HP 54750A, User Manual, Page: 424 / 478

How the Instrument W orks
Basics of Sequential Sampling
at a rate at least 40 GHz; far faster than even the fastest A/D converters
presently available .
T o overcome the limitations of Shannon's theorem, the instrument employs a
repetitive sequential sampling architecture . This approach makes use of the
principle of equivalent time to display high bandwidth waveforms using a low
bandwidth A/D converter . F or the equivalent time approach to display valid
waveforms , two conditions must be met:
1. The input signal must be repetitive .
2. A trigger signal at the same rate or a subharmonic of the input signal must
be available .
Repetitive signals are necessary because the displayed trace is built up from
samples taken over multiple repetitions of the waveform. (Eye diagrams ,
which represent statistical distributions rather than actual waveforms , are an
exception to this requirement.) The trigger signal is needed to control the
timing of the sampling process .
In the sequential sampling process , the input signal is sampled once per
trigger event. The rst trigger event in a sequence causes the input signal to
be sampled at an initial delay . On the next trigger event (at a subsequent
repetition of the waveform), the sampling instant is delayed by a small
amount relative to the initial delay . Each additional trigger event causes the
sampling instant to be delayed by greater amounts of time , so that after many
triggers , the input waveform builds on the screen from left to right.
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