SDA Operator’s Manual
Jitter Measurement
Jitter is measured by the relative variation in the location in time of the transitions of the signal
level across a specific level. For clock signals, the relative time between threshold crossings
(rising-to-rising or falling-to-falling) is measured. Data signals, on the other hand, generally
require the measurement of the relative positioning of the data signal to the sampling clock, which
is related to setup and hold time. Because of its random nature, jitter is normally described in
terms of its probability density function (PDF).
Figure 6. Setup and hold time requirement for error-free operation. Data transitions within the setup
and hold time (gray area) will result in bit errors. Time interval error (TIE) is the time difference
between clock and data edges. The PDF of TIE is a measure of the probability of an edge occurring
during the setup and hold time.
The processes that make up jitter are complex and come from many different random and non-
random (deterministic) sources. The PDF of the jitter is the convolution of all individual
component PDF’s. Measurements are able to estimate the jitter PDF but are not able to
determine the distributions of the random and deterministic parts of the overall distribution. The
lack of exact measurements for the jitter distributions of Rj and Dj has lead to the use of a
simplified model for the total jitter. Equation 1 describes this model which was first presented in
the Fibrechannel MJSQ document.
SDA-OM-E Rev H 373
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