安捷伦⽰波器使⽤⽅法
Jitter Analysis Techniques Using an Agilent Infiniium Oscilloscope
Product Note
Introduction
With higher-speed clocking and data transmission schemes in the computer and communications industries, timing margins are becoming increasingly tight.Sophisticated techniques are required to ensure that timing margins are being met and to find the source of problems if they are not.
This product note discuss
various techniques for measuring jitter and points out their
advantages and disadvantages. It describes how to t up an Agilent Infiniium oscilloscope to make effective jitter measure-ments and the accuracy of the measurements. Some of the
measurement techniques are only available on Agilent 54845A/B and 54846A/B oscilloscopes with version A.04.00 or later software.The measurement techniques are indicated as such in the text.Jitter Fundamentals
Jitter is defined as the deviation of a transition from its ideal time.Jitter can be measured relative to an ideal time or to another transition. Several factors can affect jitter. Since jitter sources are independent of each other,
a system will rarely encounter a worst-ca jitter scenario. Only when each independent jitter source is at its worst and is aligned with the other sources will this occur. As a result, jitter is statistical in nature. Predicting the worst-ca jitter in a system can take time.
Jitter can be broken down into two categories:
Random jitter is uncorrelated jitter caud by thermal or other physical, random
二四process. The shape of the jitter distribution is Gaussian.For example, a well-behaved pha lock loop (PLL) wanders randomly around its nominal clock frequency.?Deterministic or systematic jitter can be caud by inter-symbol interference,crosstalk, sub-harmonic
distortion and other spurious events such as power-supply switching. It is important to understand the nature of the jitter to help diagno its cau and, if possible, correct it. Deterministic jitter is more easily reduced or eliminated once the source has been identified.QQ怎么发说说
Jitter Measurement Techniques
This product note will discuss four methods that can be ud to characterize jitter in a system:
Infinite persistence
Histograms
Measurement statistics
Measurement functions
Infinite Persistence
Agilent’s Infiniium oscilloscopes prent veral different views of jitter. One way to measure jitter is to trigger on one waveform edge and look at another edge while infinite persistence is turned on. To
u this technique, t up the scope to trigger on a rising or falling edge and t the horizontal scale to examine the next rising or falling edge. In the Display dialog box, t persistence to infinite.
This technique measures
peak-to-peak jitter and does not provide information about jitter distribution. Infinite persistence is easy to t up and will acquire data quickly, giving you the best chance to e worst-ca jitter. However, since the tails of the jitter distribution theoretically go on forever, it will take a long time to measure the worst-ca, peak-to-peak jitter.It is important to understand
the
error sources of this measurement. This technique is subject to oscilloscope trigger jitter–the largest contributor to timing
error in an oscilloscope. Trigger jitter results from the failure to place the waveform correctly relative to when a trigger event occurs. Since the infinite persistence technique overlaps multiple waveform acquisitions onto the scope display, and each acquisition is subject to trigger jitter, the accuracy of this technique can be limited. If your jitter margins are being met using this technique, then more adv
anced measurement techniques are not necessary. This product note will discuss Infiniium’s timeba and trigger specifications in detail in a
later ction.
Histograms
This technique not only shows worst-ca jitter, but also gives a perspective on jitter distribution. Histograms do not acquire infor-mation as quickly as infinite persistence since each acquisition must be counted in the histogram measurement.
To t up a histogram, trigger on an edge and t the horizontal
scale and position so that you can悲剧性>南山忆
view the next rising or falling
edge. In the Histogram dialog box,
turn on a horizontal histogram
and t both Y window markers
to the same voltage. For example,
if a clock threshold is at 800 mV,
t both Y markers to this voltage.
Set the X markers to the left and
公共英文
right of the edge (figure 1). Figure 1. Histogram of edge showing bi-modal distribution
2
3
It is often possible to determine if the jitter is random or deter-ministic by the shape of the histogram. Random jitter will have a Gaussian distribution.Infiniium displays the percentage of points within mean +/- 1, 2,and 3 standard deviations to help in determining how Gaussian the distribution is. For
a Gaussian distribution the values should be 68%, 95%, and 99.7%, respec-tively. Non-Gaussian distributions usually indicate that the jitter has deterministic components.This technique has the same limitation on accuracy as the infinite persistence technique.Multiple acquisitions contribute to the histogram and they all contain the oscilloscope trigger jitter mentioned above. Measurement Statistics
The next method involves
computing statistics on waveform measurement results. For example,the scope can measure the period of a waveform on successive acquisitions. Simply drag the period measurement icon to the waveform that is to be measured.The statistics will indicate the mean, standard deviation, and min and max of the period measurements. You can let the scope run for a while to determine the amount of clock jitter prent. This measurement is not subject to trigger jitter becau it is a delta-time or relative measure-ment. Even if the waveform is not placed correctly relative to the trigger, the edges are measured
accurately relative to each other.
Figure 2. Setting up a jitter measurement
This measurement is subject to the timeba stability of the instrument, which is typically very good. This is a valid measurement technique but is slow to gather statistical information. Since the scope acquires a waveform, makes a measurement, and then acquires a waveform at a later time, most clock periods are not measured.With this technique, it is impossible to e how the period jitter varies over short periods of time. For example, if you have spread-spectrum clocking, this measurement will lump the slowest and fastest periods together.
The Agilent 54845A/B and
54846A/B Infiniium oscilloscopes can compute statistics on every instance of a measurement in a
single acquisition. To enable this capability, lect the Jitter tab,then check “Measure all edges” in the Measurement Definitions dia-log box (figure 2).
For example, instead of only measuring the first period on every acquisition or trigger event,every period can be measured and statistics gathered. This greatly increas the speed at which statistics are gathered and reduces the overall time to make jitter measurements. Statistics are accumulated across all
measurements in the acquisition and across acquisitions.
Pressing “Clear Display” will ret the measurement statistics.This feature is uful if you are probing the clock at different locations and want to ret the measurements.油光鱼
It is important to t up the scope correctly to make effective jitter measurements. Set the vertical scale of the channel being measured to offer the largest waveform that will fit on screen vertically. This will make the most effective u of the scope’s A/D converter.
The scope should be t to
real-time acquisition mode in
the Acquisition dialog box. Since equivalent-time sampling can combine samples from different acquisitions, the scope’s trigger jitter would adverly affect jitter measurements. The averaging function should be turned off since, again, this combines multiple acquisition data.
两头毛You may want to t the scope
to its maximum memory depth. This will make the scope less responsive to operate, but the scope c
an make many measure-ments on a single acquisition. Since jitter measurements are statistical, many measurements are desirable. Taking many acquisitions of small records will give a more random lection but will take longer than fewer large acquisitions. Having extremely deep memory is not necessary to getting good jitter measurements. Normally, measurements are
made at 10%, 50%, and 90% of the
waveform amplitude. This is
convenient for quickly making
measurements; however, when
making measurements across
acquisitions and combining
their statistics this is not the
best solution.
In the Measurement Definition
dialog box, Thresholds tab, the
measurement thresholds should
be t to absolute voltages. For
example, if you are making
cycle-cycle jitter or period
measurements, t the middle
voltage threshold to your clock
threshold. Set the upper and
lower voltage thresholds to
roughly +/- 10% of the signal
amplitude in voltage. This will
establish a band around the
threshold that the edge must go
through to be measured and will
eliminate fal edge detection.
In addition to the period jitter measurement, the cycle-cycle jitter measurement us the same technique. The cycle-cycle jitter measurement, available on Agilent 54845A/B and 54846A/B oscilloscopes, is the difference
of two concutive period measurements.猜测
Pi – P(i-1), 2 ≤i ≤n
Where
P is a period measurement and n is the number of periods in
the waveform.
Cycle-cycle jitter is a measure of the short-term stability of a clock. It may be acceptable for the clock frequency to change slowly over time but not vary from cycle to cycle. For this measurement, every period in the acquisition is measured regardless of how the “Measure all edges” lection is t. In this ca, the statistics reprent all of the cycle-cycle jitter measurements in one acquisition, or all acquisitions
if the scope is running.
If absolute clock stability is required, then a period measurement should be made. If your system can track with small changes in the clock frequency, then cycle-cycle jitter should be measured.
