STCD22X0 datasheet(16/39 Pages) STMICROELECTRONICS

16 / 39 page

Application information

STCD22x0, STCD23x0, STCD24x0

16/39

Doc ID 15400 Rev 2

4.6

Jitter

In the time domain, energy spreading can result in jitter, which is the same phenomenon as

phase noise in the frequency domain. As a sine wave passes its zero-crossing or a square

wave changes state, the real clock signal transition is not exactly the same as the ideal

case, thus causing variation in the waveform transition point. This deviation of the transition

point is known as jitter as illustrated in

Figure 8.

Jitter

Figure 8 the square wave ideal transition point should happen at points 1, 2, 3, 4 and 5,

and each "ideal" period PI1 to PI4 should be the same, thus no time jitter has occurred.

Actually, the real transition point happens at points 6, 7, 8, 9 and 10, thus causing "real"

periods PR1 to PR4 to not be the same, and exhibit visible jitter. If each of the real periods of

the cycles (PR1 to PR4) is measured, period jitter is obtained. The cycle-to-cycle jitter is

also obtained by calculating the difference between two adjacent periods (for example, PR2-

PR1, PR3-PR2 …).

These periods of jitter are described as peak-to-peak jitter and are calculated by subtracting

the minimum value from the maximum value or may also be described by the root-mean-

square (RMS) value, representing one standard deviation of the Gaussian distribution.

4.7

Output trace line

The STCD22x0, STCD23x0 and STCD24x0 is designed with maximum 50

Ω impedance

output. On the PC board, a 50

Ω transmission line with proper series termination should be

used to avoid signal distortion and reflection.

2345

PI1

PI2

PI3

PI4

PR1

PR2

PR3

PR4

Ideal transfer point

Real square wave transfer point

ai14029

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