2-6
electrical machinery appear as additional variations along the amplitude axis. When these amplitude
variations approach or exceed the variation caused by the keyed intelligence, the signal is blanked out by
interference. We have all heard this happen on our AM radios during storms or when near operating
machinery.
View (C) of figure 2-2 represents the same signal when scanned along the time and frequency axes
as it would be in an fm receiver. Variations in signal amplitude have no effect on the frequency and no
intelligence can be received. Note that the noise and interference components have also been suppressed
so that they have little effect on the received signal. Thus, if the intelligence variations were impressed as
changes along the frequency axis, and the receiving equipment were designed to respond to this type of
signal, then the effects of noise and interference would be practically eliminated. Frequency-shift keyed
circuits fulfill these conditions.
FSK SIGNALS.—In fsk the rf signal is shifted in frequency (not amplitude) between "key-open"
and "key-closed" conditions. The signal amplitude remains essentially constant. View (A) of figure 2-3
represents the letter "F" keyed as a shift in frequency between mark and space. The normal frequency
condition with the key open is a space. Recall that this may be either the lower or higher frequency. When
the key is closed, the frequency instantly changes to the mark value and remains constant during the
marking interval. Opening the key again returns the frequency to the space frequency. Midway between
the mark and space frequencies is the assigned channel frequency.
Also shown in view (A) is the variation along the amplitude axis caused by the same noise and
interference mentioned earlier. The right-hand portion of view (A) illustrates the elimination of this noise
by the receiving equipment. View (B) clearly shows that scanning the signal along the amplitude and time
axes reproduces no amplitude variations from signal interference. However, if the scanning is
accomplished along the frequency and time axes, the intelligence is reproduced, as shown in view (C). By
this system, the intelligence can be recovered at the receiving station in its original form; it will be nearly
unaffected by conditions in the radio path other than fading. As a matter of fact, fsk resists the effects of
fading better than cw.
FREQUENCY-SHIFT KEYING.—In its simplest form, frequency-shift keying of a transmitter
can be accomplished by shunting a capacitor (or an inductor) and key (in series) across the oscillator
circuit. By locking the normal key of the transmitter and operating only the oscillator circuit key, you can
change the oscillator frequency. The shift in frequency between mark (key-closed) and space (key-open)
conditions is determined by the effect of the additional capacitance (or inductance) on the oscillator
frequency. The frequency multiplication factor in the transmitter amplifiers must be taken into
consideration when determining the oscillator frequency shift. Thus, if the desired shift is the
conventional 850 hertz at the transmission frequency, and this frequency is four times the oscillator
frequency (that is, doubled in two stages), then the effect of the additional capacitance (or inductance) on
the oscillator must be limited to 212.5 hertz as shown below:
