Oscilloscope Trigger Hold Off

　　Hello everyone, sometimes we encounter such waveforms in waveform measurement. According to the waveform characteristics, the appropriate edge trigger has been set, but the waveform still cannot be displayed stably. What is going on? What should we do in this case?

　　There is a function in the trigger setting to set the trigger holdoff time. The holdoff time refers to the time the oscilloscope waits to re-enable the trigger circuit. During the holdoff period, the trigger circuit is closed and the trigger function is suspended. During the holdoff time, the oscilloscope will not trigger even if there is a waveform that meets the trigger condition.

　　The trigger holdoff function is specially designed for large-cycle repetitions where there are many non-repeating waveform points that meet the trigger conditions. If the holdoff function is not used, the trigger point is not fixed, which will cause unstable display. After the oscilloscope uses the trigger holdoff function, after the oscilloscope is triggered for the first time, the second trigger can only be performed after the trigger holdoff time has passed. In the figure, the holdoff time can be set to a value of >200 ns but <600 ns. After using trigger holdoff, the trigger is triggered at the same point each time, so the display can be stable.

　　Figure 1 Trigger holdoff time

　　We press the [Trigger] key, and in the trigger setting menu, we can set the trigger holdoff time. Turn knob A for fine adjustment, and knob B for coarse adjustment. Generally, the holdoff time should be set longer than one cycle, but it cannot enter the next cycle. Observing the waveform on the screen, we can see that we only need to set the trigger holdoff time slightly longer than 4us, but not longer than one cycle.

　　Figure 2 Screen waveform

　　It should be noted here that the trigger holdoff time does not refer to the dead time. The purpose of setting the trigger holdoff time value is to increase the dead time to stably capture and display some special signals.