Radio receiver filter options

- notes and overview of the different types of radio receiver filter that are available, detailing their salient features.

There is a wide variety of different types of RF filter used within superhet and other forms of radio receiver to provide the required selectivity.

The developer or designer of a radio receiver will need to choose the optimum type of filter for the given design, balancing the various attributes and receiver requirements.

Choice of filter type

The form of filter chosen for any given application will depend on many factors. As a result many different types of filter will be seen in use in radio receivers.

The choice of filter depends upon a number of factors which could include:

  • Performance of filter
  • Cost
  • Position within radio
  • Radio receiver topology
  • Required performance of the radio
  • Frequency of operation for the filter
  • Filter size

These and other considerations for the particular radio will have an impact on the type of filter that is chosen.

Choice of filter types

Some radio receivers will simply use RF filters in their IF stages made up from the tuned transformers (LC filters based on capacitors and inductors) linking the different intermediate frequency stages within the radios or used with an IC in the radio. Other radio receivers may incorporate highly selective crystal filters, whereas others may use mechanical filters (like those used by the Collins Radio Company some years ago) or ceramic filters. Each radio receiver will have its own requirements for its RF filter according to the form of radio communications application for which it will be used. The choice of RF filter will depend upon a variety of parameters including cost, performance frequency of operation and many other elements. Often the choice of RF filter will be a compromise, but with the technology available today, very high levels of performance can be achieved.

There is a variety of different types of RF filter that can be used. The main types that are used include the following:

  • LC tuned circuit:   The LC type of filter offers basic performance in terms of the filters that can be chosen. It is used for front end tuning. It is also used with superheterodyne radios to provide the main selectivity where the LC elements are incorporated within the inter-stage transformers. Often there are two or three stages with tuned circuits. Using them it is usually possible to achieve sufficient selectivity for a medium wave AM or VHF FM broadcast radio.

    LC filters may also be used in conjunction with other forms of higher quality filter. In these circumstances the LC filter will provide broad selectivity only in addition to any function of impedance matching of the various circuits.

    When using a single chip IF stage or broadcast radio, a single LC tuned circuit may be used dependent upon the particular.

    One of the advantages of this type of filter is that it is relatively cheap, although the cost of the manufacture of the coils or transformers can be higher than some other forms of filter.
  • Crystal filter:   This type of filter is a fixed frequency filter only, but it offers very high degrees of selectivity being based around the use of quartz crystals which offer levels of Q of upwards of 10 000 to 100 000. However they are costly as they require exacting methods of manufacture. They are also larger than some other forms of filter.
  • Monolithic crystal filter:   This form of filter is a crystal filter that has been effectively integrated onto a single quartz wafer. This saves considerably on size, although costs are not always significantly less than those of filters using discrete crystals.
  • Ceramic filter:   Ceramic filters utilise the same piezo-electric principle - that quartz crystals use. However it is possible to make them very cheaply, although, obviously not with the same levels of performance as crystal filters. Ceramic filters are widely used in conjunction with integrated circuit IF strips for commercial broadcast receivers and televisions. Requiring no transformers they are much cheaper to manufacture than LC based transformer tuned filters.
  • Mechanical filter:   The mechanical filter is a type of filter for receivers that was widely used in the 1960s and early 1970s offering very high levels of performance - comparable to, or in some cases better than those offered by crystals. As the name implies the filter uses mechanical resonators that are coupled to the electrical circuit by piezo-electric transducers.

    Typically this type of filter was used only for low frequencies, up to about 500 kHz. Their main disadvantages were size and a tendency to drift with temperature. These days mechanical filters are not as widely used, although they still appear in some radio receiver applications.
  • Roofing filter:   A roofing filter is one which is placed ahead of the main gain stages in a superheterodyne receiver. By incorporating a filter early in the gain stages of the receiver, strong off-channel signals can be removed or reduced in strength so that they do not overload the later stages prior to the main filter.

    The use of a roofing filter is generally reserved for very high performance receiver systems where strong signal handling capabilities are paramount.

The choice of the type of filter will depend upon the particular radio receiver design, its requirements including performance, cost, frequency of operation, etc.

There is a good selection of RF filters that can be used in radio receivers. The actual type that is eventually decided upon a balance of performance, cost and other factors dependent upon the radio communications application for which the receiver will be used. For many radio communications applications where the highest levels of performance are not needed, ceramic filters provide the ideal solution being very cheap and easy to use while providing levels of performance that are quite adequate for many applications. For applications where only the highest levels of performance are required, crystal filters are the most common solution either as units made from discrete crystals or as monolithic filters. However mechanical filters could be considered for some applications. These days LC filters are not widely used because the cost of winding coils is high, and often ceramic filters are more convenient, cheaper, and offer a better level of performance.

By Ian Poole

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