Device which extracts info from a modulated radio frequency current or voltage
In
radio
, a
detector
is a device or circuit that extracts
information
from a
modulated
radio frequency
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
) on an uninterrupted
carrier wave
, early radio stations transmitted information by
radiotelegraphy
. The transmitter was switched
on and off
to produce long or short periods of radio waves, spelling out text messages in
Morse code
. Therefore, early
radio receivers
could reproduce the Morse code "dots" and "dashes" by simply distinguishing between the presence or absence of a radio signal. The device that performed this function in the receiver circuit was called a
detector
.
[1]
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, were used during the wireless telegraphy era until superseded by vacuum tube technology.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 the invention of
amplitude modulation
(AM) enabled the development of AM
radiotelephony
, the transmission of sound (audio), during World War 1, the term evolved to mean a
demodulator
, (usually a
vacuum tube
) which extracted the
audio signal
from the radio frequency
carrier wave
. This is its current meaning, although modern detectors usually consist of
semiconductor diodes
,
transistors
, or
integrated circuits
.
In a
superheterodyne receiver
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
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Envelope detector
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Simple diode detector. The AM input (green) is first
rectified
by the
diode
. The output voltage (red) ripples just below the upper envelope by charging and discharging the
capacitor
.
The
envelope
of a waveform is the curve that outlines the waveform. A major category of AM demodulation technique involves
envelope detection
, since
the envelope of an AM signal is the original signal
.
The
diode detector
is a simple envelope detector. It consists of a
diode
connected between the input and output of the circuit, with a
resistor
and
capacitor
in parallel from the output of the circuit to the ground to form a
low pass filter
. Their
RC time constant
must be small enough to discharge the capacitor fast enough when the envelope is falling. Meanwhile, the filter's
cutoff frequency
should be well below the carrier wave's frequency to sufficiently attenuate the carrier.
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 rectifiers
using operational amplifiers.
Product detector
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A
product detector
is a type of
demodulator
used for
AM
and
SSB
signals, where the original carrier signal is removed by multiplying the received signal with a signal at the
carrier frequency
(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
, 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
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 essentially
ring modulators
or
synchronous detectors
and closely related to some
phase-sensitive detector
circuits. They can be implemented using something as simple as
ring of diodes
or a single dual-gate
Field Effect Transistor
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
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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
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A phase detector is a
nonlinear
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
.
The Foster?Seeley discriminator
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The Foster?Seeley discriminator
[2]
[3]
is a widely used FM detector. The detector consists of a special center-tapped
transformer
feeding two diodes in a full wave DC
rectifier
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
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The ratio detector
[4]
[5]
[6]
[7]
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
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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
, 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
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The detection process described above can also be accomplished by combining, in an
exclusive-OR
(XOR) logic gate, the
limited
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
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
of the original modulating signal.
Other FM detectors
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Less common, specialized, or obsolescent types of detectors include:
[8]
- Travis
[9]
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
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The
phase-locked loop
detector requires no frequency-selective
LC network
to accomplish demodulation. In this system, a
voltage controlled oscillator
(VCO) is
phase locked
by a
feedback loop
, 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
frequency.
See also
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References
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- ^
J. A. Fleming,
The Principles of Electric Wave Telegraphy and Telephony
, London: Longmans, Green & Co., 1919, p. 364
- ^
US 2121103
,
Seeley, Stuart W.
, "Frequency Variation Response Circuits", issued June 21, 1938
- ^
Foster, D. E.;
Seeley, S. W.
(March 1937), "Automatic tuning, simplified circuits, and design practice",
Proceedings of the Institute of Radio Engineers
,
25
(3): 289?313,
doi
:
10.1109/jrproc.1937.228940
,
S2CID
51654596
, part 1.
- ^
US 2497840
,
Seeley, Stuart William
, "Angle Modulation Detector", issued February 14, 1950
- ^
US 2561089
, Anderson, Earl I., issued July 17, 1951
- ^
Report L.B.-645: "Ratio detectors for FM receivers" (15 September 1945) issued by the Radio Corporation of America, RCA Laboratories Industry Service Division, 711 Fifth Avenue, N.Y., N.Y. Reprinted in
Radio
, pages 18-20 (October 1945).
- ^
Seeley, Stuart W.
; Avins, Jack (June 1947), "The ratio detector",
RCA Review
,
8
(2): 201?236
- ^
D. S. Evans and G. R. Jessup,
VHF-UHF Manual (3rd Edition)
, Radio Society of Great Britain, London, 1976 pages 4-48 through 4-51
- ^
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).
External links
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- Media related to
Detectors (radio)
at Wikimedia Commons
- Simple block diagrams and descriptions of key circuits for FM transmitters and receivers:
[1]