![]() Any jitter that a standard oscilloscope measures is due to trigger instability. Unfortunately, you can't use standard oscilloscope techniques to identify individual clock edges in absolute time. ![]() The most direct method of examining the placement of edges is to look at the edges using an oscilloscope. Improperly specified or measured jitter might result in unnecessary costs, or poor system performance.Ī true measure of clock jitter is the accurate position of clock edges over time. Measured jitter also varies with measurement technique and jitter bandwidth. The spectral content of clock jitter differs greatly depending on the clock-generation technique. Jitter due to 10-kHz noise can be greater than or less than jitter due to 100-kHz noise. However, the noise that causes the jitter is not necessarily uniform over all frequencies. Consequently, the edge jitter in Figure 1 also has spectral content, and the edges in the figure vary randomly with time. Noise has spectral content as well as power. The displacement of the edges in Figure 1 is a result of noise. You can also derive jitter from power-spectral-density, or phase-noise, measurements. Table 1 presents these various measures of jitter for a 155.52-MHz system clock. Expressed in decibels relative to one unit interval (dB UI ), jitter power in this case is 10log(0.0000049)=–53.1 dB UI. For the above case of 100-psec jitter, you find the rms jitter power by dividing the peak-to-peak jitter by seven and then squaring the result: 0.01555/7) 2 =0.0000049 UI 2. A reasonable expectation of peak-to-peak jitter, and a standard industry number, is approximately 7, or seven times the rms value. To define jitter power, you typically use rms, 1-sigma ( ) measurements. All three numbers describe the same amount of jitter. In this case, 100 psec of jitter=0.01555 UIs=5.598°of peak-to-peak jitter. One UI equals the period of the signal, which you can then translate to time and degree units as follows: 1/155.52X10 6 UIs=6.43 nsec=360°.
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