Acquiring and Displaying Waveforms
3–20 TDS 684A, TDS 744A, & TDS 784A User Manual
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 3–18 and Equivalent-Time Sampling on page 3–18.)
All three TDS Oscilloscopes support the following three acquisition modes:
Sample, Envelope, and Average. TDS 700A Oscilloscopes support two additional
acquisition modes: Peak Detect and Hi Res. Keep in mind which modes apply to
your model oscilloscope as you read the following descriptions.
Sample (the mode most commonly used), Peak Detect, and Hi Res modes
operate in real time on a single trigger event, provided that the oscilloscope can
acquire enough samples for each trigger event. Envelope and Average modes
operate on multiple acquisitions; the oscilloscope averages or envelopes several
waveforms on a point-by-point basis. (For TDS 700A models only, Hi Res,
Envelope, and Average modes are not available when in InstaVu mode; see
Incompatible Modes on page 3–45.)
Figure 3–13 illustrates the different modes and lists the benefits of each. It will
help you select the appropriate mode for your application. Refer to it as your
read the following descriptions of each mode.
Sample Mode. In Sample mode, the oscilloscope creates a record point by saving
the first sample (of perhaps many) during each acquisition interval. (An
acquisition interval is the time covered by the waveform record divided by the
record length.) Sample mode is the default mode.
Envelope Mode. Envelope mode lets you acquire and display a waveform record
that shows the extremes in variation over several acquisitions. You specify the
number of acquisitions over which to accumulate the data. The oscilloscope
saves the highest and lowest values in two adjacent intervals similar to the Peak
Detect mode. But Envelope mode, unlike Peak Detect, gathers peaks over many
trigger events.
After each trigger event, the oscilloscope acquires data and then compares the
min/max values from the current acquisition with those stored from previous
acquisitions. The final display shows the most extreme values for all the
acquisitions for each point in the waveform record.
Average Mode. Average mode lets you acquire and display a waveform record
that is the averaged result of several acquisitions. This mode reduces random
noise. The oscilloscope acquires data after each trigger event using Sample
mode. It then averages the record point from the current acquisition with those
stored from previous acquisitions.
The Acquisition Modes