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[[Image:5tubes-radio.jpg|thumb|A 5-tube superheterodyne receiver made in Japan around 1955]]

In [[electronics]], a '''superheterodyne receiver''' (sometimes shortened to '''superhet''') uses [[frequency mixer|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. Virtually all modern [[radio]] and [[television]] receivers use the superheterodyne principle.

==History==
[[Image:Tuning capacitor.jpg|thumb|Two-section variable capacitor, used in superheterodyne receivers]]
The word ''[[heterodyne]]'' is derived from the Greek roots ''hetero-'' "different", and ''-dyne'' "power". The original heterodyne technique was pioneered by Canadian inventor [[Reginald Fessenden]],<ref name=Sarkar/> <!-- pages 370-372 --> but it was not pursued far because [[local oscillator]]s available at the time were unstable in their frequency output.<ref name=nahin1>Nahin, Paul ''The Science of Radio'' (Chapter 7), p. 91, figure 7.10. ISBN 0-387-95150-4</ref><ref name ="luxor"/>

The superheterodyne principle was revisited in 1918 by [[U.S. Army]] [[Major]] [[Edwin Armstrong]] in [[France]] during [[World War I]].<ref name=Sarkar>Tapan K. Sarkar (ed) ''The History of Wireless'',John Wiley and Sons, 2006 ISBN 0471718149</ref><ref name="luxor">{{Cite web|url=http://www.astrosurf.com/luxorion/qsl-ham-history-landmarks.htm|title=The History of Amateur Radio|publisher=Luxorion date unknown|accessdate=19 January 2011}}</ref> <!-- page 110 -->  He invented this receiver as a means of overcoming the deficiencies of early vacuum tube [[triode]]s used as high-frequency amplifiers in radio [[direction finding]] equipment. Unlike simple radio communication, which only needs to make transmitted signals audible, direction-finders measure the received signal strength, which necessitates linear amplification of the actual [[carrier wave]].

In a triode radio-frequency (RF) amplifier, if both the plate (anode) and grid are connected to resonant circuits tuned to the same frequency, stray [[capacitive coupling]] between the grid and the plate will cause the amplifier to go into oscillation if the stage gain is much more than [[unity (mathematics)|unity]]. In early designs, dozens (in some cases over 100) low-gain triode stages had to be connected in cascade to make workable equipment, which drew enormous amounts of power in operation and required a team of maintenance engineers. The strategic value was so high, however, that the [[British Admiralty]] felt the high cost was justified.

Armstrong realized that if RDF receivers could be operated at a higher frequency, this would allow better detection of enemy shipping. However, at that time, no practical "short wave" (defined then as any frequency above 500&nbsp;kHz) amplifier existed, due to the limitations of existing triodes.

A "heterodyne" refers to a beat or "difference" frequency produced when two or more radio frequency carrier waves are fed to a [[Detector (radio)|detector]]. The term was coined by Canadian Engineer [[Reginald Fessenden]] describing his proposed method of producing an audible signal from the [[Morse Code]] transmissions of an [[Alexanderson alternator]]-type transmitter. With the [[spark gap transmitter]]s then in use, the Morse Code signal consisted of short bursts of a heavily modulated carrier wave which could be clearly heard as a series of short chirps or buzzes in the receiver's headphones. However, the signal from an Alexanderson Alternator did not have any such inherent modulation and Morse Code from one of those would only be heard as a series of clicks or thumps. Fessenden's idea was to run two Alexanderson Alternators, one producing a carrier frequency 3&nbsp;kHz higher than the other. In the receiver's [[Detector (radio)|detector]] the two carriers would beat together to produce a 3&nbsp;kHz tone thus in the headphones the morse signals would then be heard as a series of 3&nbsp;kHz beeps. For this he coined the term "[[heterodyne]]" meaning "Generated by a Difference" (in frequency).

Later, when vacuum triodes became available, the same result could be achieved more conveniently by incorporating a "[[local oscillator]]" in the receiver, which became known as a "[[beat frequency oscillator]]" (BFO). As the BFO frequency was varied, the pitch of the heterodyne could be heard to vary with it. If the frequencies were too far apart the heterodyne became [[Ultrasound|ultrasonic]] and hence no longer audible.

It had been noticed some time before that if a [[Regenerative circuit|regenerative]] receiver was allowed to go into oscillation, other receivers nearby would suddenly start picking up stations on frequencies different from those that the stations were actually transmitted on. Armstrong (and others) eventually deduced that this was caused by a "supersonic heterodyne" between the station's carrier frequency and the oscillator frequency. Thus if a station was transmitting on 300&nbsp;kHz and the oscillating receiver was set to 400&nbsp;kHz, the station would be heard not only at the original 300&nbsp;kHz, but also at 100&nbsp;kHz and 700&nbsp;kHz.

Armstrong realized that this was a potential solution to the "short wave" amplification problem, since the beat frequency still retained its original modulation, but on a lower carrier frequency. To monitor a frequency of 1500&nbsp;kHz for example, he could set up an oscillator at, for example, 1560&nbsp;kHz, which would produce a heterodyne difference frequency of 60&nbsp;kHz, a frequency that could then be more conveniently amplified by the triodes of the day. He termed this the "[[Intermediate Frequency]]" often abbreviated to "IF".

<blockquote><p>In December, 1919, Major E. H. Armstrong gave publicity to an indirect method of obtaining short-wave amplification, called the super-heterodyne. The idea is to reduce the incoming frequency which may be, say 1,500,000 cycles (200 meters), to some suitable super-audible frequency which can be amplified efficiently, then passing this current through a radio frequency amplifier and finally rectifying and carrying on to one or two stages of audio frequency amplification.<ref>(page 11 of December 1922 [[QST|QST magazine]])</ref></p></blockquote>

Early superheterodyne receivers used IFs as low as 20&nbsp;kHz, often based on the self-resonance of iron-cored [[transformer]]s. This made them extremely susceptible to [[image frequency]] interference, but at the time, the main objective was sensitivity rather than selectivity. Using this technique, a small number of triodes could be made to do the work that formerly required dozens of triodes.

In the 1920s, commercial IF filters looked very similar to 1920s audio interstage coupling transformers, had very similar construction and were wired up in an almost identical manner, and so they were referred to as "IF Transformers". By the mid-1930s however, superheterodynes were using higher intermediate frequencies, (typically around 440–470&nbsp;kHz), with tuned coils similar in construction to the aerial and oscillator coils. The name "IF Transformer" is still used. Modern receivers typically use a mixture of [[ceramic resonator]] or [[Electronic filter#SAW filters|SAW]] (surface-acoustic wave) resonators as well as traditional tuned-inductor IF transformers.

Armstrong was able to rapidly put his ideas into practice, and the technique was rapidly adopted by the military. However, it was less popular when commercial [[radio broadcasting]] began in the 1920s, mostly due to the need for an extra tube (for the oscillator), the generally higher cost of the receiver, and the level of technical skill required to operate it. For early domestic radios, [[tuned radio frequency receiver]]s ("TRF"), also called the [[Neutrodyne]], were more popular because they were cheaper, easier for a non-technical owner to use, and less costly to operate. Armstrong eventually sold his superheterodyne patent to [[Westinghouse Electric (1886)|Westinghouse]], who then sold it to [[RCA]], the latter monopolizing the market for superheterodyne receivers until 1930.<ref>{{cite web|last=Katz|first=Eugenii|title=Edwin Howard Armstrong|work=History of electrochemistry, electricity, and electronics|publisher=[http://www.geocities.com/neveyaakov/ Eugenii Katz homepage, Hebrew Univ. of Jerusalem]|url=http://www.geocities.com/neveyaakov/electro_science/armstrong.html|accessdate=2008-05-10|archiveurl=http://web.archive.org/web/20091022120609/http://geocities.com/neveyaakov/electro_science/armstrong.html|archivedate=2009-10-22}}</ref>

By the 1930s, improvements in vacuum tube technology rapidly eroded the TRF receiver's cost advantages, and the explosion in the number of broadcasting stations created a demand for cheaper, higher-performance receivers.

The development of the [[tetrode]] vacuum tube containing a [[screen grid]]  led to a multi-element tube in which the mixer and oscillator functions could be combined, first used in the so-called [[autodyne]] mixer. This was rapidly followed by the introduction of tubes specifically designed for superheterodyne operation, most notably the [[pentagrid converter]]. By reducing the tube count, this further reduced the advantage of preceding receiver designs.

By the mid-1930s, commercial production of TRF receivers was largely replaced by superheterodyne receivers. The superheterodyne principle was eventually taken up for virtually all commercial radio and TV designs.

==Design and principle of operation==
The principle of operation of the superheterodyne receiver depends on the use of [[heterodyning]] or [[frequency mixer|frequency mixing]]. The signal from the antenna is filtered sufficiently at least to reject the ''[[image frequency]]'' (see below) and possibly amplified. A [[local oscillator]] in the receiver produces a sine wave which  [[frequency mixer|mixes]] with that signal, shifting it to a specific [[intermediate frequency]] (IF), usually a lower frequency. The IF signal is itself filtered and amplified and possibly processed in additional ways. The demodulator uses the IF signal rather than the original radio frequency to recreate a copy of the original modulation (such as audio).

[[Image:Superhet2.png|frame|right|Block diagram of a typical superheterodyne receiver.]]
The diagram at right shows the minimum requirements for a single-conversion superheterodyne receiver design. The following essential elements are common to all superhet circuits:<ref name=Carr02>Joseph J. Carr ''RF Components and Circuits'' Newnes, 2002 ISBN 978-0-7506-4844-8 , Chapter 3</ref> a receiving [[Antenna (radio)|antenna]], a tuned stage which may optionally contain amplification (RF amplifier), a variable frequency [[local oscillator]], a [[frequency mixer]], a band pass filter and [[intermediate frequency]] (IF) amplifer, and a demodulator plus additional circuitry to amplify or process the original audio signal (or other transmitted information).

===Circuit description===
To receive a radio signal, a suitable [[Antenna (radio)|antenna]] is required. This is often built into a receiver, especially in the case of AM broadcast band radios. The output of the antenna may be very small, often only a few [[microvolt]]s. The signal from the antenna is tuned and may be amplified in a so-called radio frequency (RF) amplifier, although this stage is often omitted. One or more [[tuned circuit]]s at this stage block frequencies which are far removed from the intended reception frequency. In order to tune the receiver to a particular station, the frequency of the local oscillator is controlled by the tuning knob (for instance). Tuning of the local oscillator and the RF stage may use a [[variable capacitor]], or [[varicap diode]].<ref>{{Cite book|url=http://books.google.co.uk/books?id=QtJ5tNdlyYAC&pg=PA58&dq=radio+local+oscillator+tuning+varicap#v=onepage&q&f=false|title=Radio-frequency electronics: circuits and applications By Jon B. Hagen -p.58 l. 12|publisher=Cambridge University Press, 1996 - Technology & Engineering|accessdate=17 January 2011}}</ref> The tuning of one (or more) tuned circuits in the RF stage must track the tuning of the local oscillator.

===Mixer stage===
The signal is then fed into a circuit where it is mixed with a sine wave from a variable frequency oscillator known as the [[local oscillator]] (LO). The mixer uses a non-linear component to produce both sum and difference [[Beat (acoustics)#Mathematics and physics of beat tones|beat frequencies]] signals,<ref>{{Cite book|url=http://books.google.co.uk/books?id=bkOMDgwFA28C&pg=PA886&dq=sum+and+difference+signals+from+mixer#v=onepage&q=sum%20and%20difference%20signals%20from%20mixer&f=false | title= The art of electronics p.886 l.3 | publisher=Cambridge University Press 2006|accessdate=17 January 2011 }}</ref> each one containing the [[modulation]] contained in the desired signal. The output of the mixer may include the original RF signal at ''f''<small><sub>d</sub></small>, the local oscillator signal at ''f''<small><sub>LO</sub></small>, and the two new frequencies ''f''<small><sub>d</sub></small>+''f''<small><sub>LO</sub></small> and ''f''<small><sub>d</sub></small>-''f''<small><sub>LO</sub></small>. The 
mixer may inadvertently produce additional frequencies such as 3rd- and higher-order intermodulation products. The undesired signals are removed by the IF [[bandpass filter]], leaving only the desired offset IF signal at ''f''<small><sub>IF</sub></small> which contains the original modulation (transmitted information) as the received radio signal had at ''f''<small><sub>d</sub></small>.

Historically, broadcast AM receivers using vacuum tubes would save costs by employing a single tube as a mixer and also as the local oscillator. The [[pentagrid converter]]<ref>{{Cite web|url=http://v3.espacenet.com/publicationDetails/biblio?CC=GB&NR=426802&KC=&FT=E|title=Improvements in or relating to superheterodyne radio receivers|publisher=European Patent Office No.426802 filed Oct 12 1933|accessdate=17 January 2011}}</ref> tube would oscillate and also provide signal amplification as well as frequency shifting.

===Intermediate frequency stage===
The stages of an intermediate frequency amplifier are tuned to a particular frequency not dependent on the receiving frequency; this greatly simplifies optimization of the circuit.<ref name=Carr02/> The IF amplifier (or ''IF strip'') can be made highly selective around its center frequency ''f''<small><sub>IF</sub></small>, whereas achieving such a selectivity at a much higher RF frequency would be much more difficult. By tuning the frequency of the local oscillator ''f''<small><sub>LO</sub></small>, the resulting difference frequency ''f''<small><sub>LO</sub></small> - ''f''<small><sub>d</sub></small>
(or ''f''<small><sub>d</sub></small>-''f''<small><sub>LO</sub></small> when using so-called '''low-side injection''') will be matched to the IF amplifier's frequency ''f''<small><sub>IF</sub></small> for the desired reception frequency ''f''<small><sub>d</sub></small>. One section of the tuning capacitor will thus adjust the local oscillator's frequency ''f''<small><sub>LO</sub></small>  to  ''f''<small><sub>d</sub></small> +  ''f''<small><sub>IF</sub></small> (or. less often, to 
''f''<small><sub>d</sub></small> -  ''f''<small><sub>IF</sub></small>) while the RF stage is tuned to ''f''<small><sub>d</sub></small>. Engineering the multi-section tuning capacitor (or [[varactor]]s) and coils to fulfill this condition across the tuning range is known as ''tracking''. 

Other signals produced by the mixer (such as due to stations at nearby frequencies) can be very well [[Filter (signal processing)|filtered]] out in the IF stage, giving the superheterodyne receiver its superior performance. However, if  ''f''<small><sub>LO</sub></small> is set to  ''f''<small><sub>d</sub></small> +  ''f''<small><sub>IF</sub></small> , then an incoming radio signal at ''f''<small><sub>LO</sub></small> +  ''f''<small><sub>IF</sub></small> will ''also'' produce a heterodyne at ''f''<small><sub>IF</sub></small>; this is called the ''image frequency'' and must be rejected by the tuned circuits in the RF stage. The image frequency is 2''f''<small><sub>IF</sub></small> higher (or lower) than ''f''<small><sub>d</sub></small>, so employing a higher IF frequency ''f''<small><sub>IF</sub></small> increases the receiver's ''image rejection'' without requiring additional selectivity in the RF stage.

Usually the intermediate frequency is lower than the reception frequency ''f''<small><sub>d</sub></small>, but in some modern receivers (e.g. scanners and spectrum analyzers) it is more convenient to first convert an entire band to a much higher intermediate frequency; this eliminates the problem of ''image rejection''. Then a tunable local oscillator and mixer converts that signal to a second much lower intermediate frequency where the selectivity of the receiver is accomplished. In order to avoid interference to receivers, licensing authorities will avoid assigning common IF frequencies to transmitting stations. Standard intermediate frequencies used are 455&nbsp;kHz for [[medium-wave]] AM radio,<ref>{{Cite web|url=http://www.pan-tex.net/usr/r/receivers/ra02083.htm|title=The Advantages of the 455 kHz IF Strip|publisher=Pan-tex.net 7/14/08|accessdate=17 January 2011}}</ref> 10.7&nbsp;MHz for broadcast FM receivers, 38.9&nbsp;MHz (Europe) or 45&nbsp;MHz (US) for television, and 70&nbsp;MHz for satellite and terrestrial microwave equipment.

In early superhets the IF stage was often a regenerative stage providing the sensitivity and selectivity with fewer components. Such superhets were called super-gainers or regenerodynes.{{cn|date=October 2011}}

===Bandpass filter===
The IF stage includes a filter and/or multiple tuned circuits in order to achieve the desired [[selectivity (electronic)|selectivity]]. This filtering must therefore have a band pass equal to or less than the frequency spacing between adjacent broadcast channels. Ideally a filter would have a high attenuation to adjacent channels, but maintain a flat response across the desired signal spectrum in order to retain the quality of the received signal. This may be obtained using one or more dual tuned IF transformers, or a multipole ceramic crystal filter.<ref>{{Cite web|url=http://www.qsl.net/va3iul/Homebrew_RF_Circuit_Design_Ideas/Crystal_Filter_Types.gif|title=Crystal filer types|publisher=QSL RF Circuit Design Ideas Date unknown|accessdate=17 January 2011}}</ref>

===Demodulation===
The received signal is now processed by the [[demodulator]] stage where the audio signal (or other [[baseband]] signal) is recovered and then further amplified. AM demodulation requires the simple [[rectification (electricity)|rectification]] of the RF signal (so-called [[envelope detector|envelope detection]]), and a simple RC low pass filter to remove remnants of the intermediate frequency.<ref>{{Cite web|url=http://bc.inter.edu/facultad/rflores/ELEN4360/Labs/Com1_Lab3.pdf|title=Reception of Amplitude Modulated Signals - AM Demodulation |publisher=BC Internet education 6/14/2007|accessdate=17 January 2011}}</ref> FM signals may be detected using a discriminator, [[ratio detector]], or [[phase-locked loop]]. [[Continuous wave]] ([[morse code]]) and [[single sideband]] signals require a [[product detector]] using a so-called [[beat frequency oscillator]], and there are other techniques used for different types of [[modulation]].<ref>{{Cite web|url=http://www.dbugman.com/handbook/tscmh5.html|title=BASIC RADIO THEORY|publisher=TSCM Handbook Ch.5 date unknown|accessdate=17 January 2011}}</ref> The resulting audio signal (for instance) is then amplified and drives a loudspeaker.

When so-called '''high-side injection''' has been used, where the local oscillator is at a ''higher'' frequency than the received signal (as is common), then the frequency spectrum of the original signal will be reversed. This must be taken into account by the demodulator (and in the IF filtering) in the case of certain types of modulation such as [[single sideband]].

==Advanced designs==
To overcome obstacles such as [[image response]], multiple IF stages are used, and in some cases multiple stages with two IFs of different values are used. For example, the front end might be sensitive to 1&ndash;30&nbsp;MHz, the first half of the radio to 5&nbsp;MHz, and the last half to 50&nbsp;kHz. Two frequency converters would be used, and the radio would be a ''double conversion superheterodyne'';<ref name=Carr02/> a common example is a [[television]] receiver where the audio information is obtained from a second stage of intermediate-frequency conversion. Receivers which are tunable over a wide bandwidth (e.g. [[Scanner (radio)|scanners]]) may use an intermediate frequency higher than the signal, in order to improve [[Image rejection ratio|image rejection]].<ref>Ferdinand Nibler ''High-frequency circuit engineering'', IET, 1996 ISBN 0852968019, p. 263</ref>

===Other uses===
In the case of modern television receivers, no other technique was able to produce the precise [[bandpass]] characteristic needed for [[vestigial sideband]] reception, similar to that used in the [[NTSC]] system approved by the U.S. [[Federal Communications Commission]] (FCC) in 1953,<ref>{{Cite book|url=http://books.google.co.uk/books?id=VRailj6TKqUC&pg=PA386&dq=vestigial+sideband+ntsc#v=onepage&q=vestigial%20sideband%20ntsc&f=false|title=Video engineering - P.386 Chap.16|publisher=McGraw-Hill Professional 1999|accessdate=29 January 2011}}</ref> and the [[PAL]] system approved by the [[BBC]] in 1957.<ref>{{Cite book|url=http://books.google.co.uk/books?id=TOMOmmrvwCcC&pg=PA77&lpg=PA77&dq=vestigial+sideband+crystal+palace#v=onepage&q&f=false|title=The History of Television 1942 to 2000|publisher=McFarland 2007|accessdate=29 January 2011}}</ref> This originally involved a complex collection of tuneable inductors which needed careful adjustment, but since the 1970s or early 1980s<ref>{{Cite book|url=http://books.google.co.uk/books?id=dRGMkQKNjccC&pg=PA458&lpg=PA458&dq=television+tunable+i.f.+stage#v=onepage&q&f=false|title=Monochrome and Colour Television p452 onward|publisher=New Age International, 2007|accessdate=17 January 2011}}</ref> these have been replaced with precision electromechanical [[surface acoustic wave]] (SAW) [[Electronic filter|filters]]. Fabricated by precision laser milling techniques, SAW filters are cheaper to produce, can be made to extremely close tolerances, and are stable in operation. To avoid [[Machine tool|tooling costs]] associated with these components most manufacturers then tended to design their receivers around the fixed range of frequencies offered which resulted in de-facto standardization of intermediate frequencies.

===Modern designs===
Microprocessor technology allows replacing the superheterodyne receiver design by a [[software defined radio]] architecture, where the IF processing after the initial IF filter is implemented in software. This technique is already in use in certain designs, such as very low-cost FM radios incorporated into mobile phones, since the system already has the necessary [[microprocessor]].

[[Radio transmitter]]s may also use a mixer stage to produce an output frequency, working more or less as the reverse of a superheterodyne receiver.

==Advantages and drawbacks of the superheterodyne design==
Superheterodyne receivers have essentially replaced all previous receiver designs. The development of modern [[semiconductor]] electronics negated the advantages of designs (such as the [[regenerative receiver]]) which used fewer vacuum tubes. The superheterodyne receiver offers superior sensitivity, frequency stability and selectivity. Compared with the [[tuned radio frequency receiver]] (TRF) design, superhets offer better stability because a tuneable oscillator is more easily realized than a tuneable amplifier. Operating at a lower frequency, IF filters can give narrower passbands at the same [[Q factor]] than an equivalent RF filter. A fixed IF also allows the use of a [[crystal filter]]<ref name=Carr02/> or similar technologies which cannot be tuned.  [[Regenerative circuit|Regenerative]] and super-regenerative receivers offered a high sensitivity, but often suffer from stability problems making them difficult to operate.

Although the advantages of the superhet design are overwhelming, we note a few drawbacks which need to be tackled in practice.

===Image frequency (''f''<sub>image</sub>)===
One major disadvantage to the superheterodyne receiver is the problem of [[image frequency]]. In heterodyne receivers, an image frequency is an undesired input frequency equal to the station frequency plus twice the intermediate frequency. The image frequency results in two stations being received at the same time, thus producing interference. Image frequencies can be eliminated by sufficient [[attenuation]] on the incoming signal by the RF amplifier filter of the superheterodyne receiver.

:<math>f_{img} = \begin{cases} f + 2f_{IF} , & \mbox{if }  f_{LO} > f  \mbox{   (high side injection)}\\ f- 2f_{IF},  & \mbox{if } f_{LO} < f \mbox{  (low side injection)} \end{cases} </math>

For example, an AM broadcast station at 580&nbsp;kHz is tuned on a receiver with a 455&nbsp;kHz IF. The local oscillator is tuned to 580+455 = 1035&nbsp;kHz. But a signal at 580+455+455=1490&nbsp;kHz is also 455&nbsp;kHz away from the local oscillator; so both the desired signal and the image, when mixed with the local oscillator, will also appear at the intermediate frequency.   This image frequency is within the AM broadcast band.  Practical receivers have a tuning stage before the converter, to greatly reduce the amplitude of image frequency signals;  additionally, broadcasting stations in the same area have their frequencies assigned to avoid such images. 

Early [[Autodyne]] receivers typically used IFs of only 150&nbsp;kHz or so, as it was difficult to maintain reliable oscillation if higher frequencies were used. As a consequence, most Autodyne receivers needed quite elaborate antenna tuning networks, often involving double-tuned coils, to avoid image interference. Later superhets used tubes especially designed for oscillator/mixer use, which were able to work reliably with much higher IFs, reducing the problem of image interference and so allowing simpler and cheaper aerial tuning circuitry.

The unwanted frequency is called the ''image'' of the wanted frequency, because it is the "mirror image" of the desired frequency reflected <math>f_{o}\!</math>. A receiver with inadequate filtering at its input will pick up signals at two different frequencies simultaneously: the desired frequency and the image frequency.  Any noise or random radio station at the image frequency can interfere with reception of the desired signal.   

Sensitivity to the image frequency can be minimised only by (1) a filter that precedes the mixer or (2) a more complex mixer circuit [http://www.freepatentsonline.com/7227912.html] that suppresses the image.  In most receivers this is accomplished by a [[bandpass filter]] in the [[RF front end]].  In many tunable receivers, the bandpass filter is tuned in tandem with the local oscillator.<!-- must be tuned not true; only a requirement if image within or close to covered freqs; triple conv receivers often avoid tunable filters. Some use switched filters. EK-070 VLF/HF rcvr has 10 bands and 81.4 MHz IF. Some Aviation COMM/NAV rcvrs are tunable but have single fixed BPF input. Genave ALPHA 100-360. Collins 51V3 glide slope. -->  

Image rejection is an important factor in choosing the intermediate frequency of a receiver.  The farther apart the bandpass frequency and the image frequency are, the more the bandpass filter will attenuate any interfering image signal.  Since the frequency separation between the bandpass and the image frequency is <math>2f_{i}\!</math>, a higher intermediate frequency improves image rejection. 

The ability of a receiver to reject interfering signals at the image frequency is measured by the [[image rejection ratio]].  This is the ratio (in [[decibel]]s) of the output of the receiver from a signal at the received frequency, to its output for an equal-strength signal at the image frequency.

===Local oscillator radiation===
It is difficult to keep stray radiation from the local oscillator below the level that a nearby receiver can detect. The receiver's local oscillator can act like a miniature [[Continuous wave|CW]] transmitter. This means that there can be mutual interference in the operation of two or more superheterodyne receivers in close proximity. In espionage, oscillator radiation gives a means to detect a covert receiver and its operating frequency and in the United Kingdom is used to detect whether a television receiver is being used without a television license. One effective way of preventing the local oscillator signal from radiating out from the receiver's antenna is by adding a shielded and power supply [[decoupled]] stage of RF amplification between the receiver's antenna and its mixer stage. {{further|[[Electromagnetic compatibility]]}}

===Local oscillator sideband noise===
Local oscillators typically generate a single frequency signal that has negligible [[amplitude modulation]] but some random [[phase modulation]]. Either of these impurities spreads some of the signal's energy into sideband frequencies. That causes a corresponding widening of the receiver's frequency response, which would defeat the aim to make a very narrow bandwidth receiver such as to receive low-rate digital signals. Care needs to be taken to minimise oscillator phase noise, usually by ensuring that the oscillator never enters a [[non-linear]] mode.

==See also==
*[[H2X radar]]
*[[Automatic gain control]]
*[[Demodulator]]
*[[Direct conversion receiver]]
*[[VFO]]
*[[Single sideband#Demodulation|Single sideband modulation (demodulation)]]
*[[Tuned radio frequency receiver]]
*[[Reflectional receiver]]
*[[Beat frequency]]
*[[Heterodyne]]
*[[Optical heterodyne detection]]
*[[Infradyne]] - superheterodyne with IF higher than signal frequency
*[[Superheterodyne transmitter]]

==References==
{{reflist}}

==Further reading==
{{cite book |title=The Electronics Handbook |last=Whitaker |first=Jerry |authorlink= |year=1996 |publisher=CRC Press |location= |isbn=08-493834-55 |pages=1172 }}

== External links ==
{{Commons category|Superheterodyne circuits}}
*[http://antiqueradios.com/superhet/ Who Invented the Superheterodyne?] An article giving the history of the various inventors working on the superheterodyne method.
*[http://www.qsl.net/vu2msy/receiver.htm An in-depth introduction to superheterodyne receivers]
*[http://www.microwaves101.com/encyclopedia/receivers_superhet.cfm Superheterodyne receivers from microwaves101.com]

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