Understanding Receiver Selectivity

First published in Monitoring Times, December 2000

Selectivity is one of the major specifications of any receiver. Whilst the sensitivity is important to ensure that it can pick up the signals and receive them at a sufficient strength, the selectivity is also very important. It is this parameter that determines whether the receiver is able to pick out the wanted signal from all the other ones around it. The filters used in receivers these days have very high levels of performance and enable receivers to select out individual signals even on today's crowded bands.

Superhet principle

Most of the receivers that are used today are superhet radios. In these sets the incoming signal is converted down to a fixed intermediate frequency. It is within the IF stages that the main filters are to be found. It is the filter in the IF stages that defines the selectivity performance of the whole set, and as a result the receiver selectivity specification is virtually that of the filter itself.

Figure 1 Block diagram of a basic superhet receiver

In some receivers simple LC filters may be used, although ceramic filters are better and are used more widely nowadays. For the highest performance crystal or mechanical filters may be used, although they are naturally more costly and this means they are only found in high performance sets.

Filter parameters

There are two main areas of interest for a filter, the pass band where it accepts signals and allows them through, and the stop band where it rejects them. In an ideal world a filter would have a response something like that shown in Figure 2. Here it can be seen that there is an immediate transition between the pass band and the stop band. Also in the pass band the filter does not introduce any loss and in the stop band no signal is allowed through.

Figure 2 The response of an ideal filter

In reality it is not possible to realise a filter with these characteristics and a typical response more like that shown in Figure 3. It is fairly obvious from the diagram that there are a number of differences. The first is that there is some loss in the pass band. Secondly the response does not fall away infinitely fast. Thirdly the stop band attenuation is not infinite, even though it is very large. Finally it will be noticed that there is some in band ripple.

Figure 3 Typical response of a real filter

In most filters the attenuation in the pass band is normally relatively small. For a typical crystal filter figures of 2 - 3 dB are fairly typical. However it is found that very narrow band filters like those used for Morse reception may be higher than this. Fortunately it is quite easy to counteract this loss simply by adding a little extra amplification in the intermediate frequency stages and this factor is not quoted as part of the receiver specification.

It can be seen that the filter response does not fall away infinitely fast, and it is necessary to define the points between which the pass band lies. For receivers the pass band is taken to be the bandwidth between the points where the response has fallen by 6 dB, i.e. where it is 6 dB down or -6 dB.

A stop band is also defined. For most receiver filters this is taken to start at the point where the response has fallen by 60 dB, although the specification for the filter should be checked this as some filters may not be as good. Sometimes a filter may have the stop band defined for a 50 dB attenuation rather than 60 dB.

Shape factor

It can be seen that it is very important for the filter to achieve its final level of rejection as quickly as possible once outside the pass band. In other words the response should fall as quickly as possible. To put a measure on this, a figure known as the shape factor is used. This is simply a ratio of the bandwidths of the pass band and the stop band. Thus a filter with a pass band of 3 kHz at -6dB and a figure of 6 kHz at -60 dB for the stop band would have a shape factor of 2:1. For this figure to have real meaning the two attenuation figures should also be quoted. As a result the full shape factor specification should be 2:1 at 6/60 dB.

Filter types

There is a variety of different types of filter that can be used in a receiver. The older broadcast sets used LC filters. The IF transformers in the receiver were tuned and it was possible to adjust the resonant frequency of each transformer using an adjustable ferrite core.

Today ceramic filters are more widely used. Their operation is based on the piezoelectric effect. The incoming electrical signal is converted into mechanical vibrations by the piezoelectric effect. These vibrations are then affected by the mechanical resonances of the ceramic crystal. As the mechanical vibrations are then linked back to the electric signal, the overall effect is that the mechanical resonances of the ceramic crystal affect the electrical signal. The mechanical resonances of the ceramic exhibit a high level of Q and this is reflected in its performance as an electrical filter. In this way a high Q filter can be manufactured very easily.

Ceramic filters can be very cheap, some costing only a few cents. However higher performance ones are also available, and these are likely to be found in scanners and many other receivers.

For really high levels of filter performance crystal filters are used. Crystals are made from quartz, a naturally occurring form of silicon, although today's components are made from synthetically grown quartz. These crystals also use the piezoelectric effect and operate in the same way as ceramic filters but they exhibit much higher levels of Q and offer far superior degrees of selectivity. Being a resonant element they are used in many areas where an LC resonant element might be found. They are used in oscillators - many computers have crystal oscillators in them, but they are also widely used in high performance filters.

Normally crystal filters are made from a number of individual crystals. The most commonly used configuration is called the half lattice filter as shown in Figure 4. Further sections can be added to the filter to improve the performance. Often a filter will be quoted as having a certain number of poles. There is one pole per crystal, so a six pole crystal filter would contain six crystals and so forth. Many filters used in amateur communications receivers will contain either six or eight poles.

Figure 4 A basic half lattice crystal filter section

Choosing the right bandwidth

It is important to choose the correct bandwidth for a give type of signal. It is obviously necessary to ensure that it is not too wide, otherwise unwanted off-channel signals will be able to pass though the filter. Conversely if the filter is too narrow then some of the wanted signal will be rejected and distortion will occur. As different types of transmission occupy different amounts of spectrum bandwidth it is necessary to tailor the filter bandwidth to the type of transmission being received. As a result many receivers switch in different filters for different types of transmission. This may be done either automatically as part of a mode switch, or using a separate filter switch. Typically a filter for AM reception on the short wave bands will have a bandwidth of around 6 kHz, and one for SSB will be approximately 2.5 kHz. For Morse reception 500 and 250 Hz filters are often used.

Summary

Selectivity is particularly important on today's crowded bands, and it is necessary to ensure that any receiver is able to select the wanted signal as well as it can. Obviously when signals occupy the same frequency there is little that can be done, but by having a good filter it is possible to ensure that you have the best chance or receiving and being able to copy the signal you want.