The Heterodyne "Principle"

A working definition of heterodyning: To generate new frequencies by mixing two or more signals
in a nonlinear device such as a vacuum tube, transistor, or diode mixer.

Reginald Aubrey Fessenden (1866-1932) is usual credited with the invention of the heterodyne principle (reference).

As an example, we revisit our discussion of DSB-AM where we see how two frequencies are shifted by the nonlinear operation of multiplication.

Primary Example of a Nonlinear Characteristic - Diode (Rectifier) Current-Voltage Characteristics



Edwin Howard Armstrong's Superheterodyne Receiver (1917)


From A Nonlinear History of Radio

"… the way was paved for a bright engineer to devise useful circuits to exploit the audion’s potential. That bright engineer was Edwin Howard Armstrong who invented the regenerative amplifier/detector
in 1912 at the tender age of 21.

"…In a 1914 paper titled 'Operating Features of the Audion,' Armstrong published the first correct explanation for how the triode worked, and provided experimental evidence to support his claims. …He Armstrong went on to develop circuits that continue to dominate communications systems to this day. While a member of the U.S. Army Signal Corps during World War I, Armstrong became involved with the problem of detecting enemy planes from a distance, and pursued the idea of trying to home in on the signals naturally generated by their ignition systems (spark transmitters again).  Unfortunately, little useful radiation was found below about 1MHz, and it was exceedingly difficult with the tubes available at that time to get much amplification above that frequency. … so Armstrong had
his work cut out for him.  He solved the problem by employing a principle originally used by Poulsen and later elucidated by Fessenden.

"…Armstrong decided to employ Fessenden’s heterodyne principle in a different way. Rather than using it to demodulate CW directly, he used the heterodyne method to convert an incoming high frequency RF signal into one at a lower frequency, where high gain and selectivity could be obtained with relative ease. This signal, known as the intermediate frequency (IF), was then demodulated after much filtering and amplification at the IF had been achieved. The receiver could easily possess enough sensitivity so that the limiting factor was actually atmospheric noise (which is quite large in the AM broadcast band).  Furthermore, a single tuning control was made possible, since the IF amplifier works at a fixed frequency."

This page was prepared and is maintained by R. Victor Jones
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Last updated March 23, 2004