Demodulation

Source: Wikipedia, the free encyclopedia.
Passband modulation
Analog modulation
Digital modulation
Hierarchical modulation
Spread spectrum
See also

Demodulation is extracting the original information-bearing

digital signal
).

These terms are traditionally used in connection with

carrier signal which is used to carry it through a telephone line, coaxial cable, or optical fiber
.

History

Demodulation was first used in

radio receivers. In the wireless telegraphy radio systems used during the first 3 decades of radio (1884–1914) the transmitter did not communicate audio (sound) but transmitted information in the form of pulses of radio waves that represented text messages in Morse code.[citation needed] Therefore, the receiver merely had to detect the presence or absence of the radio signal, and produce a click sound. The device that did this was called a detector. The first detectors were coherers
, simple devices that acted as a switch. The term detector stuck, was used for other types of demodulators and continues to be used to the present day for a demodulator in a radio receiver.

The first type of

thermionic diode
which could also rectify an AM signal.

Techniques

There are several ways of demodulation depending on how parameters of the base-band signal such as amplitude, frequency or phase are transmitted in the carrier signal. For example, for a signal modulated with a linear modulation like AM (amplitude modulation), we can use a synchronous detector. On the other hand, for a signal modulated with an angular modulation, we must use an FM (frequency modulation) demodulator or a PM (phase modulation) demodulator. Different kinds of circuits perform these functions.

Many techniques such as

Received Signal Strength Indication, error detection and correction
, etc., are only performed by demodulators, although any specific demodulator may perform only some or none of these techniques.

Many things can act as a demodulator, if they pass the radio waves on

nonlinearly.[2]

AM radio

An AM signal encodes the information into the carrier wave by varying its amplitude in direct sympathy with the analogue signal to be sent. There are two methods used to demodulate AM signals:

  • The
    crystal set
    exploits the simplicity of AM modulation to produce a receiver with very few parts, using the crystal as the rectifier and the limited frequency response of the headphones as the filter.
  • The product detector multiplies the incoming signal by the signal of a local oscillator with the same frequency and phase as the carrier of the incoming signal. After filtering, the original audio signal will result.

carrier is reduced or suppressed entirely, which require coherent demodulation. For further reading, see sideband
.

FM radio

QPSK carrier recovery phase error causing a fixed rotational offset of the received symbol constellation
, X, relative to the intended constellation, O.

Frequency modulation (FM) has numerous advantages over AM such as better fidelity and noise immunity. However, it is much more complex to both modulate and demodulate a carrier wave with FM, and AM predates it by several decades.

There are several common types of FM demodulators:

  • The quadrature detector, which phase shifts the signal by 90 degrees and multiplies it with the unshifted version. One of the terms that drops out from this operation is the original information signal, which is selected and amplified.
  • The signal is fed into a PLL and the error signal is used as the demodulated signal.
  • The most common is a Foster–Seeley discriminator. This is composed of an electronic filter which decreases the amplitude of some frequencies relative to others, followed by an AM demodulator. If the filter response changes linearly with frequency, the final analog output will be proportional to the input frequency, as desired.
  • A variant of the Foster–Seeley discriminator called the ratio detector[3]
  • Another method uses two AM demodulators, one tuned to the high end of the band and the other to the low end, and feed the outputs into a difference amplifier. [citation needed]
  • Using a digital signal processor, as used in software-defined radio.

PM

Main article: Phase modulation
Receiver structure for QPSK. The matched filters can be replaced with correlators. Each detection device uses a reference threshold value to determine whether a 1 or 0 is detected.

QAM

Main article:
QAM § Receiver

QAM demodulation requires a coherent receiver.

See also

References

  1. ^ "Demodulator - Definitions from Dictionary.com". dictionary.reference.com. Retrieved 2008-05-16.
  2. ^ Ghostly voices, New Scientist, 2 October 1999, retrieved 2017-04-25
  3. ^ "The ratio detector"

External links

Wikibooks has a book on the topic of: Communication Systems/Coherent Receivers
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