is is to modulate the amplitude of the carrier in line with the audio signal. While this method is not as efficient as many other methods, it is still widely used for broadcast stations in the long, medium and short wave bands as well as being used by aircraft at VHF. A basic AM transmitter requires the carrier to be generated and this is modified by the modulating signal. Figure 6.11 shows a basic AM transmitter. In this circuit a system called high level modulation is used, in other words the modulation is applied to the carrier in the final amplifier. The carrier is generated by the oscillator circuit. This may be a variable frequency oscillator, or possibly a crystal oscillator. Today frequency synthesizers are often likely to be used, for convenience and flexibility. Once the basic signal has been generated it is buffered and amplified. This brings it to the correct level for the signal to drive the final power amplifier. Both the drive amplifiers and the final amplifier will operate in class C to maintain the optimum efficiency. Buffer and Final Oscillator driver Matching power amplifier amplifier and filtering Microphone Audio Modulation amplifier amplifier Figure 6.11 A basic AM transmitter 196 Newnes Guide to Radio and Communications Technology The audio signal from the microphone or other source enters the transmitter and is amplified. These stages may also limit or process the signals to improve their intelligibility, or reduce the bandwidth occupied. For most communications purposes frequencies above 3 kHz and below 300 Hz are attenuated. For broadcast purposes top limits above this are used dependent upon the channel spacing in use. On the short wave bands this is 5 kHz and on the medium wave band it is 9 kHz in Europe and 10 kHz in North America. The audio signal is further amplified by a driver amplifier if required and applied to the modulation amplifier. This is a high power audio amplifier that must be capable of developing an audio power equal to half the input power requirements for the final 