tor and produces a voltage output that is proportional to the phase difference and hence to the level of deviation on the signal. The detector is able to operate with relatively low input levels, typically down to levels of around 100 microvolts and it is very easy to set up requiring only the phase shift network to be tuned to the centre frequency of the expected signal. It also provides good linearity and this results in low levels of distortion. Often the analogue multiplier is replaced by a logic AND gate and the input signal is hard limited to produce a variable frequency pulse waveform. The operation of the circuit is fundamentally the same, but it is known as a coincidence detector. Also the output of the AND gate has an integrator to 'average' the output waveform to provide the required audio output, otherwise it would consist of a series of square wave pulses. Receivers 161 AND gate Integrator Incoming frequency Audio modulated output signal Phase shifter Figure 5.45 A coincidence detector Phase locked loops are widely used for demodulating FM. Apart from not using a coil they also give a very linear voltage to frequency conversion. As a result of this they are often used in hi-fi tuners. The way in which they operate is very simple. The circuit is set up to operate as shown in Figure 5.46. The FM signal from the IF stages of the set is connected to one of the phase detector inputs as shown, and the output from the VCO is connected to the other. With no modulation applied and the carrier in the centre position of the pass-band the voltage on the tune line to the VCO is set to the mid position. However, if the carrier deviates in frequency, the loop will try to keep the loop in lock. For this to happen the VCO frequency must follow the incoming signal, and for this to occur the tune line voltage must vary. Monitoring the tune line shows that the variations in voltage correspond to the modulation applied to the signal. By amplifying the variations in Input from Phase Voltage IF stages controlled of the rec