Superheterodyne Receiver

The Supersonic Heterodyne receiver, or Superheterodyne receiver uses frequency mixing to convert a received signal to a fixed intermediate frequency (IF) which can be more conveniently processed than the original carrier frequency. At the cost of an extra frequency converter stage, the superheterodyne receiver provides superior selectivity and sensitivity compared with simpler designs. Superheterodyne receivers have better performance because the components can be optimized to work a single intermediate frequency, and can take advantage of arithmetic selectivity.

So, the incoming radio signal is mixed with a local oscillator to produce sum and difference frequency components. The lower frequency difference component called the intermediate frequency (IF), is separated from the other components by fixed tuned amplifier stages set to the intermediate frequency. The tuning of the local oscillator is mechanically ganged to the tuning of the signal circuit or radio frequency (RF) stages so that the difference intermediate frequency is always the same fixed value.

How it works

The antenna collects the radio signal. The tuned RF stage with optional RF amplifier provides some initial selectivity and prevent strong out-of-passband signals from saturating the initial amplifier. A local oscillator provides the mixing frequency. The oscillator is typically a variable frequency oscillator which is used to tune the receiver to different stations. The frequency mixer then changes the incoming radio frequency signal to a higher or lower, fixed, intermediate frequency (IF). The IF band-pass filter and amplifier supply most of the gain and the narrowband filtering for the radio. The demodulator extracts the audio or other modulation from the IF radio frequency. Finally, the extracted signal is then amplified by the audio amplifier.

Superheterodyning

Heterodyning is the mixing of two frequencies together so as to produce a beat frequency. With AM (Amplitude Modulation) the information signal is mixed with the carrier to produce the side-bands. The side-bands occur at precisely the sum and difference frequencies of the carrier and information.

Superheterodyning is creating a beat frequency that is lower than the original signal. The lower side band and the difference between the other of the mixed frequencies is superheterodyning. What superheterodying does is to purposely mix in another frequency in the receiver, so as to reduce the signal frequency prior to processing.

History

Major Edwin Howard Armstrong is credited with developing the first practical superheterodyne receiver design. While conducting military work in France during WWI Armstrong developed an approach that first converted the signal to a fixed intermediate frequency, which was then processed and amplified. His work began in 1918 and by 1920 the superheterodyne design started to replace Tuned Radio Frequency receivers. His approach first converted the signal to a fixed intermediate frequency, which was then processed and amplified which allowed reception of a much broader frequency range, in particular providing superior reception of signals higher than 1500 kHz.

The development of high quality vacuum-tubes during WW1 made possible great improvements to the superheterodyne receiver.

Alternatives

TRF

Before radios implemented the superheterodyne design most of the available radios on the market were Tuned Radio Frequency (TRF) receivers. In an early TRF receiver there were tuned circuits separated by the radio frequency (RF) amplifier stages and the last tuned circuit feeded the AM detector stage. The individual tuning capacitors were attached to separate tuning dials. Each had to be reset each time a different station was selected.

Homodyne

The homodyne is a direct conversion front-end design that eliminates 1/3 of the components necessary for the superheterodyne receiver design. ZIF, or Zero-IF is the homodyne attracting industry attention as a means to greatly reduce cost. However, due to problems with ZIF, it is typically not considered a beneficial tradeoff.

Direct conversion receivers perform the RF to baseband frequency translation in a single step. The RF signal is mixed with a local oscillator at the carrier frequency thus eliminating any image components to cause signal corruption.

Today these homodyne or direct conversion receivers are typically only used where modulation methods do not put any significant signal energy near direct current.