9.1 TRANSMITTERS

A radio communications system transfers information from one location to another across a radio-frequency (RF) channel. At the transmitter, the information signal is used to modulate a suitable carrier frequency, thereby translating the signal to an allocated portion of the electromagnetic spectrum. The modulated signal is then amplified to a power level commensurate with the required range of the system. The two principal functions of a transmitter are therefore frequency translation and power amplification.

In modern systems, these functions are frequently performed by digital signal-processing (DSP) and software-defined radio (SDR) techniques. Digital baseband data are generated or modulated in the digital domain, converted to analogue form by a high-speed digital-to-analogue converter (DAC), and then up-converted and amplified by a chain of RF stages. Power-control loops maintain compliance with spectral-emission limits defined by international standards such as those of the ITU and the ETSI.

This section outlines the principal features of amplitude-modulated (AM) and frequency-modulated (FM) transmitters and then examines frequency-generation and performance considerations.