A detector is an older term for an electronic component in a radio receiver that recovers
information contained in a modulated radio wave.
The term dates from the first three decades of radio.
Unlike modern radio stations which transmit sound on the radio carrier wave, the first
radio transmitters transmitted information by wireless telegraphy, using different length
pulses of radio waves to spell out text messages in Morse code.
So radio receiving equipment didn't have to extract a sound wave from the incoming radio
signal, but only "detect" the presence or absence of the radio signal, to produce clicks
in the receiver's earphones representing the Morse code symbols.
The device that did this was called a detector.
A variety of different detector devices, such as the coherer, electrolytic detector, and
magnetic detector, were used during the wireless telegraphy era.
After sound transmission took over around 1920, the term evolved to mean a demodulator,
a nonlinear rectifier 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.
This is called the first detector, while the demodulator that extracts the audio signal
from the intermediate frequency is called the second detector.
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 and capacitor
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
nearly identical 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.
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 plate detector, grid-leak detector and
transistor equivalents of them, infinite-impedance detectors, and precision rectifiers.
Product detector 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.
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 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.
Product detector circuits are analog multipliers and so 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.
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 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
The Foster-Seeley discriminator 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
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.
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, 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 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, the output pulses
from the XOR gate become longer or shorter.
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 loop detector requires no
frequency-selective LC network to accomplish demodulation.
In this system, a voltage controlled oscillator 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
Coherer Tuner
Electrolytic detector Foster-Seeley discriminator
Grid-leak detector Hot wire barretter
Magnetic detector Plate detector
Demodulation Tikker
Wunderlich detector References
External links Simple block diagrams and descriptions of
key circuits for FM transmitters and receivers: [1]