360-degree carrier sine wave to the baseline of thesine wave. The carrier signal starts on the baseline, asillustrated in figure 1-6, and continues to form a curvecalled the sine wave. When the sine wave reaches itsmaximum positive amplitude, it is at the 90-degreepoint. When it returns to the baseline, it is at 180degrees. When it reaches its maximum negativeamplitude, it is at 270 degrees; and when it returns tothe baseline, it is at 360 degrees or the 0-degree pointfor the start of the next cycle. This process occursover a period, with the number of full cycles persecond (Hz) being the frequency of the signal. A fullcycle is the transition from the 0-degree point to the360-degree point.Figure 1-6.—Carrier sine wave,For a particular frequency this process continueswithout interruption.Phase modulation involvesinterrupting the cycle at one or more degree pointsand instantaneously changing the direction oramplitude of the sine wave. Figure 1-7 shows how a180-degree phase shift is used to indicate two discretestates. The third cycle of the carrier is interrupted atthe 180-degree point. Instead of continuing in thenegative direction, the sine starts at the 0-degree pointagain. The resultant signal has the same frequencyand amplitude as the original signal but is 180 degreesout of phase. This phase shift can be directly relatedto a digital input at a modulator in which oneparticular phase represents the 0 bit and the otherphase represents the 1 bit.Multibit ModulationWhile the 180-degree phase shift can be used toindicate two discrete states, many points on the sinewave can be defined to represent different bitconfigurations.Individual phase changes of 0degrees, 90 degrees, 180 degrees, and 270 degreesfrom a reference phase can each represent twoseparate data bits. For example, a 0-degree phaseshift or no phase shift could indicate a binary 00; a90-degree phase shift, a binary 01; a 180-degree phaseshift, a binary 10; and a 270-degree phase shift, abinary 11. This type of modulation is known as amultibit, or quadrature (four-state) phase-shiftmodulation, as shown in figure 1-8. Keep in mindthat only one continuous frequency and amplitudesignal is being phase-modulated to transmit two bitsof data for each phase shift.Figure 1-8.—Multibit phase modulation.A modification of the quadrature phase-shiftmodulation,calleddifferentialquadraturephase-shift keying, uses the difference between aphase-shifted signal and its preceding sine wave torepresent data. When a phase shift is detected, thecurrent signal is compared with the previouslytransmitted phase signal. The difference between thetwo signals is computed to determine the amount ofphase shift. The previously transmitted signal is usedas the reference phase for demodulating the data bits.Two binary digits are represented by phase changes of-45, -135, -225, and -315 degrees. The -45 degreeshift indicates a binary 11; the -135 degree shift, abinary 01; the -225 degree shift, a binary 00; and the-316 degree shift, a binary 10.Figure 1-7.—Phase modulation.1-7
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