reject Filter, rejects frequencies lying within a certain frequency band and allows other frequencies to pass through. This article will explain about Band Stop or Band Reject Filter, it’s types, how it works, applications, advantages and disadvantages.

What is Band Stop Filter

Filters are crucial circuits that eliminate unwanted frequencies in the field of electronics. There are four types of Filters namely Low Pass Filter, High Pass Filter, Band Pass Filter, and Band Stop Filter. In this post, we will discuss more about Band Reject Filter. It is also called Rejection Filter/ Elimination Filter or second order filter. When a low pass filter and high pass filter are connected in parallel to each other, Band Reject Filter is formed.

Introduction to Band Stop Filter

Fig. 1 – Introduction to Band Reject Filter

The band-Reject Filter rejects a particular band of frequencies and allows the rest of the frequencies to pass through. The basic characteristic of this Filter is exactly opposite to Band Pass Filter. Band Reject Filter with Wide Stop Band is known as a Wide Band Reject Filter and the one with Narrow Stop Band is known as a Notch Filter.

Fig. 2 shows a Band Elimination Filter using a basic RLC circuit. { R = Resistor, (L) = Inductor, and C = Capacitor } This circuit acts as an open circuit at very high and very low frequencies. At mid-range frequencies, it acts as a short circuit. The Basic RLC circuit attenuates only mid-range frequencies and allows all other frequencies. The lower and higher cut-off frequencies depend on the filter design and they can be found using the formula :

image

 RLC CircuitFig. 2 – Band Reject Filter using RLC Circuit

Types of Band Stop Filters

They are of two types:

  • Wide Band Stop Filter
  • Narrow Band (Low Band) Stop Filter

Wide Band Stop Filter

Wide Band Reject Filter uses a low pass filter, a high pass filter, and an amplifier. To get the actual result of the wide band stop filter, the low cut-off frequency of the Low Pass Filter has to be larger than the high cut-off frequency of the high pass filter. Also, the pass band of the high and low pass filters has to be equal. Here F1 > F2. So we should design HPF with F1 and LPF with F2. Depending upon the band of frequencies to be suppressed, the Wide Band Stop Filter is designed. The sample circuit of the Wide Band Stop Filter is shown in Fig. 3.

Wide Band Filter (1)

Fig. 3 – Sample Circuit Diagram of Wide Band Stop Filter

Earlier, this type of filter was implemented using shunt stubs or blocking stepped microstrips in line with large circuits. But, these filters have a narrow band stop response. As the demand for Wide Band Stop Filters is gaining momentum, some slow wave structures like the open loop resonators are getting used. Also, the photonic band gap (PBG) and electronic bandgap (EBG) are surveyed to fulfill the demand.

Narrow Band (Low Band) Stop Filter

Narrow Band Filters are also called Notch Filters. The circuit is composed of two T-shaped networks where One of the T-networks is made up of two resistors and a capacitor, while the other is constructed using two capacitors and a resistor. Hence this network is also called Twin-T Network.

It is generally used in force reduction in a single band of frequency of about 60 Hz. The Band Reject Filters with Q > 10 are called the Narrow Band Reject Filters. Q is ‘Figure of Merit’ or measure of Selectivity in passing Centre Frequency and rejecting other frequencies. The relationship between Q, Bandwidth, and center frequency is defined by the equation.

image

The frequency of maximum rejection is calculated as fc = 1/2ΠRC. A notch Filter is used in communication, control, instrumental and biomedical systems to remove 50/60 Hz power line noisiness. Notch Filter is specially used to reduce the humming noise in electric guitars and also eradicates disturbances and provides the accurate capacity to air conditioners, DC motor drives, inverters, and converters.

Narrow Band Filter

Fig. 4 – Sample Circuit Diagram of Narrow Band Stop Filter

The Software-defined Radios (SDR) of today’s dynamic world of operations get easily saturated by a strong FM broadcast signal. The FM signals stop an SDR from handling other signals and a Notch Filter is helpful and hence is popular in SDR applications. Spectroscopy, an instrument used for life science research and technology also uses these filters to understand the uniqueness of molecular and crystal structures.

How does Reject Filter Work

This circuit consists of High Pass Filter and Low Pass Filter which are parallel to each other. Band Rejects Filter functions to eliminate a particular band of frequencies. A high pass and a low pass filter have only one cut-off frequency while a Band Stop Filter has two cut-off frequencies. This filter rejects a particular range of frequencies between the two cut-off frequencies and hence it is also known as the Band Elimination Filter. Both Band Pass Filter and Band Reject Filter have two cut-off frequencies known as half power points/ -3dB that produces a Wide Stop bandwidth between these points.

How Band Stop Filter Works (1)

Fig. 5 – Basic Structure of Band Elimination Filter

Let us assume the cut-off frequency of low-pass filter fL is 400 Hz and that of the High-pass filter is 600 Hz. These two Filters are connected in parallel and the input signal is applied to the circuit. A low-Pass filter allows frequencies below 400 Hz to pass through. Similarly, High-Pass filter allows frequencies above 600 Hz to pass unattenuated. The signal frequencies between 200 Hz and 600 Hz i.e. between fL to fH will be rejected forming a notch in the output response. Therefore, the Filters bandwidth, BW is defined as: (fH – fL). The output from the two Filter circuits is then summed using an Adder to get the final output.

Frequency Response (1)

Fig. 6 – Frequency Response Curve of Band Reject Filter

Applications of Band Reject Filter

The applications include:

  • Band Reject Filter is used in various speaker systems to enable and enhance audio quality.
  • They are used in Telecommunications (Telephone, Digital Subscriber Line) to reduce the noise while signal transmission.
  • It is reduced to clear static in radio devices.
  • They are also used in instrument amplifiers, especially for acoustic instruments like Electronic Guitar in reducing the humming sound produced at 60 Hz.
  • In the field of medicine, Electrocardiogram (ECG) machine also has Band Reject Filters for hassle-free operations.
  • They are a vital part of signal and image processing units.
  • Optical Filters also have Band Reject Filters for use in laser eye protection applications.
  • It filters out unwanted noises from the duplexes and switches.

Advantages of Band Reject Filter

The advantages include:

  • It offers Low Passband insertion loss and group delay.
  • Unwanted Signals are effectively suppressed while Transmission.
  • These Filters are preferred while transmitting high-power signals.

Disadvantages of Band Reject Filter

Notch Filter has a relatively low figure of Merit Q but can be increased by incorporating a Voltage follower in the circuit.

Also Read:
Filters – Classification, Characteristics, Types, Applications & Advantages
High Pass Filter – Types, Applications, Advantages & Disadvantages
Band Pass Filter – Types, How it Works, Applications and Advantages
Laxmi Ashrit
Laxmi Ashrit
Laxmi is a B.E (Electronics & Communication) and has work experience in RelQ Software as Test Engineer and HP as Technical support executive. She is an author, editor and partner at Electricalfundablog.
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