synthesizer will generate wideband noise extending either side of the signal. This will be transmitted and may cause interference to other users nearby if the signal is strong enough. For data transmissions using forms of phase modulation, high levels of phase noise may introduce data errors, increasing the bit error rate. As a result care must be taken in the design to ensure that this does not occur. Mixers The basic mixing process and the circuits used have been described in Chapter 5. Not only are mixers used widely for receiver applications, but they are also widespread in transmitter circuitry as well. Here they are also used in frequency conversion applications to change a signal from one frequency to another. In addition to this they are used in the modulation process. For example, they can be used in the generation of the basic signal being used when phase modulation or single sideband is required. When mixers are used in frequency changers, care must be taken to ensure that the unwanted mix products are removed SO that these are not transmitted otherwise spurious signals will be transmitted and these are likely to cause interference to other users of the radio spectrum. 182 Newnes Guide to Radio and Communications Technology Amplifiers Once the signal leaves the area of the transmitter where it is generated it is normally necessary to amplify it to increase the power to the required level to be applied to the antenna and be transmitted. After the signal has been generated its power level may only be a few milliwatts, whereas the required level is likely to be much higher than that. For example, high power broadcast stations may have output powers of tens or even hundreds of kilowatts. Many other transmitters run much lower powers. Automobile-based systems like those used for commercial point to point communications may have output powers of around 10 watts, whereas mobile phones may develop a power of a watt or even much less. To achieve these power levels amplifiers are required to bring the output pow