User`s guide

PCI Express Theory of Operation
This section will give a basic overview of the methodology behind the KPWM measurement. In addition to
the data signal to be analyzed, this tool requires a pattern marker to be connected to the Arm Channel.
The Marker signal has an edge relative to the same bit of the pattern each time the marker occurs. Since
no bit-clock is used, analysis of jitter is independent of clock-jitter effects, and because the Arm is not a
trigger, any jitter on the marker will not transfer to the measurement of the Data.
The following steps are performed automatically when a measurement is acquired using this tool. The
first step is to accurately measure the Unit Interval (UI). This is done by making a series of pattern length
measurements, calculating the mean and dividing that time by the length of the pattern in UI. This
results in the Average or ideal UI and an accurate bit rate. Subsequent measurements and analysis will
compare to this ideal UI and the jitter is displayed as the deviation from this ideal bit time.
Pattern Marker
Data Pattern
After the UI measurement, a pattern match of the data must be done to identify the measured data
stream relative to the expected bit sequence as well as the phase relationship to the pattern marker. This
eliminates the need to have the pattern marker at the beginning of the expected pattern. The expected
pattern is rotated against the measured pattern until it matches.
EXPECTED
IDEAL
PATTERN
MEASURED
PATTERN
PATTERN
MARKER
Next, the total DCD+ISI is calculated. Histograms are made of every edge in the pattern. The mean values
of these histograms are compared to the ideal edge locations. The measured mean location is subtracted
from the calculated ideal edge location. The worst-case positive edge location is added to the worst-case
negative edge location giving the total DCD+ISI. This can also be thought of as comparing the earliest
edge to the latest edge.
© WAVECREST Corporation 2005 Section 4 - GigaView
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