Waveform Differentation
TDS 620A, 640A, & 644A User Manual
When using the vertical scale knob to scale the source waveform, note
that it also scales your derivative waveform.
Because of the method the oscilloscope uses to scale the source waveform
before differentiating that waveform, the derivative math waveform may be
too large vertically to fit on screen — even if the source waveform is only a
few divisions on screen. You can use Zoom to reduce the size of the wave-
form on screen (see

Zoom

that follows), but if your waveform is clipped
before zooming, it will still be clipped after it is zoomed.
If your math waveform is a narrow differentiated pulse, it may not appear to
be clipped when viewed on screen. You can detect if your derivative math
waveform is clipped by expanding it horizontally using Zoom so you can see
the clipped portion. Also, the automated measurement Pk-Pk will display a
clipping error message if turned on (see

Automated Measurements of a

Derivative Waveform

on page 3-151).
If your derivative waveform is clipped, try either of the following methods to
eliminate clipping:
Reduce the size of the source waveform on screen. (Select the source
channel and use the vertical SCALE knob.)
Expand the waveform horizontally on screen. (Select the source channel
and increase the horizontal scale using the horizontal SCALE knob.) For
instance, if you display the source waveform illustrated in Figure 3-80 on
page 3-151 so its rising and falling edges are displayed over more hori-
zontal divisions, the amplitude of the corresponding derivative pulse will
decrease.
Whichever method you use, be sure Zoom is off and the zoom factors are
reset (see

Zoom

below).
Zoom
Once you have your waveform optimally displayed, you can also magnify (or
contract) it vertically and horizontally to inspect any feature. Just be sure the
differentiated waveform is the selected waveform. (Press MORE, then select
the differentiated waveform in the More main menu. Then use the Vertical and
Horizontal SCALE knob to adjust the math waveform size.)
If you wish to see the zoom factor (2X, 5X, etc.), you need to turn zoom on:
press ZOOM ➞ ON (side). The vertical and horizontal zoom factors appear
on screen.
Whether zoom is on or off, you can press Reset Zoom Factors (side) to
return the zoomed derivative waveform to no magnification.
See
Waveform Integration
, on page 3-154.
See
Fast Fourier Transforms
, on page 3-38.
See
Waveform Math
, on page 3-159.
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