Superheterodyne receiver

From Wikipedia, the free encyclopedia

Jump to: navigation, search
 This article needs additional citations for verification. Please help improve this article by adding reliable references (ideally, using inline citations). Unsourced material may be challenged and removed. (December 2008)

In electronics, the superheterodyne receiver (also known as the supersonic heterodyne receiver, or by the abbreviated form superhet) is a receiver which uses the principle of frequency mixing or heterodyning to convert the received signal to a lower (sometimes higher) "intermediate" frequency, which can be more conveniently processed than the original carrier frequency. Virtually all modern radio and TV receivers use the Superheterodyne principle.

Contents

[edit] History

The word heterodyne is derived from the Greek roots hetero- "different", and -dyne "power". The original heterodyne technique was pioneered by Canadian inventor-engineer Reginald Fessenden but was not pursued very far because local oscillators were not very stable at the time.[1]

Later, the superheterodyne (superhet) principle was conceived in 1918 by Edwin Armstrong during World War I, as a means of overcoming the deficiencies of early vacuum triodes used as high-frequency amplifiers in radio direction finding (RDF) equipment. Unlike simple radio communication, which only needs to make transmitted signals audible, RDF requires actual measurements of received signal strength, which necessitates linear amplification of the actual carrier wave.

In a triode RF amplifier, if both the plate 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 had realized that if RDF could be operated at a higher frequency, it would allow detection of enemy shipping much more effectively, but at the time, no practical "short wave" amplifier existed, (defined then as any frequency above 500 kHz) due to the limitations of triodes of the day.

A "heterodyne" refers a beat or "difference" frequency produced when two or more radio frequency carrier waves are fed to a detector. The term was originally coined by Canadian Engineer Reginald Fessenden describing his proposed method of making Morse Code transmissions from an Alexanderson Alternator type transmitter audible. With the Spark Transmitters then in wide 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.

The signal from an Alexanderson Alternator on the other hand, 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 3kHz higher than the other. In the receiver's detector the two carriers would beat together to produce a 3kHz tone and so in the headphones the morse signals would then be heard as a series of 3kHz 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" or BFO. As the BFO frequency was varied, the pitch of the heterodyne could be heard to vary with it. If the frequences were too far apart the heterodyne became supersonic and hence no longert 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, for example, 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 moduation, but on a lower carrier frequency. To monitor a frequency of 1500 kHz for example, he could set up an oscillator to say, 1560 kHz, which would produce a heterodyne of 60kHz, a frequency that could then be much more conveniently amplified by the triodes of the day. He termed this the "Intermediate Frequency" often abbreviated to "IF"

Early Superheterodyne receivers actually used IFs as low as 20 kHz, often based around 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 triodes could be made to do work that formerly required dozens or even hundreds.

1920s commercial IF transformers actually look very similar to 1920s audio interstage coupling transformers, and were wired up in an almost identical manner. By the mid-1930s superhets were using much higher intermediate frequencies, (typically around 440-470kHz), using tuned coils very similar in construction to the aerial and oscillator coils. However the term "Intermediate Frequency Transformer" or "IFT" still persists to this day.

Modern receivers typically use a mixture of Ceramic Filters and/or Saw Resonators as well as traditional tuned-inductor IF transformers

Armstrong was able to put his ideas into practice quite quickly, and the technique was rapidly adopted by the military. However, it was less popular when commercial radio broadcasting began in the 1920s. There were many factors involved,but the main issues were 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 RFs ("TRF"), also called the Neutrodyne, were much 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.[2]

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.

First, the development of practical indirectly-heated-cathode tubes allowed the mixer and oscillator functions to be combined in a single Pentode tube, in the so-called Autodyne mixer. This was rapidly followed by the introduction of low-cost multi-element tubes specifically designed for superheterodyne operation. These allowed the use of much higher Intermediate Frequencies (typically around 440-470kHz) which eliminated the problem of image frequency interference. By the mid-30s, for commercial receiver production the TRF technique was obsolete.

The superheterodyne principle was eventually taken up for virtually all commercial radio and TV designs.

[edit] Overview

The superhet receiver consists of three principle parts, the local oscillator, a mixer that mixes the local oscillator's signal with the received signal, and a tuned amplifier.

Reception starts with an antenna signal, optionally amplified, including the frequency the user wishes to tune, fd. The local oscillator is tuned to produce a frequency close to fd, fLO. The received signal is mixed with the local oscillator's, producing four frequencies in the output; the original signal, the original fLO, and the two new frequencies fd+fLO and fd-fLO. The output signal also generally contains a number of undesirable mixtures as well. (These are 3rd- and higher-order intermodulation products. If the mixing were performed as a pure, ideal multiplication, the original fd and fLO would also not appear; in practice they do appear because mixing is done by a nonlinear process that only approximates true ideal multiplication.)

The amplifier portion of the system is tuned to be highly selective at a single frequency, fIF. By changing fLO, the resulting fd-fLO (or fd+fLO) signal can be tuned to the amplifier's fIF. In typical amplitude modulation ("AM radio" in the U.S., or MW) receivers, that frequency is 455 kHz; for FM receivers, it is usually 10.7 MHz; for television, 45 MHz. Other signals from the mixed output of the heterodyne are filtered out by the amplifier.

[edit] Design and its evolution

The diagram below shows the basic elements of a single conversion superhet receiver. The essential elements of a local oscillator and a mixer followed by a fixed-tuned filter and IF amplifier are common to all superhet circuits. Cost-optimized designs may use one active device for both local oscillator and mixer—this is sometimes called a "converter" stage. One such example is the pentagrid converter.

The advantage to this method is that most of the radio's signal path has to be sensitive to only a narrow range of frequencies. Only the front end (the part before the frequency converter stage) needs to be sensitive to a wide frequency range. For example, the front end might need to be sensitive to 1–30 MHz, while the rest of the radio might need to be sensitive only to 455 kHz, a typical IF. Only one or two tuned stages need to be adjusted to track over the tuning range of the receiver; all the intermediate-frequency stages operate at a fixed frequency which need not be adjusted.

To overcome obstacles such as image response, multiple IF stages are used, and in some case multiple stages with two IFs of different values. 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 Super Heterodyne"—a common example is a television receiver where the audio information is obtained from a second stage of intermediate frequency conversion. Occasionally special-purpose receivers will use an intermediate frequency much higher than the signal, in order to obtain very high image rejection.

Superheterodyne receivers have superior characteristics to simpler receiver types in frequency stability and selectivity. They offer much better stability than Tuned radio frequency receivers (TRF) because a tuneable oscillator is more easily stabilized than a tuneable amplifier, especially with modern frequency synthesizer technology. IF filters can give much narrower passbands at the same Q factor than an equivalent RF filter. A fixed IF also allows the use of a crystal filter when exceptionally high selectivity is necessary. Regenerative and super-regenerative receivers offer better sensitivity than a TRF receiver, but suffer from stability and selectivity problems.

In the case of modern television receivers, no other technique was able to produce the precise bandpass characteristic needed for vestigial sideband reception, first used with the original NTSC system introduced in 1941. This originally involved a complex collection of tuneable inductors which needed careful adjustment, but since the early 1980s these have been replaced with precision electromechanical surface acoustic wave (SAW) filters. Fabricated by precision laser milling techniques, SAW filters are much cheaper to produce, can be made to extremely close tolerances, and are extremely stable in operation.

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 where the necessary microprocessor is already present in the system.

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

[edit] Drawbacks

Drawbacks to the superheterodyne receiver include interference from signal frequencies close to the intermediate frequency. To prevent this, IF frequencies are generally controlled by regulatory authorities, and this is the reason most receivers use common IFs. Examples are 455 kHz for AM radio, 10.7 MHz for FM, and 38.9 MHz (Europe) 45 MHz (US) for television.

(For AM radio, a variety of IFs have been used, but most of the Western World settled on 455kHz, in large part because of the almost universal transition to Japanese-made ceramic resonators which used the US standard of 455kHz. In more recent digitally tuned receivers, this was changed to 450kHz as this figure simplifies the design of the synthesizer circuitry).

Additionally, in urban environments with many strong signals, the signals from multiple transmitters may combine in the mixer stage to interfere with the desired signal.

[edit] High-side and low-side injection

The amount that a signal is down-shifted by the local oscillator depends on whether its frequency f is higher or lower than fLO. That is because its new frequency is |f − fLO| in either case. Therefore, there are potentially two signals that could both shift to the same fIF one at f = fLO + fIF and another at f = fLO − fIF. One or the other of those signals, called the image frequency, has to be filtered out prior to the mixer to avoid aliasing. When the upper one is filtered out, it is called high-side injection, because fLO is above the frequency of the received signal. The other case is called low-side injection. High-side injection also reverses the order of a signal's frequency components. Whether or not that actually changes the signal depends on whether it has spectral symmetry or not. The reversal can be undone later in the receiver, if necessary.

[edit] Image Frequency (fimage)

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.

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

Early Autodyne receivers typically used IFs of only 150kHz 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 especialy designed for oscillator/mixer use, which were able work reliably with much higher IFs, reducing th eproblem of image interference and so allowing simpler and cheaper aerial tuning circuitry.


[edit] Local oscillator radiation

It is difficult to keep stray radiation from the local oscillator below the level that a nearby receiver can detect. 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.

Further information: Electromagnetic compatibility

[edit] 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 minimize osicllator phase noise, usually by ensuring that the oscillator never enters a non-linear mode.

[edit] See also

[edit] References

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

[edit] Footnotes

  1. ^ Nahin, Paul. The Science of Radio. Page 91. Figure 7.10. Chapter 7. ISBN 0-387-95150-4.
  2. ^ Katz, Eugenii. "Edwin Howard Armstrong". History of electrochemistry, electricity, and electronics. Eugenii Katz homepage, Hebrew Univ. of Jerusalem. http://www.geocities.com/neveyaakov/electro_science/armstrong.html. Retrieved on 2008-05-10. 

[edit] External links

Sister project Wikimedia Commons has media related to: Radio electronic diagrams

Retrieved from "http://en.wikipedia.org/wiki/Superheterodyne_receiver"
Personal tools