What is Band Stop Filter : Theory & Its Applications

There are different types of filters like High pass filter, low pass filter, bandpass filter, and bandstop filter. High pass filter will only allow frequencies which are higher than cut off frequency and low pass filter allows the frequencies which are lower than cut off frequencies. The bandpass filter will allow a particular band of frequencies and a band stop filter will reject a particular band of frequencies. This article discusses an overview of the band-stop filter and its working.

What is the Band Stop Filter?

The bandstop filter is formed when a low pass filter and a high pass filter are connected in parallel with each other. The main function of the bandstop filter is eliminating or stopping the particular band of frequencies. The bandstop filter is also referred with some other names like band-reject or notch or band elimination filter. As discussed previously, For high pass filter there will be one cut off frequency, low pass filter also has one cut off frequency but this bandpass and bandstop filters have two cut off frequencies.

This band stop filter will reject a particular range of frequencies which are there in between the two cut off frequencies. It allows the frequencies which are above the high cut off frequency and the below the low cut off frequencies. These two cut off frequencies are determined based on the value of components used in the design of the circuit. This filter has a stopband and two passbands.

The ideal characteristics of the bandstop filter are clearly demonstrated in this figure

‘fL’= cut off frequency of low pass filter

‘fH’= cut off frequency of high pass filter

The working and characteristics of bandpass and bandstop filters are completely opposite to each other.

Band Stop Filter Theory

When the signal is given an input, a low pass filter allows the low frequencies to pass through the circuit and a high pass filter allows the high frequencies to pass through the circuit.

This is the block diagram of the bandstop filter. Low pass filter and high pass filter are connected in parallel. There is some difference between ideal and practical conditions while working with the filter. This difference is due to the switching mechanism of a capacitor. The frequency response can be clearly explained in the above figure.

Band Stop Filter using R, L & C

Here in the circuit resistor, inductor and capacitor are connected. The output is taken across the inductor and capacitor which are connected in series. The circuit will become a short circuit or open circuit basing on the frequency given at input. For high frequency, capacitor become short circuit and inductor will be an open circuit and for low frequencies, inductors act like short circuit and capacitor as an open circuit.

Due to this parallel connection of capacitor and inductor, we can say that at low and high frequencies, it becomes an open circuit and during mid-range frequencies. It behaves as a short circuit. So that’s why mid ranges are not allowed through the circuit and thus acts as a band-reject filter.

The set of frequency for which the filter acts as a short circuit is depending on the lower and higher cut off frequencies. These cut off frequencies are dependent on the components and its value used while designing. According to the design, the transfer function determines the component values.

Notch Filter

The narrow stop band filter is referred to as the NOTCH filter. For the elimination of single frequency, this notch filter is used. It is also called as twin T network due to its two T shaped networks. At center frequency fC = 1/2πRC, maximum elimination takes place.

The capacitor and resistor are used in the notch filter circuit. Capacitor value must be less than or equal to 1µF. the value of the resistor can be calculated by using the equation of center frequency.

This notch filter is very much useful in eliminating single frequency at 50 or 60Hz.

The frequency response of the bandstop filter can be obtained by recording gain and frequency.
Across lower and higher cut off frequencies, bandwidth is obtained. The stopband should have again o0f zero and the passband should have a gain of Amax according to the ideal stop-band filter.

Applications

The applications of the band-stop filter include the following.

• In electric guitar amplifiers, the bandstop filters are effectively used. Normally guitar produces hum at 60Hz frequency. The bandstop filter used is helpful in reducing the hum for amplifying the signal. Not only in this guitar but also the filter is used in acoustic applications like base instrument amplifiers and mandolin.
• A bandstop filter is used in the image and signal processing for reduction of noise
• For the reduction of static on a radio, these bandstop filters are used.
• The bandstop filter is used in medical field applications like biomedical instruments for the removal of noise.
• In DSL internet services and noise reducers, these bandstop filters are used for removing interference on the line.
• If the noise occurs in the communication, the signal will get distorted which will rise to errors in the output. So to reduce this unwanted harmonics and errors, the bandstop filters are used efficiently
• In audio applications like PA systems i.e. Public Address Systems, this filter is used.
• In optical communication technologies for eliminating the distortions, these bandstop filters are used. One of the examples of this is Raman spectroscopy.

Thus, this is all about a complete view of the bandstop filter. This band stop filter consists of one stopband and two passbands. Bandpass filter and band stop filter characteristics are completely opposite. This filter is also referred to as a band rejection filter or notch filter. It used a low pass filter and high pass filter in its design. Both filters are connected in parallel to each other. It will have two cut off frequencies i.e. low cut off frequency and high cut of frequency. These mid frequencies are rejected and all other frequencies will be allowed. This is the complete description of band-stop filter
Here is a question for you, what is a high pass RC filter?