h can be amplified by an audio amplifier before being passed into headphones or a loudspeaker. Receivers 155 To enable the best detection to take place the signal should be centred about the middle of the curve. If it moves off too far then the characteristic becomes less linear and higher levels of distortion result. Often the linear region is designed to extend well beyond the bandwidth of a signal SO that this does not occur. In this way the optimum linearity is achieved. Typically the bandwidth of a circuit for receiving VHF FM broadcasts may be about 1 MHz whereas the signal is only 200 kHz wide. In many receivers it is necessary to ensure that the receiver stays on the correct frequency. Any drift in the set may mean that the signal may not remain on the linear portion of the demodulator curve. Some sets incorporate a circuit called an automatic frequency control or AFC. This enables the set to remain on tune, even when the local oscillator is prone to drifting. The circuit operates by taking the output from the demodu- lator and removing the audio using a simple filter. The remaining voltage is then an indication of where the carrier is on the demodulator curve. Ideally it should be in the centre. The voltage can be applied to the local oscillator in the set as a correction signal to pull its frequency slightly SO that the signal remains in the centre of the pass-band. Steady DC proportional Demodulator Recovered to frequency offset used response audio to tune oscillator Filter Radio frequency signal deviates over this range Figure 5.39 Operation of an AFC One of the advantages of FM is its resilience to noise. This is one of the main reasons why it is used for high quality audio broadcasts. However, when no signal is present, a high noise level is present at the output of the receiver. If a low level FM signal is introduced and its level slowly increased it will be found that the noise level reduces. From this the quieting level can be deduced. It is the reduction in noise level expressed in decibels when