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Acquisition
TDS 620A, 640A, & 644A User Manual
If you focus on only one channel at the maximum possible real-time rate, the
TDS 524A and TDS 620A oscilloscopes can acquire at 500 Megasamples/
second using both its digitizers, while the TDS 644A and TDS 640A oscillo-
scopes can combine all four digitizers and acquire at 1 Gigasample/second.
Depending on how many channels you are using and the speed of the time
base, at some point the digitizing oscilloscope
will not be able to get enough
samples to create a waveform record. (See the discussion on page 2-22 for
more details about when that happens.) At that point, the digitizing oscillo-
scope will create the waveform record in one of two ways depending on
whether you have limited the oscilloscope to real-time sampling or enabled
equivalent-time sampling (you make that choice in the Acquisition menu).
If you have restricted it to real-time sampling, the digitizing oscilloscope uses
a process called interpolation to create the intervening points in the waveform
record. There are two options for interpolation: linear or sin(x)/x.
Linear interpolation
computes record points between actual acquired samples
by using a straight line fit. It assumes all the interpolated points fall in their
appropriate point in time on that straight line. Linear interpolation is useful for
many waveforms such as pulse trains.
Sin(x)/x interpolation
computes record points using a curve fit between the
actual values acquired. It assumes all the interpolated points fall along that
curve. That is particularly useful when acquiring more rounded waveforms
such as sine waves. Actually, it is appropriate for general use, although it may
introduce some overshoot or undershoot in signals with fast rise times.
NOTE
When using either type of interpolation, you may wish to set the
display style so that the real samples are displayed intensified
relative to the interpolated samples. The instructions under
Display
Style
on page 3-28 explain how to turn on intensified samples.
Equivalent-Time Sampling — The digitizing oscilloscope only uses
equivalent-time sampling if you have enabled the equivalent-time option in the
Acquisition menu
and the oscilloscope is not able to get enough samples with
which to create a waveform record.
In equivalent-time (ET) sampling the oscilloscope acquires samples over
many repetitions of the event (Figure 2-7). It should only be used on repetitive
signals.