ple diode detector circuit like that shown in Figure 3.4. In this circuit the diode rectifies the signal, only allowing the one half of the alternating radio frequency waveform through. A capacitor is used to remove the radio frequency parts of the signal, leaving the audio waveform. This can be fed into an amplifier after which it can be used to drive a loudspeaker. As the circuit used for demodulating AM is very cheap, it enables the cost of receivers for AM to be kept low. 54 Newnes Guide to Radio and Communications Technology Modulating waveform Envelope of modulated signal Radio frequency signal Figure 3.3 An amplitude modulated signal Radio frequency Rectified signal. signal The capacitor removes the radio frequency element Figure 3.4 A simple diode detector circuit AM has advantages of simplicity, but it is not the most efficient mode to use, both in terms of the amount of spectrum it takes up and the usage of the power. It is for this reason that it is rarely used for communications purposes. Its only major communications use is for VHF aircraft communications. However, it is still widely used on the long, medium, and short wave bands for broadcasting because of its simplicity, enabling the cost of radio receivers to be kept to a minimum. Many radio stations can be heard using AM. For example, within the UK the BBC broadcasts Modulation 55 many of its domestic networks on the long and medium wave bands. BBC Radio 4 broadcasts on 198 kilohertz, and then on the short wave bands there are stations including the BBC World Service, Voice of America and many more that use AM. To find out why it is inefficient it is necessary to look at a little theory behind the operation of AM. When a radio frequency signal is modulated by an audio signal the envelope will vary. The level of modulation can be increased to a level where the envelope falls to zero and then rises to twice the unmodulated level. Any increase on this will cause distortion because the envelope cannot fall below zero. As this is the maximum amount of