The intermediate frequency stages of a superheterodyne radio are where the main amplification is provided and where adjacent channels filtering is found.
In this way the IF stages of a superhet radio are some of the most exacting stages in terms of design. Gain, overload performance, filtering and a host of other requirements are key to the performance of the whole radio.
IF stage choices
Lie all the other areas of a superheterodyne radio, there are many choices for the configuration of this stage. There are many different outlines for the IF elements of the superheterodyne radio dependent upon the level of performance required, cost, and of course the operational needs for the circuit.
A variety of different choices are available:
- Single frequency IF with LC tuned filters: This form of superheterodyne receiver IF stage is the most basic form and was traditionally used for many broadcast, and even some communications receivers. The IF stage uses a single frequency (most likely 455 kHz or thereabouts) and the transformers are tuned to provide the filtering.The circuit of the simple superheterodyne radio shows how this is achieved. In this case the collector circuit of the mixer device has an IF transformer tuned to the intermediate frequency and this then passes the signal on to further IF amplifiers, each with a tuned IF transformer. The transformers are tuned to provide the required selectivity and bandwidth. Sometimes each IF transformer may be tuned to a slightly different frequency to provide the required bandwidth.
- Single frequency IF with crystal or ceramic filter: LC filers will never be able to provide an exceedingly sharp filter response. Where better responses are required ceramic or better still quartz crystal filters may be used.
With modern receiver integrated circuits, like those used in mass produced broadcast radios, ceramic filters are widely used. A single ceramic filter in the IF stages of the receiver will provide adequate selectivity for broadcast reception.
High performance radios may use a crystal filter to give much better performance . but crystal filters are more costly and therefore they will be used in professional radios or those intended for amateur radio, etc.
- Multiple conversion superhet IF: One of the big issues with the superheterodyne radio is that of the image response. raising he intermediate frequency increases the frequency separation between the IF and the image and therefore many receivers need as high an intermediate frequency as possible. However high intermediate frequency IFs can cause other issues, especially with filtering. To overcome this, an additional conversion may be added. In this way a high intermediate frequency first IF can be used to ensure the image response is sufficiently attenuated, and a subsequent stage at a lower frequency is used to provide the main selectivity. Although ceramic and crystal filters can be made much higher in frequency these days, it is still better to provide the main filtering at a lower frequency.
- Multiple conversion IF with roofing filter: With the intermediate frequency stages of superheterodyne receivers having large amounts of gain, it is quite possible for strong off-channel signals to cause overloading in stages prior tot he main filter. As multiple conversion IF stages have a lot more circuitry before the main filter, strong signals that are off channel but are able to enter the IF can cause overloading before they are filtered. To overcome this issue, a filter known as a roofing filter is added early in the early stages of the IF. Its job is not to provide the main filtering, but it is just there to attenuate any strong off channel signals to prevent overloading the subsequent stages. Specialised filters for use as IF roofing filters are available for this type of application.
Choice of filter typesSome 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 for the IF stages. 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 and often used in the IF stages of high performance superheterodyne receivers, 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 IF stags of superhet 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 IF stages of 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.
IF and AGC
The automatic gain control voltage is applied to points within he IF amplifier chain. Typically it is applied to the earlier stages of the IF, and in this way it reduces the likelihood of overload.
However, there are considerations associated with the noise figure of the whole receiver. If the gain reduction is applied too strongly to the earlier stages only, then it can affect the noise figure, so typically an ideal AGC would reduce the signal slightly later into e chain first, where there is no likelihood of overload and then progressively reduce the gain easier int he chain.
The IF stages of any receiver are a key section of the receiver. Both the majority of the gain and the adjacent signal selectivity are provided int these stages, and they are two of the most important functions of any receiver. Accordingly the IF needs to be carefully designed to give the optimum performance across all signal conditions.