ver. However, this term is normally reserved for commercial items where 'traditional' radio communications are required and items such as mobile phones that also contain both transmitter and receiver are not referred to by this term. Whatever the use of the transmitter, the fundamental concepts are the same, and the same building blocks are used and the same specifications apply. Transmitter building blocks Like all electronics the complexity of transmitters is increasing, and their flexibility is improving to enable them to fulfil the increasing require- ments placed on them. From low power UHF transmitters in cellular phones to the high power broadcast stations, they all use a number of the same basic building blocks to achieve the output of the required signal. Oscillator The heart of any transmitter is its master oscillator. This generates the carrier onto which the modulation is superimposed. Many transmitters utilize a number of oscillators to mix the signal to its final required 180 Newnes Guide to Radio and Communications Technology frequency. However, this processing often takes place after the modula- tion has been applied. The requirements of the oscillator wherever it is used in the equipment are that it should be stable and not drift. If it does then the output signal will change. In some instances drift on an oscillator may cause the characteristics of the signal to change as in the case when single sideband is used. The oscillators used in transmitters take many forms. Those trans- mitters that do not need a frequency change may use a crystal oscillator. This will ensure a high degree of frequency stability. Typically this approach is adopted where small low power transmitters are used. In some cases where high degrees of accuracy and stability are required, crystal ovens may be employed. For signals which need to change in frequency a variable frequency oscillator (VFO) is required. The simplest form of VFO uses an inductor and variable capacitor. An example of a suitable oscillator is shown in Ch