iced under normal conditions. Being just a mix product between two signals this is called a second order effect. The major problem occurs when harmonic generation and mixing occur together. It is possible for the harmonic of one signal to mix with the harmonic or fundamental of another to give a third signal which is within the pass-band of the receiver, i.e. 2f1 - f2. It is only the difference products that cause a problem, as the sum products fall well away from the received frequencies. To give an example, signals on 100.00 and 100.1 MHz may enter the front end of a receiver. The harmonic of the first will be at 200.00 MI and this can mix with the second (200 - 100.1) to give a signal at 99.99 MHz. All of these signals are well within the pass-band of the front end and the unwanted signal at 99.9 MHz will appear as a real signal. This is a third order effect. Other higher order products can be calculated and it can be seen that a comb like that shown in Figure 5.58 is produced. Note that only the odd order effects cause problems. 176 Newnes Guide to Radio and Communications Technology Amplitude 2f f2 2f2 -f1 3f2 -2f 4f1 3f2 4f2-3f1 Frequency Figure 5.58 Intermodulation products from two signals Third order intercept To give an indication of the performance of an amplifier a figure known as the third order intercept point is often quoted. It is found that the levels of distortion in a circuit are very small under normal operating conditions, but rise very rapidly as the level of the input signal increases. For every 1 dB increase in signal level a third order signal will rise by 3 dB and a fifth order signal by 5 dB, and SO forth. Normally the amplifier would run into saturation well before the third order effect became comparable with the wanted signal. However, it is possible to plot the curves the levels of the two signals would take. At a certain point they intersect as shown in Figure 5.59. This is known as the Output Third order intercept point Third order intercept Extrapolated responses Wanted signals A