Acquiring and Displaying Waveforms
TDS 684A, TDS 744A, & TDS 784A User Manual 3–19
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 684A). (The setting varies with
the number of channels for TDS 700A models; see Tables 3–4 and 3–5 begin-
ning on page 3–24.) 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 700A models can also equivalent time sample to acquire more
samples; see Equivalent-Time Sampling on page 3–18.)
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 3–28 explain
how to turn on intensified samples.
A 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.
Table 3–2: How Interleaving Affects Sample Rate
No. of Channels
Turned On
Digitizers Used
Per Channel
Maximum Digitizing Rate1
No. of Channels
Turned On
Digitizers Used
Per Channel
TDS 744A TDS 784A
One Four 2 Gs/second 4 Gs/second
Two Two 1 Gs/second 2 Gs/second
Three or Four One 500 Ms/second 1 Gs/second
1When real-time sampling. (Gs = Gigasamples; Ms = Megasamples.)
Interleaving