Introduction to Bandstop Filter

Catalog

I. General Introduction

1. Working principle

When a low pass filter and a high pass filter are connected parallel to each other, the bandstop filter is created. The key purpose of the bandstop filter is to remove the same frequency band or to stop it. Some other names, such as band-reject or notch or band elimination filter, often apply to the bandstop filter. As discussed earlier, there will be one cut-off frequency for high pass filters, low pass filters also have one cut off frequency, but there are two cut off frequencies for this bandpass and bandstop filters.

A specific spectrum of frequencies between the two cut off frequencies would be refused by this band stop filter. It enables frequencies above the high cut-off frequency and the low cut-off frequencies below. Based on the value of components used in the circuit design, these two cut off frequencies are calculated. It has a stopband and two passbands for this filter.

The figure above clearly shows the ideal features of the bandstop filter.

- ‘fL’= cut off frequency of low pass filter

- ‘fH’= cut off frequency of high pass filter

Bandpass and bandstop filter operations and characteristics are totally opposite to each other.

Band Stop Filter Theory

A low pass filter allows the low frequencies to pass through the circuit when an input is provided to the signal, and a high pass filter enables the high frequencies to pass through the circuit. The low-pass filter and the high-pass filter are connected simultaneously. When operating with the filter, there is some distinction between ideal and practical situations. This distinction is attributable to a capacitor's switching mechanism. In the figure above, the frequency response can be illustrated clearly.

2. Different Bandstop Filters

Band Stop Filter using R, L & C

Here, the inductor and capacitor are attached to the circuit resistor. The output is collected via the inductor and capacitor that are linked in sequence. The circuit, based on the frequency given at the input, will become a short or open circuit. For high frequency, the capacitor becomes a short circuit and the inductor is an open circuit and inductors operate as an open circuit for low frequencies, such as a short circuit and capacitor.

Because of this parallel relation between the capacitor and the inductor, we may assume that it becomes an open circuit at low and high frequencies, and at mid-range frequencies. It acts as a short circuit. So that's why the circuit does not allow mid-ranges and thus acts as a filter for band-reject. 

Depending on the lower and greater cut off frequencies, the range of frequencies for which the filter serves as a short circuit. These cut-off frequencies depend on the parts used during the design and their value. The transfer function, according to the specification, specifies the component values.

Notch Filter

In Raman spectroscopy, live sound reproduction (public address systems or PA systems) and instrument amplifiers (particularly amplifiers or preamplifiers for acoustic instruments such as acoustic guitar, mandolin, bass instrument amplifiers, etc.), narrow notch filters (optical) are used to reduce or prevent audio feedback, while having little noticeable effect on the remainder of the frequency spectrum. Other names include 'band cap filter,' 'T-notch filter,' 'band-removal filter,' and 'band-reject filter.'

The filter for the narrow stop band is referred to as the filter NOTCH. This notch filter is used for the removal of a single frequency. Because of its two T formed networks, it is also called the twin T network. Maximal removal takes place at center frequency fC = 1/2πRC. 

Inside the notch filter circuit, the capacitor and resistor are included. The capacitor value must be equal to or less than 1μF. By using the center frequency equation, the value of the resistor can be determined. 

This notch filter is very useful for 50 or 60Hz single frequency removal.

By recording gain and frequency, the frequency response of the bandstop filter can be obtained. 

Bandwidth is obtained through the lower and higher cut off frequencies. The stopband should have o0f zero again, and according to the ideal stopband filter, the passband should have an Amax gain.

II. Applications

The following are the applications of the band-stop filter:

- The bandstop filters are effectively used in electric guitar amplifiers. Guitar typically generates hum at 60Hz frequency. For amplifying the signal, the bandstop filter used is helpful in reducing the hum. The filter is not only used in this guitar, but also in acoustic applications such as base instrument amplifiers and mandolins. 

- To minimize noise, a bandstop filter is used in image and signal processing. 

- These bandstop filters are used for the reduction of static on a radio. 

- The bandstop filter is used in applications in the medical field, such as biomedical noise reduction devices.

- These bandstop filters are used in DSL internet services and noise reducers to eliminate interference on the line. 

- If noise occurs in the communication, the signal will be skewed, resulting in output errors. The bandstop filters are therefore used effectively to reduce these unnecessary harmonics and errors. 

- In audio apps such as PA systems, i.e. This filter is used for Public Address Systems. 

- These bandstop filters are used in optical communication technologies to remove distortions. Raman spectroscopy is one example of this.

This is all about a full view of the bandstop filter, therefore. It consists of one stopband and two passbands for this band stop filter. The features of bandpass filters and band stop filters are totally opposite. This filter is often referred to as a filter or notch filter for band rejection. In its growth, it used a low pass filter and a high pass filter. Both filters are linked to each other in parallel. It will have two frequencies of cut off, i.e. a low frequency of cut off and a high frequency cut. These intermediate frequencies are refused and all other frequencies are permitted.