The Use of Band Stop Filters in Laboratories

Filters are used in radio communications to enable radio receivers to receive the desired signal while rejecting all other signals. The filters are also commonly used in DC power supplies to eliminate undesired high frequencies as well as on a power supply’s output to reduce ripple. The filters are also used in audio electronics and in analog-to-digital conversion. The filters work extracting important frequencies from signals that also contain irrelevant or undesirable frequencies. The filters can be categorized into four major types that include the high-pass filter, the low-pass filter, the band-pass filter and the band-reject filter also referred to as the band stop or the notch filter.

Band Stop Filters

A band pass filter is a simple form of a filter that has high pass filters and low pass filters passing a band or range of frequencies on either side of two cut-off frequency points. The band stop filter functions exactly the opposite way the band pass filter works. It is a type of a frequency selective circuit that combines the low and high pass filter sections enabling it to severely attenuate or block a band of frequencies within the two cut-off frequency points.

A look at how the band stop filter functions

The band stop filter passes all frequencies with the exception of greatly attenuated frequencies within a specified stop band. In a situation where the stop band is very narrow and highly attenuated over a few hertz, the filter is referred to as a notch filter. The notch filters have a frequency response of a deep notch with high selectivity instead of a flattened wider band. The filter has two cut-off frequencies that include the -3dB or the half-power points that produce a wide stop band bandwidth between the points. The band stop filter work is to pass the frequencies from zero (DC) to the lower cut-off point and then to the second cut-off frequency while also rejecting or blocking the frequencies in-between.

In a wide-band band stop filter, the stop band filter is between the lower and upper -3dB points for it to reject or attenuate frequency between the cut-off points. There are several ways the band stop filter can be designed but the common design features a combination of low pass filter and high pass filter in a parallel type configuration. The design can also include operational amplifiers that introduce voltage gain into the circuit. Depending on an individual needs, the circuit can be designed with a much narrower frequency response by increasing the selectivity of the filter. The filters with narrower bandwidths require thicker coatings. The coatings can be expensive and may also exhibit high levels of stress, which makes it beneficial to use band stop filter – Evaporated Coatings.

The band stop filters are widely used in the electronics and communication circuit. They can be used to eliminate a band of unwanted frequencies while at the same time enabling other frequencies to pass with minimum loss. When the filter is designed properly, it can produce attenuation of more than -60dB at the notch frequency.

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