Oscilloscope Trigger Controls

Suppose you were looking at this waveform in an oscilloscope display:

This is a difficult waveform to trigger, because there are so many identical leading and trailing edges to trigger from. No matter where the trigger level control is set, or whether it is set for rising- or falling-edge, the waveform will tend to “jitter” back and forth horizontally on the screen because these controls cannot discriminate the first pulse from the other pulses in each cluster of pulses. At the start of each “sweep,” any of these pulses are adequate to initiate triggering.

One triggering control that is helpful in stabilizing such a waveform is the trigger holdoff control. Explain what this control does, and how it might work to make this waveform more stable on the screen.

A technician is measuring two waveforms of differing frequency at the same time on a dual-trace oscilloscope. The waveform measured by channel Ä” seems to be triggered just fine, but the other waveform (channel “B”) appears to be untriggered: the waveshape slowly scrolls horizontally across the screen as though the trace were free-running.

This presents a problem for the technician, because channel B’s waveform is the more important one to have “locked” in place for viewing. What should the technician do to make channel B’s display stable?

Another challenging sort of waveform to “lock in” on an oscilloscope display is one where a high-frequency waveform is superimposed on a low-frequency waveform. If the two frequencies are not integer multiples (harmonics) of each other, it will be impossible to make both of them hold still on the oscilloscope display.

However, most oscilloscopes have frequency-specific rejection controls provided in the trigger circuitry to help the user discriminate between mixed frequencies. Identify these controls on the oscilloscope panel, and explain which would be used for what circumstances.