Risetime Measurements Risetime measurements employ basically the same techniques as time-duration measurements. The main difference is the points between which the measurement is made. The following procedure gives the basic method of measuring risetime between the 10% and 90% points of the waveform. Falltime can be measured in the same manner on the trailing edge of the waveform. 1. Connect the signal to be displayed to the input of the Amplifier plug-in unit. 2. Set the Vertical and Horizontal Mode switches on the oscilloscope to display the plug-in units used. 3. Set the Volts/Division switch and the Variable Volts/ Division control of the Amplifier plug-in unit to produce a signal an exact number of divisions in amplitude. 4. Center the display about the center horizontal graticule line with the Amplifier plug-in unit Position control. 5. Set the MAIN TRIGGERING controls to obtain a stable display. 6. Set the TIME/DIV OR DLY TIME switch to the fastest sweep rate that displays less than 8 divisions between the 10% and 90% points on the waveform. 7. Determine the 10% and 90% points on the rising portion of the waveform. The figures given in Table 2-1 are for the points 10% up from the start of the rising portion and 10% down from the top of the rising portion (90% point).
Example: Assume that the horizontal distance between the 10% and 90% points is 4 divisions (see figure 2-8) and the TIME/DIV OR DLY TIME switch is set to 20 ns. Applying the time duration formula to risetime: Time Duration = (Risetime) horizontal distance (divisions) TIME/DIV OR x DLY TIME setting
Substitute the given values: Risetime = 4 x 20 nanoseconds = 80 nanoseconds
TABLE 2-1 Risetime Measurements
Vertical display (divisions) 4 5 6 7 8 10% and 90% points (divisions) 0.4 and 3.6 0.5 and 4.5 0.6 and 5.4 0.7 and 6.3 0.8 and 7.2 Divisions vertically between 10% and 90% points 3.2 4.0 4.8 5.6 6.4
Figure 2-8. Measuring risetime.
Delayed Sweep Measurements The delayed sweep mode can be used to make accurate time measurements. The following measurement determines the time difference between two pulses displayed on the same trace. This application may also be used to measure time difference from two different sources (dual-trace) or to measure time duration of a single pulse. 1. Connect the signal to be displayed to the Amplifier plug-in unit. 2. Set the Vertical and Horizontal Mode switches on the oscilloscope to display the plug-in units used. 3. Set the Volts/Division switch of the Amplifier plug-in unit to produce a display about 4 divisions in amplitude. 4. Adjust the MAIN TRIGGERING controls for a stable display.
8. Measure the horizontal distance between the 10% and 90% points. Be sure the VARIABLE control is set to CAL. 9. Multiply the distance measured in step 8 by the setting of the TIME/DIV OR [)LY TIME switch.
