Acquiring and Displaying Waveforms
3–20
TDS 500B, TDS 600B, & TDS 700A User Manual
Eventually the time period established by scale setting does not allow enough
time to get all the real samples needed to fill the record.
The situation just described occurs if you set the Horizontal SCALE knob to a
time base setting that is faster than 10 ns (TDS 600B). (The setting varies with
the number of channels for TDS 500B and TDS 700A models; see Tables 3–4
and 3–5 beginning on page 3–26.) The oscilloscope then interpolates to create
the intervening points in the waveform record. There are two options for
interpolation: linear or sin(x)/x. (TDS 500B and TDS 700A models can also
equivalent-time sample to acquire more samples; see Equivalent-Time Sampling
on page 3–19.)
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 want to set the display
style so that the real samples are displayed intensified relative to the interpolated
samples. The instructions under Select the Display Style on page 3–29 explain
how to turn on intensified samples.
A TDS 500B or TDS 700A Oscilloscope can interleave its channels to attain
higher digitizing rates without equivalent time sampling or interpolating. The
oscilloscope applies the digitizing resources of unused channels (that is, channels
that are turned off) to sample those that are in use (turned on). Table 3–2 lists
how interleaving more than one digitizer to sample a channel extends the
maximum digitizing rate.
Once you set horizontal scale to exceed the maximum digitizing rate for the
number of channels in use (see Table 3–2), the oscilloscope will not be able to
get enough samples to create a waveform record. At that point, the oscilloscope
will either interpolate to calculate additional samples or it will switch from real
to equivalent time sampling to obtain additional samples. (See Interpolation on
page 3–19 and Equivalent-Time Sampling on page 3–19.)
Interleaving
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