The concept of jitter and its measurement method

The concept of jitter and its measurement method
http: // 2007-7-20 Beijing Ocean Industrial Technology Co., Ltd.

Abstract: In digital communication systems, especially synchronous systems, as the system clock frequency continues to increase, time jitter becomes a key factor affecting communication quality. This article introduces the concept and analysis method of time jitter.
Keywords: time jitter, jitter, phase noise, measurement

1. Introduction As the clock rate in communication systems enters the GHz level, jitter, a very critical factor in analog design, has begun to gain increasing attention in the digital design field. In high-speed systems, the timing error of the clock or oscillator waveform will limit the maximum rate of a digital I / O interface. Not only that, it also leads to an increase in the bit error rate of the communication link, and even limits the dynamic range of the A / D converter. Some data indicate that in systems above 3GHz, time jitter will cause inter-symbol interference (ISI), resulting in an increase in transmission error rate.
In this trend, designers of high-speed digital devices have begun to pay more attention to timing factors. This article introduces the basic concept of jitter to digital designers, analyzes its impact on system performance, and gives common circuit techniques that minimize phase jitter.
Second, the concept of time jitter

In an ideal situation, the duration of a perfect pulse signal with a fixed frequency (take 1MHz as an example) should be exactly 1us, with a transition edge every 500ns. Unfortunately, this signal does not exist. As shown in Figure 1, the length of the signal cycle will always change to a certain extent, resulting in an uncertain arrival time of the next edge. This uncertainty is jitter.
Jitter is a measurement result of the time-domain change of a signal. It essentially describes how much the signal period deviates from its ideal value. In most documents and specifications, time jitter is defined as the deviation of the arrival time of the edge of a high-speed serial signal from the ideal time. The difference is that in some specifications, the slowly changing component of this deviation is called time. Wander, and the component that changes more quickly is defined as time jitter. 1. Classification of time jitter There are two main types of jitter: deterministic jitter and random jitter.
Deterministic jitter is caused by identifiable interfering signals. This jitter is usually limited in amplitude, has a specific (rather than random) cause, and cannot be statistically analyzed.
Random jitter refers to changes in timing caused by factors that are more difficult to predict. For example, temperature factors that can affect the mobility of semiconductor crystal materials may cause random changes in carrier flow. In addition, changes in semiconductor processing technology, such as uneven doping density, may also cause jitter.
2. Time jitter description method The characteristics of jitter can be determined by many basic measurement indicators. The basic jitter parameters include:
1) Period jitter
Measure the width of each clock and data period in the real-time waveform. This is the earliest and most direct way to measure jitter. This indicator illustrates the change of the clock signal every cycle.
2) Cycle-cycle jitter
Measure how much the cycle width of any two adjacent clocks or data fluctuates. By applying a first-order differential operation to the cycle jitter, the cycle jitter can be obtained. This indicator has obvious meaning when analyzing the nature of trivial phases.
3) Time interval error (timer interval error, TIE)
It measures how much each active edge of the clock or data deviates from its ideal position. It uses the reference clock or clock recovery to provide the ideal edge. TIE is particularly important in communication systems because he illustrates the cumulative effect of periodic jitter in various periods.
3. Frequency domain representation of time jitter-phase noise Phase noise is another measure of the timing variation of a signal. Its jitter is displayed in the frequency domain. Figure 2 uses an oscillator signal to explain the phase noise.

If there is no phase noise, then the entire power of the oscillator should be concentrated at the frequency f = fo. However, the appearance of phase noise spreads part of the power of the oscillator to adjacent frequencies, creating a sideband. As can be seen from Figure 2, at an offset frequency that is a reasonable distance from the center frequency, the sideband power rolls off to 1 / fm, where fm is the difference between the frequency and the center frequency.
Phase noise is usually defined as the dBc / Hz value at a given offset frequency, where dBc is the ratio of the power at that frequency to the total power in dB. The phase noise of an oscillator at a certain offset frequency is defined as the ratio of the signal power to the total signal power within a 1 Hz bandwidth at that frequency.

3. Analysis method of time jitter Statistical characteristics and statistical histograms Since all signals containing jitter have random components, statistical calculations are widely used in the evaluation of jitter performance. Commonly used statistical parameters are average, standard deviation, maximum, minimum, peak-to-peak, etc. Usually use the form of histogram to describe these statistical characteristics of jitter.
The abscissa of the statistical histogram is the size of the jitter, and the ordinate is the frequency of the jitter in a certain interval. When the number of measurements is sufficient, the histogram is a good estimate of the probability density function of the size of jitter. Therefore, when estimating the bit error rate of the system through jitter, the statistical histogram plays an important role.

Figure 4 Statistical histogram of period jitter

It should be noted that the histogram does not contain the order in which each jitter point occurs, so it cannot be used to display the periodic information that exists in the jitter.
2. Jiiter—Time curve and frequency spectrum of Jitter Because the statistical histogram cannot display the modulation or periodic component information present in Jitter, the Jitter-time curve can be used to describe the trend of Jitter with time. The abscissa of the curve is the moment when the Jitter is measured, and the ordinate is the size of the Jitter. In this way, you can clearly see the change of Jitter with time

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