Measure jitter three ways

Whether you test datacom ICs that exchange data with other chips on a board or you test telecom networks that send data many miles, you need to measure jitter—the difference between when a digital signal's edges should occur and when they actually occur. Jitter in clocks can cause misaligned bits in both electrical and optical data streams, leading to bit errors. By measuring jitter on clock and data signals, you can uncover the sources of bit errors.

Three types of instruments can help you measure and analyze jitter: bit-error-rate (BER) testers, jitter analyzers, and oscilloscopes (both digitizing scopes and sampling scopes). Figure 1 shows the bit rate that each instrument can handle.

Figure 1 The data rate you use can determine which type of instrument you need for measuring jitter.

You most likely will need a combination of instruments to track down jitter problems. Tommy Cook, R&D section manager at Agilent Technologies (South Queensferry, Scotland), says that many engineers start with a BER tester, then move to a jitter analyzer or oscilloscope to isolate the cause of the errors. (The application described in "BERT and scope " highlights one such instance.)

Manufacturers need to measure their products' BER to ensure the products comply with telecom standards. BER testing is also essential for testing high-speed serial data communications equipment when you need to characterize datacom components and systems.

A BER tester sends a predefined data stream, called a pseudorandom bit sequence (PRBS), through a system or device under test. It then samples each bit in a received data stream and checks incoming bits against the expected PRBS pattern. BER testers, therefore, can give you an exact BER measurement, something you can't get with jitter analyzers or scopes. (See "BER measurements reveal network health," Ref. 1.)

Figure 2 When looking for jitter and for bit errors, you get the best performance if you sample bits at the center of the eye.

Figure 2 shows how the scan works using the familiar eye diagram as a reference. You can program BER testers to sample incoming bits at any point in a bit's duration (called a "unit interval" or "UI"). The graph below the eye diagram (often called a "bathtub" curve) shows BER as a function of the sample location. If a BER tester checks bits at the center of a bit period (0.5 UI), then the probability that jitter will cause a bit error should be small. But if the BER tester checks bits at locations closer to the eye's crossover points, it will increase its likelihood of finding a bit error caused by jitter.

BER testers, however, may not provide enough information about the characteristics or sources of jitter. Jitter analyzers (often called timing-interval analyzers or signal-integrity analyzers) can measure jitter in any clock signal and provide you with information that can help you troubleshoot jitter. These instruments also use jitter characteristics to predict BER in considerably less time than a BER tester.

You'll find jitter analyzers useful for testing devices used in high-speed datacom buses such as Fibre Channel, SerialATA, Infiniband, and RapidIO at data rates up to 3.125 Gbits/s per channel (Ref. 2). Because jitter analyzers predict BER in just seconds, they are useful for production line testing, and many ATE manufacturers will install a jitter analyzer—specified by the customer—into their test systems.

Figure 3 Jitter analyzers and oscilloscopes can separate jitter into its components. Here, the jitter’s sinusoidal component is clearly visible, which can reveal clues as to the source of the jitter. Courtesy of Anritsu.

Figure 3 shows a type of deterministic jitter, which has a specific source. An interfering signal phase modulates the reference signal in the upper trace to produce the jitter in the measured signal in the lower trace. Jitter analyzers can calculate the frequencies present in jitter (Phases 1–4). Once you know the jitter frequency, you can often isolate the jitter source and mitigate its effects. If the interfering signal's frequency corresponds to another clock frequency, for example, you may be able to solve the problem by adding EMI shielding or otherwise isolating the source.

Two types of oscilloscopes prove useful for jitter measurement and analysis. To test devices, cables, subsystems, or systems that communicate at speeds up to 3.125 Gbits/s (the current highest speeds possible for transmitting data over copper), you can use a real-time sampling oscilloscope. Like jitter analyzers, these scopes can measure jitter in any clock signal, not just those used in communications. (See "Not just a communications problem " for an application example.)

To make measurements on optical signals such as OC-192 and 10 Gigabit Ethernet (9.952 Gbits/s) or OC-768 (39.808 Gbits/s), you need the 50-GHz to 75-GHz bandwidth of a sampling scope, such as the Agilent digital communications analyzer or the Tek communications signal analyzer. You can use these scopes on electrical data signals, too.

The high bandwidth of sampling scopes makes them useful for measuring jitter at the highest bit rates in use today. Because of their low sampling rates (150 ksamples/s or less), they require repetitive signals such as PRBS patterns to build eye diagrams from which they can build jitter histograms.

Figure 4 Oscilloscopes can display a plot of a time-interval error, which can take on periodic characteristics. A FFT of the time-interval error reveals the frequency, and a histogram reveals the jitter distribution. Courtesy of LeCroy.

The timing-error plot is effectively an instantaneous phase plot of the data stream. It shows that the jitter contains a periodic component. A fast Fourier transform of the timing-error plot (third trace, in blue), scaled to 1 MHz/div, reveals the frequency of the jitter. That frequency could correspond to a switching power supply's clock frequency or it could come from crosstalk in the system's data cables.

Figure 5 shows another example of data that a scope can provide. This histogram of an eye-diagram's crossover point reveals a distribution with two peaks. The twin peaks indicate deterministic jitter, which comes from an outside source of interference such as a switching power supply. You can often trace deterministic jitter back to its source. "Jitter Fundamentals Brochure," listed in "Jitter lessons," above, provides a good explanation of deterministic jitter. The other type of jitter—random jitter—follows a Gaussian distribution, and you can't determine its source.

Figure 5 The twin peaks in this jitter histogram indicate deterministic jitter. The distance between teh peaks indicates the deterministic jitter's amplitude. Courtesy of Tektronix.

When you need to equip a lab or production facility with jitter-measuring equipment, you must decide how many of these instruments to purchase. You may think you can forgo BER testers and jitter analyzers and purchase just a scope. Because they take a series of samples on a waveform, oscilloscopes can display more information about a signal than BER testers and jitter analyzers without scope displays can. By looking at the entire waveform, and not just the edges, a scope can provide information about a signal's amplitude as well as its timing.

On the downside, taking all those samples means that, when measuring jitter, you'll get samples containing amplitude information (although today's scopes contain sufficient memory for many jitter-measurement applications). You won't get all that extra information with jitter analyzers or BER testers, but these other instruments give you more edge samples.

You'll hear conflicting claims from equipment manufacturers about which instruments to use. BER tester manufacturers will tell you that only their equipment gives you enough samples. Jitter-analyzer manufacturers will tell you that scopes can't give you enough samples to accurately measure jitter. Scope manufacturers will argue that scopes have enough memory to do the job and that only scopes let you see a signal's amplitude. Tim Margeson, product manager at Tektronix (Beaverton, OR), states that "in some labs, all three types of equipment have roles to play due to their unique capabilities."

Recently, some test-equipment makers have developed hybrid instruments. BER testers, which traditionally reported bit errors only, now perform some jitter analysis, and some even include sampling oscilloscopes. Jitter analyzers may now also contain sampling oscilloscopes; one example is the Wavecrest SIA-3000 (see Product Update ). These sampling scopes let you view eye diagrams but they don't have the bandwidth that you get from stand-alone sampling scopes. The scope bandwidth of a hybrid instrument currently tops at 6 GHz. Real-time and equivalent-time sampling scopes now offer software that measures jitter and estimates BER. They key word here is "estimate," for you can get a true measure of BER with a BER tester only.