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."
