A direct-conversion receiver (DCR), also known as a homodyne, synchrodyne, zero intermediate frequency receiver (zero-IF receiver), is a

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 (radio), antenna in order to produce usable information such as audio. The complexity of a ...

that demodulates the incoming radio signal using synchronous detection driven by a

local oscillator In electronics, the term local oscillator (LO) refers to an electronic oscillator when used in conjunction with a Frequency mixer, mixer to change the frequency of a signal. This frequency conversion process, also called Heterodyne, heterodyning ...

whose

frequency Frequency is the number of occurrences of a repeating event per unit of time. Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio ...

is identical to, or very close to the

carrier frequency In telecommunications, a carrier wave, carrier signal, or just carrier, is a periodic waveform (usually sinusoidal) that conveys information through a process called ''modulation''. One or more of the wave's properties, such as amplitude or fre ...

of the intended signal. This contrasts with the standard

superheterodyne A superheterodyne receiver, often shortened to superhet, is a type of radio receiver that uses frequency mixing to convert a received signal to a fixed intermediate frequency (IF) which can be more conveniently processed than the original car ...

receiver, which uses an initial conversion to an

intermediate frequency In communications and electronic engineering, an intermediate frequency (IF) is a frequency to which a carrier wave is shifted as an intermediate step in Transmission (telecommunications), transmission or reception. The intermediate frequency is ...

(IF).mwrf.com: The Differences Between Receiver Types, Part 1
Quote: "...A direct-conversion receiver, also known as a homodyne or zero-IF receiver, is one type of receiver architecture (Fig. 1). Direct-conversion receivers convert an RF signal to a 0-Hz signal in one stage..."
backup
/ref> The simplification of performing only a single frequency conversion reduces the basic circuit complexity but other issues arise, for instance, regarding

dynamic range Dynamics (from Greek δυναμικός ''dynamikos'' "powerful", from δύναμις ''dynamis'' " power") or dynamic may refer to: Physics and engineering * Dynamics (mechanics), the study of forces and their effect on motion Brands and ent ...

. In its original form it was unsuited to receiving AM and FM signals without implementing an elaborate

phase locked loop A phase-locked loop or phase lock loop (PLL) is a control system that generates an output signal whose phase is fixed relative to the phase of an input signal. Keeping the input and output phase in lockstep also implies keeping the input and ou ...

. Although these and other technical challenges made this technique rather impractical around the time of its invention (1930s), current technology, and software radio in particular, have revived its use in certain areas including some consumer products.

Principle of operation

The conversion of the modulated signal to

baseband In telecommunications and signal processing, baseband is the range of frequencies occupied by a signal that has not been modulated to higher frequencies. Baseband signals typically originate from transducers, converting some other variable into ...

is done in a single frequency conversion. This avoids the complexity of the

's two (or more) frequency conversions, IF stage(s), and image rejection issues. The received radio frequency signal is fed directly into a

frequency mixer In electronics, a mixer, or frequency mixer, is an electrical circuit that creates new frequencies from two signals applied to it. In its most common application, two signals are applied to a mixer, and it produces new signals at the sum and di ...

, just as in a

receiver. However unlike the superheterodyne, the frequency of the local oscillator is not offset from, but identical to, the received signal's frequency. The result is a demodulated output just as would be obtained from a superheterodyne receiver using synchronous detection (a ''

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 ...

'') following an

(IF) stage.

Technical issues

To match the performance of the

receiver, a number of the functions normally addressed by the IF stage must be accomplished at

. Since there is no high gain IF amplifier utilizing

automatic gain control Automatic gain control (AGC) is a closed-loop feedback regulating circuit in an amplifier or chain of amplifiers, the purpose of which is to maintain a suitable signal amplitude at its output, despite variation of the signal amplitude at the inpu ...

(AGC), the

output level may vary over a very wide range dependent on the received signal strength. 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 An envelope detector (sometimes called a peak detector) is an electronic circuit that takes a (relatively) high-frequency signal as input and outputs the ''envelope'' of the original signal. Diode detector A simple form of envelope detecto ...

of AM signals. Thus direct demodulation of AM or FM signals (as used in broadcasting) requires phase locking the local oscillator to the

, a much more demanding task compared to the more robust envelope detector or ratio detector at the output of an IF stage in a

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 (DSP) is the use of digital processing, such as by computers or more specialized digital signal processors, to perform a wide variety of signal processing operations. The digital signals processed in this manner are a ...

. 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

, 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

(''two stage conversion model''). The design was later renamed the "synchrodyne". Not only did it have superior performance due to the single conversion stage, but it also had reduced circuit complexity and power consumption. The design suffered from the thermal drift of the local

oscillator Oscillation is the repetitive or periodic variation, typically in time, of some measure about a central value (often a point of equilibrium) or between two or more different states. Familiar examples of oscillation include a swinging pendulum ...

which changed its frequency over time. To counteract this drift, the frequency of the local oscillator was compared with the broadcast input signal by a

phase detector A phase detector or phase comparator is a frequency mixer, analog multiplier or Digital logic, logic circuit that generates a signal which represents the difference in phase between two signal inputs. The phase detector is an essential elemen ...

. This produced a correction

voltage Voltage, also known as (electrical) potential difference, electric pressure, or electric tension, is the difference in electric potential between two points. In a Electrostatics, static electric field, it corresponds to the Work (electrical), ...

which would vary the local oscillator frequency keeping it in lock with the wanted signal. This type of

feedback Feedback occurs when outputs of a system are routed back as inputs as part of a chain of cause and effect that forms a circuit or loop. The system can then be said to ''feed back'' into itself. The notion of cause-and-effect has to be handle ...

circuit evolved into what is now known as a ''

phase-locked loop A phase-locked loop or phase lock loop (PLL) is a control system that generates an output signal whose phase is fixed relative to the phase of an input signal. Keeping the input and output phase in lockstep also implies keeping the input and ou ...

''. While the method has existed for several decades, it had been difficult to implement due largely to component

tolerances Engineering tolerance is the permissible limit or limits of variation in: # a physical dimension; # a measured value or physical property of a material, manufactured object, system, or service; # other measured values (such as temperature, hu ...

, 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 filter A low-pass filter is a filter that passes signals with a frequency lower than a selected cutoff frequency and attenuates signals with frequencies higher than the cutoff frequency. The exact frequency response of the filter depends on the filt ...

at the audio output stage. The receiver design has the additional advantage of high

selectivity Selectivity may refer to: Psychology and behaviour * Choice, making a selection among options * Discrimination, the ability to recognize differences * Socioemotional selectivity theory, in social psychology Engineering * Selectivity (radio), a ...

, and is therefore a precision demodulator. The design principles can be extended to permit separation of adjacent channel broadcast signals whose

sideband In radio communications, a sideband is a band of frequencies higher than or lower than the carrier frequency, that are the result of the modulation process. The sidebands carry the information transmitted by the radio signal. The sidebands c ...

s may overlap the wanted transmission. The design also improves the detection of pulse-modulated transmission mode signals.

Disadvantages

Signal leakage paths can occur in the receiver. The high audio frequency gain required can result in difficulty in rejecting mains hum. Local-oscillator energy can leak through the mixer stage to the antenna input and then reflect back into the mixer stage. The overall effect is that the local oscillator energy will self-mix and create a

DC offset In signal processing, when describing a periodic function in the time domain, the DC bias, DC component, DC offset, or DC coefficient is the mean value of the waveform. A waveform with zero mean or no DC bias is known as a ''DC balanced'' or ''DC ...

signal. The offset may be large enough to overload the

amplifiers and prevent receiving the wanted signal. There are design modifications that deal with this issue, but they add to the complexity of the receiver. The additional design complexity often outweighs the benefits of a direct-conversion receiver.

Modern usage

Wes Hayward and Dick Bingham's 1968 article brought new interest in direct-conversion designs. The development of the

integrated circuit An integrated circuit (IC), also known as a microchip or simply chip, is a set of electronic circuits, consisting of various electronic components (such as transistors, resistors, and capacitors) and their interconnections. These components a ...

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 finds use in processing more complex modulation methods. Direct-conversion receivers are now incorporated into many receiver applications, including cellphones,

pagers A pager, also known as a beeper or bleeper, is a wireless telecommunications device that receives and displays alphanumeric or voice messages. One-way pagers can only receive messages, while response pagers and two-way pagers can also acknow ...

, televisions,

avionics Avionics (a portmanteau of ''aviation'' and ''electronics'') are the Electronics, electronic systems used on aircraft. Avionic systems include communications, Air navigation, navigation, the display and management of multiple systems, and the ...

medical imaging Medical imaging is the technique and process of imaging the interior of a body for clinical analysis and medical intervention, as well as visual representation of the function of some organs or tissues (physiology). Medical imaging seeks to revea ...

apparatus and

software-defined radio Software-defined radio (SDR) is a radio communication system where components that conventionally have been implemented in analog hardware (e.g. mixers, filters, amplifiers, modulators/ demodulators, detectors, etc.) are instead implemented ...

systems.

References

External links

The History of the Homodyne and Syncrodyne
''The Journal of the British Institution of Radio Engineers, April 1954'' * {{US Patent, 706740, "Wireless Signaling" (

heterodyne A heterodyne is a signal frequency that is created by combining or mixing two other frequencies using a signal processing technique called ''heterodyning'', which was invented by Canadian inventor-engineer Reginald Fessenden. Heterodyning is us ...

principle) – 12 August 1902 - by

Reginald Fessenden Reginald Aubrey Fessenden (October 6, 1866 – July 22, 1932) was a Canadian-American electrical engineer and inventor who received hundreds of List of Reginald Fessenden patents, patents in fields related to radio and sonar between 1891 and 1936 ...

Radio electronics Receiver (radio)