Under normal conditions the amplifiers in a set must remain linear. In other words the output is directly proportional to the input. However, when strong signals are received there comes a point where the output of the amplifier starts to overload and cannot give out the required level. At this point the amplifier is said to be in compression. Output Input Figure 5.57 Characteristic of a typical amplifier Receivers 175 Compression itself is not a problem as the absolute values of a signal are rarely important and in any case the AGC acts on the signal level to alter the gain of the set, it is the effects associated with compression and non-linearity which cause the problems. Intermodulation, blocking, cross-modulation are three problems associated with overload, and these can noticeably reduce the performance of a receiver in some circumstances. Intermodulation distortion One of the main problems associated with overload is intermodulation distortion. When this occurs, new signals are generated within the receiver, sometimes giving the perception that it is receiving a lot of signals and it is very sensitive. Two effects occur to give rise to intermodulation distortion when an amplifier becomes overloaded. One is that harmonics of signals being received are generated, and the second is that the amplifier, being non- linear, acts as a mixer. On their own these effects are unlikely to degrade the performance very much because the RF tuning would remove signals which would pass through the rest of the set. If a signal at frequency f is generating harmonics, these will fall at frequencies of 2f, 3f, 4f, and SO forth. For even the lowest harmonic to enter the set, the fundamental at half the frequency must enter the front end. The RF tuning will reduce this to a low level and it is unlikely to cause a problem for most applications. Similarly if two signals are to mix together to form a signal which is within the receiver pass-band they will be outside the acceptance range of the front end and their effects are not not