2-3
Figure 2-1C.—Comparison of ON-OFF and frequency-shift keying. TRANSMITTED ON-OFF KEYED CW
SIGNAL.
Figure 2-1D.—Comparison of ON-OFF and frequency-shift keying. TRANSMITTED FREQUENCY-SHIFT
KEYED SIGNAL (FSK).
In fsk the output is abruptly changed between two differing frequencies by opening and closing the
key. This is shown in view (D). For illustrative purposes, the spacing frequency in view (D) is shown as
double the marking frequency. However, in practice the difference is usually less than 1,000 hertz, even
when operating at several megahertz. You should also note that the limit of frequency shift is determined
without reference to the amplitude of the keying signal in the fsk system. The frequency shift may be set
at plus or minus 425 hertz from the allocated channel frequency. The total shift between mark and space
would be 850 hertz. Either the mark or space may use the higher of the two frequencies. The upper
frequency of the transmitted signal is usually the spacing interval and the lower frequency is the marking
interval.
COMPARING FSK AND CW SIGNALS.—A comparison of on-off keyed cw (figure 2-2), (view
(A), view (B), view (C)), and fsk (figure 2-3), (view (A), view (B), view (C)), signals will show clearly
the principal features of fsk and give us a basis on which frequency modulation can be discussed. Let's
use views (A), (B), and (C) of both figures to show the Morse code character "F" for an example. Figures
2-2 and 2-3 are graphic drawings of the two types of keying. Time and amplitude are known dimensions
of AM; but to explain fsk properly, we have added the third dimension of frequency.
