Detector (radio)
Encyclopedia
A detector is a device that recovers information
of interest contained in a modulated wave. The term dates from the early days of radio when all transmissions were in Morse code
, and it was only necessary to detect the presence (or absence) of a radio wave using a device such as a coherer
without necessarily making it audible. A more up-to-date term is demodulator
, but "detector" has a history of many decades of use, even if it is a misnomer.
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 and capacitor in parallel from the output of the circuit to the ground. If the resistor and capacitor are correctly chosen, the output of this circuit will approximate a voltage-shifted version of the original signal.
An early form of envelope detector was the cat's whisker, which was used in the crystal set radio receiver.
is a type of demodulator used for AM
and SSB
signals. Rather than converting the envelope of the signal into the decoded waveform like an envelope detector, the product detector takes the product of the modulated signal and a local oscillator
, hence the name. At least partially, it multiplies the signal by the output of the local oscillator. This can be accomplished by heterodyning. The received signal is mixed, in some type of nonlinear device, with a signal from the local oscillator, to produce an intermediate frequency
, referred to as the beat frequency, from which the modulating signal
is detected and recovered.
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
.
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.
, however the output is only 50% of the output of a discriminator for the same input signal.
, 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 that's imposed by the capacitor and the positive or negative phase change that's 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.
This detection process can also be accomplished by combining, in an exclusive-OR
(XOR) logic gate, the original FM signal and a square wave
whose frequency equals the FM signal's center frequency. The XOR gate produces an output pulse whose duration equals the difference between the times at which the square wave and the received FM signal pass through zero volts. As the FM signal's frequency varies from its unmodulated center frequency (which is also the frequency of the square wave), the output pulses from the XOR gate become longer or shorter. (In essence, this quadrature detector converts an FM signal into a pulse-width modulated (PWM) signal.) When these pulses are filtered, 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.
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.
Information
Information in its most restricted technical sense is a message or collection of messages that consists of an ordered sequence of symbols, or it is the meaning that can be interpreted from such a message or collection of messages. Information can be recorded or transmitted. It can be recorded as...
of interest contained in a modulated wave. The term dates from the early days of radio when all transmissions were in Morse code
Morse code
Morse code is a method of transmitting textual information as a series of on-off tones, lights, or clicks that can be directly understood by a skilled listener or observer without special equipment...
, and it was only necessary to detect the presence (or absence) of a radio wave using a device such as a coherer
Coherer
The coherer was a primitive form of radio signal detector used in the first radio receivers during the wireless telegraphy era at the beginning of the twentieth century. Invented around 1890 by French scientist Édouard Branly, it consisted of a tube or capsule containing two electrodes spaced a...
without necessarily making it audible. A more up-to-date term is demodulator
Demodulation
Demodulation is the act of extracting the original information-bearing signal from a modulated carrier wave.A demodulator is an electronic circuit that is used to recover the information content from the modulated carrier wave.These terms are traditionally used in connection with radio receivers,...
, but "detector" has a history of many decades of use, even if it is a misnomer.
Envelope detector
Envelope detector
An envelope detector is an electronic circuit that takes a high-frequency signal as input and provides an output which is the "envelope" of the original signal. The capacitor in the circuit stores up charge on the rising edge, and releases it slowly through the resistor when the signal falls...
is the diode detector that consists of a diode
Diode
In electronics, a diode is a type of two-terminal electronic component with a nonlinear current–voltage characteristic. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material connected to two electrical terminals...
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. If the resistor and capacitor are correctly chosen, the output of this circuit will approximate a voltage-shifted version of the original signal.
An early form of envelope detector was the cat's whisker, which was used in the crystal set radio receiver.
Product detector
A product detectorProduct detector
A product detector is a type of demodulator used for AM and SSB signals. Rather than converting the envelope of the signal into the decoded waveform like an envelope detector, the product detector takes the product of the modulated signal and a local oscillator, hence the name...
is a type of demodulator used for AM
Amplitude modulation
Amplitude modulation is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. AM works by varying the strength of the transmitted signal in relation to the information being sent...
and SSB
Single-sideband modulation
Single-sideband modulation or Single-sideband suppressed-carrier is a refinement of amplitude modulation that more efficiently uses electrical power and bandwidth....
signals. Rather than converting the envelope of the signal into the decoded waveform like an envelope detector, the product detector takes the product of the modulated signal and a local oscillator
Local oscillator
A local oscillator is an electronic device used to generate a signal normally for the purpose of converting a signal of interest to a different frequency using a mixer. This process of frequency conversion, also referred to as heterodyning, produces the sum and difference frequencies of the...
, hence the name. At least partially, it multiplies the signal by the output of the local oscillator. This can be accomplished by heterodyning. The received signal is mixed, in some type of nonlinear device, with a signal from the local oscillator, to produce an intermediate frequency
Intermediate frequency
In communications and electronic engineering, an intermediate frequency is a frequency to which a carrier frequency is shifted as an intermediate step in transmission or reception. The intermediate frequency is created by mixing the carrier signal with a local oscillator signal in a process called...
, referred to as the beat frequency, from which the modulating signal
Modulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...
is detected and recovered.
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 detectorPhase detector
A phase detector or phase comparator is a frequency mixer, analog multiplier or logic circuit that generates a voltage signal which represents the difference in phase between two signal inputs...
is a nonlinear device whose output represents the phase
Phase (waves)
Phase in waves is the fraction of a wave cycle which has elapsed relative to an arbitrary point.-Formula:The phase of an oscillation or wave refers to a sinusoidal function such as the following:...
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
Carrier wave
In telecommunications, a carrier wave or carrier is a waveform that is modulated with an input signal for the purpose of conveying information. This carrier wave is usually a much higher frequency than the input signal...
.
The Foster-Seeley discriminator
The Foster-Seeley discriminatorFoster-Seeley discriminator
The Foster-Seeley discriminator is a common type of FM detector circuit, invented in 1936 by Dudley E. Foster and Stuart William Seeley. The circuit was envisioned for automatic frequency control of receivers, but also found application in demodulating an FM signal.It uses a tuned RF transformer...
is a widely used FM detector. The detector consists of a special center-tapped transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
feeding two diodes in a full wave DC rectifier
Rectifier
A rectifier is an electrical device that converts alternating current , which periodically reverses direction, to direct current , which flows in only one direction. The process is known as rectification...
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. 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. While distinct from the Foster-Seeley discriminator, the ratio detector will similarly not respond to AM signalsAmplitude modulation
Amplitude modulation is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. AM works by varying the strength of the transmitted signal in relation to the information being sent...
, however the output is only 50% of the output of a discriminator for the same input signal.
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 capacitorCapacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
, 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
Phase detector
A phase detector or phase comparator is a frequency mixer, analog multiplier or logic circuit that generates a voltage signal which represents the difference in phase between two signal inputs...
; 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
Electronic filter
Electronic filters are electronic circuits which perform signal processing functions, specifically to remove unwanted frequency components from the signal, to enhance wanted ones, or both...
; 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 that's imposed by the capacitor and the positive or negative phase change that's 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.
This detection process can also be accomplished by combining, in an exclusive-OR
XOR gate
The XOR gate is a digital logic gate that implements an exclusive or; that is, a true output results if one, and only one, of the inputs to the gate is true . If both inputs are false or both are true , a false output results. Its behavior is summarized in the truth table shown on the right...
(XOR) logic gate, the original FM signal and a square wave
Square wave
A square wave is a kind of non-sinusoidal waveform, most typically encountered in electronics and signal processing. An ideal square wave alternates regularly and instantaneously between two levels...
whose frequency equals the FM signal's center frequency. The XOR gate produces an output pulse whose duration equals the difference between the times at which the square wave and the received FM signal pass through zero volts. As the FM signal's frequency varies from its unmodulated center frequency (which is also the frequency of the square wave), the output pulses from the XOR gate become longer or shorter. (In essence, this quadrature detector converts an FM signal into a pulse-width modulated (PWM) signal.) When these pulses are filtered, 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.
Other FM detectors
Less common, specialized, or obsolescent types of detectors include :- Travis 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 loopPhase-locked loop
A phase-locked loop or phase lock loop is a control system that generates an output signal whose phase is related to the phase of an input "reference" signal. It is an electronic circuit consisting of a variable frequency oscillator and a phase detector...
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.
See also
- Cat's whisker detector
- CohererCohererThe coherer was a primitive form of radio signal detector used in the first radio receivers during the wireless telegraphy era at the beginning of the twentieth century. Invented around 1890 by French scientist Édouard Branly, it consisted of a tube or capsule containing two electrodes spaced a...
- Electrolytic detectorElectrolytic detectorThe electrolytic detector, or the bare-point electrolytic detector as it was also called, was a type of wet demodulator used in early radio receivers. This form of detector was in extensive use between the years 1902 and 1913, after which the superior vacuum tube diode became available...
- Foster-Seeley discriminatorFoster-Seeley discriminatorThe Foster-Seeley discriminator is a common type of FM detector circuit, invented in 1936 by Dudley E. Foster and Stuart William Seeley. The circuit was envisioned for automatic frequency control of receivers, but also found application in demodulating an FM signal.It uses a tuned RF transformer...
- Grid-leak detectorGrid-leak detectorA grid-leak detector is a combination diode rectifier and audio amplifier used as a detector in vacuum tube AM radio receivers.The grid leak is the small current through the grid of a vacuum tube caused by the small negative EMF which is present in all vacuum tubes as a result of a space charge...
- Hot wire barretterHot wire barretterThe 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 developed to permit the reception of amplitude modulated signals, something that the coherer could not...
- Magnetic detectorMagnetic detectorThe magnetic detector was one of the first practical devices able to make radio signals audible through a pair of headphones. It was not as sensitive as some detectors but despite that, it was favored for early maritime use because of its reliability....
- Plate detectorPlate detector (radio)A plate detector is a vacuum tube detector circuit used in A.M. radios. This circuit employs a tube with an indirectly heated cathode, typically a medium-mu triode, or a tetrode or pentode with a sharp cut-off control grid. Rectification of R.F. signals occurs in the plate of the detector tube. ...
- TikkerTikkerA tikker was a device that allowed early crystal radio circuits to detect continuous-wave telegraphy. Ordinary crystal detectors cannot demodulate continuous waves; the direct current signal is inaudible in headphones...
- Wunderlich detectorWunderlich (vacuum tube)Wunderlich refers to a series of vacuum tubes introduced in the early 1930s. Wunderlichs were designed to be used as full-wave detectors in AM radio receivers. However, because of their unusual design, they were rarely used in commercially manufactured receivers. The tube is named for its...