Teledyne LeCroy PeRT3 Eagle Systems - Manual (Page 34)

Version 1.6 PeRT 3 Eagle User Manual
30
LeCroy Corporation
Random Jitter
The Random Jitter panel allows the user to introduce and control
random jitter into the signal.
Random jitter can be low frequency (LF), high frequency (HF) or a
combination of the two. Random jitter (either LF or HF) can be
controlled over the range from a minimum available value
(established by calibration of the system) to 12 ps in increments of
0.1 ps. The value selected is the root-mean-square (RMS) of the
jitter introduced.
When used in this context, jitter is the variation in pulse position in
the time domain. In an ideal jitter-free system, each electrical transition would occur at
precise intervals defined by the data rate of the bit stream (e.g., 6 Gb/sec). In practice,
there is always some level of jitter as subsequent pulse edges vary slightly from the ideal,
and as this variation becomes appreciable relative to the length of each pulse, the
receiver has more challenge to correctly determine the value of the bit.
Since random jitter is specified in picoseconds (ps), the degree to which the bit stream is
degraded through the introduction of random jitter will also depend on the data rate. For
a data rate of 1.5 Gb/sec, the width of each bit is 667 ps, so an RMS jitter of 6 ps would
introduce an average variation of approximately 1% of the pulse width (or unit interval).
For a data rate of 6 Gb/sec, the pulse width is 167 ps, and the same 6 ps RMS jitter is an
average variation of approximately 4% of the pulse width (or unit interval).
The random jitter is generated by a physical noise source with a roughly Gaussian
distribution, so while the RMS of the random jitter to 4% of the unit interval, a small
number of transitions will have considerably larger jitter. On average one in a trillion
(10
12
) transitions will have 14 times as much jitter (or 56% of the unit interval in the case
of 4% RMS).
The Low Frequency Random Jitter is generated by passing the random source through
a low pass filter as called for in the PCIe compliance specification. The resulting jitter is
bandwidth limited and will not necessarily be measured as "random" jitter on many
scopes or other jitter measurement tools and packages that define random jitter as jitter
with a flat frequency spectrum and use frequency analysis to separate random jitter from
deterministic jitter. The High Frequency Random Jitter source, on the other hand, is not
frequency limited and will appear as random jitter when measured by such instruments.