Receiver Dynamic Range
By Ian Poole, www.radio-electronics.com
First published in Monitoring Times, November 2000
Sensitivity is one of the main specifications people look at when buying a receiver. However the sensitivity of a set is by no means the whole story. The specification for a set may show it to have an exceedingly good level of sensitivity, but when it is connected to an antenna its performance may be very disappointing because it is easily overloaded when strong signals are present, and this may impair its ability to receive weak signals.
The overall dynamic range of the receiver is very important. It is just as important for a set to be able to handle strong signals well as it is to be able to pick up weak ones. This becomes very important when trying to pick up weak signals in the presence of nearby strong ones. Under these circumstances a set with a poor dynamic range may not be able to hear the weak stations picked up by a less sensitive set with a better dynamic range. Problems like blocking, inter-modulation distortion and the like within the receiver may mask out the weak signals, despite the set having a very good level of sensitivity.
What is dynamic range?
Strong signal handling
In the ideal world the output of an amplifier would be proportional to the input for all signal levels. However amplifiers only have a limited output capability and it is found that beyond a certain level the output falls below the required level because it cannot handle the large levels required of it. This gives a characteristic like that shown in Fig. 1. From this it can be seen that amplifiers are linear for the lower part of the characteristic, but as the output stages are unable to handle the higher power levels the signals start to become compressed as seen by the curve in the characteristic.
Fig. 1 A typical amplifier characteristic
The fact that the amplifier is non-linear does not create a major problem in itself. However the side effects do. When a signal is passed through a non-linear element there are two main effects which are noticed. The first is that harmonics are generated. Fortunately these are unlikely to cause a major problem. For a harmonic to fall near the frequency being received, a signal at half the received frequency must enter the amplifier. The front end tuning should reduce this by a sufficient degree for it not to be a noticeable problem under most circumstances.
The other problem that can be noticed is that signals mix together to form unwanted products. These again are unlikely to cause a problem because any signals which could mix together should be removed sufficiently by the front end tuning. Instead problems occur when harmonics of in-band signals mix together.
Third order products
It is simple to calculate the frequencies where the spurious signals will fall. If the input frequencies are f1 and f2, then the new frequencies produced will be at 2f1 - f2, 3f1 - 2f2, 4f1 - 3f2 and so forth. On the other side of the two main or original signals products are produced at 2f2 - f1, 3f2 - 2f2, 4f2 - 3f1 and so forth as shown in the diagram. These are known as odd order inter-modulation products. Two times one signal plus one times another makes a third order product, three times one plus two times another is a fifth order product and so forth. It can be seen from the diagram that the signals either side of the main signals are first the third order product, then fifth, seventh and so forth.
To take an example with some real figures. If large signals appear at frequencies of 30.0 MHz and 30.01 MHz, then the inter-modulation products will appear at 30.02, 30.03, 30.4 ...MHz and 29.99, 29.98, 29.97 ..... MHz.
Fig. 2 Inter-modulation products
The amount of blocking is obviously dependent upon the level of the signal. It also depends on how far off channel the strong signal is. The further away, the more it will be reduced by the front end tuning and the less the effect will be. Normally blocking is quoted as the level of the unwanted signal at a given offset (normally 20 kHz) to give a 3 dB reduction in gain.
Dynamic range definition
To gain a feel for the figures which may be obtained where inter-modulation is the limiting factor figures of between 80 and 90 dB range are typical, and where blocking is the limiting factor figures around 115 dB are generally achieved in a good receiver.
Designing for optimum performance
A receiver with a good dynamic range will be able to give a far better account of itself under exacting conditions than one designed purely for optimum sensitivity.
© Ian Poole, www.radio-electronics.com