Scope Measurement Hints

Capture Fast, Low-Duty-Cycle Pulses Using Peak Detect

Capturing narrow pulses such as the 2.5 ns pulse shown in Figure 1 is not a problem for high speed digitizing scopes.

However, this pulse is part of a pulse train with a duty cycle of just .002%. In order to view the long time span between the narrow pulses, the sweepspeed must be reduced. With conventional sampling techniques, the scopeís sample rate is reduced such that the scope is no longer able to capture points on the narrow pulses. In the example shown in Figure 2, the scope samples at 500 KSa/s on the 100 microseconds/div timebase range and only randomly catches a point on one of the narrow pulses. The statistical odds of capturing a 2.5 ns pulse with a 2 microsecond sample interval is just 0.125% per acquisition.

However, by using peak detect mode, you can capture pulse trains with widths as narrow as 1 ns with 100% confidence, regardless of your scopeís sweepspeed. See Figure 3. Peak detect is a display mode that maintains the maximum sample rate of the scope independent of the sweepspeed. The max and min values are saved and displayed on the scope. Without peak detect, many of these max and min values would have been thrown away. The minimum pulse width that can be captured is a function of the sample rate of your scope.