Causes of Jitter

Jitter—Understanding it, Measuring It, Eliminating It; Part 3: Causes of Jitter

By Johnnie Hancock Agilent Technologies

his final article of separate and time-correlate specific determin- Understanding and meas- this three-article istic jitter components. uring jitter are important, Tseries we discuss but the ultimate objective identifying jitter in the Characteristics of Individual Jitter is finding out what is caus- real world. In Part 1 we Components ing excessive jitter, then introduced jitter and its In order to interpret measurement results reducing or eliminating it to various kinds, in Part 2 and waveform views performed by real-time achieve the desired overall we discussed jitter mea- jitter analysis, you must first understand the system performance surements, jitter mea- characteristics and likely causes of individual surements at high data jitter components. Knowing that Random rates and issues relating to the accuracy of jit- Jitter (RJ) is Gaussian in distribution and ter measurements. Now we move on to the Deterministic Jitter is non-Gaussian is a good task of identifying the causes of jitter. beginning, but there is more. Some real-time instruments can separate Total Jitter is composed of a Random Jitter random and deterministic jitter components component and a Deterministic Jitter compo- to predict/extrapolate worst-case total jitter nent, discussed in Article 1, and denoted in (TJ) and eye-opening based on a user-specified Figure 1. Random Jitter is unbounded, and it Bit Error Ratio (BER), typically 10-12. But is for this reason (unlimited peak-to-peak) when jitter measurements fail to meet a par- that it is usually measured in terms of an ticular minimum standard, or if the results RMS value. In addition, Random Jitter is pre- are “too close for com- fort,” then measuring the amount of component or system jitter is just half of the jitter test equation. Determining the root-cause of jitter is the other half. The focus of this article is to provide practical “tips & tricks.” In particular, we discuss how to employ real-time oscilloscopes with jitter analysis and high-speed pulse/pattern generators to Figure 1 · Total Jitter is the sum of several components.

28 High Frequency Electronics High Frequency Design CAUSES OF JITTER

exhibit a duty cycle of less than 50%. If the threshold level is shifted negatively, then the output of the transmitter will exhibit a duty cycle that is greater than 50%. Measuring Time Interval Error (TIE) relative to the software-generated best-fit clock (red waveform), results in a positive timing error on the rising edge of each data bit and a negative timing error on the falling edge of each data bit. The resultant TIE trend waveform (purple waveform) will possess a fundamental frequency equal to one- half the data rate. The phase of the TIE trend waveform relative to the data signal will depend on whether the threshold shift is positive or negative. With no other sources of jitter in the system, the peak-to-peak ampli- Figure 2 · Duty Cycle Distortion (DCD): The dashed tude of DCD jitter will be constant across the entire data green line represents a distorted output of a transmitter signal, at least theoretically. Unfortunately, other sources due to positive shift. of jitter, such as ISI, are almost always present, often making it difficult to isolate the DCD component. One technique to test for DCD is to stimulate your sys- dictable in terms of distribution. Its probability distribu- tem/components with a repeating 1-0-1-0... data pattern. tion function is always Gaussian in shape. This technique will eliminate ISI jitter and make viewing Unfortunately, predicting the cause of random jitter is the DCD within both the trend and spectrum waveform a more difficult task and is not within the scope of this displays much easier. Using the jitter spectrum display, article. Random Jitter is often caused by thermal effects the DCD component of jitter will show up as a frequency of semiconductors and requires a deeper understanding of spur equal to one-half the data rate. device physics. However, one piece of advice is to pay close Another cause of DCD is asymmetry in rising and attention to the amount of vertical noise in your system. falling edge speeds. A slower falling edge speed relative to Random vertical noise will directly translate into random the rising edge will result in a duty cycle of more than timing jitter. 50% for a repeating 1-0-1-0... pattern, and slower rising On the other hand, deterministic jitter is bounded and edge speeds relative to the falling edge will result in a is always measured in terms of a peak-to-peak value. duty cycle of less than 50%. Although not graphically Although the distribution of deterministic jitter can be shown in this paper, the results of jitter analysis and the unpredictable, the likely causes and characteristics of the TIE trend waveform will look similar to the results of the individual sub-components of measured deterministic jit- example illustrated in Figure 2. ter are very predictable. The sub-components of Deterministic Jitter consist of Duty Cycle Distortion Inter-Symbol Interference (DCD), Inter-Symbol Interference (ISI), and Periodic ISI, sometimes called data dependent jitter, is usually Jitter, as shown in Figure 1. the result of a bandwidth limitation problem in either the Let’s now take a closer look at possible causes and transmitter or physical media. With a reduction in trans- characteristics of each of the sub-components of deter- mitter or media bandwidth, limited rise and fall times of ministic jitter. the signal will result in varying amplitudes of data bits dependent on not only repeating-bit lengths, but also Duty Cycle Distortion dependent on preceding bit states. There are two primary causes of DCD jitter. If the data In addition, improper impedance termination and input to a transmitter is theoretically perfect, but the physical media discontinuities will also result in ISI due transmitter threshold is offset from its ideal level, then to reflections that cause signal distortions. Although we the DCD at the output of the transmitter will be a func- will address these two phenomena—BW limitations and tion of the slew rate of the data signal’s edge transitions. reflections—separately in this article as contributors to Referring to Figure 2, the waveform represented by the ISI jitter, in reality, waveform distortions due to reflec- solid green line shows the ideal output of a transmitter tions are also a bandwidth limitation problem. Shown in with an accurate threshold level set at 50% and with a Figure 3 is an example of ISI due to bandwidth limitation duty cycle of 50%. The dashed green waveform represents problems. Limited bandwidth produces limited edge a distorted output of a transmitter due to a positive shift speeds, and limited edge speeds will result in varying in the threshold level. With a positive shift in threshold pulse amplitudes at high-speed data rates. Varying pulse level, the resultant output signal of the transmitter will amplitudes will then result in transition timing errors.

30 High Frequency Electronics High Frequency Design CAUSES OF JITTER

Figure 3 · Inter-symbol interference (ISI) due to band- Figure 4 · Intersymbol Interference (ISI) due to signal width limitation problems. reflections.

Let’s examine this more closely: trated at point D on the jitter trend begin on and where pulse distortion With a long series of repeating waveform follows the same logic as (reflection) occurs as illustrated by “1s,” the amplitude of the data signal the positive timing error at point B the end of each of the arrows in will eventually rise to a full steady- previously discussed. With a long Figure 4. state high level as illustrated by the string of “0s” the data signal has suf- If the amplitude of the signal long-high pulse at point A in Figure ficient time to settle to a full steady- becomes distorted on or near a data 3. When the state of the data changes state low level. When this signal then transition edge due to reflections, to a “0,” the signal will exhibit a rela- transitions back to a high level, it then a timing error may occur. If a tively long transition time to reach again has a longer transition time to signal reflection causes signal atten- the threshold level, resulting in a reach the threshold level, and hence uation near the data edge, then a positive timing error. This will be produces a positive timing error. negative timing error will be detected manifested as a positive peak of tim- since the signal will have less dis- ing error in the jitter trend waveform The Unique Signature of tance to travel when transitioning to (purple) at point B. Note that this Inter-Symbol Interference the threshold level. This is illustrated point on the jitter trend waveform is Once you understand how band- at point A on the jitter trend wave- time-aligned with the negative data width limitations produce ISI timing form in Figure 4. If a signal reflection crossing point on the data signal. errors, it becomes more intuitive to causes a boost in signal amplitude, The negative peak amplitude of understand the unique signature of then the result will be a positive tim- the next “0” bit preceded by a long the jitter trend waveform due to ISI ing error since the signal will have string of “1s” will be attenuated for and how it relates to the time-corre- farther to transition to reach the two reasons: First of all, the preced- lated serial data signal being mea- threshold level. This is illustrated at ing long string of “1s” means that the sured. point B on the jitter trend waveform. signal will take longer to transition In addition to bandwidth limita- ISI due to signal reflections can be to a true low level since the data sig- tions, another common cause of ISI is very difficult to isolate and interpret. nal starts from a higher initial level. signal reflections due to improper But if you have signal reflection prob- Secondly, the following “1” bit causes terminations or impedance anoma- lems in your system, it is likely that the signal to reverse direction before lies within the physical media. Signal there is also a bandwidth limitation it even reaches a solid low level. This reflections will produce distortions in problem. reduction in signal amplitude will the amplitude of the data signal as produce a negative timing error on shown in Figure 4. Depending on the Periodic Jitter the next transition to a “1” since the physical distances between Usually the result of a cross-cou- signal has a very short distant to impedance anomalies, reflections pling or EMI problem in your system travel to reach the threshold level. produced by one pulse may not show PJ can be either correlated or uncor- This is illustrated at point C on the up on a high-speed data signal until related to the data signal. An exam- jitter trend waveform in Figure 3. several bits later in the serial pat- ple of uncorrelated PJ would be sig- The positive timing error illus- tern. Notice which pulse the arrows nals from a switching power supply

32 High Frequency Electronics Figure 5 · Periodic Jitter (PJ) caused by capacitive coupling. Figure 6 · Isolating Duty-Cycle Distortion (DCD). coupling into the data or system clock interpret. Fortunately, there are some also a measurement technique to signals. This kind of jitter is consid- novel stimulus-response techniques eliminate both random jitter and ered to be uncorrelated because it is you can employ to isolate, measure uncorrelated periodic jitter in the jit- not time-correlated with either the and then view individual jitter com- ter measurement results. Shown in clock or data signal since it would be ponents. Once you successfully iso- Figure 6 is a captured serial pattern based on a different clock source. An late individual components, you can (green trace) of repeating ones and example of correlated PJ would be then often time-correlate worst-case zeros. The jitter analysis results coupling from an adjacent data signal peaks of jitter to specific data bit appear immediately below as the TIE based on the same clock—or a clock of transitions and then use common- trend waveform (pink trace). the same frequency. sense debug techniques to solve your To eliminate the random compo- Shown in Figure 5 is an example jitter problems one jitter component nents (RJ and PJ), waveform math of a “corrupter” signal (red trace) at a time. has been employed to average the jit- capacitively coupled to a serial data The primary tool to isolate jitter ter trend waveform. Before averag- signal (green trace). This coupling components is a real-time oscillo- ing, the TIE trend waveform would be will result in amplitude distortions scope with responsive and interactive “bouncing” vertically due to the ran- on the data signal. Just like ISI due jitter analysis such as the 6-GHz dom components with repetitive to reflections, if these amplitude dis- Agilent 54855A. In addition, a high- acquisitions. But averaging has elim- tortions occur at or near a data signal speed pulse/pattern generator such inated the random jitter components transition, a timing error may occur. as the 3.3-Gb/s Agilent 81134A can to disclose a very stable trend wave- Since most PJ will be uncorrelat- be very useful for generating known form with an fairly constant level of ed with the data signal, any attempts serial patterns of high-speed differ- peak-to-peak amplitude of jitter from to time-correlate the jitter trend ential stimulus. cycle to cycle. We can then use the waveform with the data waveform Let’s now turn to some real jitter scope’s manual markers or the will be futile. As we will show later in measurement examples using these scope’s automatic parametric mea- this article, uncorrelated PJ can often two measurement tools. surement capability to measure the be detected using the jitter spectrum Isolating and measuring DCD— peak-to-peak amplitude of duty cycle view. One technique for isolating and mea- distortion. In this case, we measured suring DCD is to stimulate your sys- approximately 10 picoseconds of Isolating Jitter Components tem/component with a repeating 1-0- DCD. in the Real World 1-0... serial pattern. This stimulus In addition to determining the In the real world, jitter compo- pattern will eliminate most of the level of DCD, we can also glean addi- nents are rarely isolated. If there are ISI. Although ISI is eliminated with tional information about our mea- multiple jitter components contribut- this repeating pattern, RJ and any surement results. With the time-cor- ing to the total system jitter in your PJ will still be present in the signal, related display of the jitter trend system, you end up viewing compos- which will contribute to convoluted waveform and data signal, we can see ite results—which can be difficult to measurement results. But there is that the trend waveform is in-phase

June 2004 33 High Frequency Design CAUSES OF JITTER with the data signal. This is an indi- ter analysis packages are described Agilent Technologies Product Note cation that the duty cycle of our pulse in [2]). This can be important for 5988-9592EN, www.agilent.com is less than 50%. Rising edges always determining if your high-speed digi- occur late (+error), and falling edges tal system meets a particular worst- Author Information always occur early (–error). case jitter and eye-opening specifica- Johnnie Hancock is a Signal Perhaps our transmitter thresh- tion. But knowing how much random Integrity Applications Engineer with- old level is too high, or perhaps the jitter and deterministic jitter is in in Agilent Technologies Electronic output of the our transmitter gener- your system usually doesn’t give you Products Group. He is responsible for ates slower falling edge speeds as a clue as to where it is coming from. worldwide application support activi- compared to faster rising edge The key to finding sources of jitter ties for Agilent’s high-performance speeds. At this point if we believe the lies in the ability to time-correlate jit- digitizing oscilloscopes. He has a peak-to-peak level of DCD is exces- ter measurement results with high- degree in Electrical Engineering from sive, we can set up additional charac- speed serial data signals, as well as the University of South Florida and terization tests to measure the duty with other possible sources of uncor- he holds a patent on digital oscillo- cycle of each pulse in the data stream related periodic jitter. A real-time scope amplifier calibration. He can be using jitter analysis. oscilloscope with jitter analysis along reached at johnnie_hancock@agi- In addition, if we suspect that the with the stimulus-response tech- lent.com DCD is caused by asymmetry in the niques described in this article meet rising and falling edge speeds, we can that critical time-correlation require- Parts 1 and 2 of this series on jitter then set up the instrument and jitter ment to relate jitter trend measure- appeared in the April 2004 and May analysis to characterize each rising ment results to measured signals. 2004 issues of High Frequency and falling edge in the data stream. Once you are able to time-correlate Electronics. These articles, as well Additional Techniques—Refer- particular real-time timing error as all past technical articles and ence [1] contains an example of iso- measurements to particular bits columns, are available online at: lating ISI that is caused by a system within a serial data pattern, it usual- www.highfrequencyelectronics.com bandwidth problem. There is also a ly becomes a routine troubleshooting From the web site main page, just discussion of how to measure shape task to solve your deterministic jitter select “Article Archives” and choose and frequency of designed-in modula- problems. the desired article from the contents tion of serial data with spread-spec- of the various past issues. trum clocking. References 1. Finding Sources of Jitter with Archived articles are intended for the personal use of our readers and their professional col- Conclusions Real-Time Jitter Analysis, Agilent leagues and may not be incorporated into other Some real-time jitter analysis Technologies Application Note publications or used in any commercial man- packages give answers in terms of AN1448-2, www.agilent.com ner without the express written permission of Summit Technical Media, LLC, publishers of the amount of total jitter that may be 2. Jitter Solutions for Telecom, High Frequency Electronics. present in your system. (Agilent’s jit- Enterprise and Digital Designs,