Direct-conversion receiver


Direct-conversion receiver

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.

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Principle of operation

The direct-conversion receiver feeds the radio frequency signal into a frequency mixer, 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.

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 baseband. 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.

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 detector. 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.

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 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 sidebands 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 antenna 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.

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 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.[1] Direct-conversion receivers are now incorporated into many receiver applications, including cellphones, televisions, avionics, medical imaging apparatus and Software-defined radio systems.[2] 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. [3]


References

  1. ^ "Quad Demodulators Arm Direct-Conversion Receivers". Microwaves & RF 2004. http://www.mwrf.com/Article/ArticleID/7470/7470.html. Retrieved 9 February 2011. 
  2. ^ "Direct Conversion Receiver". Qsl Network. http://www.qsl.net/5z4ft/dcrx.html. Retrieved 9 February 2011. 
  3. ^ Medina, Antonio. "Three Dimensional Camera and Rangefinder". January 1992. United States Patent 5081530. 

See also

External links


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