It is a kind of
modulation in which two different
carriers, each with a slightly different
frequency, are alternatively transmitted.
Usually the frequency shift is equal to 170 Hertz and the two carriers
are associated with the binary digits, 0 and 1. FSK is thus used to
send digital data, with many different codes and protocols, such as
RTTY,
Packet, etc.
It is also possibile to send an FSK signal using two audio tones on a
AM or
FM carrier, and in this case the mode is
called AFSK, Audio Frequency Shift Keying. Audio tones applied to an
SSB transmitter result in the same signal
of true FSK, but of course the latter is better, because it does not
require a modulation amplifier, filters, etc.
