Notes#8, ECE594I, Fall 2009, E.R. Brown
Heterodyne and Homodyne Conversion
The heterodyne technique goes back to the early days of radio (World War I) when
amplifiers were in their infancy and all made from vacuum tubes, meaning that it was difficult to
boost the amplitude of incoming signals, even at the ~1 MHz or lower carrier frequencies that
were being used at that time.
Taken from two Greek roots, “hetero”
“force”, the basic idea is to couple the
incoming signal to a nonlinear “mixer” that is
simultaneously driven by a “local oscillator” (LO), thereby creating a beat note at an intermediate
frequency (IF) between the incoming signal and baseband.
In the early days, baseband was often
just the human audible range since the information being transmitted was imposed by a human
So the IF band was usually in the “supersonic” region between ~20 KHz (approximate
upper end of audible range) and 1000 KHz, leading to the descriptor “supersonic heterodyne” or
super heterodyne for short.
Today, superheterodyne continues to be the descriptor, even when the
incoming radiation is in the THz region, and the IF band is in the UHF or microwave regions,
typically between 0.1 and 10 GHz.
Arguably, super heterodyne has been one of the most
valuable, if not the most valuable, developments in the history of communications, RF sensors,
and more recently THz systems.
The heterodyne technique generally utilizes a three-port nonlinear device called a mixer.
incoming signal X
is coupled to one port, and the local oscillator X
to a second port.
is taken from the third (intermediate frequency) port.
The mixer design and the amplitude of the LO are chosen to impart on X
a beat note at the
intermediate frequency through product terms of the form
A quadratic or “square law” term in the mixer transfer function is an effective way to do this:
for a fascinating story behind the roots of the heterodyne technique, see Wikipedia entry
This is in large part the personal story of the creative genius,
Edwin Armstrong, who later invented and developed FM radio in the 1930s.