In
radio
Radio is the technology of signaling and communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transm ...
, a detector is a device or circuit that extracts
information
Information is an abstract concept that refers to that which has the power to inform. At the most fundamental level information pertains to the interpretation of that which may be sensed. Any natural process that is not completely random, ...
from a
modulated radio frequency
Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the uppe ...
current or voltage. The term dates from the first three decades of radio (1888-1918). Unlike modern radio stations which transmit sound (an
audio signal
An audio signal is a representation of sound, typically using either a changing level of electrical voltage for analog signals, or a series of binary numbers for digital signals. Audio signals have frequencies in the audio frequency range of ro ...
) on an uninterrupted
carrier wave
In telecommunications, a carrier wave, carrier signal, or just carrier, is a waveform (usually sinusoidal) that is modulated (modified) with an information-bearing signal for the purpose of conveying information. This carrier wave usually has ...
, early radio stations transmitted information by ''
radiotelegraphy
Wireless telegraphy or radiotelegraphy is transmission of text messages by radio waves, analogous to electrical telegraphy using cables. Before about 1910, the term ''wireless telegraphy'' was also used for other experimental technologies for ...
''. The transmitter was switched
on and off to produce long or short periods of radio waves, spelling out text messages in
Morse code
Morse code is a method used in telecommunication to encode text characters as standardized sequences of two different signal durations, called ''dots'' and ''dashes'', or ''dits'' and ''dahs''. Morse code is named after Samuel Morse, one ...
. Therefore, early
radio receivers had only to distinguish between the presence or absence of a radio signal. The device that performed this function in the receiver circuit was called a ''detector''. A variety of different detector devices, such as the
coherer,
electrolytic detector,
magnetic detector and the
crystal detector, were used during the wireless telegraphy era until superseded by vacuum tube technology.
After sound (
amplitude modulation, AM) transmission began around 1920, the term evolved to mean a
demodulator, (usually a
vacuum tube
A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric voltage, potential difference has been applied.
The type kn ...
) which extracted the
audio signal
An audio signal is a representation of sound, typically using either a changing level of electrical voltage for analog signals, or a series of binary numbers for digital signals. Audio signals have frequencies in the audio frequency range of ro ...
from the radio frequency
carrier wave
In telecommunications, a carrier wave, carrier signal, or just carrier, is a waveform (usually sinusoidal) that is modulated (modified) with an information-bearing signal for the purpose of conveying information. This carrier wave usually has ...
. This is its current meaning, although modern detectors usually consist of
semiconductor diodes,
transistor
upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink).
A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch ...
s, or
integrated circuits.
In a
superheterodyne receiver
A superheterodyne receiver, often shortened to superhet, is a type of radio receiver that uses frequency mixing to convert a received signal to a fixed intermediate frequency (IF) which can be more conveniently processed than the original carr ...
the term is also sometimes used to refer to the
mixer, the tube or transistor which converts the incoming radio frequency signal to the
intermediate frequency. The mixer is called the first detector, while the demodulator that extracts the audio signal from the intermediate frequency is called the second detector.
In microwave and millimeter wave technology the terms ''detector'' and ''crystal detector'' refer to waveguide or coaxial transmission line components, used for power or
SWR measurement, that typically incorporate point contact diodes or surface barrier Schottky diodes.
Amplitude modulation detectors
Envelope detector
One major technique is known as envelope detection. The simplest form of
envelope detector is the diode detector that consists of a
diode connected between the input and output of the circuit, with a
resistor
A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias activ ...
and
capacitor
A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals.
The effect of a ...
in parallel from the output of the circuit to the ground to form a
low pass filter. If the resistor and capacitor are correctly chosen, the output of this circuit will be a nearly identical voltage-shifted version of the original signal.
An early form of envelope detector was the
crystal detector, which was used in the
crystal set radio receiver. A later version using a crystal diode is still used in crystal radio sets today. The limited frequency response of the headset eliminates the RF component, making the low pass filter unnecessary.
More sophisticated envelope detectors include the
grid-leak detector, the
plate detector, the
infinite-impedance detector, transistor equivalents of them and
precision rectifier
The precision rectifier is a configuration obtained with an operational amplifier in order to have a circuit behave like an ideal diode and rectifier.Paul Horowitz and Winfield Hill, ''The Art of Electronics''. 2nd ed. Cambridge University Press, ...
s using operational amplifiers.
Product detector
A
product detector is a type of
demodulator used for
AM and
SSB
SSB may refer to:
Organizations
* Scandinavian School of Brussels
* Social Security Board (disambiguation)
* , the Society of Saint Bridget
* Society of the Sisters of Bethany, an Anglican order of sisters
* Society of Systematic Biologists
* , ...
signals, where the original carrier signal is removed by multiplying the received signal with a signal at the
carrier frequency
In telecommunications, a carrier wave, carrier signal, or just carrier, is a waveform (usually sinusoidal) that is modulated (modified) with an information-bearing signal for the purpose of conveying information. This carrier wave usually has ...
(or near to it). Rather than converting the envelope of the signal into the decoded waveform by rectification as an envelope detector would, the product detector takes the product of the modulated signal and a
local oscillator
In electronics, a local oscillator (LO) is an electronic oscillator used with a mixer to change the frequency of a signal. This frequency conversion process, also called heterodyning, produces the sum and difference frequencies from the freque ...
, hence the name. By
heterodyning, the received signal is mixed (in some type of nonlinear device) with a signal from the local oscillator, to give sum and difference frequencies to the signals being mixed, just as a ''first mixer'' stage in a
superhet would produce an
intermediate frequency; the beat frequency in this case, the low frequency
modulating signal
In electronics and telecommunications, modulation is the process of varying one or more properties of a periodic waveform, called the ''carrier signal'', with a separate signal called the ''modulation signal'' that typically contains informatio ...
is recovered and the unwanted high frequencies
filtered out from the output of the product detector. Because the
sidebands of an amplitude-modulated signal contain all the information in the carrier displaced from the center by a function of their frequency, a product detector simply mixes the sidebands down into the audible range so that the original audio may be heard.
Product detector circuits are and so essentially
ring modulator
In electronics, ring modulation is a signal processing function, an implementation of frequency mixing, in which two signals are combined to yield an output signal. One signal, called the carrier, is typically a sine wave or another simple ...
s or
synchronous detectors and closely related to some
phase-sensitive detector
A lock-in amplifier is a type of amplifier that can extract a signal with a known carrier wave from an extremely noisy environment. Depending on the dynamic reserve of the instrument, signals up to a million times smaller than noise components, p ...
circuits. They can be implemented using something as simple as
ring of diodes or a single dual-gate
Field Effect Transistor
The field-effect transistor (FET) is a type of transistor that uses an electric field to control the flow of current in a semiconductor. FETs ( JFETs or MOSFETs) are devices with three terminals: ''source'', ''gate'', and ''drain''. FETs con ...
to anything as sophisticated as an
Integrated Circuit containing a
Gilbert cell. Product detectors are typically preferred to envelope detectors by
shortwave listeners and radio amateurs as they permit the reception of both AM and SSB signals. They may also demodulate
CW transmissions if the beat frequency oscillator is tuned slightly above or below the carrier.
Frequency and phase modulation detectors
AM detectors cannot demodulate
FM and
PM signals because both have a
constant amplitude. However an AM radio may detect the sound of an FM broadcast by the phenomenon of slope detection which occurs when the radio is tuned slightly above or below the nominal broadcast frequency. Frequency variation on one sloping side of the radio tuning curve gives the amplified signal a corresponding local amplitude variation, to which the AM detector is sensitive. Slope detection gives inferior distortion and noise rejection compared to the following dedicated FM detectors that are normally used.
Phase detector
A phase detector is a
nonlinear
In mathematics and science, a nonlinear system is a system in which the change of the output is not proportional to the change of the input. Nonlinear problems are of interest to engineers, biologists, physicists, mathematicians, and many other ...
device whose output represents the
phase difference between the two oscillating input signals. It has two inputs and one output: a reference signal is applied to one input and the phase or frequency modulated signal is applied to the other. The output is a signal that is proportional to the phase difference between the two inputs.
In phase demodulation the information is contained in the amount and rate of phase shift in the
carrier wave
In telecommunications, a carrier wave, carrier signal, or just carrier, is a waveform (usually sinusoidal) that is modulated (modified) with an information-bearing signal for the purpose of conveying information. This carrier wave usually has ...
.
The Foster–Seeley discriminator
The
Foster–Seeley discriminator is a widely used FM detector. The detector consists of a special center-tapped
transformer
A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...
feeding two diodes in a full wave DC
rectifier
A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction. The reverse operation (converting DC to AC) is performed by an inve ...
circuit. When the input transformer is tuned to the signal frequency, the output of the discriminator is zero. When there is no deviation of the carrier, both halves of the center tapped transformer are balanced. As the FM signal swings in frequency above and below the carrier frequency, the balance between the two halves of the center-tapped secondary is destroyed and there is an output voltage proportional to the frequency deviation.
Ratio detector
The ratio detector is a variant of the Foster–Seeley discriminator, but one diode conducts in an opposite direction, and using a tertiary winding in the preceding transformer. The output in this case is taken between the sum of the diode voltages and the center tap. The output across the diodes is connected to a large value capacitor, which eliminates AM noise in the ratio detector output. The ratio detector has the advantage over the Foster–Seeley discriminator that it will not respond to
AM signals, thus potentially saving a limiter stage; however the output is only 50% of the output of a discriminator for the same input signal. The ratio detector has wider bandwidth but more distortion than the Foster–Seeley discriminator.
Quadrature detector
In quadrature detectors, the received FM signal is split into two signals. One of the two signals is then passed through a
high-reactance capacitor
A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals.
The effect of a ...
, which shifts the phase of that signal by 90 degrees. This phase-shifted signal is then applied to an LC circuit, which is resonant at the FM signal's unmodulated, "center," or "carrier" frequency. If the received FM signal's frequency equals the center frequency, then the two signals will have a 90-degree
phase difference and they are said to be in "phase quadrature" — hence the name of this method. The two signals are then multiplied together in an analog or digital device, which serves as a phase detector; that is, a device whose output is proportional to the phase difference between two signals. In the case of an unmodulated FM signal, the phase detector's output is — after the output has been
filtered; that is, averaged over time — constant; namely, zero. However, if the received FM signal has been modulated, then its frequency will vary from the center frequency. In this case, the resonant LC circuit will further shift the phase of the signal from the capacitor, so that the signal's total phase shift will be the sum of the 90 degrees imposed by the capacitor, and the positive or negative phase change imposed by the LC circuit. Now the output from the phase detector will differ from zero, and in this way, one recovers the original signal that was used to modulate the FM carrier.
XOR gate detector
The detection process described above can also be accomplished by combining, in an
exclusive-OR (XOR) logic gate, the
limited
Limited may refer to:
Arts and media
*''Limited Inc'', a 1988 book by Jacques Derrida
*Limited series (comics), a comic book series with predetermined length
Businesses
*Limited Brands, an American company - owners of Victoria's Secret, Bath & Bo ...
original FM signal and either a copy of that signal passed through a network which imposes a phase shift that varies with frequency, e.g. an
LC circuit (and then limited as well), or a fixed-frequency square wave carrier at the center frequency of the signal. The XOR gate produces a stream of output pulses the duty cycle of which corresponds to the phase difference between the two signals. Due to the varying phase difference between the two inputs, a
pulse-width modulated (PWM) signal is produced. When a
low-pass filter
A low-pass filter is a filter that passes signals with a frequency lower than a selected cutoff frequency and attenuates signals with frequencies higher than the cutoff frequency. The exact frequency response of the filter depends on the filt ...
is applied to those pulses, the filter's output rises as the pulses grow longer and its output falls as the pulses grow shorter. In this way, one recovers the original signal that was used to modulate the FM carrier.
When a phase-shifted version of the original signal is used, the result is a frequency demodulation, as the frequency difference between the inputs of the XOR gate remains zero and thus does not affect their phase relationship.
With a fixed-frequency carrier, the result is a
phase demodulation, which, in this case is an
integral
In mathematics, an integral assigns numbers to functions in a way that describes displacement, area, volume, and other concepts that arise by combining infinitesimal data. The process of finding integrals is called integration. Along with ...
of the original modulating signal.
Other FM detectors
Less common, specialized, or obsolescent types of detectors include:
* Travis
[Charles Travis, "Automatic oscillator frequency control system" U.S. patent: 2,294,100 (filed: 4 February 1935; issued: August 1942). See also: Charles Travis, "Automatic frequency control," ''Proceedings of the Institute of Radio Engineers'', vol. 23, no. 10, pages 1125-1141 (October 1935).] or double tuned circuit discriminator using two non-interacting tuned circuits above and below the nominal center frequency
* Weiss discriminator which uses a single LC tuned circuit or crystal
* Pulse count discriminator which converts the frequency to a train of constant amplitude pulses, producing a voltage directly proportional to the frequency.
Phase-locked loop detector
The
phase-locked loop
A phase-locked loop or phase lock loop (PLL) is a control system that generates an output signal whose phase is related to the phase of an input signal. There are several different types; the simplest is an electronic circuit consisting of a ...
detector requires no frequency-selective LC network to accomplish demodulation. In this system, a
voltage controlled oscillator (VCO) is
phase lock
In mathematics, particularly in dynamical systems, Arnold tongues (named after Vladimir Arnold) Section 12 in page 78 has a figure showing Arnold tongues. are a pictorial phenomenon that occur when visualizing how the rotation number of a dynami ...
ed by a
feedback loop
Feedback occurs when outputs of a system are routed back as inputs as part of a chain of cause-and-effect that forms a circuit or loop. The system can then be said to ''feed back'' into itself. The notion of cause-and-effect has to be handled ...
, which forces the VCO to follow the frequency variations of the incoming FM signal. The low-frequency error voltage that forces the VCO's frequency to track the frequency of the modulated FM signal is the demodulated audio output. The phase-locked loop detector should not be confused with the phase-locked loop frequency synthesizer, which is often used in digitally-tuned AM and FM radios to generate the
local oscillator
In electronics, a local oscillator (LO) is an electronic oscillator used with a mixer to change the frequency of a signal. This frequency conversion process, also called heterodyning, produces the sum and difference frequencies from the freque ...
frequency.
See also
*
Cat's whisker detector
*
Coherer
*
Tuner (radio)
A tuner is a subsystem that receives radio frequency (RF) transmissions, such as FM broadcasting, and converts the selected carrier frequency and its associated bandwidth into a fixed frequency that is suitable for further processing, usually b ...
*
Electrolytic detector
*
Foster–Seeley discriminator
*
Grid-leak detector
*
Hot wire barretter The hot-wire barretter was a demodulating detector, invented in 1902 by Reginald Fessenden, that found limited use in early radio receivers. In effect, it was a highly sensitive thermoresistor, which could demodulate amplitude-modulated signals, s ...
*
Magnetic detector
*
Plate detector
*
Demodulation
*
Tikker
*
Wunderlich detector
References
External links
*{{Commons category inline
*Simple block diagrams and descriptions of key circuits for FM transmitters and receivers:
'
Detectors
Radio electronics
Demodulation
ja:ラジオ受信機