Transmitter and Oscillator Systems
A transmitter is an important subsystem in a wireless system. In any active wireless system, a signal will be generated and transmitted through an antenna. The signal’s generating system is called a transmitter. The specifications for a transmitter depend on the applications. For long-distance transmission, high power and low noise are important. For space or battery operating systems, high efficiency is essential. For communication systems, low noise and good stability are required. A transmitter can be combined with a receiver to form a transceiver. In this case, a duplexer is used to separate the transmitting and receiving signals. The duplexer could be a switch, a circulator, or a diplexer, as described in Chapter 4.
A transmitter generally consists of an oscillator, a modulator, an upconverter, filters, and power amplifiers. A simple transmitter could have only an oscillator, and a complicated one would include a phase-locked oscillator or synthesizer and the above components. Figure 6.1 shows a typical transmitter block diagram. The information will modulate the oscillator through AM, FM, phase modulation (PM), or digital modulation. The output signal could be unconverted to a higher frequency. The power amplifiers are used to increase the output power before it is transmitted by an antenna. To have a low phase noise, the oscillator or local oscillator can be phase locked to a low-frequency crystal ...