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These choices can be found on the Jitter. The two choices for jitter calculation method are shown . This opens the main configuration screen, which operates as a flow chart. See Figure 1. It is important to understand that the jitter measurements taken with the SDA II package as well as the SDA package are estimates based on the dual-Dirac jitter model and are not a simple calculation such as and RMS or standard deviation measurement. The importance of this fact cannot be understated!
The dual-Dirac jitter model describes jitter as having a dual-Dirac shape, with two Gaussian functions that grow with the bit error ratio. See Figure 2 and the suggested readings for additional details. Since it is virtually impossible to acquire enough events to directly measure total jitter at extremely small bit error ratios such as the commonly used value of 10 , a model must be employed to extrapolate beyond the measurements actually taken.
In the dual-Dirac model, total jitter Tj is the sum of deterministic jitter Dj and random jitter Rj , with Rj weighted by a multiplier corresponding to the bit error ratio. A variety of methods for estimating jitter have been developed and refined over the years, and the methods employed by the SDA II package are the sum of nearly a decade of effort to devise the best algorithms possible. The first step in the analysis flow is to calculate data-dependent jitter DDj.
With DDj removed, these traces are much more amenable to jitter analysis. Figure 3 shows the effect of removing DDj from the jitter Spectrum. The peak can be seen easily in the first division. The dual-Dirac Spectral method aims to quickly converge on jitter results that are accurate and that correlate well to other instruments.
These peaks are removed, and the remaining spectrum is associated with random jitter. This corresponds to the sigma value for the dual-Dirac Gaussian. Note: this algorithm is used beginning with firmware version 6.
Rj and Dj values are determined using the fitting method previously described in the "Dual Dirac Spectral Method" section. The results are posted to the Jitter Measurements table using the notation Tj 1e , Dj sp and Rj sp. The "1e " indicates that the user selected the BER level of for the analysis ". The mean positions of the Gaussians are also determined as in the Dual-Dirac Spectral method.
The results are posted to the Jitter Measurements table using the notation Tj 1e , Dj nq and Rj nq. The two methods offered by the SDA II package make different assumptions about the Gaussian distributions in the dual-Dirac model, and thus calculate different values for Tj, Rj and Dj. The NQ-Scale will typically converge on a more accurate result given its greater flexibility.
More statistics are also required with NQ-Scale in order to return a stable result. The Dual-Dirac Spectral method is known to return incorrect results when there is wide-band bounded uncorrelated jitter BUj. In other words, deterministic jitter with energy that is spread across a wide frequency band such that it cannot be detected as a peak in the RjBUjSpect trace will be incorrectly interpreted as Rj.
Examples include crosstalk from neighboring data channels and non-stationary periodic jitter sources like spreadspectrum clocks. The Dual-Dirac Spectral method also is hindered by its assumption that both Gaussians have the same underlying sigma and population.
Nonetheless, the Dual-Dirac Spectral method is fast and returns correct results in the majority of cases. In general when the results for the models are markedly different, using NQ-Scale is highly recommended. It is important to keep in mind that both methods return estimates of the total, random and deterministic jitter since the algorithms are model-based and are not a simple calculation based on the measured data. Summary The LeCroy SDA II analysis package contains two methods for estimating total jitter and for breaking down jitter into random and deterministic components.
This document describes the methods in non-mathematical language. Additional Formats PDF.
Understanding SDA II Jitter Calculation Methods
These choices can be found on the Jitter. The two choices for jitter calculation method are shown . This opens the main configuration screen, which operates as a flow chart. See Figure 1.
[8023-10GEPON] UIp-p in jitter budget tables
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