3-22
Figure 3-21.—Basic tone modulated (afts) system.
RADIO-FREQUENCY-CARRIER SHIFT.—For frequency-shift (fsk) systems, the transmitter
provides a source of radio-frequency excitation. Figure 3-22 illustrates a basic frequency-shift keyed
system. In modern systems, the keyer is built into the transmitter. The keyer shifts the signal box below or
above the assigned frequency to correspond with the mark or space required to transmit tty characters.
Normally the keyer is adjusted for an 850-hertz spread, 425 hertz above and 425 hertz below the assigned
frequency. A spacing impulse will be 425 hertz above the operating frequency, and a marking impulse
will appear 425 hertz below.
