Superheterodyne receiver

File:5tubes-radio.jpg

A 5-tube superheterodyne receiver made in Japan around 1955

In electronics, a superheterodyne receiver (sometimes shortened to superhet) uses 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

File:Tuning capacitor.jpg

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, but it was not pursued far because local oscillators available at the time were unstable in their frequency output.

The superheterodyne principle was revisited in 1918 by U.S. Army Major Edwin Armstrong in France during World War I. He invented this receiver as a means of overcoming the deficiencies of early vacuum tube triodes 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. 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 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. 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 transmitters 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 kHz higher than the other. In the receiver's detector the two carriers would beat together to produce a 3 kHz tone thus in the headphones the morse signals would then be heard as a series of 3 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 ultrasonic and hence no longer audible.

It had been noticed some time before that if a 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 kHz and the oscillating receiver was set to 400 kHz, the station would be heard not 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 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 that could 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 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.

Early superheterodyne receivers used IFs as low as 20 kHz, often based on the self-resonance of iron-cored transformers. 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 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 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 receivers ("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, who then sold it to RCA, the latter monopolizing the market for superheterodyne receivers until 1930.

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 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 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).

File:Superhet2.png

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: a receiving 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 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 tuned circuits 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. 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 frequencies signals, each one containing the modulation contained in the desired signal. The output of the mixer may include the original RF signal at f_d, the local oscillator signal at f_LO, and the two new frequencies f_d+f_LO and f_d-f_LO. 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_IF which contains the original modulation (transmitted information) as the received radio signal had at f_d.

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 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. The IF amplifier (or IF strip) can be made highly selective around its center frequency f_IF, 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_LO, the resulting difference frequency f_LO - f_d (or f_d-f_LO when using so-called low-side injection) will be matched to the IF amplifier's frequency f_IF for the desired reception frequency f_d. One section of the tuning capacitor will thus adjust the local oscillator's frequency f_LO to f_d + f_IF (or. less often, to f_d - f_IF) while the RF stage is tuned to f_d. Engineering the multi-section tuning capacitor (or varactors) 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 filtered out in the IF stage, giving the superheterodyne receiver its superior performance. However, if f_LO is set to f_d + f_IF , then an incoming radio signal at f_LO + f_IF will also produce a heterodyne at f_IF; this is called the image frequency and must be rejected by the tuned circuits in the RF stage. The image frequency is 2f_IF higher (or lower) than f_d, so employing a higher IF frequency f_IF 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_d, 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 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.

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.

Bandpass filter

The IF stage includes a filter and/or multiple tuned circuits in order to achieve the desired 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.

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 of the RF signal (so-called envelope detection), and a simple RC low pass filter to remove remnants of the intermediate frequency. 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. 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 1–30 MHz, the first half of the radio to 5 MHz, and the last half to 50 kHz. Two frequency converters would be used, and the radio would be a double conversion superheterodyne; 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. scanners) may use an intermediate frequency higher than the signal, in order to improve image rejection.

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, and the PAL system approved by the BBC in 1957. This originally involved a complex collection of tuneable inductors which needed careful adjustment, but since the 1970s or early 1980s these have 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 stable in operation. To avoid 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 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 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 or similar technologies which 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, we note a few drawbacks which need to be tackled in practice.

Image frequency (f_image)

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 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+455+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 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 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 [1] 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.

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 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 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 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 information: 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 modulation (demodulation)
• Tuned radio frequency receiver
• Reflectional receiver
• Beat frequency
• Heterodyne
• Optical heterodyne detection
• Infradyne - superheterodyne with IF higher than signal frequency
• Superheterodyne transmitter

Further reading

Whitaker, Jerry (1996). The Electronics Handbook. CRC Press. p. 1172. ISBN 08-493834-55. 

External links

• Who Invented the Superheterodyne? An article giving the history of the various inventors working on the superheterodyne method.
• An in-depth introduction to superheterodyne receivers
• Superheterodyne receivers from microwaves101.com

bg:Суперхетеродинен радиоприемник ca:Receptor superheterodí cs:Superheterodyn da:Superheterodynmodtager de:Überlagerungsempfänger es:Receptor superheterodino fa:گیرنده سوپرهترودین fr:Récepteur superhétérodyne hi:सुपरहेटरोडाइन संग्राही it:Supereterodina he:מקלט סופרהטרודין lt:Superheterodinas nl:Superheterodyne ja:スーパーヘテロダイン受信機 no:Superheterodynmottaker pl:Superheterodyna pt:Super-heteródino ru:Супергетеродинный радиоприёмник sv:Superheterodynmottagare

zh:超外差接收机