Direct-conversion receiver
Encyclopedia
A direct-conversion receiver (DCR), also known as homodyne, synchrodyne, or zero-IF receiver, is a radio receiver design
that demodulates
the incoming radio signal using synchronous detection driven by a local oscillator
whose frequency
is identical to, or very close to the carrier
frequency of the intended signal.This is in contrast to the standard superheterodyne receiver where this is accomplished only after an initial conversion to an intermediate frequency
.
The simplification of performing only a single frequency conversion reduces the basic circuit complexity but other issues arise, for instance, regarding dynamic range. In its original form it was unsuited to receiving AM and FM signals without implementing an elaborate phase locked loop. Although these and other technical challenges made this technique rather impractical around the time of its invention (1930's), current technology and software radio in particular have revived its use in certain areas including some consumer products.
, just as in a superheterodyne receiver. However unlike the superheterodyne, the frequency of the local oscillator is not offset from but right at the received signal's frequency. The result is a demodulated output just as you would obtain from a superheterodyne receiver using synchronous detection (a product detector
) following an intermediate frequency
(IF) stage. In other words, the conversion to baseband is done in a single frequency conversion. This avoids the complexity of the superheterodyne's two (or more) frequency conversions, IF stage(s), and image rejection issues.
. Since there is no high gain IF amplifier utilizing automatic gain control
(AGC), the output level at baseband
varies over a very wide dynamic range
. This is one major technical challenge which limited the practicability of the design. Another issue is the inability of this design to implement envelope detection
of AM signals. Thus direct reception of AM or FM signals (as used in broadcasting) requires phase locking the local oscillator to the carrier frequency, a much more demanding task compared to the more robust envelope detector
or frequency discriminator at the output of an IF stage in a superheterodyne design. However this can be avoided in the case of a direct-conversion design using quadrature detection followed by digital signal processing
. Using software radio techniques, the two quadrature outputs can be processed in order to perform any sort of demodulation and filtering on down-converted signals from frequencies close to the local oscillator frequency. The proliferation of digital hardware, along with refinements in the analog components involved in the frequency conversion to baseband
, has thus made this simpler topology practical in many applications.
. This produced a correction voltage
which would vary the local oscillator frequency keeping it in lock with the wanted signal. This type of feedback
circuit evolved into what is now known as a phase-locked loop
. While the method has existed for several decades, it had been difficult to implement due largely to component tolerances, which must be of small variation for this type of circuit to function successfully.
at the audio output stage. The receiver
design has the additional advantage of high selectivity
, and is therefore a precision demodulator. The design principles can be extended to permit separation of adjacent channel broadcast signals whose sideband
s may overlap the wanted transmission. The design also improves the detection of pulse-modulated transmission mode signals.
input and then re-enter the mixer stage. The overall effect is that the local oscillator energy would self-mix and create a DC offset signal. The offset could be large enough to overload the baseband
amplifiers and overcome the wanted signal reception. There were subsequent modifications to deal with this issue but added to the complexity of the receiver. Ultimately the higher production costs were found to outweigh the benefits.
The development of the integrated circuit
and incorporation of complete phase-locked loop devices in low-cost IC packages made this design widely accepted. Usage is no longer limited to the reception of AM radio signals, but also find use in processing more complex modulation methods. Direct-conversion receivers are now incorporated into many receiver applications, including cellphones, television
s, avionics
, medical imaging
apparatus and Software-defined radio
systems. The principle applies as well to optical signals. The state of the art 3D cameras and lidars, invented by Medina, use homodyne detection by means of an optical shutter synchronized with the frequency of the illuminating source.
Radio receiver design
Radio receiver design includes the electronic design of different components of a radio receiver which processes the radio frequency signal from an antenna in order to produce usable information such as audio. This article only concentrates on the historical configurations leading up to and...
that demodulates
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,...
the incoming radio signal using synchronous detection driven by 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...
whose frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
is identical to, or very close to the carrier
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...
frequency of the intended signal.This is in contrast to the standard superheterodyne receiver where this is accomplished only after an initial conversion to 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...
.
The simplification of performing only a single frequency conversion reduces the basic circuit complexity but other issues arise, for instance, regarding dynamic range. In its original form it was unsuited to receiving AM and FM signals without implementing an elaborate phase locked loop. Although these and other technical challenges made this technique rather impractical around the time of its invention (1930's), current technology and software radio in particular have revived its use in certain areas including some consumer products.
Principle of operation
The direct-conversion receiver feeds the radio frequency signal into a frequency mixerFrequency 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 -...
, just as in a superheterodyne receiver. However unlike the superheterodyne, the frequency of the local oscillator is not offset from but right at the received signal's frequency. The result is a demodulated output just as you would obtain from a superheterodyne receiver using synchronous detection (a product detector
Product 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...
) following 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...
(IF) stage. In other words, the conversion to baseband is done in a single frequency conversion. This avoids the complexity of the superheterodyne's two (or more) frequency conversions, IF stage(s), and image rejection issues.
Technical issues
To match the performance of the superheterodyne receiver, a number of the functions normally addressed by the IF stage must be accomplished at basebandBaseband
In telecommunications and signal processing, baseband is an adjective that describes signals and systems whose range of frequencies is measured from close to 0 hertz to a cut-off frequency, a maximum bandwidth or highest signal frequency; it is sometimes used as a noun for a band of frequencies...
. Since there is no high gain IF amplifier utilizing automatic gain control
Automatic gain control
Automatic gain control is an adaptive system found in many electronic devices. The average output signal level is fed back to adjust the gain to an appropriate level for a range of input signal levels...
(AGC), the output level at baseband
Baseband
In telecommunications and signal processing, baseband is an adjective that describes signals and systems whose range of frequencies is measured from close to 0 hertz to a cut-off frequency, a maximum bandwidth or highest signal frequency; it is sometimes used as a noun for a band of frequencies...
varies over a very wide dynamic range
Dynamic range
Dynamic range, abbreviated DR or DNR, is the ratio between the largest and smallest possible values of a changeable quantity, such as in sound and light. It is measured as a ratio, or as a base-10 or base-2 logarithmic value.-Dynamic range and human perception:The human senses of sight and...
. This is one major technical challenge which limited the practicability of the design. Another issue is the inability of this design to implement envelope detection
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...
of AM signals. Thus direct reception of AM or FM signals (as used in broadcasting) requires phase locking the local oscillator to the carrier frequency, a much more demanding task compared to the more robust 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...
or frequency discriminator at the output of an IF stage in a superheterodyne design. However this can be avoided in the case of a direct-conversion design using quadrature detection followed by digital signal processing
Digital 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...
. Using software radio techniques, the two quadrature outputs can be processed in order to perform any sort of demodulation and filtering on down-converted signals from frequencies close to the local oscillator frequency. The proliferation of digital hardware, along with refinements in the analog components involved in the frequency conversion to baseband
Baseband
In telecommunications and signal processing, baseband is an adjective that describes signals and systems whose range of frequencies is measured from close to 0 hertz to a cut-off frequency, a maximum bandwidth or highest signal frequency; it is sometimes used as a noun for a band of frequencies...
, has thus made this simpler topology practical in many applications.
History and applications
The homodyne was developed in 1932 by a team of British scientists searching for a design to surpass the superheterodyne (two stage conversion model). It was later renamed the "synchrodyne". Not only did it have superior performance due to the single conversion stage, also had reduced circuit complexity and power consumption. The design suffered with thermal drift of the local oscillator which changed its frequency after a period of time. To counteract this drift, the frequency of the local oscillator was compared with the broadcast input signal by 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...
. This produced a correction voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
which would vary the local oscillator frequency keeping it in lock with the wanted signal. This type of feedback
Feedback
Feedback describes the situation when output from an event or phenomenon in the past will influence an occurrence or occurrences of the same Feedback describes the situation when output from (or information about the result of) an event or phenomenon in the past will influence an occurrence or...
circuit evolved into what is now known as 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...
. While the method has existed for several decades, it had been difficult to implement due largely to component tolerances, which must be of small variation for this type of circuit to function successfully.
Advantages
Unwanted by-product beat signals from the mixing stage do not need any further processing, as they are completely rejected by use of a 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...
at the audio output stage. The receiver
Receiver (radio)
A radio receiver converts signals from a radio antenna to a usable form. It uses electronic filters to separate a wanted radio frequency signal from all other signals, the electronic amplifier increases the level suitable for further processing, and finally recovers the desired information through...
design has the additional advantage of high selectivity
Electronic selectivity
Selectivity is a measure of the performance of a radio receiver to respond only to the radio signal it is tuned to and reject other signals nearby in frequency, such as another broadcast on an adjacent channel....
, and is therefore a precision demodulator. The design principles can be extended to permit separation of adjacent channel broadcast signals whose sideband
Sideband
In radio communications, a sideband is a band of frequencies higher than or lower than the carrier frequency, containing power as a result of the modulation process. The sidebands consist of all the Fourier components of the modulated signal except the carrier...
s may overlap the wanted transmission. The design also improves the detection of pulse-modulated transmission mode signals.
Disadvantages
The design was not without other problems. Signal leakage paths can occur in the receiver. Local-oscillator energy can leak through the mixer stage back and feed back to the antennaAntenna (radio)
An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...
input and then re-enter the mixer stage. The overall effect is that the local oscillator energy would self-mix and create a DC offset signal. The offset could be large enough to overload the baseband
Baseband
In telecommunications and signal processing, baseband is an adjective that describes signals and systems whose range of frequencies is measured from close to 0 hertz to a cut-off frequency, a maximum bandwidth or highest signal frequency; it is sometimes used as a noun for a band of frequencies...
amplifiers and overcome the wanted signal reception. There were subsequent modifications to deal with this issue but added to the complexity of the receiver. Ultimately the higher production costs were found to outweigh the benefits.
Modern usage
The modern design techniques were based on the publication of 'Direct Conversion - A Neglected Technique QST, Nov, 1968'. It appeared as an article in 1968 November's QST publication written by Wes Hayward and Dick Bingham.The development of the integrated circuit
Integrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...
and incorporation of complete phase-locked loop devices in low-cost IC packages made this design widely accepted. Usage is no longer limited to the reception of AM radio signals, but also find use in processing more complex modulation methods. Direct-conversion receivers are now incorporated into many receiver applications, including cellphones, television
Television
Television is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...
s, avionics
Avionics
Avionics are electronic systems used on aircraft, artificial satellites and spacecraft.Avionic systems include communications, navigation, the display and management of multiple systems and the hundreds of systems that are fitted to aircraft to meet individual roles...
, medical imaging
Medical imaging
Medical imaging is the technique and process used to create images of the human body for clinical purposes or medical science...
apparatus and Software-defined radio
Software-defined radio
A software-defined radio system, or SDR, is a radio communication system where components that have been typically implemented in hardware are instead implemented by means of software on a personal computer or embedded computing devices...
systems. The principle applies as well to optical signals. The state of the art 3D cameras and lidars, invented by Medina, use homodyne detection by means of an optical shutter synchronized with the frequency of the illuminating source.
See also
- Crystal radio
- Reflectional receiverReflectional receiverA reflectional radio receiver is a radio receiver design in which the same amplifier is used for both the high-frequency radio and low-frequency sound signals. The radio signal from the output of the amplifier passes detection and then re-enters the input of the amplifier...
- Homodyne detectionHomodyne detectionHomodyne detection is a method of detecting frequency-modulated radiation by non-linear mixing with radiation of a reference frequency, the same principle as for heterodyne detection....
- Low IF receiverLow IF receiverIn a low-IF receiver, the RF signal is mixed down to a non-zero low or moderate intermediate frequency, typically a few megahertz. Low-IF receiver topologies have many of the desirable properties of zero-IF architectures, but avoid the DC offset and 1/f noise problems.The use of a non-zero IF...
- NeutrodyneNeutrodyneThe Neutrodyne was a particular type of Tuned Radio Frequency radio receiver, in which the instability-causing inter-electrode capacitance of the triode RF tubes is cancelled out or "neutralized"...
- Regenerative radio receiver
- Superheterodyne receiverSuperheterodyne receiverIn 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...
- Tuned radio frequency receiverTuned radio frequency receiverA tuned radio frequency receiver is a radio receiver that is usually composed of several tuned radio frequency amplifiers followed by circuits to detect and amplify the audio signal. Prevalent in the early 20th century, it can be difficult to operate because each stage must be individually tuned...
External links
- The History of the Homodyne and Syncrodyne The Journal of the British Institution of Radio Engineers, April 1954