Specifications
34
The acquisition time
is determined by the sampling rate, memory depth, and the details of how the
scope works. The scope takes a fixed amount of time
to perform the basic calculations needed to
display the acquired signal on the screen. If the user has certain features enabled, such as making
waveform measurements or placing cursors on the waveform, extra computing time
will be needed to
make the requisite calculations (a non-minimum holdoff time can contribute to
also). Then it takes a
time of
to write the information to the screen (one could argue that the time to write to the screen
could be included in the fixed processing time).
The maximum rate that the scope can display waveforms is determined by the total processing time
where:
=
+
+
+
=
+
Thus, assuming the times are measured in seconds, the maximum display rate is 1/
Hz.
The three times that are hatched in the figure comprise the dead time
of the scope. Any changes
to the signal that occur during the dead time are invisible to the user of the oscilloscope. The
dead time of a scope is typically not specified by manufacturers for basic general-purpose
oscilloscopes, so please don't assume the relative times as shown in Figure 22 are meaningful.
Here's another aspect of viewing waveforms on a scope. Suppose your scope's timebase is set to
1 µs/division. Further suppose that the scope displays a trace every 1 ms (i.e., one thousand traces per
second). Since the typical screen width is about 10 divisions, the screen represents 10 µs of time. Then
the "duty cycle" of the display is
s
ms
or 0.01. This “duty cycle” could vary because of the scope’s
architecture, deep memory, processing and display strategies, etc. Thus, on the surface, there's a 99%
chance that a single transient event will be missed by the scope (unless you triggered the scope on the
transient event). You could calculate this duty cycle more correctly if you knew the waveform display
rate, but this is typically not specified.
Analog scopes have a dead time too. This is typically the retrace time of the electron beam and any
holdoff time set by the user. It will, however, typically be shorter than a digital scope's dead time at a
given sweep speed. But there's a more subtle problem with analog scopes: if a waveform feature
occurs for only a short period, you may not be able to see it on the screen because of the limited
amount of light. This can be affected by the electron beam's intensity, the ambient light in the room, and
the user's attention level.
Every scope will thus have the notion of dead time and this dead time determines the events you can
and can't see. Since the dead time is typically not specified, you would have to experimentally measure
it to determine it.
Operation and features
This section gives an overview of using a digital oscilloscope and uses the B&K 254xB scopes as
examples. The control numbers will be as indicated in Figure 16.
The basic method of operating a digital oscilloscope is the same as the analog oscilloscope: you need
to set the vertical coupling, set the vertical gain to display the whole signal, set the timebase to a
suitable value to display one or more periods of the waveform, and choose trigger settings to allow the
scope to trigger. While this can be done in the same fashion as is done with an analog scope, the digital
scope provides an automatic measurement button.
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