Superheterodyne receiver

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The superheterodyne receiver was invented by Edwin Armstrong. It superseded the tuned radio frequency receiver TRF also invented by Edwin Armstrong.

The superheterodyne principle as used in radio receivers allows certain obstacles to high performance radio design to be overcome. 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.

In radios using the principle, all signal frequencies are converted to a constant frequency before detection. This constant frequency is called the Intermediate Frequency, or IF.

Superheterodyne receivers subtract a set frequency from an input signal. The user tuned the radio by adjusting the set frequency. The resulting lower IF frequency was then amplified and demodulated. Almost all receivers in use today utilize this method.

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.

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 Superheterodyne".

Superheterodyne receivers have superior characteristics in both frequency stability and selectivity. It is much easier to stabilize an oscillator than a filter, especially with modern frequency synthesizer technology, and IF filters can give much narrower passbands at the same Q factor than an equivalent RF filter.

Radio transmitters also use superheterodyne technology. The design of a superheterodyne transmitter is similar to that of a superheterodyne receiver which each stage of the signal path reversed.

The next evolution of superheterodyne receiver design is the software defined radio architecture, where the IF processing after the initial IF filter is implemented in software.