Engineering:Superheterodyne receiver

Short description: Type of radio receiver

A 5-tube superheterodyne receiver manufactured by Toshiba circa 1955

Superheterodyne transistor radio circuit circa 1975

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 processed more efficiently and selectively than the original carrier frequency. Edwin Howard Armstrong developed the concept, though Lucien Lévy, Walter Schottky, Henry Round, and John Renshaw Carson explored related ideas. The superheterodyne design is used in most radio receivers today and can be used for many modulation schemes, including amplitude modulation (AM).

According to Nahin, any receiver that shifts the antenna signal frequencies to new locations in the spectrum is a heterodyne receiver. The prefix super is reserved for those receivers that include both a RF input filter for image rejection, and a fixed-frequency, narrow-band intermediate-frequency amplifier for adjacent-channel suppression.

Precursors

Early receivers evolved through several approaches. Crystal detectors provided simple rectification without amplification and were widely used in inexpensive sets produced by companies such as Crosley. The introduction of the audion made vacuum-tube amplification practical and was adopted in commercial receivers by firms including RCA and Atwater Kent. Regenerative receivers increased sensitivity and selectivity through feedback, while reflex receivers reused a single tube for multiple amplification stages to reduce cost. The Neutrodyne addressed instability in tuned radio-frequency designs, and combinations of these techniques were used in receivers such as the Trirdyne.

A superregenerative receiver is a radio receiver that achieves high sensitivity by periodically varying the stability of a resonant circuit. Edwin H. Armstrong introduced the technique in 1922 as an extension of the regenerative receiver. Superregenerative receivers saw extensive use during the Second World War, particularly in identification friend or foe (IFF) systems.

Westinghouse entered broadcsting when Frank Conrad starts an amature transmiter from his home. U.S.stations grew from 5 in 1921 to 530 in 1924.

These developments took place alongside extensive patent activity and licensing arrangements among manufacturers, which influenced which circuits could be commercially produced. In parallel, early heterodyne methods combined signals to produce an audible beat frequency for continuous-wave reception, but had not yet been developed into a consumer product.

By the late 1920s, changes in broadcasting station density rendered earlier receiver designs increasingly inadequate. As noted in a contemporary review, “receivers which were giving satisfactory service at the beginning of that period are now obsolete.” Increasing station density imposed stricter selectivity requirements, while higher transmitter powers and limited antenna installations increased the need for sensitivity. Improvements in program quality and the use of loudspeakers also raised expectations for fidelity.

These requirements were not readily met by existing receiver types, which could not simultaneously provide ease of use, high gain, stable tuning, and selective frequency response. The introduction of features such as automatic gain control to handle large variations in signal strength further favored receiver architectures in which gain and selectivity were distributed and controlled at different stages.

Radio direction finding

There was one role where the regenerative system was not suitable, even for Morse code sources, and that was the task of radio direction finding, RDF.

The regenerative system was highly non-linear, amplifying any signal above a certain threshold by a huge amount, sometimes so large it caused it to turn into a transmitter (which was the entire basis of the original IFF system). In RDF, the strength of the signal is used to determine the location of the transmitter, so one requires linear amplification to allow the strength of the original signal, often very weak, to be accurately measured.

To address this need, RDF systems of the era used triodes operating below unity. To get a usable signal from such a system, tens or even hundreds of triodes had to be used, connected together anode-to-grid. These amplifiers drew enormous amounts of power and required a team of maintenance engineers to keep them running. Nevertheless, the strategic value of direction finding on weak signals was so high that the British Admiralty felt the high cost was justified.

History

Conceptualisation

By the late 1910s, heterodyne reception was understood as a method of frequency conversion, and the use of vacuum tubes made it possible to generate local oscillations within the receiver. It was also recognized that amplification was easier to achieve at lower frequencies with the tubes then available, and that stable, high-gain amplification was difficult when the tuned frequency had to be varied across a wide range.

In 1922, C.R. Leutz described the super-heterodyne as "reduce the incoming frequency which may be, say 1,500,00 cycles (200 meters) to some super-audible frequency which can be amplified efficiently, then passing this current through a radio frequency amplifier and finally rectifying and carrying on with one or two stages of audio frequency amplification if desired. Transformation of the incoming sign frequency is usally accomplished by a heterodyne oscillator and rectifier."

The solution was to convert the received signal to a fixed intermediate frequency before amplification. This was accomplished by combining the incoming signal with a locally generated oscillation (LO) in a detector or mixer stage (first detector), producing sum and difference frequencies. The difference frequency, later termed the intermediate frequency (IF), retained the original modulation while shifting the signal to a frequency more suitable for amplification.

The use of a local oscillator and detector for heterodyne reception was already present in earlier vacuum-tube practice. The distinguishing feature of the superheterodyne was the deliberate selection of a fixed intermediate frequency above the audio range, allowing subsequent stages to operate at a single frequency. Early work had noted that heterodyne frequencies need not be audible, but had not developed this into a complete receiver architecture.

Following conversion, the signal could be filtered and amplified at the intermediate frequency, then detected to recover the audio, and further amplified for output. This separated tuning from amplification, allowing most of the receiver to operate as a fixed-frequency system without the need to retune high-selectivity stages. The selectivity was best achieved in the intermediate-frequency stages. More elaborate tuned circuits were used at the output of the first detector, providing controlled bandwidth and improved selectivity at a single frequency. A 1921 article in QST showed how to use a 55 kHz tuned radio frequency receiver as a super-heterodyne by adding a tube as an oscillator (LO) and one as a detector. Frequencies between 375 kHz and 2Mhz could be received.

Development

The first commercial superheterodyne receiver, the RCA Radiola AR-812, released on March 4, 1924, priced at $286 (equivalent to $5,250 in 2024). It used 6 triodes: a mixer, local oscillator, two IF and two audio amplifier stages, with an IF of 45 kHz. It was a commercial success, with better performance than competing receivers.

Armstrong put his ideas into practice, and the technique was soon adopted by the military. 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 skill required to operate it. For early domestic radios, tuned radio frequency receivers (TRF) 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, which then sold it to Radio Corporation of America (RCA), the latter monopolizing the market for superheterodyne receivers until 1930.

Because the original motivation for the superhet was the difficulty of using the triode amplifier at high frequencies, there was an advantage in using a lower intermediate frequency. During this era, many receivers used an IF frequency of only 30 kHz. These low IF frequencies, often using IF transformers based on the self-resonance of iron-core transformers, had poor image frequency rejection, but overcame the difficulty in using triodes at radio frequencies in a manner that competed favorably with the less robust neutrodyne TRF receiver. Higher IF frequencies (455 kHz was a common standard) came into use in later years, after the invention of the tetrode and pentode as amplifying tubes, largely solving the problem of image rejection. Even later, however, low IF frequencies (typically 60 kHz) were again used in the second (or third) IF stage of double or triple-conversion communications receivers to take advantage of the selectivity more easily achieved at lower IF frequencies, with image-rejection accomplished in the earlier IF stage(s) which were at a higher IF frequency.

In the 1920s, at these low frequencies, commercial IF filters looked very similar to 1920s audio interstage coupling transformers, had similar construction, and were wired up in an almost identical manner, so they were referred to as "IF transformers". By the mid-1930s, superheterodynes using much higher intermediate frequencies (typically around 440–470 kHz) used tuned transformers more similar to other RF applications. The name "IF transformer" was retained, however, now meaning "intermediate frequency". Modern receivers typically use a mixture of ceramic resonators or surface acoustic wave resonators and traditional tuned-inductor IF transformers.

The superheterodyne receiver played a significant role in the complex patents and licensing in early radio.

"All American Five" vacuum-tube superheterodyne AM broadcast receiver from 1940s was cheap to manufacture because it only required five tubes.

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 introduction of an additional grid in a vacuum tube, but before the more modern screen-grid tetrode, included the tetrode with two control grids; this tube combined the mixer and oscillator functions, 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 (with each tube stage being the main factor affecting cost in this era), this further reduced the advantage of TRF and regenerative receiver designs.

By the mid-1930s, commercial production of TRF receivers was largely replaced by superheterodyne receivers. By the 1940s, the vacuum-tube superheterodyne AM broadcast receiver was refined into a cheap-to-manufacture design called the "All American Five" because it used five vacuum tubes: usually a converter (mixer/local oscillator), an IF amplifier, a detector/audio amplifier, audio power amplifier, and a rectifier. Since this time, the superheterodyne design was used for almost all commercial radio and TV receivers.

Patent battles

The following section outlines how heterodyne reception was understood in the years leading up to the superheterodyne, followed by the associated patent claims and legal outcomes.

Early heterodyne theory and practice

The heterodyne method of reception was described by Fessenden in 1905, 1908 and 1912 patents. It was described as a way of making continuous signals audible by offsetting the frequencies of two ends of a communications link, and using a generator at each end as both a transmitter and a heterodyne source for receiving. In 1913 it was described as a method of achieving apparent signal amplification. In 1913, John L. Hogan Jr. described heterodyne reception in terms of generating beat frequencies. When asked about using heterodyning for telephony by Robert Marriott, Hogan responded that the beat frequency would be at a frequency above hearing.

By 1915, the vacuum tube used as an oscillator expanded the use of heterodyne methods. Hogan described receivers in which a single vacuum tube could simultaneously generate oscillations, detect signals, and provide amplification, while noting that the combined action of these processes made the underlying behavior difficult to analyze. Patents also described the use of locally generated oscillations to improve detection, but did not include selective intermediate-frequency stages or amplification at the converted frequency.

In a 1916 patent application, Langmuir describes using a single triode as both the oscillator and the detector. Use of a heterodyne frequency above the audio band was known, as he states "Since in most cases it will be desirable to employ frequencies beyond the range of audibility of a telephone receiver". This reflects an early recognition that the heterodyne process need not produce an audible output directly.

A central question in this period was whether heterodyne reception itself provided amplification. Benjamin Liebowitz analyzed the method mathematically in 1915 and concluded that no gain was inherent in the process itself, attributing observed effects to detector nonlinearity rather than frequency conversion alone.

Subsequent work clarified the conditions under which gain could occur. In 1917, Edwin Howard Armstrong showed that heterodyne reception could produce amplification when the detector operated in a square-law region, with conversion gain increasing as the amplitude of the local oscillator was raised until limited by tube characteristics. This result was further analyzed by G. W. O. Howe in 1918, who confirmed mathematically that gain depended on detector nonlinearity and local oscillator level. In 1919, John R. Carson provided a more general theoretical treatment of the three-element vacuum tube, showing that the apparent amplification in heterodyne reception arises from nonlinear mixing and is proportional to the amplitude of the locally generated signal within the limits of the device.

By the end of this period, the heterodyne process was understood as a form of nonlinear frequency conversion in which gain could be obtained under specific operating conditions. Conversion to super-audio frequencies had been discussed, but the use of a fixed intermediate frequency for further processing as the basis of a receiver architecture had not yet been developed. These developments form the technical background to later patent claims.

Parallel inventions

French engineer Lucien Lévy filed a patent application for the superheterodyne principle in August 1917 with brevet n° 493660. Armstrong also filed his patent in 1917. Levy filed his original disclosure about seven months before Armstrong's. German inventor Walter H. Schottky also filed a related patent in 1918.

At first the United States recognized Armstrong as the inventor, and his U.S. Patent No. 1,342,885 was issued on 8 June 1920. After various changes and court hearings Lévy was awarded U.S. patent No. 1,734,938 that included seven of the nine claims in Armstrong's application, while the two remaining claims were granted to Alexanderson of GE and Kendall of AT&T.

Later commentary distinguished between patent disclosure and practical development, and in practice the superheterodyne was adopted through cross-licensing among major manufacturers.

Contemporary technical commentary reflected a distinction between legal priority and practical development. Walter Schottky, writing in 1926, stated that Lévy’s patent described the essential elements of the method and therefore represented its origin from a patent-law perspective, while attributing the practical development of the superheterodyne receiver to Armstrong and his collaborators.

After the suicide of Armstrong in 1954, Lévy wrote "If I may be permitted, in remembering with emotion the memory of E. H. Armstrong whom I knew towards the end of the war of 1914, of deeply regretting the loss to humanity and to radio of a spirit as original and of as great value as that of his."

Principle of operation

Block diagram of a typical single-conversion superheterodyne receiver. The diagram has blocks that are common to superheterodyne receivers, with only the RF amplifier being optional. Red parts are those that handle the incoming radio frequency (RF) signal; green are parts that operate at the intermediate frequency (IF), while blue parts operate at the modulation (audio) frequency. The dotted line indicates that the local oscillator and RF filter must be tuned in tandem.

How a superheterodyne radio works. The horizontal axes are frequency f. The blue graphs show the voltage spectra of the radio signals at various points in the circuit. The red graphs show the transfer functions of the filters in the circuit; the thickness of the red bands shows the fraction of signal from the previous graph that passes through the filter at each frequency. The incoming radio signal from the antenna (top graph) consists of the desired radio signal S1 plus others at different frequencies. The RF filter (2nd graph) removes any signal such as S2 at the image frequency LO - IF, which would otherwise pass through the IF filter and interfere. The remaining composite signal is applied to the mixer along with a local oscillator signal (LO) (3rd graph). In the mixer the signal S1 combines with the LO frequency to create a heterodyne at the difference between these frequencies, the intermediate frequency (IF), at the mixer output (4th graph). This passes through the IF bandpass filter (5th graph) and then is amplified and demodulated (demodulation is not shown). The unwanted signals create heterodynes at other frequencies (4th graph), which are filtered out by the IF filter .

The antenna collects the radio signal. The tuned RF stage with optional RF amplifier provides some initial selectivity; it is necessary to suppress the image frequency, and may also serve to prevent strong out-of-passband signals from saturating the initial amplifier. A local oscillator provides the mixing frequency; it is usually a variable frequency oscillator which is used to tune the receiver to different stations. The frequency mixer does the actual heterodyning that gives the superheterodyne its name; it changes the incoming radio frequency signal to a higher or lower, fixed, intermediate frequency (IF). The IF band-pass filter and amplifier supply most of the gain and the narrowband filtering for the radio. The demodulator extracts the audio or other modulation from the IF radio frequency. The extracted signal is then amplified by the audio amplifier.

Circuit description

To receive a radio signal, a suitable antenna is required. 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 tuned circuits at this stage block frequencies that are far removed from the intended reception frequency. 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. The tuning of one (or more) tuned circuits in the RF stage must track the tuning of the local oscillator.

Local oscillator and mixer

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 frequency signals, each one containing the modulation in the desired signal. The output of the mixer may include the original RF signal at f_RF, the local oscillator signal at f_LO, and the two new heterodyne frequencies f_RF + f_LO and f_RF − f_LO. The mixer may inadvertently produce additional frequencies such as third- and higher-order intermodulation products. Ideally, the IF bandpass filter removes all but the desired IF signal at f_IF. The IF signal contains the original modulation (transmitted information) that the received radio signal had at f_RF.

The frequency of the local oscillator f_LO is set so the desired reception radio frequency f_RF mixes to f_IF. There are two choices for the local oscillator frequency because of the correspondence between positive and negative frequencies. If the local oscillator frequency is less than the desired reception frequency, it is called low-side injection (f_IF = f_RF − f_LO); if the local oscillator is higher, then it is called high-side injection (f_IF = f_LO − f_RF).

The mixer will process not only the desired input signal at f_RF, but also all signals present at its inputs. 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 tuned amplifier; that gives the superheterodyne receiver its superior performance. However, if f_LO is set to f_RF + f_IF, then an incoming radio signal at f_LO + f_IF will also produce a heterodyne at f_IF; the frequency f_LO + f_IF is called the image frequency and must be rejected by the tuned circuits in the RF stage. The image frequency is 2 f_IF higher (or lower) than the desired frequency f_RF, so employing a higher IF frequency f_IF increases the receiver's image rejection 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" each other. 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 than its IF center frequency. For example, a typical AM broadcast band receiver covers 510 kHz to 1655 kHz (a roughly 1160 kHz input band) with a 455 kHz IF frequency; an FM broadcast band receiver covers 88 MHz to 108 MHz band with a 10.7 MHz IF frequency. In that situation, the RF amplifier must be tuned so the IF amplifier does not see two stations at the same time. If the AM broadcast 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 stations that are twice the IF frequency away are significantly attenuated. The tracking can be done with a multi-section variable capacitor or some varactors driven by a common control voltage. An RF amplifier may have tuned circuits at both its input and its output, so three or more tuned circuits may be tracked. In practice, the RF and LO frequencies need to track closely but not perfectly.

In the days of tube (valve) electronics, it was common for superheterodyne receivers to combine the functions of the local oscillator and the mixer in a single tube, leading to a savings in power, size, and especially cost. A single pentagrid converter tube would oscillate and also provide signal amplification as well as frequency mixing.

The mixer tube or transistor is sometimes called the first detector, while the demodulator that extracts the modulation from the IF signal is called the second detector. In a dual-conversion superhet there are two mixers, so the demodulator is called the third detector.

IF amplifier

The stages of an intermediate frequency amplifier ("IF amplifier" or "IF strip") are tuned to a fixed frequency that does not change as the receiving frequency changes. The fixed frequency simplifies optimization of the IF amplifier. The IF amplifier is selective around its center frequency f_IF. The fixed center frequency allows the stages of the IF amplifier to be carefully tuned for best performance (this tuning is called "aligning" the IF amplifier). If the center frequency changed with the receiving frequency, then the IF stages would have had to track their tuning. That is not the case with the superheterodyne.

Normally, the IF center frequency f_IF is chosen to be less than the range of desired reception frequencies f_RF. That is because it is easier and less expensive to get high selectivity at a lower frequency using tuned circuits. The bandwidth of a tuned circuit with a certain Q is proportional to the frequency itself (and what's more, a higher Q is achievable at lower frequencies), so fewer IF filter stages are required to achieve the same selectivity. Also, it is easier and less expensive to get high gain at a lower frequencies.

However, in many modern receivers designed for reception over a wide frequency range (e.g. scanners and spectrum analyzers) a first IF frequency higher than the reception frequency is employed in a double conversion configuration. For instance, the Rohde & Schwarz EK-070 VLF/HF receiver covers 10 kHz to 30 MHz. It has a band switched RF filter and mixes the input to a first IF of 81.4 MHz and a second IF frequency of 1.4 MHz. The first LO frequency is 81.4 to 111.4 MHz, a reasonable range for an oscillator. But if the original RF range of the receiver were to be converted directly to the 1.4 MHz intermediate frequency, the LO frequency would need to cover 1.4-31.4 MHz which cannot be accomplished using tuned circuits (a variable capacitor with a fixed inductor would need a capacitance range of 500:1). Image rejection is never an issue with such a high IF frequency. The first IF stage uses a crystal filter with a 12 kHz bandwidth. There is a second frequency conversion (making a triple-conversion receiver) that mixes the 81.4 MHz first IF with 80 MHz to create a 1.4 MHz second IF. Image rejection for the second IF is not an issue as the first IF has a bandwidth of much less than 2.8 MHz.

To avoid interference to receivers, licensing authorities will avoid assigning common IF frequencies to transmitting stations. Standard intermediate frequencies used are 455 kHz for medium-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 costs associated with these components, most manufacturers then tended to design their receivers around a fixed range of frequencies offered, which resulted in a worldwide de facto standardization of intermediate frequencies.

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. This is also called a Q multiplier, involving a small modification to an existing receiver especially for the purpose of increasing selectivity.

IF bandpass filter

The IF stage includes a filter and/or multiple tuned circuits to achieve the desired selectivity. This filtering must 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, a quartz crystal filter, or a multipole ceramic crystal filter.

In the case of television receivers, no other technique was able to produce the precise bandpass characteristic needed for vestigial sideband reception, such as that used in the NTSC system first approved by the US in 1941. By the 1980s, multi-component capacitor-inductor filters had been replaced with precision electromechanical surface acoustic wave (SAW) filters. Fabricated by precision laser milling techniques, SAW filters are cheaper to produce, can be made to extremely close tolerances, and are very stable in operation.

Demodulator

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 envelope detection, which can be achieved by means of rectification and a low-pass filter (which can be as simple as an RC circuit) to remove remnants of the intermediate frequency. FM signals may be detected using a discriminator, ratio detector, or phase-locked loop. Continuous wave 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. 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.

Multiple conversion

Double conversion superheterodyne receiver block diagram

To overcome obstacles such as image response, some receivers use multiple successive stages of frequency conversion and multiple IFs of different values. A receiver with two frequency conversions and IFs is called a dual conversion superheterodyne, and one with three IFs is called a triple conversion superheterodyne.

The main reason that this is done is that with a single IF there is a tradeoff between low image response and selectivity. The separation between the received frequency and the image frequency is equal to twice the IF frequency, so the higher the IF, the easier it is to design an RF filter to remove the image frequency from the input and achieve low image response. However, the higher the IF, the more difficult it is to achieve high selectivity in the IF filter. At shortwave frequencies and above, the difficulty in obtaining sufficient selectivity in the tuning with the high IFs needed for low image response impacts performance. To solve this problem two IF frequencies can be used, first converting the input frequency to a high IF to achieve low image response, and then converting this frequency to a low IF to achieve good selectivity in the second IF filter. To improve tuning, a third IF can be used.

For example, for a receiver that can tune from 500 kHz to 30 MHz, three frequency converters might be used. With a 455 kHz IF it is easy to get adequate front end selectivity with broadcast band (under 1600 kHz) signals. For example, if the station being received is on 600 kHz, the local oscillator can be set to 1055 kHz, giving an image on (-600+1055=) 455 kHz. But a station on 1510 kHz could also potentially produce an image at (1510-1055=) 455 kHz and so cause image interference. However, because 600 kHz and 1510 kHz are so far apart, 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 a 455 kHz IF, but a station on 30.910 would also produce a 455 kHz beat, so both stations would be heard at the same time. But it is virtually impossible to design an RF tuned circuit that can adequately discriminate between 30 MHz and 30.91 MHz, so one approach is to "bulk downconvert" 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 to 2 MHz to 3 MHz, there they can be tuned more conveniently. This is often done by first converting each "block" up to a higher frequency (typically 40 MHz) and then using a second mixer to convert it down to the 2 MHz to 3 MHz range. The 2 MHz to 3 MHz "IF" is basically another self-contained superheterodyne receiver, most likely with a standard IF of 455 kHz.

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

Advantages and disadvantages

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) that 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 or similar technologies that cannot be tuned. 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, there are a few drawbacks that need to be tackled in practice.

Image frequency (f_IMAGE)

Graphs illustrating the problem of image response in a superheterodyne. The horizontal axes are frequency and the vertical axes are voltage. Without an adequate RF filter, any signal S2 (green) at the image frequency $f_{\text{IMAGE}}$ is also heterodyned to the IF frequency $f_{\text{IF}}$ along with the desired radio signal S1 (blue) at $f_{\text{RF}}$, so they both pass through the IF filter (red). Thus S2 interferes with S1.

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 (or minus) twice the intermediate frequency. The image frequency results in two stations being received at the same time, thus producing interference. Reception at the image frequency can be combated through tuning (filtering) at the antenna and RF stage of the superheterodyne receiver.

$f_{\mathrm{I}\mathrm{M}\mathrm{A}\mathrm{G}\mathrm{E}}=\{\begin{array}{ll}{f}_{\mathrm{R}\mathrm{F}}+2f_{\mathrm{I}\mathrm{F}},& \text{if }{f}_{\mathrm{L}\mathrm{O}}>f_{\mathrm{R}\mathrm{F}}\text{ (high side injection)}\\ f_{\mathrm{R}\mathrm{F}}-2f_{\mathrm{I}\mathrm{F}},& \text{if }{f}_{\mathrm{L}\mathrm{O}}<f_{\mathrm{R}\mathrm{F}}\text{ (low side injection)}\end{array}$

For example, an AM broadcast station at 580 kHz is tuned on a receiver with a 455 kHz IF. The local oscillator is tuned to 580 + 455 = 1035 kHz. But a signal at 580 + (2 × 455) = 1490 kHz is also 455 kHz away from the local oscillator; so both the desired signal and the image, when mixed with the local oscillator, will 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.

The unwanted frequency is called the image of the wanted frequency, because it is the "mirror image" of the desired frequency reflected about $f_{LO}$. A receiver with inadequate filtering at its input will pick up signals at two different frequencies simultaneously: the desired frequency and the image frequency. A radio reception which happens to be at the image frequency can interfere with reception of the desired signal, and noise (static) around the image frequency can decrease the receiver's signal-to-noise ratio (SNR) by up to 3dB.

Early Autodyne receivers typically used IFs of only 150 kHz or so. As a consequence, most Autodyne receivers required greater front-end selectivity, often involving double-tuned coils, to avoid image interference. With the later development of tubes able to amplify well at higher frequencies, higher IF frequencies came into use, reducing the problem of image interference. Typical consumer radio receivers have only a single tuned circuit in the RF stage.

Sensitivity to the image frequency can be minimized only by (1) a filter that precedes the mixer or (2) a more complex mixer circuit to suppress the image; this is rarely used. In most tunable receivers using a single IF frequency, the RF stage includes at least one tuned circuit in the RF front end whose tuning is performed in tandem with the local oscillator. In double (or triple) conversion receivers in which the first conversion uses a fixed local oscillator, this may rather be a fixed bandpass filter which accommodates the frequency range being mapped to the first IF frequency range.

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 $2f_{\mathrm{I}\mathrm{F}}$, a higher intermediate frequency improves image rejection. It may be possible to use a high enough first IF that a fixed-tuned RF stage can reject any image signals.

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 decibels) 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 can be difficult to keep stray radiation from the local oscillator below the level that a nearby receiver can detect. If the receiver's local oscillator can reach the antenna it will act as a low-power CW transmitter. Consequently, what is meant to be a receiver can itself create radio interference.

In intelligence operations, local oscillator radiation gives a means to detect a covert receiver and its operating frequency. The method was used by MI5 during Operation RAFTER. This same technique is also used in radar detector detectors used by traffic police in jurisdictions where radar detectors are illegal.

Local oscillator radiation is most prominent in receivers in which the antenna signal is connected directly to the mixer (which itself receives the local oscillator signal) rather than from receivers in which an RF amplifier stage is used in between. Thus it is more of a problem with inexpensive receivers and with receivers at such high frequencies (especially microwave) where RF amplifying stages are difficult to implement.

Local oscillator sideband noise

Local oscillators typically generate a single frequency signal that has negligible amplitude modulation but some random phase modulation which 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 minimize oscillator phase noise, usually by ensuring{{dubious|date=July 2021} r enters a non-linear mode.

See also

• Automatic gain control
• Demodulator
• Direct conversion receiver
• H2X radar
• Optical heterodyne detection – Information method in electromagnetic radiation
• Reflex receiver
• RF front end
• Single sideband
• Superheterodyne transmitter
• Tuned radio frequency receiver
• VFO

Further reading

• Whitaker, Jerry (1996). The Electronics Handbook. CRC Press. pp. 1172. ISBN 0-8493-8345-5. 
• Fessenden, Reginald A., "Wireless Signaling", US patent 706740, published September 28, 1901, issued August 12, 1902
• Fessenden, Reginald A., "Electric Signaling Apparatus", US patent 1050441, published July 27, 1905, issued January 14, 1913
• Fessenden, Reginald A., "Method of Signaling", US patent 1050728, published August 21, 1906, issued January 14, 1913
• Witts, Alfred T. (1936). The Superheterodyne Receiver (2nd ed.). London, UK: Sir Isaac Pitman & Sons. https://archive.org/details/TheSuperheterodyneReceiver. 

External links

• Douglas, Alan (November 1990). "Who Invented the Superheterodyne?". Proceedings of the Radio Club of America 64 (3): 123–142. http://antiqueradios.com/superhet/. . An article giving the history of the various inventors working on the superheterodyne method.
• Hogan, John L. Jr. (September 1915). "Developments of the Heterodyne Receiver". Proceedings of the IRE 3 (3): 249–260. doi:10.1109/jrproc.1915.216679. Bibcode: 1915PIRE....3..249H. https://books.google.com/books?id=AF0AAAAAMAAJ&pg=PA249. 
• Champeix (March–April 1979). "Qui a Inventé le Superhétérodyne?" (in fr). La Liaison des Transmissions 116. 
  Champeix (April–May 1979). "Qui a Inventé le Superhétérodyne?" (in fr). La Liaison des Transmissions 117.  Raises Paul Laüt published six months before Lévy; Étienne published the memo.
• Schottky, Walter H. (October 1926). "On the Origin of the Super-Heterodyne Method". Proceedings of the I.R.E. 14 (5): 695–698. doi:10.1109/JRPROC.1926.221074. ISSN 0731-5996. Bibcode: 1926PIRE...14..695S. 
• Morse, A. M. (July 31, 1925). "needed". Electrician.  Describes English efforts.
• 29F(2d)953. Armstrong v. Lévy, decided Dec. 3, 1928 http://www.leagle.com/decision/192898229F2d953_1614/ARMSTRONG%20v.%20LEVY
• An in-depth introduction to superheterodyne receivers
• Superheterodyne receivers from microwaves101.com
• Multipage tutorial describing the superheterodyne receiver and its technology