2-27
Figure 2-24A.—Phase modulator.
Figure 2-24B.—Phase modulator.
In practice, a circuit like this could not provide enough phase shift to produce the desired results in
the output. Several of these circuits are arranged in cascade to provide the desired amount of phase shift.
Also, since the output of this circuit will vary in amplitude, the signal is fed to a limiter to remove
amplitude variations.
The major advantage of this type modulation circuit over frequency modulation is that this circuit
uses a crystal-controlled oscillator to maintain a stable carrier frequency. In fm the oscillator cannot be
crystal controlled because it is actually required to vary in frequency. That means that an fm oscillator
will require a complex automatic frequency control (afc) system. An afc system ensures that the oscillator
stays on the same carrier frequency and achieves a high degree of stability. The afc circuit will be covered
in a later module.
Phase-Shift Keying
Phase-shift keying (psk) is similar to ON-OFF cw keying in AM systems and frequency-shift keying
in fm systems. Psk is most useful when the code elements are all of equal length; that is, all marks and
spaces, whether message elements or synchronizing signals, occupy identical elements of time. It is not
fully suitable for use on start-stop teletypewriter circuits where the stop pulse is 1.42 times longer than the
