 to lock onto this. This is known as single sideband reduced carrier. As either sideband can be used equally well, a convention is needed for which sideband to use. In this way receivers can be set up to expect the received sideband. If this is not done then the receiver has to be continually switched between upper and lower sideband. For commercial operations the upper sideband is adopted as standard. Although it is perfectly possible to use SSB for frequencies above MHz it is infrequently used on these frequencies, although it is widely used on frequencies below 30 MHz. Signal propagation and the way the bands are used mean that other modes are often more suitable. It is often necessary to define the output power of a single sideband transmitter. As the output is continually varying and dependent upon the level of modulation at any instant, a measure known as the peak envelope power (PEP) is used. This is the peak level of power of the transmitted signal and includes the sideband plus any pilot carrier that may be included. Frequency modulation The most obvious method of applying modulation to a signal is to superimpose the audio signal onto the amplitude of the carrier. However, this is by no means the only method which can be employed. It is also possible to vary the frequency of the signal to give frequency modulation or FM. It can be seen from Figure 3.9 that the frequency of the signal varies as the voltage of the modulating signal changes. The amount by which the signal frequency varies is very important. This is known as the deviation and is normally quoted as the number of Modulation 59 Modulating signal Radio frequency signal Figure 3.9 A frequency modulated signal kilohertz deviation. As an example the signal may have a deviation of +3 kHz. In this case the carrier is made to move up and down by 3 kHz. Broadcast stations in the VHF portion of the frequency spectrum between 88.5 and 108 MHz use large values of deviation, typically +75 kHz. This is known as wideband FM (WBFM). These signals are capable 