HP 54645A, User'S And Service Manual

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To use holdoff
By setting the Holdoff, you can synchronize triggers. The oscilloscope will
trigger on one edge of the waveform, and ignore further edges until the
holdoff time expires. The oscilloscope will then re-arm the trigger circuit to
search for the next edge trigger. This allows the oscilloscope to trigger on a
repeating pattern in a waveform.
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Turn the holdoff knob in the trigger section to increase and decrease
trigger holdoff time.
Holdoff Operating Hints
Holdoff keeps a trigger from occurring until after a certain amount of time has
passed since the last trigger. This feature is valuable when a waveform crosses
the trigger level multiple times during one period of the waveform. Without
holdoff, the scope could trigger on each of the crossings, producing a confusing
waveform.
With holdoff set correctly, the scope always triggers on the same crossing.
The correct holdoff setting is typically slightly less than one period. Set the
holdoff to this time to generate a unique trigger point. This action works even
though many waveform periods pass between triggers, because the holdoff
circuit operates on the input signal continuously.
The HP 54645A/D uses digital holdoff independent of time base settings.
The advantage of digital holdoff is that it is a fixed number. As a result, changing
the time base settings does not affect the holdoff number; so, the oscilloscope
remains triggered. In contrast, the holdoff in analog oscilloscopes is a function
of the time base setting, making it necessary to re-adjust the holdoff each time
you change the time base setting.
The rate of change of the holdoff adjustment knob depends on the time base
setting you have selected. If you need a lengthy holdoff setting, increase the
time/div setting on the time base, then make your coarse holdoff adjustment.
Now switch back to the original time/div setting and make the fine adjustment to
reach the exact amount you want.
With HP’s MegaZoom technology, you can press Stop, then pan and zoom
through the data to find where it repeats. Measure this time using the cursors,
then set this number to the holdoff.
Triggering HP 54645A/D Oscilloscopes
To use holdoff
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