Nov 16, 2008

what is heterodyne?

What is Heterodyne?

A heterodyne is a circuit that transfers a signal from one carrierwave to another with a different frequency. It mixes the input signalwith a wave generated by an oscillator to create two new signals,called beats. While heterodyning is a simple process governed by thelaws of trigonometry, most heterodynes are complex devices with severalfilters and amplifiers.

Heterodyne Beats

A beat is a signal produced by two input signals with differentfrequencies. A heterodyne produces two beats, one with a frequency thatis the sum of the mixed frequencies, while the other beat has afrequency that is the difference between the mixed frequencies. Forexample, an input signal with a 10MHz carrier wave is mixed with a15MHz carrier wave to create two output beats. The upper beat has afrequency of 25MhHz, and the lower beat has one of 5MHz.

Superheterodyne Receivers

The superheterodyne, also called the superhet, uses the heterodyneprinciple to allow high frequency signals to be detected by lowfrequency receivers. It was invented by Edwin Armstrong in 1918 so thatshort-wave radar could be used to detect enemy ships during the firstworld war. When a signal enters a superheterodyne receiver, it isamplified and mixed with the local oscillator wave before beingfiltered to produce an intermediate frequency. It is usually amplifiedand filtered again before reaching the output. The receiver is tuningby varying the frequency of the local oscillator wave.

Heterodyne Applications

Virtually all radios and televisions made today use asuperheterodyne in their receiver. An analogue video system, such asthe VCR, compresses the video signal before transferring it to tape tosave bandwidth. The signal is passed through a superheterodyne tocreate a new signal that has a lower frequency, and expanded again whentransferred from tape. These frequencies are defined by video standardssuch as NTSC and PAL.

Neutrodyne

This was a short-lived alternative to the heterodyne used in radiosduring the 1920s. It was cheaper and simpler than the heterodyne at thetime, but advances in valve technology made the neutrodyne obsolete bythe 1930's.