Superheterodyne receiver

The superheterodyne receiver (or to give it its full name, the supersonic heterodyne receiver – often abbreviated superhet) was invented by Edwin Armstrong in 1918.

The superheterodyne receiver principle overcomes certain limitations of previous receiver designs. Tuned radio frequency (TRF) receivers suffered from poor selectivity, since even filters with a high Q factor have a wide bandwidth at radio frequencies. Regenerative and super-regenerative receivers offer better sensitivity but suffer from stability and selectivity problems.

In receivers using the superheterodyne principle, all signal frequencies are converted typically to a constant lower frequency before detection. This constant frequency is called the intermediate frequency, or IF. In typical AM (Medium Wave) home receivers, that frequency is 455 kHz; for FM VHF receivers, it is usually 10.7 MHz.

Heterodyne receivers mix a signal from a local oscillator (within the receiver) with all the incoming signals. The user tunes the radio by adjusting the set's oscillator frequency. In the mixer stage of a receiver, the local oscillator signal multiplies with the incoming signal, producing beat frequencies at both the sum of the two input frequencies and at the difference. The signal at the difference frequency is passed on by tuned circuits, amplified, and then demodulated to recover the original audio signal.

Most receivers now use this method. The diagram below shows the basic elements of a single conversion superhet receiver. In practice not every design will have all these elements, nor does this convey the complexity of other designs, but the essential elements of a local oscillator and a mixer followed by a 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 frequency. 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.

Sometimes, to overcome obstacles such as image response, more than one IF is used. In such a case, 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.

Super Heterodyne receivers have superior characteristics to simpler receiver types in frequency stability and selectivity. It is much easier to stabilize an oscillator than a filter, especially with modern frequency synthesiser technology, and 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 in very critical designs such as radiotelephone receivers which have exceptionally high selectivity.

In the case of modern television receivers, no other technique was able to produce the precise bandpass characteristic needed for vestigal 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" (or "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.

The next evolution of Super Heterodyne receiver design is the software defined radio architecture, where the IF processing after the initial IF filter is implemented in software. This technique is already in use in the latest design analog television receivers and digital set top boxes, where there are no coils or other resonant circuits used at all. The antenna simply connects via a small capacitor to a pin on an integrated circuit and all the signal processing is carried out digitally. Similar techniques are used in the tiny FM radios incorporated into Mobile phones and MP3 players.

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

Drawbacks to the superheterodyne receiver include the cost of the mixer and local oscillator stages. Receivers become vulnerable to interference from signals other than the desired signal. A strong signal at the intermediate frequency may overcome the desired signal; regulatory authorities will prevent licensed transmitters from operating on these frequencies. In urban environments with many strong signals, the signals from multiple transmitters may combine in the mixer stage to interfere with the desired signal. A superheterodyne receiver may pick up a so-called "image frequency" signal that also produces a mixer output at the desired intermediate frequency; this phenomenon is sometimes used for scanner reception of transmissions outside of the receiver's official capabilities.

History

The superheterodyne principle was originally conceived by Edwin Armstrong during World War 1 as a means of overcoming the deficiencies of early vacuum triodes used as high-frequency amplifiers in radio direction finding (RDF) equipment. 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 of low-gain triode stages sometimes had to be connected in cascade to make workable designs, which drew enormous amounts of power in operation. However the strategic value was so high that British Admiralty felt it was money well spent.

Armstrong had realized that higher frequency equipment would allow them to detect enemy shipping much more effectively, but at the time no practical "short wave" amplifier existed. (In those days "Short Wave" meant anything above 500KHz)

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 to those they were actually transmitted on. Armstrong (and others) soon realized that this was caused by a "supersonic" heterodyne (or beat) between the station's carrier frequency and the oscillator frequency.

If a station was transmitting on 300kHz for example, and the oscillator was set to 400kHz, as well as the original 300kHz, the same station would be also heard on 100kHz and 700kHz.

In a flash of insight, Armstrong suddenly realized that this was a potential solution to the "short Wave" amplification problem. To monitor a frequency of 1500kHz, he could set up an oscillator to say, 1560kHz, which would down-convert the signal to a 60kHz carrier, which was far more amenable to high gain amplification using triodes.

He 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 radio broadcasting began in the 1920s, due both to the need for an extra tube for the oscillator, and the amount of technical knowledge required to operate it. For domestic radios, an alternative approach to Short Wave "Tuned RF" ("TRF") amplification called the Neutrodyne became more popular for reasons of simplicity and economy.

However by the 1930s, improvements in vacuum tube technology rapidly eroded these advantages. First, the development of practical indirectly-heated cathodes 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 and by the mid-30s the TRF technique was rendered obsolete. Just about all radio receivers (including the receiver sections of television sets), now use the superheterodyne principle.

See also

• H2X radar
• Automatic gain control
• Demodulator
• Direct conversion receiver
• VFO
• Single sideband modulation (demodulation)
• Directly amplifying receiver
• Reflectional receiver

External links

• Radio Receiver Technology A selection of articles describing various aspects of the superhet radio
• Who Invented the Superheterodyne? An article giving the history of the various inventors working on the superheterodyne method.
• Introduction to Communication Systems - A Multimedia workbook chapter on analog carrier modulation; excellent illustrations