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Superheterodyne receiver

A 5-tube superheterodyne receivermade in Japan around 1955

From Wikipedia, the free encyclopedia

In electronics, a superheterodyne receiver (oftenshortened to superhet) uses frequency mixing to converta received signal to a fixed intermediate frequency (IF)which can be more conveniently processed than theoriginal radio carrier frequency. It was invented by USengineer Edwin Armstrong in 1918 during World War I.[1]

Virtually all modern radio receivers use thesuperheterodyne principle. At the cost of an extrafrequency converter stage, the superheterodyne receiverprovides superior selectivity and sensitivity compared withsimpler designs.

Contents [hide]

1 History1.1 Background

1.2 Invention

1.3 Development

2 Design and principle of operation

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Superheterodyne transistor radio circuitaround 1975

2.1 Circuit description

2.2 Local oscillator and mixer

2.3 Intermediate frequency amplifier

2.4 Bandpass filter

2.5 Demodulation

3 Advanced designs3.1 Other uses

3.2 Modern designs

4 Advantages and drawbacks of the superheterodynedesign

4.1 Image frequency (fimg)

4.2 Local oscillator radiation

4.3 Local oscillator sideband noise

5 See also

6 References

7 Further reading

8 External links

History [edit]

Background [edit]

"Superheterodyne" is a contraction of"supersonic heterodyne", where"supersonic" indicates frequencies abovethe range of human hearing. The wordheterodyne is derived from the Greek

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One of the prototype superheterodyne receivers built atArmstrong's Signal Corps laboratory in Paris during WorldWar I. It is constructed in two sections, the mixer and localoscillator (left) and three IF amplification stages and adetector stage (right). The intermediate frequency was 75kHz.

roots hetero- "different", and -dyne"power". In radio applications the termderives from the "heterodyne detector"pioneered by Canadian inventorReginald Fessenden in 1905, describinghis proposed method of producing anaudible signal from the Morse code transmissions of the new continuous wave transmitters. Withthe older spark gap transmitters then in use, the Morse code signal consisted of short bursts of aheavily modulated carrier wave, which could be clearly heard as a series of short chirps or buzzesin the receiver's headphones. However, the signal from a continuous wave transmitter did not haveany such inherent modulation and Morse Code from one of those would only be heard as a seriesof clicks or thumps. Fessenden's idea was to run two Alexanderson alternators, one producing acarrier frequency 3 kHz higher than the other. In the receiver's detector the two carriers would beattogether to produce a 3 kHz tone thus in the headphones the Morse signals would then be heardas a series of 3 kHz beeps. For this he coined the term "heterodyne" meaning "generated by adifference" (in frequency).

Invention [edit]

The superheterodyne principle was devised in 1918 by U.S. Army Major Edwin Armstrong inFrance during World War I.[2][3] He invented this receiver as a means of overcoming thedeficiencies of early vacuum tube triodes used as high-frequency amplifiers in radio directionfinding equipment. Unlike simple radio communication, which only needs to make transmittedsignals audible, direction-finders measure the received signal strength, which necessitates linearamplification of the actual carrier wave.

In a triode radio-frequency (RF) amplifier, if both the plate

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One of the first amateursuperheterodyne receivers, built in1920 even before Armstrong publishedhis paper. Due to the low gain of earlytriodes it required 9 tubes, with 5 IFamplification stages, and used an IF ofaround 50 kHz.

(anode) and grid are connected to resonant circuits tunedto the same frequency, stray capacitive coupling betweenthe grid and the plate will cause the amplifier to go intooscillation if the stage gain is much more than unity. Inearly designs, dozens (in some cases over 100) low-gaintriode stages had to be connected in cascade to makeworkable equipment, which drew enormous amounts ofpower in operation and required a team of maintenanceengineers. The strategic value was so high, however, thatthe British Admiralty felt the high cost was justified.

Armstrong realized that if radio direction-finding (RDF)receivers could be operated at a higher frequency, thiswould allow better detection of enemy shipping. However,at that time, no practical "short wave" (defined then as anyfrequency above 500 kHz) amplifier existed, due to thelimitations of existing triodes.

It had been noticed some time before that if a regenerativereceiver was allowed to go into oscillation, other receivers nearby would suddenly start picking upstations 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 wastransmitting on 300 kHz and the oscillating receiver was set to 400 kHz, the station would be heardnot only at the original 300 kHz, but also at 100 kHz and 700 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

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The first commercial superheterodynereceiver,[5] the RCA Radiola AR-812, brought outMarch 4, 1924 priced at $286. It used 6 triodes: a

monitor a frequency of 1500 kHz for example, he could set up an oscillator at, for example,1560 kHz, which would produce a heterodyne difference frequency of 60 kHz, a frequency thatcould then be more conveniently amplified by the triodes of the day. He termed this the"Intermediate Frequency" often abbreviated to "IF".

In December 1919, Major E. H. Armstrong gave publicity to an indirect method ofobtaining short-wave amplification, called the super-heterodyne. The idea is to reducethe incoming frequency, which may be, say 1,500,000 cycles (200 meters), to somesuitable super-audible frequency that can be amplified efficiently, then passing thiscurrent through a radio frequency amplifier and finally rectifying and carrying on toone or two stages of audio frequency amplification.[4]

Development [edit]

Armstrong was able to put his ideas intopractice, and the technique was soon adoptedby the military. However, it was less popularwhen commercial radio broadcasting began inthe 1920s, mostly due to the need for an extratube (for the oscillator), the generally highercost of the receiver, and the level of technicalskill required to operate it. For early domesticradios, tuned radio frequency receivers("TRF"), also called the Neutrodyne, were morepopular because they were cheaper, easier fora non-technical owner to use, and less costly to

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March 4, 1924 priced at $286. It used 6 triodes: amixer, local oscillator, two IF and two audio amplifierstages, with an IF of 45 kHz. It was a commercialsuccess, with better performance than competingreceivers. In an apparent attempt to preventcompetitors from "reverse engineering" it, theinnards were encased in solid wax.

By the 1940s the vacuum-tube superheterodyne AMbroadcast receiver was refined into a cheap-to-

operate. Armstrong eventually sold hissuperheterodyne patent to Westinghouse, whothen sold it to RCA, the latter monopolizing themarket for superheterodyne receivers until1930.[6]

Early superheterodyne receivers used IFs aslow as 20 kHz, often based on the self-resonance of iron-cored transformers. This made themextremely susceptible to image frequency interference, but at the time, the main objective wassensitivity rather than selectivity. Using this technique, a small number of triodes could be made todo the work that formerly required dozens of triodes.

In the 1920s, commercial IF filters looked very similar to 1920s audio interstage couplingtransformers, had very similar construction and were wired up in an almost identical manner, andso they were referred to as "IF Transformers". By the mid-1930s however, superheterodynes wereusing much higher intermediate frequencies, (typically around 440–470 kHz), with tuned coilssimilar in construction to the aerial and oscillator coils. However, the name "IF Transformer" wasretained and is still used today. Modern receivers typically use a mixture of ceramic resonator orSAW (surface-acoustic wave) resonators as well as traditional tuned-inductor IF transformers.

By the 1930s, improvements in vacuum tubetechnology rapidly eroded the TRF receiver'scost advantages, and the explosion in thenumber of broadcasting stations created ademand for cheaper, higher-performancereceivers.

The development of the tetrode vacuum tube

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manufacture design called the "All American Five",because it only used five vacuum tubes: usually aconverter (mixer/local oscillator), an IF amplifier, adetector/audio amp, audio power amp, and a rectifier.

containing a screen grid led to a multi-element tube in which the mixer and oscillatorfunctions could be combined, first used in theso-called autodyne mixer. This was rapidlyfollowed by the introduction of tubesspecifically designed for superheterodyne operation, most notably the pentagrid converter. Byreducing the tube count, this further reduced the advantage of preceding receiver designs.

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

Design and principle of operation [edit]

The principle of operation of the superheterodyne receiver depends on the use of heterodyning orfrequency mixing. The signal from the antenna is filtered sufficiently at least to reject the imagefrequency (see below) and possibly amplified. A local oscillator in the receiver produces a sinewave, which mixes with that signal, shifting it to a specific intermediate frequency (IF), usually alower frequency. The IF signal is itself filtered and amplified and possibly processed in additionalways. The demodulator uses the IF signal rather than the original radio frequency to recreate acopy of the original information (such as audio).

The diagram

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Block diagram of a typical superheterodyne receiver

The diagramat right showsthe minimumrequirementsfor a single-conversion

superheterodyne receiver design. The following essential elements are common to allsuperheterodyne circuits:[7] a receiving antenna; a tuned stage, which may optionally containamplification (RF amplifier); a variable frequency local oscillator; a frequency mixer; a band passfilter and intermediate frequency (IF) amplifier; and a demodulator plus additional circuitry toamplify or process the original audio signal (or other transmitted information).

Circuit description [edit]

To receive a radio signal, a suitable 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 microvolts. 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 tunedcircuits at this stage block frequencies that are far removed from the intended receptionfrequency. In order to tune the receiver to a particular station, the frequency of the local oscillatoris controlled by the tuning knob (for instance). Tuning of the local oscillator and the RF stage mayuse a variable capacitor, or varicap diode.[8] The tuning of one (or more) tuned circuits in the RF

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stage must track the tuning of the local oscillator.

Notice that the accompanying diagram shows a fixed-frequency local oscillator, as the symbol is fora fixed-frequency crystal frequency-determining device. A tuneable receiver would show avariable-frequency oscillator with operational connection to the tuned circuits of the antenna andradio-frequency amplifier stages.

Local oscillator and mixer [edit]

The signal is then fed into a circuit where it is mixed with a sine wave from a variable frequencyoscillator known as the local oscillator (LO). The mixer uses a non-linear component to produceboth sum and difference beat frequencies signals,[9] each one containing the modulationcontained in the desired signal. The output of the mixer may include the original RF signal at fRF,the local oscillator signal at fLO, and the two new heterodyne frequencies fRF + fLO and fRF − fLO.The mixer may inadvertently produce additional frequencies such as third- and higher-orderintermodulation products. Ideally, the IF bandpass filter removes all but the desired IF signal at fIF.The IF signal contains the original modulation (transmitted information) that the received radiosignal had at fRF.

Historically, vacuum tubes were expensive, so broadcast AM receivers would save costs byemploying a single tube as both a mixer and also as the local oscillator. The pentagrid convertertube would oscillate and also provide signal amplification as well as frequency shifting.[10]

The frequency of the local oscillator fLO is set so the desired reception radio frequency fRF mixes tofIF. There are two choices for the local oscillator frequency because the dominant mixer productsare at fRF ± fLO. If the local oscillator frequency is less than the desired reception frequency, it iscalled low-side injection (fIF = fRF − fLO); if the local oscillator is higher, then it is called high-sideinjection (fIF = fLO − fRF).

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The mixer will process not only the desired input signal at fRF, but also all signals present at itsinputs. There will be many mixer products (heterodynes). Most other signals produced by the mixer(such as due to stations at nearby frequencies) can be filtered out in the IF amplifier; that gives thesuperheterodyne receiver its superior performance. However, if fLO is set to fRF + fIF, then anincoming radio signal at fLO + fIF will also produce a heterodyne at fIF; this is called the imagefrequency and must be rejected by the tuned circuits in the RF stage. The image frequency is 2 fIFhigher (or lower) than fRF, so employing a higher IF frequency fIF increases the receiver's imagerejection without requiring additional selectivity in the RF stage.

To suppress the unwanted image, the tuning of the RF stage and the LO may need to "track" eachother. In some cases, a narrow-band receiver can have a fixed tuned RF amplifier. In that case,only the local oscillator frequency is changed. In most cases, a receiver's input band is wider thanits IF center frequency. For example, a typical AM broadcast band receiver covers 510 kHz to1655 kHz (a roughly 1160 kHz input band) with a 455 kHz IF frequency; an FM broadcast bandreceiver covers 88 MHz to 108 MHz band with a 10.7 MHz IF frequency. In that situation, the RFamplifier must be tuned so the IF amplifier does not see two stations at the same time. If the AMbroadcast band receiver LO were set at 1200 kHz, it would see stations at both 745 kHz(1200−455 kHz) and 1655 kHz. Consequently, the RF stage must be designed so that any stationsthat are twice the IF frequency away are significantly attenuated.. The tracking can be done with amulti-section variable capacitor or some varactors driven by a common control voltage. An RFamplifier may have tuned circuits at both its input and its output, so three or more tuned circuitsmay be tracked. In practice, the RF and LO frequencies need to track closely but notperfectly.[11][12]

Intermediate frequency amplifier [edit]

The stages of an intermediate frequency amplifier ("IF amplifier" or "IF strip") are tuned to a fixed

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frequency that does not change as the receiving frequency changes. The fixed frequencysimplifies optimization of the IF amplifier.[7] The IF amplifier is selective around its center frequencyfIF. The fixed center frequency allows the stages of the IF amplifier to be carefully tuned for bestperformance (this tuning is called "aligning" the IF amplifier). If the center frequency changed withthe receiving frequency, then the IF stages would have had to track their tuning. That is not thecase with the superheterodyne.

Typically, the IF center frequency fIF is chosen to be less than the desired reception frequency fRF.The choice has some performance advantages. First, it is easier and less expensive to get highselectivity at a lower frequency. For the same bandwidth, a tuned circuit at a lower frequencyneeds a lower Q. Stated another way, for the same filter technology, a higher center frequency willtake more IF filter stages to achieve the same selectivity bandwidth. Second, it is easier and lessexpensive to get high gain at a lower frequency. When used at high frequencies, many amplifiersshow a constant gain–bandwidth product (dominant pole) characteristic. If an amplifier has a gain–bandwidth product of 100 MHz, then it would have a voltage gain of 100 at 1 MHz but only 10 at10 MHz. If the IF amplifier needed a voltage gain of 10,000, then it would need only two stages withan IF at 1 MHz but four stages at 10 MHz.

Usually the intermediate frequency is lower than the reception frequency fRF, but in some modernreceivers (e.g. scanners and spectrum analyzers) a higher IF frequency is used to minimizeproblems with image rejection or gain the benefits of fixed-tuned stages. The Rohde & SchwarzEK-070 VLF/HF receiver covers 10 kHz to 30 MHz.[13] It has a band switched RF filter and mixesthe input to a first IF of 81.4 MHz. The first LO frequency is 81.4 to 111.4 MHz, so the primaryimages are far away. The first IF stage uses a crystal filter with a 12 kHz bandwidth. There is asecond frequency conversion (making a triple-conversion receiver) that mixes the 81.4 MHz first IFwith 80 MHz to create a 1.4 MHz second IF. Image rejection for the second IF is not a majorproblem because the first IF provides adequate image rejection and the second mixer is fixed

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tuned.

In order to avoid interference to receivers, licensing authorities will avoid assigning common IFfrequencies to transmitting stations. Standard intermediate frequencies used are 455 kHz formedium-wave AM radio, 10.7 MHz for broadcast FM receivers, 38.9 MHz (Europe) or 45 MHz (US)for television, and 70 MHz for satellite and terrestrial microwave equipment. To avoid tooling costsassociated with these components, most manufacturers then tended to design their receiversaround a fixed range of frequencies offered, which resulted in a worldwide de facto standardizationof intermediate frequencies.

In early superhets, the IF stage was often a regenerative stage providing the sensitivity andselectivity with fewer components. Such superhets were called super-gainers orregenerodynes.[citation needed]

Bandpass filter [edit]

The IF stage includes a filter and/or multiple tuned circuits in order to achieve the desiredselectivity. This filtering must therefore have a band pass equal to or less than the frequencyspacing between adjacent broadcast channels. Ideally a filter would have a high attenuation toadjacent channels, but maintain a flat response across the desired signal spectrum in order toretain the quality of the received signal. This may be obtained using one or more dual tuned IFtransformers, a quartz crystal filter, or a multipole ceramic crystal filter.[14]

Demodulation [edit]

The received signal is now processed by the demodulator stage where the audio signal (or otherbaseband signal) is recovered and then further amplified. AM demodulation requires the simplerectification of the RF signal (so-called envelope detection), and a simple RC low pass filter toremove remnants of the intermediate frequency.[15] FM signals may be detected using a

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discriminator, ratio detector, or phase-locked loop. Continuous wave (Morse code) and singlesideband signals require a product detector using a so-called beat frequency oscillator, and thereare other techniques used for different types of modulation.[16] The resulting audio signal (forinstance) is then amplified and drives a loudspeaker.

When so-called high-side injection has been used, where the local oscillator is at a higherfrequency than the received signal (as is common), then the frequency spectrum of the originalsignal 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 [edit]

To overcome obstacles such as image response, in some cases multiple stages with two or moreIFs of different values are used. For example, for a receiver that can tune from 500 kHz to 30 MHz,three frequency converters might be used, and the radio would be referred to as a tripleconversion superheterodyne;[7]

The reason that this is done is the difficulty in obtaining sufficient selectivity in the front-end tuningwith higher shortwave frequencies.

With a 455 kHz IF it is easy to get adequate front end selectivity with broadcast band (under1600 kHz) signals. For example, if the station being received is on 600 kHz, the local oscillator willbe set to 600 + 455 = 1055 kHz. But a station on 1510 kHz could also potentially produce an IF of455 kHz and so cause image interference. However because 600 kHz and 1510 kHz are so farapart, it is easy to design the front end tuning to reject the 1510 kHz frequency.

However at 30 MHz, things are different. The oscillator would be set to 30.455 MHz to produce a455 kHz IF, but a station on 30.910 would also produce a 455 kHz beat, so both stations would beheard at the same time. But it is virtually impossible to design an RF tuned circuit that can

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adequately discriminate between 30 MHz and 30.91 MHz, so one approach is to "bulkdownconvert" whole sections of the shortwave bands to a lower frequency, where adequate front-end tuning is easier to arrange.

For example the ranges 29 MHz to 30 MHz; 28 MHz to 29 MHz etc. might be converted down to2 MHz to 3 MHz, there they can be tuned more conveniently. This is often done by first convertingeach "block" up to a higher frequency (typically 40 MHz) and then using a second mixed to convertit down to the 2 MHz to 3 MHz range. The 2 MHz to 3 MHz "IF" is basically another self-containedsuperheterodyne receiver, most likely with a standard IF of 455 kHz.

Other uses [edit]

In the case of modern television receivers, no other technique was able to produce the precisebandpass characteristic needed for vestigial sideband reception, similar to that used in the NTSCsystem first approved by the U.S. in 1941. By the 1980s these had been replaced with precisionelectromechanical surface acoustic wave (SAW) filters. Fabricated by precision laser millingtechniques, SAW filters are cheaper to produce, can be made to extremely close tolerances, andare very stable in operation.

Modern designs [edit]

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

Radio transmitters may also use a mixer stage to produce an output frequency, working more orless as the reverse of a superheterodyne receiver.

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Advantages and drawbacks of the superheterodyne design [edit]

Superheterodyne receivers have essentially replaced all previous receiver designs. Thedevelopment of modern semiconductor electronics negated the advantages of designs (such asthe regenerative receiver) that used fewer vacuum tubes. The superheterodyne receiver offerssuperior sensitivity, frequency stability and selectivity. Compared with the tuned radio frequencyreceiver (TRF) design, superhets offer better stability because a tuneable oscillator is more easilyrealized than a tuneable amplifier. Operating at a lower frequency, IF filters can give narrowerpassbands at the same Q factor than an equivalent RF filter. A fixed IF also allows the use of acrystal filter[7] or similar technologies that cannot be tuned. Regenerative and super-regenerativereceivers offered a high sensitivity, but often suffer from stability problems making them difficult tooperate.

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

Image frequency (fimg) [edit]

One major disadvantage to the superheterodyne receiver is the problem of image frequency. Inheterodyne receivers, an image frequency is an undesired input frequency equal to the stationfrequency plus twice the intermediate frequency. The image frequency results in two stations beingreceived at the same time, thus producing interference. Image frequencies can be eliminated bysufficient attenuation on the incoming signal by the RF amplifier filter of the superheterodynereceiver.

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

The unwanted frequency is called the image of the wanted frequency, because it is the "mirrorimage" of the desired frequency reflected . A receiver with inadequate filtering at its input willpick up signals at two different frequencies simultaneously: the desired frequency and the imagefrequency. Any noise or random radio station at the image frequency can interfere with receptionof the desired signal.

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

Sensitivity to the image frequency can be minimised only by (1) a filter that precedes the mixer or(2) a more complex mixer circuit [1] that suppresses the image. In most receivers this isaccomplished by a bandpass filter in the RF front end. In many tunable receivers, the bandpassfilter is tuned in tandem with the local oscillator.

Image rejection is an important factor in choosing the intermediate frequency of a receiver. Thefarther apart the bandpass frequency and the image frequency are, the more the bandpass filter

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will attenuate any interfering image signal. Since the frequency separation between the bandpassand the image frequency is , a higher intermediate frequency improves image rejection. Itmay be possible to use a high enough first IF that a fixed-tuned RF stage can reject any imagesignals.

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

Local oscillator radiation [edit]

Further information: Electromagnetic compatibility

It is difficult to keep stray radiation from the local oscillator below the level that a nearby receivercan detect. The receiver's local oscillator can act like a low-power CW transmitter. Consequently,there can be mutual interference in the operation of two or more superheterodyne receivers inclose proximity.

In intelligence operations, local oscillator radiation gives a means to detect a covert receiver andits operating frequency. The method was used by MI-5 during Operation RAFTER.[17] This sametechnique is also used in radar detector detectors used by traffic police in jurisdictions where radardetectors are illegal.

A method of significantly reducing the local oscillator radiation from the receiver's antenna is touse an RF amplifier between the receiver's antenna and its mixer stage.

Local oscillator sideband noise [edit]

Local oscillators typically generate a single frequency signal that has negligible amplitudemodulation but some random phase modulation. Either of these impurities spreads some of the

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signal's energy into sideband frequencies. That causes a corresponding widening of the receiver'sfrequency response, which would defeat the aim to make a very narrow bandwidth receiver suchas to receive low-rate digital signals. Care needs to be taken to minimize oscillator phase noise,usually by ensuring that the oscillator never enters a non-linear mode.

See also [edit]

H2X radar

Automatic gain control

Demodulator

Direct conversion receiver

VFO

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 [edit]

1. ^ Armstrong, Edwin H. (February 1921). "A new system of short wave amplification" . Proc. of theIRE (New York: Institute of Radio Engineers) 9 (1): 3–11. Retrieved 22 October 2013.

2. ^ "The History of Amateur Radio" . Luxorion date unknown. Retrieved 19 January 2011.

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3. ^ Sarkar, Tapan K.; Mailloux, Robert J.; Oliner, Arthur A.; Salazar-Palma, Magdalena; Sengupta,Dipak L. (2006), History of Wireless, John Wiley and Sons, ISBN 0-471-71814-9, p 110?

4. ^ (page 11 of December 1922 QST magazine)

5. ^ Malanowski, Gregory (2011). The Race for Wireless: How Radio Was Invented (or Discovered?) .Authorhouse. p. 69. ISBN 1463437501.

6. ^ Katz, Eugenii. "Edwin Howard Armstrong" . History of electrochemistry, electricity, andelectronics. Eugenii Katz homepage, Hebrew Univ. of Jerusalem . Archived from the original on2009-10-22. Retrieved 2008-05-10.

7. ̂a b c d Joseph J. Carr RF Components and Circuits Newnes, 2002 ISBN 978-0-7506-4844-8,Chapter 3

8. ^ Radio-frequency electronics: circuits and applications By Jon B. Hagen -p.58 l. 12 . CambridgeUniversity Press, 1996 - Technology & Engineering. Retrieved 17 January 2011.

9. ^ The art of electronics . Cambridge University Press. 2006. p. 886. Retrieved 17 January 2011.

10. ^ GB 426802 , "Improvements in or relating to superheterodyne radio receivers", published 12October 1933

11. ^ Terman, Frederick Emmons (1943), Radio Engineers' Handbook, New York: McGraw-Hill. Pages649–652 describes design procedure for tracking with a pad capacitor in the Chebyshev sense.

12. ^ Rohde, Ulrich L.; Bucher, T. T. N. (1988), Communications Receivers: Principles & Design, NewYork: McGraw-Hill, ISBN 0-07-053570-1. Pages 155–160 discuss frequency tracking. Pages 160–164 discuss image rejection and include an RF filter design that puts transmission zeros at both thelocal oscillator frequency and the unwanted image frequency.

13. ^ Rohde & Bucher 1988, pp. 44–55

14. ^ "Crystal filer types" . QSL RF Circuit Design Ideas Date unknown. Retrieved 17 January 2011.

15. ^ "Reception of Amplitude Modulated Signals - AM Demodulation" (PDF). BC Internet education6/14/2007. Retrieved 17 January 2011.

16. ^ "Basic Radio Theory" . TSCM Handbook Ch.5 date unknown. Retrieved 17 January 2011.

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[show]V · T · E ·

Wikimedia Commons hasmedia related toSuperheterodyne circuits.

17. ^ Wright, Peter (1987), Spycatcher: The Candid Autobiography of a Senior Intelligence Officer,Penguin Viking, ISBN 0-670-82055-5

Further reading [edit]

Whitaker, Jerry (1996). The Electronics Handbook. CRC Press. p. 1172. ISBN 0-8493-8345-5.

US 706740 , Fessenden, Reginald A., "Wireless Signaling", published September 28, 1901,issued August 12, 1902

US 1050441 , Fessenden, Reginald A., "Electric Signaling Apparatus", published July 27,1905, issued January 14, 1913

US 1050728 , Fessenden, Reginald A., "Method of Signaling", published August 21, 1906,issued January 14, 1913

External links [edit]

Who Invented the Superheterodyne? An articlegiving the history of the various inventors working onthe superheterodyne method.

An in-depth introduction to superheterodyne receivers

Superheterodyne receivers from microwaves101.com

Telecommunications

Categories: Radio electronics Communication circuits Electronic design History of radioReceiver (radio)

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