Synchronous demodulation

from Wikipedia, the free encyclopedia
Shortwave radio reception without synchronous demodulation
Shortwave radio reception with synchronous demodulation

Under the synchronous demodulation , or even coherent demodulation called, is meant in the communications engineering a special form of amplitude demodulator . The phase position of the non-transmitted carrier frequency is reconstructed in the receiver.

The functional unit that carries out this process in a receiving device is called a synchronous demodulator .

description

In the following, the principle of synchronous demodulation will be illustrated using the example of amplitude modulation , since this results in particularly simple facts.

With amplitude modulation, the information to be transmitted is often only transmitted through the sidebands of the carrier signal. As it were randomly - provided that the degree of modulation m is less than one - the result of vector addition is that the amplitude of the carrier signal is directly proportional to the instantaneous signal deflection of the information signal. Therefore (and only because of this) an envelope demodulator is sufficient to demodulate AM .

If m  > 1 or a sideband is missing, as in single sideband modulation , a synchronous demodulator can still demodulate without distortion. As in the modulator, the received signal is mixed with the carrier frequency generated in the receiver in a local oscillator . Sometimes a sample-and-hold circuit is also used, which records the peak value of the sinusoidal voltage shortly after the zero crossing.

This results in two spectra of the useful signal: one in the baseband position and one with twice the carrier frequency. The original information signal can be obtained directly from the signal in the baseband position by means of low-pass filtering .

The carrier frequency generated in the receiver by an oscillator must have the same phase position as the carrier frequency used in the modulation, from which the term synchronous demodulator is derived. This temporal alignment is ensured in the demodulator by an oscillator with adjustable frequency and phase position, a VCO or VCXO , usually in a control loop ( PLL ). In order to control this, information about the phase reference must be transmitted. This can be the transmitted carrier frequency - possibly with a lowered level - or another frequency with a phase position fixed to the carrier frequency and a known division ratio (e.g. pilot tone for the stereo decoder in the pilot tone multiplex method ).

The coherence demodulator can easily be implemented in integrated circuits in that analog switches switch or sample the signal to be demodulated, consisting of one or both sidebands, in time with the synthesized carrier.

Coherent modulation also allows two information signals to be modulated onto only one carrier signal without mutual interference: One information signal is modulated "in phase" with the carrier, the second carrier signal is modulated on the carrier signal, which is phase-shifted by 90 °. These two components are also called the I and Q signal components of a complex signal. The two components are orthogonal to one another and therefore do not have any mutual influence. This is used in quadrature amplitude modulation , for the demodulation of which synchronous demodulation is absolutely necessary.

Applications

  • In the field of digital signal processing and digital communications technology, where coherent demodulators are used almost without exception.
  • In analog application areas, synchronous demodulation is found in the area of PAL color television and the radio data system in VHF broadcasting, where it is used to transmit additional information for radio receivers such as traffic announcements.
  • Another application is the PLL - stereo decoder .
  • The so-called synchronous detector of a world receiver reconstructs a very weak carrier signal - for example through fading - with the help of a locally generated synchronous carrier and thus considerably facilitates the reception of weak stations.
  • The lock-in amplifier can determine extremely low voltages of a given frequency from highly noisy signals and ignores interference from neighboring frequencies.
  • Impedance bridges evaluate the measurement signals according to phase and amplitude. Double the measuring frequency serves as a reference for the signal sampling.

literature