Product detector
Encyclopedia
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. A product detector is a frequency mixer
.
Product detectors can be designed to accept either IF
or RF
frequency inputs. A product detector which accepts an IF signal would be used as a demodulator block in a superheterodyne receiver
, and a detector designed for RF can be combined with an RF amplifier and a low-pass filter into a direct-conversion receiver.
Multiplying the AM signal x(t) by an oscillator at the same frequency as and in phase with the carrier yields
which can be re-written as
After filtering out the high-frequency component based around cos(2ωt) and the DC component C, the original message will be recovered.
Frequency of an AM carrier can be accurately determined with a phase-locked loop
, but for SSB, the only solution is to construct a highly stable oscillator.
equipment, one may experiment with a wide range of interesting techniques. For instance, it is possible to multiply the incoming signal by the carrier, times the square of another carrier 90° out of phase with it. This will produce a copy of the original message, and another AM signal at the fourth harmonic, by means of the trigonometric identity
The high-frequency component can again be filtered out, leaving the original signal.
Multiplying the AM signal by the new set of frequencies yields
After filtering out the component based around cos(4ωt) and the DC component C, the original message will be recovered.
ed. The second copy is mixed with a 90° phase-shifted copy of the oscillator and the output of this mixer is also 90° phase-shifted and then low-pass filtered. These copies are then combined to produce the original message.
The only real disadvantage of a product demodulator is the increased complexity.
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
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...
, 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. A product detector is a frequency mixer
Frequency mixer
In electronics a mixer or frequency mixer is a nonlinear electrical circuit that creates new frequencies from two signals applied to it. In its most common application, two signals at frequencies f1 and f2 are applied to a mixer, and it produces new signals at the sum f1 + f2 and difference f1 -...
.
Product detectors can be designed to accept either IF
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...
or RF
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
frequency inputs. A product detector which accepts an IF signal would be used as a demodulator block in a superheterodyne receiver
Superheterodyne receiver
In electronics, a superheterodyne receiver uses frequency mixing or heterodyning to convert a received signal to a fixed intermediate frequency, which can be more conveniently processed than the original radio carrier frequency...
, and a detector designed for RF can be combined with an RF amplifier and a low-pass filter into a direct-conversion receiver.
A simple product detector
The simplest form of product detector multiplies an incoming signal by its carrier, to produce a copy of the original message, and another AM signal at twice the original carrier frequency. This high-frequency component can then be filtered out, leaving the original signal.Mathematical model of the simple product detector
If m(t) is the original message, the AM signal can be shown to beMultiplying the AM signal x(t) by an oscillator at the same frequency as and in phase with the carrier yields
which can be re-written as
After filtering out the high-frequency component based around cos(2ωt) and the DC component C, the original message will be recovered.
Drawbacks of the simple product detector
Although this simple detector works, it has two major drawbacks:- The frequency of the local oscillator must be the same as the frequency of the carrier, or else the output message will fade in and out in the case of AM, or be frequency shifted in the case of SSB
- Once the frequency is matched, the phase of carrier must be obtained, or else the demodulated message will be attenuated, but the noise will not be.
Frequency of an AM carrier can be accurately determined with a phase-locked loop
Phase-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...
, but for SSB, the only solution is to construct a highly stable oscillator.
Another example
There are many other kinds of product detectors as well. If one has access to digital signal processingDigital signal processing
Digital signal processing is concerned with the representation of discrete time signals by a sequence of numbers or symbols and the processing of these signals. Digital signal processing and analog signal processing are subfields of signal processing...
equipment, one may experiment with a wide range of interesting techniques. For instance, it is possible to multiply the incoming signal by the carrier, times the square of another carrier 90° out of phase with it. This will produce a copy of the original message, and another AM signal at the fourth harmonic, by means of the trigonometric identity
The high-frequency component can again be filtered out, leaving the original signal.
Mathematical model of the detector
If m(t) is the original message, the AM signal can be shown to beMultiplying the AM signal by the new set of frequencies yields
After filtering out the component based around cos(4ωt) and the DC component C, the original message will be recovered.
A more sophisticated product detector
A more sophisticated product detector can be constructed in a way much like a single-sideband modulator. Two copies of the modulated input signals are created. The first copy is mixed with a local oscillator and low-pass filterLow-pass filter
A low-pass filter is an electronic filter that passes low-frequency signals but attenuates signals with frequencies higher than the cutoff frequency. The actual amount of attenuation for each frequency varies from filter to filter. It is sometimes called a high-cut filter, or treble cut filter...
ed. The second copy is mixed with a 90° phase-shifted copy of the oscillator and the output of this mixer is also 90° phase-shifted and then low-pass filtered. These copies are then combined to produce the original message.
Advantages and disadvantages
The product demodulator has several advantages over an envelope detector.- The product demodulator can decode overmodulatedOvermodulationOvermodulation is the condition that prevails in telecommunication when the instantaneous level of the modulating signal exceeds the value necessary to produce 100% modulation of the carrier. In the sense of this definition, it is almost always considered a fault condition. In layman's terms, the...
AM, AM with suppressed carrier, and SSB in addition to regular AM - A signal demodulated with a product detector will have a higher signal to noise ratio than the same signal demodulated with an envelope detector.
The only real disadvantage of a product demodulator is the increased complexity.