Teledyne LeCroy JTA2 - Manual (Page 11)

JTA2-OM-E Rev A ISSUED: December 2003
9
CLOCK OR DATA?
For most waveform attributes, JitterTrack offers the choice of Clock or Data modes for measuring
clock signals or data streams. "Data" should be used (where available) when the pulse widths,
intervals, periods or other significant instants being measured are randomly distributed and
contain multiples of the clock period.
On the one hand, apart from jitter, clock signals ought to be regular. On the other hand, data
streams by their very nature have irregular pulse widths.
A clock signal is normally required to characterize jitter. But such a signal will not be available if
the waveform being measured is a data stream, whose very randomness hides the clock signal.
To overcome this, JitterTrack provides both Clock and Data modes. Selecting Data from the
VClock dialog gives the superior timing resolution through normalization (see table) required for
correctly measuring jitter in data signals.
The diagram on the previous page shows a data stream in relation to its clock signal. It illustrates
how data pulses contain, within themselves, multiples of their clock-signal pulse widths. Analyzing
the positive pulses in the data stream, we observe a great variance between each sample in, for
instance, the range
Τ to 3Τ . In fact, it is the small variations (the jitter) that are important. And
they could be normalized if clock frequency, and clock frequency over pulse width, were known.
This normalization, provided by JitterTrack, reduces pulse variations and increases timing
resolution so that errors (
ε ) can be clearly observed. It does this by reducing the jitter range,
dividing each measurement equal to n
× Τ by n.