2-7
Frequency-shift keyers are, of course, more complicated than this simple illustration would seem to
show, but the basic principles are the same. Still, the keyer does change the oscillator frequency by a
certain number of cycles. Further, this change must be correlated with the multiplication factor of the
transmitter to cause the desired shift between mark and space frequencies.
METHODS OF FREQUENCY SHIFTING.—Frequency-shift keyers operate on either of two
general principles. First, the keyer may take the output of the transmitter's master oscillator and modulate
it with the output of another oscillator that is frequency-shift keyed. This action will result in two
frequencies that are used to excite the first amplifier stage of the transmitter. This system is illustrated in
view (A) of figure 2-4. View (B) illustrates the second method of frequency-shift keyer operation. In this
method the transmitter's master oscillator is itself shifted in frequency by the mark and space impulses
from the keyer unit.
Figure 2-4A.—Two methods of frequency-shift keying (fsk). FREQUENCY-SHIFT KEYING BY MODULATING
MASTER OSCILLATOR OUTPUT.
