from
The Free On-line Dictionary of Computing (8 July 2008)
continuous wave
CW
<communications> (CW) A term from early radio history, when
the {spark gap} method of transmission was replaced by
vacuum-tube oscillators. A spark gap initiates a ringing,
damped sinusoidal wave in a tuned circuit consisting of an
inductor and capacitor. The energy in this circuit is
constantly changing between the capacitor's electrostatic
field and the inductor's magnetic field. The energy is then
coupled, loosely (so as not to dampen the wave too quickly),
to the radiating antenna.
In contrast, a vacuum-tube oscillator constantly adds energy
to the tuned circuit, compensating for the amount coupled to
the antenna, and the transmitted energy or "wave," is
therefore "continuous".
Many (especially radio amateurs) continue to understand "CW"
to mean transmission by means a signal of a single frequency
which is either on or off (e.g. {Morse code}), as opposed to a
carrier which varies continuously in amplitude, frequency or
phase. Some would even call the former "unmodulated" even
though turning on and off is actually an extreme form of
amplitude modulation.
(1995-03-15)