Jitter in Clock Sources

Continuous advances in high-speed communication and measurement systems require higher levels of performance from system clocks and references. Performance acceptable in the past may not be sufficient to support high-speed synchronous equipment. Perhaps the most important and least understood measure of clock performance is jitter.

The purpose of this discussion is fourfold.

1. To define jitter intuitively, and discuss it's properties.
2. To explain how jitter degrades system performance.
3. To describe various practical methods of measuring jitter, including the relevance and ease of each method.
4. Offer guidelines for specifying high-speed clocks and related devices.

• Jitter: "Short-term variations of the significant instants of a digital signal from their ideal positions in time" (ITU).

The expected edges in a digital datastream never occur exactly where desired. Defining and measuring the timing accuracy of those edges (jitter) is critical to the performance of synchronous communication systems.

Digital Waveform with Jittered Edges


Figure 1

Definition of terms:

1. Jitter expressed in Unit Intervals: A single unit interval is one cycle of the clock frequency. This is the normalized clock period. Jitter expressed in Unit Intervals describes the magnitude of the jitter as a decimal fraction of one unit interval.
2. Jitter expressed in degrees (deg.): Jitter expressed in degrees describes the magnitude of the jitter in units of deg. where one cycle equals 360 deg.
3. Jitter expressed in absolute time: Jitter expressed in units of time describes the magnitude of the jitter in appropriate orders of magnitude, usually picoseconds.
4. Jitter expressed as a power measurement is described in units of radians or unit intervals squared. Often expressed in dB relative to one cycle-squared (From Bellamy) [1]
5. Pattern Jitter: Pattern dependent jitter. Sometimes referred to as flanging. Not random in nature. Generally a result of subharmonics. When viewed in the time domain, it is seen as multiple modes of jitter. Pattern jitter is deterministic, it is a phenomenon that may be attributed to a unique source. All other jitter referred to in this discussion is stochastic in nature, and may only be described as a random variable with respect to time.

For example:

Assume a clock rate of 155.52 MHz. One unit interval would be equal to the period of the signal, 1/155.52 MHz = 6.43 nsec. = 360 deg.
Assume 100 ps Pk-Pk of jitter.
100 Ps of jitter = .01555 unit intervals (UI) of jitter = 5.598 deg. of jitter. (All Pk-Pk) All three measurements describe the same amount of jitter.

For jitter power, rms (one sigma, ) measurements are used. For the above case, we approximate Pk-Pk as 7, or 7 times the RMS value, placing the rms. jitter power at .0000049 UI². {(.01555/7)² . Expressed in dB, relative to one unit interval, jitter power in this case would be 10log (.0000049)= -53.1 dB ui. As will be seen later, jitter can be derived from power spectral density (phase noise) measurements. Table 1 relates various measures of jitter in a 155.52 MHz system clock.

Table 1

Jitter Bandwidth and Spectral Content

The displacement of edges in Fig.1 is a result of noise. Noise has spectral content as well as power. Consequently, the edge jitter in Fig.1 also has spectral content. The edges in Figure 1 vary randomly with time, however the noise that causes the jitter is not necessarily uniform over all frequencies. Jitter due to 10 kHz noise could be greater or less than jitter due to 100 kHz noise. Spectral content of clock jitter differs greatly depending on the technique used to generate the clock. Measured jitter also varies with measurement technique and jitter bandwidth. Improperly specified or measured jitter might result in unnecessary costs, or poor system performance. See references [2,3] for additional information on defining and specifying jitter in telecom systems. Jitter characteristics of various clock sources is discussed later in this article.

How Does Jitter Effect System Performance

The effects of jitter on communication systems are well beyond the scope of this discussion. Refer to references [1,4] for a more thorough treatment. A simple discussion may help to understand the deleterious effects of jitter in digital systems.
Every bit of data transmitted over synchronous communication systems is sampled for its value at the receiver. The sampled data can only have the value of logical one or zero. The optimum point for sampling data is at the center of each transmit clock cycle. In order to perform this function, the receiver aligns its own clock with the clock used to transmit the data. Figs.2, a, b, and c represent ideal, typical, and corrupted datastreams respectively. Commonly referred to as an "eye diagram" each graph is a cumulative graphical portrait of the edge placement due to noise or jitter. Ideally, sampling occurs at the center of the "eye". As edge jitter increases, the apparent eye begins to close. As a result, the likelihood of an error, i.e.…. mistaking a logical one for a zero is more likely. Jitter due to oscillator noise is only one source of jitter in a telecom system. System designers must consider many sources of noise in telecom systems. The jitter introduced by clock sources is one component of noise, and becomes only one part of an "error budget" that must be weighed against performance requirements and cost.

Fig. 2a

Fig. 2b

Fig. 2c

Measurement Techniques

Time Domain Measurements
Edge to Edge Jitter Using a Delay Line

A true measure of clock jitter is the accurate position of clock edges over time. The most direct method of examining the placement of edges would be to look at the edges using an oscilloscope. Unfortunately, using standard oscilloscope techniques it is impossible to identify individual clock edges in absolute time. Any jitter measured with a standard oscilloscope is due to trigger instability. As a result, direct waveform measurements using an oscilloscope (even a very good oscilloscope) are not valid measurements of jitter. An additional technique is used to locate the reference edge, discriminate with time, and examine the jitter on following edges. Figure 3 illustrates this method with a typical configuration.


Figure 3

The output of the unit under test is fed into splitter/delay line. The non-delayed output of the splitter is fed to the external trigger input of the oscilloscope (a CSA-803 in this case). The delayed output of the DL-11 is connected to the input of the oscilloscope. By examining the clock-stream at a time after the trigger equal to the delay used (in this case, 47 nsec), the trigger-edge is located. After the triggered edge has been identified, the next edge is examined. A histogram plot is then produced of the measured jitter of the second edge.

A CSA-803 is used for its statistical and histogram capabilities. This is a useful technique limited by the length of the delay line and the speed/sensitivity of the oscilloscope. For all frequencies greater than 1/(2d), the measurement is limited by the noise of the oscilloscope. Below 1/(2d), the sensitivity drops approximately 20 dB/decade. For the 47-nsec delay shown in fig 3, the corner frequency occurs at 3.3 MHz. All jitter due to frequencies above 3.3 MHz can be resolved to approximately 5 ps using the CSA-803. Jitter at 330 kHz can not be resolved below 50 ps. In a similar manner, jitter at 33 kHz can not be measured below 500 ps. Figure 4 is a plot of RMS jitter sensitivity using a 47-nsec delay line. It is critical to understand the advantages and limitations of this measurement method. For the numerical example given, low frequency jitter below 300 kHz would not be seen. Conversely, jitter due to sidebands 3.0 MHz offset or more could easily be identified. This test method is appropriate when measuring oscillators that employ direct frequency multiplication or where low frequency jitter is not considered. (See previous description of Pattern Jitter).


Sens(f)= Where:
Sens(f)=jitter sensitivity
d=amount of delay used
f=the jitter frequency to be measured
r=oscilloscope resolution

Fig. 4

Jitter Measurements Using Phase Locked Loops

It was noted that the length of the delay line limits resolution when measuring edge jitter. In order to measure jitter below 100 Hz offset, one needs merely to order up about three hundred miles of very low loss delay line. In lieu of such a device, phase locked loops are used for a variety of noise measurements.


Fig. 5

Figure 4 shows the basic elements of a phase locked loop (PLL) used to measure the noise of a clock source. Gardner [5], Best [6], and Woolover [7] are three excellent references for understanding PLLs. Some key loop requirements follow:

• The PLL loop bandwidth is a critical parameter for successful measurements. The system will only measure jitter frequencies outside (higher than) the loop bandwidth. It is recommended that the loop bandwidth be set to a maximum of 1/10 the lowest jitter frequency of interest.
• Loop damping must be set to at least 5 in order to reduce jitter peaking in the PLL. Jitter peaking will increase measured jitter.
• The measurement filter corresponds to jitter bandwidths recommended in Bellcore and ITU specifications. Refer to [2,3] for a list of jitter bandwidths and specifications. Band limiting needs to be defined as a pre-condition for any valid measurement.
• The output of the phase detector (PD) is a varying DC signal that is proportional to the varying phase due to jitter. It is necessary to know the gain constant (Kd) of the PD in volts/radian in order to quantify the detected jitter. For example, a phase detector with a Kd equal to 1 millivolt per degree will have a peak to peak output of 10 mV for an oscillator with 10 deg. Pk-Pk jitter. It may be necessary to inject a known amount of jitter in order to calibrate the system for accurate measurements.

Jitter Measurements

Using Phase Locked Loops: Interpreting the Data.

• In The Time Domain, the output signal of the phase detector in Figure 5 contains a wealth of information about the jitter of the measured clock. Direct examination of the signal using an oscilloscope can show Pk-Pk jitter. A true RMS voltmeter can be used to measure RMS (one sigma) jitter. For these measurements, it is critical that the measurement filters used represent the band of jitter frequencies of interest. It would make no sense to measure noise from dc to 10 MHz when a bandwidth of 10 kHz to 1 MHz is required. Oscilloscopes with histogram and statistical capabilities are useful for characterizing the measured jitter.
• In the Frequency Domain, the spectrum of the output signal from the phase detector in Figure 4 represents the spectrum and relative amplitude of jitter in the frequency domain. Examining the spectrum with a low frequency or FFT analyzer gives the most intuitive picture of clock jitter in terms of spectrum. By integrating the signal jitter spectrum over the frequency of interest, it is possible to derive the RMS jitter of the clock. This is the most accurate and unfortunately the most cumbersome method for characterizing jitter, requiring specialized test equipment. A numerical example is shown below and included in Appendix A.

Specifying Jitter Performance

Good jitter performance and low cost are not mutually exclusive as long as:

• The system requirements for jitter are defined in terms of amplitude and spectrum.
• The method used to generate the clock output frequency is optimal for the application.

System Requirements:

Although it is impossible to address all possible variations, some general recommendations based on years of oscillator manufacturing may be helpful.
While not a complete survey of all applications, Table 1 is a starting point for specifying oscillator performance. Jitter above 1 kHz is considered high frequency jitter.

Application Performance Requirements