an automatic volume control (AVC) although this term is not particularly common these days and the more correct description of AGC is used. This circuit is used to help compensate for the enormous variation in signal levels that are encountered. From one station to the next there could be a difference of 90 dB or more. Even when the receiver is tuned to one station the signal may still vary. One example of this occurs with car radios where the vehicle is travelling, and hills and other obstructions mean that the signal strength is always varying. Signal strengths on the short wave bands also vary considerably and the AGC can be used to compensate for this. The AGC operates by sensing the level of the signal when it is demodulated. The voltage generated is used to adjust the gain of some of 134 Newnes Guide to Radio and Communications Technology amplifier Mixer amplifier and filter detector amplifier AGC line to apply Local bias to the IF and RF amplifiers oscillator to control their gain Figure 5.17 An AGC system in a superhet radio the earlier stages. The greater the level of the signal being received, the greater the voltage produced. This enables the gain of the earlier stages to be reduced to a greater extent, thereby keeping the demodulated signal almost constant. All portable broadcast receivers have AGC circuits that utilize a minimum number of components. However, for more sophisticated receivers like communications sets the design of the AGC system can become considerably more complicated. The first requirement of the system is that it should not reduce the gain of the set until the signal has achieved a sufficiently high signal to noise ratio. Time constants also have to be taken into consideration. For amplitude modulated signals it is essential that the AGC system does not act SO quickly that the modulation is removed from the carrier. To ensure that this does not happen a simple filter is normally used. This may be a simple capacitor resistor network, giving a time constant of about a quarter of a se