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Analyzing jitter, timing in the presence of crosstalk

Posted: 28 Dec 2011 ?? ?Print Version ?Bookmark and Share

Keywords:Serial data standards? bit error ratio? jitter?

Serial data standards continue to proliferate, providing drastic enhancements in PC and server system performance. Testing these higher speed standards for evidence of jitter is critical for long-term stability and to achieving the objective of a good bit error ratio (BER) in the design. Effective analysis begins with selecting the right instruments and have a good understanding of instrument noise, rise time and factors such 3rd, 4th, 5th harmonic performance.

However, it's more than just taking the measurementthe proper instruments need to be paired with the proper analysis tools. And other factors such as jitter separation, and de-embedding/embedding are also important considerations when testing serial data rates beyond 8 Gb/sec. For this article, we will focus on a new approach to jitter separation in the presence of crosstalk, a growing problem as the number of lanes increases to boost computing system throughput.

All electrical systems that use voltage transitions to represent timing information have timing jitter. Historically, electrical systems have lessened the ill effects of timing jitter (or, simply jitter) by employing relatively low signaling rates. As data climb above 8 Gb/sec., jitter has become a significant percentage of the signaling interval, and understanding the types and sources of jitter is vital to successfully deploying high-speed serial technologies.

At its simplest, jitter is a deviation of an edge from where it should be as shown in figure 1. As the ITU defines it, jitter is "short-term variations of the significant instants of a digital signal from their ideal positions in time."

Figure 1: Jitter is the deviation of an edge from where it should be.

There are several ways in which jitter can be measured on a single waveform including period jitter, cycle-cycle jitter, and time interval error (TIE), and the design will often dictate which measurement is appropriate.

In the case of a standalone oscillator, the signal is a clock and it can be hopping or swept. Here period jitter is an appropriate measurement. In the case of a transmitter for a serial data stream, the signal is a data stream and ISI (inter-symbol interference) is a key problem. Here TIE jitter is the appropriate measurement.

The engineer on the prowl for jitter issues has a number of instruments available, each with unique strengths and weaknesses:

???A real-time digital storage oscilloscope (DSO) recovers the whole waveform and can measure anything and can be used for TIE, cycle-to-cycle and period jitter measurements. It has limitations, however, around frequency (or bit rate) and resolution of spectra, minute jitter and multilevel modulation.
???A BER Tester (BERT) is well suited for TIE jitter, particularly TJ or total jitter, a form of TIE. The advantage of the BERT is that is counts every bit, but the tests can be time consuming to perform.
???A real-time spectrum analyzer (RTSA) can be used for cycle-to-cycle and period measurements with complex modulations for mobile devices, looking at clocks, PLLs and understanding their dynamic performance. Limitations include span (sub-100MHz) and bandwidth signals with large modulation spectrum
???Equivalent-time sampling oscilloscopes offer the best bandwidth and can be used for all jitter measurements for serial data. Currently, these are the only instruments with noise analysis and a BER eye. Limitations include no real-time capture and can only be used for repetitive patterns and some jitter spectra are aliased.

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