Frequency shift keying employs two different
carrier frequencies which are switched ON and OFF alternately
by the mark and space signal. A mark signal switches
ON one carrier while the other carrier remains OFF.
Under the space conditions the first carrier is switched
OFF and the second carrier is switched ON.
Frequency shift keying is used for signaling speeds
up to 1200 bauds with carrier frequencies of 1300 Hz
and 2100 Hz for mark and space respectively. The system
may be considered as frequency modulation with a centre
frequency of 1700 Hz and frequency deviation of.
The above illustration
represents a FSK signal with the binary signal bandwidth
of 600 Hz, the modulation index
For the given
case, the bandwidth of the FSK signals, is approx. twice
the highest frequency present in the modulating signal.
The system cannot, however, be used for higher signaling
rates, because the FSK signal output demands larger
bandwidth and cannot be accommodated in a speech channel
and attempt to reduce frequency deviation and hence
the bandwidth results in excessive noise.
The system is commonly termed as FM
VFT.
In case of digital signal,
the logic 1 corresponds to frequency
and logic 0 corresponds to frequency .
The required peak frequency deviation is
given by
(Minimum)
where is
given by
and
(
is the bit duration in seconds).
The expression for FSK
signal are given as
and in terms of bit
rate expression is given as
It is seen that FSK can
be considered as consisting of two pulsed sinusoidal
waves of frequencies
and .
Digital FSK generation
Frequency shifting keying
generation is an FM technique. This technique is employed
to improve signal to noise and reduces the effects of
interfering signals.
.
and logic 0 corresponds to frequency 
.
The required peak frequency deviation 
is
given by 
(Minimum)
and 
(
is the bit duration in seconds). 
