 modulation possible it is called 100 per cent modulation. Level when no Peak amplitude is twice modulation is that when no modulation present is present Figure 3.5 Fully modulated signal Even with 100 per cent modulation the utilization of power is very poor. When the carrier is modulated sidebands appear at either side of the carrier in its frequency spectrum. Each sideband contains the information about the audio modulation. To look at how the signal is made up and the relative powers take the simplified case where the 1 kHz tone is modulating the carrier. In this case two signals will be found 1 kHz either side of the main carrier as shown in Figure 3.6. When the carrier is fully modulated, i.e. 100 per cent, the amplitude of the modulation is equal to half that of the main carrier, i.e. the sum of the powers of the sidebands is equal to half that of the carrier. This means that each sideband is just a quarter of the total power. In other words for a transmitter with a 100 watt carrier, the total sideband power would be 50 watts and each individual sideband would be 25 watts. During the modulation process the carrier power remains constant. It is only needed as a reference during the demodulation process. This means that the sideband power is the useful section of the signal, and this corresponds 56 Newnes Guide to Radio and Communications Technology Carrier Lower sideband Upper sideband (quarter the power level (quarter the power level of the carrier at 100% of the carrier at 100% modulation) modulation) f 1 kHz f + 1 kHz Frequency Figure 3.6 Spectrum of a signal modulated with a 1 kHz tone to (50/150) X 100 per cent, or only 33 per cent of the total power transmitted. Not only is AM wasteful in terms of power, it is also not very efficient in its use of spectrum. If the 1 kHz tone is replaced by a typical audio signal made up of a variety of sounds with different frequencies then each frequency will be present in each sideband. Accordingly the sidebands spread out either side of the carrier as shown and the