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Measurement System
TDS 620A, 640A & 644A User Manual
3Ć89
NOTE
Cursors are displayed relative to the selected waveform. If you are
making a measurement using two waveforms, this can be a source
of confusion. If you turn off horizontal locking and adjust the horiĆ
zontal position of one waveform independent of the other, the
cursors appear at the requested position with respect to the seĆ
lected waveform. Gated measurements remain accurate, but the
displayed positions of the cursors change when you change the
selected waveform.
HighĆLow Setup
The HighĆLow Setup item provides two choices for how the oscilloscope
determines the High and Low levels of waveforms. These are histogram and
minĆmax.
H Histogram sets the values statistically. It selects the most common value
either above or below the mid point (depending on whether it is defining
the high or low reference level). Since this statistical approach ignores
short term aberrations (overshoot, ringing, etc.), histogram is the best
setting for examining pulses.
H MinĆmax uses the highest and lowest values of the waveform record.
This setting is best for examining waveforms that have no large, flat
portions at a common value, such as sine waves and triangle waves Ċ
almost any waveform except for pulses.
To use the highĆlow setup:
Press MEASURE ➞ HighĆLow Setup (main) ➞ Histogram or MinĆMax
(side). If you select MinĆMax, you may also want to check and/or revise
values using the Reference Levels main menu.
Reference Levels
Once you define the reference levels, the digitizing oscilloscope will use
them for all measurements requiring those levels. To set the reference levels:
1. Press MEASURE ➞ Reference Levels (main) ➞ Set Levels (side) to
choose whether the References are set in % relative to High (100%) and
Low (0%) or set explicitly in the units of the selected waveform (typically
volts). See Figure 3Ć47. Use the general purpose knob or keypad to
enter the values.
H % is the default selection. It is useful for general purpose applicaĆ
tions.
H Units is helpful for setting precise values. For example, if you are
trying to measure specifications on an RSĆ232ĆC circuit, you can set
the levels precisely to RSĆ232ĆC specification voltage values by
defining the high and low references in units.