Editing Satellite modem (section)

=== Modulator and demodulator ===
A digital modulator transforms a digital stream into a radio signal at the intermediate frequency (IF). A modulator is generally simpler than a demodulator because it doesn't have to recover symbol and carrier frequencies.

A demodulator is one of the most important parts of the receiver. The exact structure of the demodulator is defined by a [[modulation]] type. However, the fundamental concepts are similar. Moreover, it is possible to develop a demodulator that can process signals with different modulation types.

Digital demodulation implies that a ''symbol clock'' (and, in most cases, an intermediate frequency generator) at the receiving side has to be synchronous with those at the transmitting side. This is achieved by the following two circuits:
* timing recovery circuit, determining the borders of symbols;
* carrier recovery circuit, which determines the actual meaning of each symbol. There are modulation types (like [[frequency-shift keying]]) that can be demodulated without carrier recovery, however, this method, known as ''noncoherent demodulation'', is generally worse.

There are also additional components in the demodulator such as the [[intersymbol interference]] [[Equalization (communications)|equalizer]].

If the analog signal was digitized without a four-quadrant multiplier, the complex envelope has to be calculated by a digital ''complex mixer''.

Sometimes a digital [[automatic gain control]] circuit is implemented in the demodulator.