RF filters of all types are required in a variety of applications from audio to RF and across the whole spectrum of frequencies. As such RF filters form an important element within a variety of scenarios, enabling the required frequencies to be passed through the circuit, while rejecting those that are not needed.

The ideal filter, whether it is a low pass, high pass, or band pass filter will exhibit no loss within the pass band, i.e. the frequencies below the cut off frequency. Then above this frequency in what is termed the stop band the filter will reject all signals.

In reality it is not possible to achieve the perfect pass filter and there is always some loss within the pass band, and it is not possible to achieve infinite rejection in the stop band. Also there is a transition between the pass band and the stop band, where the response curve falls away, with the level of rejection rises as the frequency moves from the pass band to the stop band.

Basic types of RF filter

There are four types of filter that can be defined. Each different type rejects or accepts signals in a different way, and by using the correct type of RF filter it is possible to accept the required signals and reject those that are not wanted. The four basic types of RF filter are:

    Low pass filter
    High pass filter
    Band pass filter
    Band reject filter

As the names of these types of RF filter indicate, a low pass filter only allows frequencies below what is termed the cut off frequency through. This can also be thought of as a high reject filter as it rejects high frequencies. Similarly a high pass filter only allows signals through above the cut off frequency and rejects those below the cut off frequency. A band pass filter allows frequencies through within a given pass band. Finally the band reject filter rejects signals within a certain band. It can be particularly useful for rejecting a particular unwanted signal or set of signals falling within a given bandwidth.

RF filter frequencies

A filter allows signals through in what is termed the pass band. This is the band of frequencies below the cut off frequency for the filter.

The cut off frequency of the filter is defined as the point at which the output level from the filter falls to 50% (-3 dB) of the in band level, assuming a constant input level. The cut off frequency is sometimes referred to as the half power or -3 dB frequency.

The stop band of the filter is essentially the band of frequencies that is rejected by the filter. It is taken as starting at the point where the filter reaches its required level of rejection.

Filter classifications

Filters can be designed to meet a variety of requirements. Although using the same basic circuit configurations, the circuit values differ when the circuit is designed to meet different criteria. In band ripple, fastest transition to the ultimate roll off, highest out of band rejection are some of the criteria that result in different circuit values. These different filters are given names, each one being optimised for a different element of performance. Three common types of filter are given below:

Butterworth Filter: This type of filter provides the maximum in band flatness, although it provides a lower stop-band attenuation than a Chebyshev filter. However it is also able to provide better group delay performance, and hence lower overshoot.
Bessel: This filter provides the optimum in-band phase response and therefore also provides the best step response. It is often used where signals incorporate square waves, etc as the shape is maintained best of all.
Chebyshev: This filter provides fast roll off after the cut off frequency is reached. However this is at the expense of in band ripple. The more in band ripple that can be tolerated, the faster the roll off.
Elliptic: This filter, also known as the Cauer filter has significant levels of in band and out of band ripple, and as expected the higher the degree of ripple that can be tolerated, the steeper it reaches its ultimate roll off.

These are some of the main types of RF filter. Other types are available, although they tend to be sued in more specialized or specific applications.

RF filters are widely used in RF design and in all manner of RF and analogue circuits in general. As they allow though only particular frequencies or bands of frequencies, they are an essential tool for the RF design engineer.

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