Specifications

33
Since this measured waveform will be a high frequency, thousands of sampling windows can be used
to generate thousands of sample points per period -- and thus, the periodic waveform can be accurately
displayed by the oscilloscope.
Thus, equivalent-time sampling effectively increases the scope's sampling frequency for periodic
waveforms. However, be aware that the scope's vertical amplifier bandwidth will likely be the deciding
factor on what you're able to see with the scope.
Another important component of the digital oscilloscope is the waveform memory. The amount of this
memory determines the length of a sample that can be captured and displayed by the oscilloscope. The
relationship is:
=


where
t = captured time, seconds
S = sampling rate of scope, Hz
M = number of points of waveform storage memory
For example, when displaying a 1 kHz square wave, the B&K 2542B scope has a sampling rate of 25
MSa/s (million samples per second) when the timebase is set to 2 ms/division. Since the scope’s
memory can store 1.2 million points at this sampling rate, the time extent of a single captured waveform
by pressing the SINGLE button is (1.2 × 10

samples)/(25 × 10

samples/s) = 48ms.
The reciprocal of the sampling frequency gives you the time between points in the waveform. In the
previous example, the time between sample points is 1/(25 × 10

samples/s) = 40ns.
Dead time
A scope user should be cognizant of the notion of "dead time" (also called blind time). This is the time
during which the signal is not being sampled and displayed by the oscilloscope. Let's look at an
example situation. The scope acquires the waveform at a particular sampling rate during the time
occupied by the black area in the following diagram:
Figure 22
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