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Signal chain basics: Clock jitter unveiled (Part 2)

Posted: 25 Jan 2012 ?? ?Print Version ?Bookmark and Share

Keywords:jitter? phase noise? ADCs? DACs?

In the previous signal chain basics article, we discussed the fundamental relationships between jitter and phase noise. In this part, we discuss practical aspects of jitter transfer and capabilities as well as the limitations of phase noise measurement techniques.

Clock jitter and edge rate
Figure 1 shows three waveforms represented by a generic equation. The equation includes a phase noise term "(t)" and an amplitude noise term "(t). " For each of the three frequencies evaluated, (t) = 0 and (t) is a pseudo random function generating a constant envelope of noise for each waveform.

Figure 1 illustrates an exploded view of the crossing of Vth for each of the three waveforms, along with distributions of locations where Vth could be crossed.

Figure 1: Time-jitter induction versus signal-edge rate.

Figure 1 emphasizes that noise sources other than intrinsic jitter can cause timing jitter errors. Faster edge rates diminish the effect that voltage noise on a clock signal has on clock jitter performance. This phenomenon is not solely a characteristic of the clock signal. Rather, this mechanism is apparent in devices that receive the clock signal or measure jitter performance.

Clock performance measurements
Engineers often evaluate lab results that don't make sense; and clock performance measurement is particularly problematic. For example, you can use an oscilloscope and a phase noise analyser (PNA) to measure random jitter. However, the results are sometimes radically different. To better understand these results, a few experiments are identified in figure 2. The results are tabulated in table.

Figure 2: Random-jitter measurement experiments.

Table: Results of random-jitter measurement experiments from figure 2.

For example, the ONET1191P is a limiting amplifier which is optimised for high-speed applications. We used this amplifier to decrease the rise and fall time of the clock signal, which in this case is from about 200 ps to 35 ps.

Notice that the application of this amplifier has different results for the scope versus the PNA. The PNA results are as expected (for example, adding the limiting amplifier (LA) adds noise). However, the LA improves the results delivered by the oscilloscope significantly.

Equation: Oscilloscope jitter noise floor.

These results are related to the edge rate and jitter transfer mechanism shown in figure 1. Instead of the clock signal containing the noise component, the front end of the oscilloscope provides the noise (error) source. The equation shows an expression for the jitter noise floor of an oscilloscope. It comprises the jitter of the scope's time base, the noise of the scope's front end (Vnoise), and the slew rate of the signal under test.

About the author
John Johnson is the Manager of Market Development and Systems Engineering for the Clocks and Timing Group of Texas Instruments. John has 30 years of experience in the electronics industry and has worked in the fields of product development, marketing, systems engineering, and business management. He holds a MSEE from Purdue University.

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