ctual responses Third order products Input Figure 5.59 Third order intercept point Receivers 177 third order intercept point, and it gives an indication about its strong signal handling capacity. Typically a good professional high-performance receiver may have an intercept point of around 25 dBm (i.e. a signal which is 25 dB above a milliwatt). Blocking Often a very strong signal that is off channel can reduce the sensitivity of a receiver. This happens because the RF amplifier stage is being driven into compression. When this occurs it has the effect of only allowing the main signal through the amplifier, and all the others are reduced in strength. When blocking is quoted it is usually given as the level of signal 20 kHz away from the received channel which gives a 3 dB reduction in the wanted signal. Usually a good set will be able to withstand levels of several milliwatts before this happens. Cross-modulation Another effect caused by overloading is called cross-modulation. When this occurs the modulation or amplitude variations on a strong signal are superimposed on signals close by. It is particularly noticeable on AM signals where the modulation of a strong signal is superimposed onto weaker signals either side. The problem is a third order effect, and often occurs as a result of poor mixer performance. However, any set with a high third order intercept point should give good cross-modulation performance as well. Dynamic range The dynamic range of the most important parameter of its specification as it outlines the range over which it can operate. In some instances it may be necessary for a set to be able to accommodate signals with enormous differences in strength, especially if a good antenna is used. There are a number of ways in which the dynamic range may be determined. Essentially it is the difference between the weakest signal that it can hear and the strongest one it can tolerate without any noticeable degradation in performance. As these two end points can be specified in a number of different ways 