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other pulses. Neither is it applicable to those pulsed systems in which the duration or position of the
pulses are varied by the modulation frequency. In its simplest form, psk operates on the principle of phase
reversal of the carrier. Each time a mark is received, the phase is reversed. No phase reversal takes place
when a space is received. In binary systems, marks and spaces are called ONES and ZEROS,
respectively, so that a ONE causes a 180-degree phase shift, and a ZERO has no effect on the incoming
signal. Figure 2-25 shows the application of phase-shift keying to an unmodulated carrier [view (A)] in
the af range. For transmission over other than a conductive path, the wave shown in view (D) must be
used as the modulating signal for some other system of modulating an rf carrier.
Figure 2-25A.—Phase-shift keying. UNMODULATED CARRIER.
Figure 2-25B.—Phase-shift keying. MODULATION SIGNAL - DATA ELEMENTS.
Figure 2-25C.—Phase-shift keying. MODULATED CARRIER.
Figure 2-25D.—Phase-shift keying. MODULATED CARRIER AFTER FILTERING.
The modulating signal in view (B) consists of a bit stream of ZEROS and ONES. A ZERO does not
affect the carrier frequency which is usually set to equal the bit rate. For example, a data stream of 1,200
bits per second would have a carrier of 1,200 hertz. When a data bit ONE occurs, the phase of the carrier
frequency is shifted 180 degrees. In view (C) we find that the third, fifth, and sixth cycles (all ONE) have
been reversed in phase. This phase reversal produces CUSPS (sharp phase reversals) which are usually
