RF & Microwave Filters: the basics

RF filters are a key part of RF design as the filters enable the required signals to be selected and unwanted ones removed.

RF Filters Includes:
RF filters - the basics     Filter specifications     RF filter design basics     High & low pass filter design     Constant-k filter     Butterworth filter     Chebychev filter     Bessel filter     Elliptical filter     Crystal filter    

Filters are used in many areas of electronics. One of the main areas where they are used is within the radio frequency or RF domain.

RF filters are used to remove or accept signals that fall in certain areas of the radio spectrum.

There are many different instances where they can be used - the list of applications is almost infinite. They are sued within radio receivers to provide the selectivity, as well as only enabling the right band of frequencies to enter the latter parts of the set. They are used within transmitters to ensure that unwanted or spurious signals are not transmitted. RF filters are used to ensure that the required mix products from mixers are passed on to the next stages . . . the list of RF filter applications goes on.

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:   As the name indicates the low pass filter is a form of filter that only allows through the lower frequencies. Typically it is nominally flat until the cut-off point, and then it rolls off.
    Generic low pass RF filter response
    Generic low pass filter response
    The actual rate of roll off is dependent mainly upon what is termed the order of the filter.
    Read more about . . . . Low pass filters.

  • High pass filter:   The high pass filter is in many ways the inverse of the low pass filter. It only allows signals through that are higher than the cut-off frequency. Above this point it is nominally flat, and below the RF filter cut-off frequency the response falls away at a rate determined by the order of the filter.
    Generic high pass RF filter response
    Generic low pass filter response
    Read more about . . . . High pass filters.

  • Band pass filter:   The band pass RF filter only allows through signals within certain frequencies. Above and below the cut-off frequencies, the signals will be attenuated and within the accepted band of radio frequencies, signals will be passed through.
    Generic band pass RF filter response
    Generic band pass filter response
  • Band reject filter:   The band reject filter is the opposite of a band pass filter, as it rejects signals within a certain RF band. This form of RF filter is often used to remove unwanted signals that are know to exist in a system.
    Generic band reject RF filter response
    Generic band reject filter response

RF filter characteristics

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.

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.

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:

  • Constant-k: The constant-k filter has the advantage of it being very easy to calculate values for the different components. This enables it to be easily designed with a minimum of theoretical knowledge about the mathematics as in the case of many other filters. However its performance does not quite match that of other filter types, although for many applications it is more than adequate.
    Read more about . . . . Constant-k filter.

  • 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.
    Read more about . . . . Butterworth filter.

  • 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.
    Read more about . . . . Bessel filter.

  • 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.
    Read more about . . . . Chebychev filter.

  • 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.
    Read more about . . . . Elliptic / Cauer filter.

There are many different types or models for RF filters. These ones mentioned above are some of the more commonly used ones, although there are very many other different types of RF filter.

RF filters are an essential element in virtually all RF designs. Filters are required within the systems as well as on the input and output. By using filters, the right signals are enabled to reach the required parts of the circuit and in this way the fidelity of the final signal is maintained to the highest standards - interference is reduced and the system performance is kept as high as possible.

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