tem known as synchronous detection. Essentially it uses a beat frequency oscillator and product detector or mixer for AM. For this to work the beat frequency oscillator must be kept on the same frequency or zero beat with the carrier. There are a number of methods of doing this, but the one which is most commonly used employs a high gain limiting amplifier. This takes the AM signal and removes the modulation to leave only the carrier as shown in Figure 5.36. This is used as the beat frequency oscillation to demodulate the signal. Incoming signal Mixer Audio from IF stages (product output detector) filter High gain limiting amplifier Figure Synchronous detector Synchronous detection of AM gives a far more linear method of demodulation. It also gives more immunity against the effects of selective fading on the short wave bands. FM demodulation In order to be able to demodulate an FM signal it is necessary to convert the frequency variations into voltage variations. In order to be able to achieve this the circuit of the demodulator needs to be frequency dependent. The ideal response is a perfectly linear voltage to frequency characteristic like that shown in Figure 5.37. Here it can be seen that the centre frequency is in the middle of the response curve and this is where 154 Newnes Guide to Radio and Communications Technology Output voltage Input frequency Design centre frequency Figure 5.37 An ideal FM demodulator response curve Output voltage Frequency Figure 5.38 The typical characteristic curve of a real FM demodulator the unmodulated carrier would be located when the receiver is correctly tuned into the signal. In other words there would be no offset DC voltage present. In most cases the demodulator has a response like that shown in Figure 5.38. For obvious reasons the response curve is known as an 'S' curve. From this it can be seen that as the signal moves up in frequency a higher voltage is produced and vice versa. In this way the frequency variations of the signal are converted into voltage variations whic