Superheterodyne receiver
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The superheterodyne receiver (or to give it its full name, the supersonic heterodyne receiver – usually these days shortened to superhet) was invented by Edwin Armstrong in 1918.
The super heterodyne principle, as used in radio receivers, allows certain obstacles in high-performance radio design to be overcome. Tuned radio frequency (TRF) receivers suffered from poor frequency stability, and poor selectivity, as 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 radios using the 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 "beat" or heterodyne a frequency 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 a mixer stage of the receiver, the local oscillator signal multiplies with the incoming signal, producing beat frequencies both above and below the incoming signal. The mixer stage produces outputs at both the sum of the two input frequencies and at the difference. Either the higher or the lower (typically) is chosen as the IF, which is amplified and then demodulated (reduced to just audio frequencies through a speaker).
Almost all receivers in use today utilize 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.
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.
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.
Radio transmitters may also use a mixer stage to produce an output frequency.
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.
See also
- H2X radar
- Automatic gain control
- Demodulator
- Direct conversion receiver
- VFO
- Single sideband#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 illustrationsda:Superheterodynmodtager
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