HP 54620A, User'S And Service Manual

Page: 134 / 243

The minimum duration trigger setting is two sample periods or 16 ns,
whichever is greater. And, the resolution of the duration trigger is one
sample period. How might this affect your measurement? Suppose you have
the sweep speed set to 2
µ
s/div, and have a duration trigger set up for
duration > 48 ns (three sample periods of 16 ns). If you then set the sweep
speed to 10
µ
s/div, the trigger will be lost, because the minimum duration
trigger value at the new sweep speed is 160 ns (two sample periods of 80 ns).
See chapter 7, “Performance Characteristics,” for more information.
Delayed Sweep and the Time Reference
When you enable the delayed sweep view, the analyzer divides the window
into two vertical portions. The upper part shows the waveform according to
the main time base; the lower shows a small portion of the waveform
magnified according to a second time base (the delayed sweep). In delayed
sweep, the Time/Div knob adjusts the magnification of the delayed sweep
portion. The delayed sweep time base must be at least twice as fast the main
time base and can vary up to the maximum sweep speed of 5 ns/div. For
example, at a main time base setting of 500
µ
s/div, the delayed sweep time
base will be adjustable from 200
µ
s/div to 5 ns/div.
You must be careful not to expand the delayed sweep time base such that no
useful data is displayed. If the main time base setting is 500
µ
s/div, the data
was obtained at a sampling interval of 4
µ
s. Expanding the delayed time base
so that no original samples appear in the delayed sweep will not yield any
useful information. For this example, expanding the delayed sweep time base
to 1
µ
s/div yields two or three samples in the delayed sweep window.
Expanding the delayed sweep time base beyond 1
µ
s/div will not be useful.
There are two different delays: Main and Delayed. The horizontal mode
determines which delay is used. When the horizontal mode is set to Main, the
delay positions the main sweep relative to the trigger. This delay is a fixed
number of samples in the positive and negative directions, so the time value
of this delay is a function of sampling interval and therefore a function of
sweep speed. Changing this delay value does not affect the sampling interval
or sweep speed, however. When the horizontal mode is set to Delayed, the
delay controls the position of the delayed sweep window inside the main
sweep display. This delay value is independent of sampling interval and
sweep speed.
Ensuring Accurate Measurements
Time base and Acquisition
133