3. The last line of the display will show the prompt:
Enter Start %.
4. Enter the desired Start % setting through the numeric keypad.
5. Press [UNITS].
6. The Enter Start % indication will change to:
Enter End %.
7. Enter the desired End % value and press [UNITS].
(Repeated pressing of any Units key will toggle the display between the Start and End values.)
8. To change the start and stop percentages for rise and fall times, move the cursor to the
appropriate line and repeat Steps 3 through 7.
The relationship of 10% to 90% rise or fall time equals 2.198 time constants for a single pole
circuit. Similarly, a 10% to 90% power rise or fall time equals 1.098 time constants.
For some applications it is desirable to measure rise, fall, and width in terms of the voltage
waveform instead of the power waveform. This can be done by simply modifying the appropriate
start and stop percentages. The power waveform is proportional to the square of the voltage
waveform. Therefore, to measure the voltage pulse parameters you would change the default
percentages to the following:
Measurement Start % Stop %
Rise time 1% rising edge 81% rising edge
Fall time 81% falling edge 1% falling edge
Width 25% rising edge 25% falling edge
The pulse rise time, fall time, and width are displayed with the appropriate time units. A display
consisting of all dashes indicates that:
a. the instrument failed to find the peak power point, or
b. the instrument failed to find a start percentage point, or
c. the instrument failed to find a stop percentage point.
The method used to find pulse parameters is as follows. On each trace sweep, the instrument:
A. finds the 100% pulse level. Starting from the trailing edge of the pulse, the instrument looks earlier
and finds the largest flat section (±1 pixel on the screen) on the top of the pulse. This level is chosen
as the 100% level. If no area is flat, (such as with gaussian shaped pulses) the maximum level found
is used as the 100% level. This algorithm helps ignore the ringing and overshoot that sometimes
accompanies rectangular pulses. Note that the display scaling (i.e., the REFerence PoWeR) is not
reset and is independent of the 100% level found in this step.
B. finds the rise time, fall time and width of the pulse. In order to improve the accuracy of timing
measurements. You should set the delay window so that the pulse fills at least 20% of the screen.
Operation
Manual No. 20790, Rev C, November 1998
2-27
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