Pattern Sequence

The Jitter/Noise analysis software can analyze the jitter on any clock waveform or NRZ serial data waveform, no matter what binary sequence is present in the data. It analyzes clock waveforms by treating them as data waveforms with alternating 1/0 data patterns. It uses two different analysis techniques depending on whether you specify that the data pattern is periodic or non-periodic. The periodic method is usually preferable for periodic data applications because it runs considerably faster than the arbitrary method. Note, however, that there is a maximum pattern length limitation when you use the periodic analysis method.

• Periodic — Periodic should be used on waveforms with repeating data patterns such as Pseudorandom Bit Sequences (PRBS).

  In order to isolate the component of jitter that is correlated to the data pattern, the TIE function of the jitter is calculated and each TIE value is associated with a specific bit in the source waveform's logical bit sequence. This is done by extracting the logical bit sequence from the source waveform and determining the length in bits of its periodic pattern.

  The original TIE function is decimated into sub-sampled TIE functions, where all of the values in each sub-sampled function correspond to a specific bit within the pattern. The number of original samples that are skipped when decimating depends on the RJ Bandwidth or RN Bandwidth setting in the Jitter Measurements and Noise Measurements tabs. The Narrow (Pink) selection maximizes the decimation ratio, while the Wide (White) selection minimizes it.

  RJ/DJ measurements are performed on a data pattern that repeats itself over some number of bits in a waveform. To specify the pattern, click Specify Pattern... to open the Signal Type Setup dialog box. See Signal Type Setup.

• Arbitrary — Arbitrary should be used on waveforms with non-repeating patterns such as those carrying live traffic. In arbitrary data, the pattern is unknown.

  When Arbitrary is selected, the RJ/PJ separation algorithm relies on the randomness of the pattern to spread the amplitude-modulated PJ broadly across the frequency spectrum. If you see the "pattern too periodic" message, too many of the modulated PJ spurs remain too large to be reliably subtracted from the RJ spectra.

  When Arbitrary is selected, the TIE values cannot simply be averaged and associated with specific locations within the bit sequence. Instead, a formula determined for calculating the DDJ from the surrounding data bits is used. The formula used is a transitional ISI (inter-symbol interference) filter. This filter works like a conventional FIR (finite impulse response) digital filter, except that it calculates the DDJ value of each transition from the polarity of the transitions that surround it. The ISI filter actually consists of four separate sets of weighted-coefficients; rising victim-rising aggressor, rising victim-falling aggressor, falling victim-rising aggressor, and falling victim-falling aggressor. The four different sets of coefficients enable EZJIT Plus to accommodate non-linear effects in the DDJ.

  These controls let you specify how many leading and lagging coefficients are contained in the ISI filter. The filter coefficient values are calculated which minimizes the squared-error between the measured TJ and the calculated DDJ. The larger the number of coefficients the longer it takes to display the graph. The following picture shows a graph where the ISI Filter Lead is set to -5 and the ISI Filter Lag is set to 20.

  

  Notice that from -5 to -2 all of data are zero which means that the ISI Filter Lead could be set to -1. Also, from 15 to 20 the data are zero which means that the ISI Filter Lag could be set 15. Making the filter range from -2 to 15 would speed up the display of the graph and still contain all of the information you need. Therefore, by increasing the filter range until zeros appear is a good way to set the ISI Filter Lead and ISI Filter Lag.