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Capture Time
Long Capture Time At Maximum Sample Rate Yields Better
Measurements
LAB
in PDF format
Capture time, the usable signal
duration shown on an oscilloscope screen, is related to the scope's maximum sampling rate
and the length of the acquisition memory. LeCroy's WaveSurfer oscilloscope has a 250 kpoint per
channel acquisition memory which, in two channel mode, provides 100 times the capture
time at its maximum sampling rate compared to a similar scope which only offers a 10 kpoint
memory. For example, WaveSurfer provides up to 200 µs capture at its maximum sampling
rate of 2 Gigasamples per second (GS/s) compared with only a 2 µs capture time for a scope
with only a 10 kpoint acquisition memory.
Lets look at the real consequence
of this. Consider the serial peripheral
interface (SPI) signal shown in figure 1. The waveform is captured using a LeCroy
WaveSurfer scope with the standard 250 kpoint per channel maximum acquisition memory.
Figure 1 - A measurement of a 1 ms capture time sampled at 500MS/s using
500 kpoints of memory. The high sample rate produces excellent fidelity.
Figure 1 the poor performance of a scope with only 10 kpoint of acquisition
memory due to a lower sampling rate of only 10 MS/s
The memory channels are interleaved
to provide dual channel with 500 kpoint memories. This display uses a capture time of
1 ms to see two adjacent data bursts on trace C2. Trace C3 shows the corresponding clock
signal. Expanded (zoom) traces of both waveforms are shown in
the lower traces marked Z2 and Z3, respectively. The resulting
sample rate of 500 MS/s provides excellent time resolution and sufficient sample rate to resolve
the signal. Now consider Figure 3. This shows the same
signal acquired using a scope with only a 10 kpoint memory.
Figure 3 - A chart showing sampling rate as a function of capture time
For the same capture time the
sampling rate is only 10 MS/s. Note the loss of detail in the clock pulses and the poorly defined
transition times of the data signal. This is caused by the low
sample rate which effectively limits time resolution of the measurement. Although the unzoomed
view of both scopes look similar, it is obvious that the LeCroy WaveSurfer provides
more information about this signal.
From this example you can see
why capture time is important. You will need long capture time
for several reasons:
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If you require the ability
to capture a mix of signals that are spaced far apart in time
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You need to view a long
pre-trigger time interval.
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You need the ability to
capture both high frequency and low frequency signals in a single
measurement
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There is a long delay between
the trigger event and the measurement point.
To effectively use a scope over a
wide range of capture times it needs to have adequate acquisition
memory to do the job. Figure 3 shows the relationship between capture time and sampling
rate for the WaveSurfer and another scope in its class with 10kpoints per channel memory.
For a capture time of 1 ms, marked by a dotted red line, the WaveSurfer has a sampling rate
that is 50 times higher MS/s vs.10 Ms/s. This means that the scope with the longer memory
has a time resolution 50 times
higher than the scope with the shorter memory.
It's not enough for a scope to
have a high maximum sampling rate. Without a long enough memory the capture time at the
high sample rate will be limited to very short captures.
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