APPENDIX II
UNIQUE CHARACTERISTICS OF DIGITAL STORAGE OSCILLSCOPES
Digital Storage Oscilloscopes (DSO’s) use a digital sam-
pling technique to convert analog input signals to a series of
digital words that can be stored in memory. Since digital
sampling has disadvantages as well as advantages, it is
important to be aware of these unique characteristics of
DSO’s.
ALIASING
This DSO uses Real Time Sampling when sweep
TIME/DIV settings of 50 sec/div to 20
µ
s/div are selected.
Real Time Sampling simply means that samples of the input
signals are taken at equal spaces (e.g., every 0.25 ms when
the 50 ms/div range is selected). With Real Time Sampling,
a phenomena called “Aliasing” can occur when the input
signal is not sampled often enough. This causes the digitized
signal to appear to be of a lower frequency than that of the
input signal. Unless you have an idea what the input signal
is supposed to look like, you will usually be unaware that
Aliasing is occurring.
Aliasing Example
To see what actually occurs when a Digital Storage Oscil-
loscope is Aliasing, perform the following example.
1.
Apply a 10 kHz signal to the input jack and set the
sweep TIME/DIV control to 50
µ
s/div. You should see
about 5 cycles of the waveform on the display. Since
the DSO samples the input waveform 200 times per
division, each cycle is sampled 400 times.
2.
Now change the sweep TIME/DIV control to 2 ms/div.
The display should look crowded. Because the DSO
takes 200 samples per division, the sample points are
10
µ
s apart. Since the input signal is at a frequency of
10 kHz, it is being sampled 10 times per cycle. The
resulting display is too crowded to be useful, however,
it is not incorrect (it is very similar to what you would
see on a conventional analog oscilloscope).
3.
Change the setting of the sweep TIME/DIV control to
20 ms/div. Vary the frequency a slight amount (until
the display is readable) to obtain as few cycles as
possible on the CRT. If you were to calculate the
frequency of the signal from the display, you would
come up with a much lower frequency than that of the
actual frequency of the signal at the input jack. As an
example, if three cycles are displayed, the calculated
frequency would be approximately 15 Hz. This is
obviously incorrect. This occurs because the DSO is
taking one sample every 0.1 ms and a 10 kHz signal
has one cycle every 0.1 ms. What is actually happening
is that the frequency is off (not perfectly 10 kHz) by
just enough to cause the DSO to take one sample at a
slightly different place on each cycle of the waveform.
Avoiding Aliasing
Aliasing is not limited to the above example. This phe-
nomenon can occur anytime that at least two samples per
cycle are not taken (whenever the sweep TIME/DIV setting
is much too slow for the waveform being applied to the
input). Whenever the frequency of the signal is unknown,
always begin with the fastest real-time sweep speed
(20
µ
s/div) or by viewing the waveform in the analog mode
of operation first.
NOTE
Viewing one-time events or glitches is not
possible when sweep TIME/DIV settings
higher than 20
µ
s/div are selected. Viewing
one-time events poses no problem with Ali-
asing because Aliasing can occur only with
repetitive waveforms.
EQUIVALENT TIME SAMPLING
This oscilloscope uses a sampling method called Equiva-
lent Time Sampling when sweep speeds higher than
20
µ
s/div are selected. This method permits viewing of
repetitive waveforms to 20 MHz, although the maximum
digital sampling rate is 10 Msamples/sec. When the Equiva-
lent Time Sampling mode is activated, one sample is taken
during each cycle. Of course if one sample is taken during
each cycle at the trigger point (the same point on each cycle),
only a flat trace would be produced. Therefore, it is neces-
sary to take each sample further (in time) from the trigger
point than the last sample. This incremental delay is deter-
mined by the sweep TIME/DIV control setting. Because
1024 (1 k) samples are needed to fill the display, the oscil-
loscope must sample 1024 cycles of the waveform.
Therefore, only repetitive waveforms should be observed
in this mode. Irregularities that are present on an otherwise
repetitive waveform are not likely to show up when the
Equivalent Time Samplling method is used. With only one
sample being taken during each cycle, it is very likely that
glitches and other irregularities will be skipped over.
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