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2-4
IM 701210-05E
2.2 Setting the Horizontal and Vertical Axes
• Zooming Horizontally and Drawing Waveforms
The DL750/DL750P is capable of expanding (zooming) the waveform horizontally
(see page 2-41). When the zoom rate of the waveform is increased, the number of
displayed points decreases. The waveform is displayed P-P compressed until the
number of display points falls to 2002 points. When the number of displayed points
falls below 1001, the waveform can no longer be displayed using continuous lines. In
this case, the display data is interpolated using the display interpolation function (see
page 2-37) so that the number of displayed points is 1001 points.
• Dot Display
By default, the display interpolation function works automatically. However, you can
also disable the display interpolation in which case the waveform is displayed using
dots. In this setting, up to 2002 points or 100100 points (select either 2 k or 100 k
points) of the acquired data are displayed without using P-P compression. For
example, if the number of displayed points is set to 100 k and the display record
length is 10 kW (the number of acquired data points is 10010 points), all the points of
the waveform are drawn by aligning 10 points vertically at the same time axis
positions. If the number of acquired data points exceeds 2002 or 100100, the data is
decimated down to 2002 or 100100 points (select either 2 k or 100 k points) for
displaying the waveform.
Relationship between the Time Axis Setting and Sample Rate/Record Length
If you change the time axis setting, the sample rate and the record length acquired to the
acquisition memory also change. For details, see appendix 1, “Relationship between the
Time Axis Setting, Sample Rate and Record Length.”
Sample Rate
If you change the time axis setting, the sample rate (the maximum sample rate varies
depending on the module) changes. The DL750/DL750P can only display waveforms
correctly up to one-half the frequency of the sample rate (the number of samples per
second, unit is S/s) as defined by the Nyquist sampling theorem.
1
1. If the sample rate is comparatively low with respect to the input signal frequency, the
harmonics contained in the signal are lost. In this case, some of the harmonics will appear at
low frequencies due to the effects described by the Nyquist sampling theorem. This
phenomenon is called aliasing. You can prevent aliasing by acquiring waveforms with the
acquisition mode set to envelope.
Aliasing signal
Input signal
Sampling point
Time Axis Setting and Roll Mode Display
If the trigger mode is set to Auto, Auto Level, Single, or Log and the time axis is set greater
than or equal to 100 ms/div, the waveform is displayed in roll mode. In roll mode, the
displayed waveform is not updated using triggers (update mode). Rather, the oldest data is
deleted as new data is acquired, and the waveform is shifted from right to left on the screen.
Roll mode display allows waveforms to be observed in the same way as on a pen
recorder. It is useful in observing signals with a long period or signals that change slowly.
It is also useful in detecting glitches (spikes in the waveform) that occur intermittently.