AC Electric Circuits

Oscilloscope Trigger Controls

15 questions By Tony R. Kuphaldt

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Question 1 of 15

A very useful tool for observing rotating objects is a strobe light. Basically, a strobe light is nothing more than a very bright flash bulb connected to an electronic pulse generating circuit. The flash bulb periodically emits a bright, brief pulse of light according to the frequency set by the pulse circuit. By setting the period of a strobe light to the period of a rotating object (so the bulb flashes once per revolution of the object), the object will appear to any human observer to be still rather than rotating:

One problem with using a strobe light is that the frequency of the light pulses must exactly match the frequency of the object’s rotation, or else the object will not appear to stand still. If the flash rate is mismatched, even by the slightest amount, the object will appear to slowly rotate instead of stand still.

Analog (CRT-based) oscilloscopes are similar in principle. A repetitive waveform appears to “stand still” on the screen despite the fact that the trace is made by a bright dot of light constantly moving across the screen (moving up and down with voltage, and sweeping left to right with time). Explain how the sweep rate of an oscilloscope is analogous to the flash rate of a strobe light.

If an analog oscilloscope is placed in the “free-run” mode, it will exhibit the same frequency mismatch problem as the strobe light: if the sweep rate is not precisely matched to the period of the waveform being displayed (or some integer multiple thereof), the waveform will appear to slowly scroll horizontally across the oscilloscope screen. Explain why this happens.

Reveal answer

The best “answer” I can give to this question is to get an analog oscilloscope and a signal generator and experiment to see how “free-run” mode works. If your oscilloscope does not have a “free-run” mode, you may emulate it by setting the trigger control to “EXTERNAL” (with no probe connected to the “EXTERNAL TRIGGER” input. You will have to adjust the sweep control very carefully to get any waveform “locked” in place on the display. Set the signal generator to a low frequency (10 Hz is good) so that the left-to-right sweeping of the dot is plainly visible, and use the “vernier” or “fine” timebase adjustment knob to vary the sweep rate as needed to get the waveform to stand still.

Notes:
Really, the best way I’ve found for students to learn this principle is to experiment with a real oscilloscope and signal generator. I highly recommend setting up an oscilloscope and signal generator in the classroom during discussion time so that this may be demonstrated live.
Question 2 of 15

Suppose a metal-detecting sensor were connected to a strobe light, so that the light flashed every time a fan blade passed by the sensor. How would this setup differ in operation from one where the strobe light is free-running?

Reveal answer

In this system, the fan would always appear to “stand still” in a position where a fan blade is near the sensor.

Follow-up question: how would the strobe light respond if the fan speed were changed? Explain your answer in detail.

Notes:
This question previews the concept of oscilloscope triggering: waiting until an event occurs before plotting the shape of a moving waveform. Often I find new students relate better to such mechanical analogies than directly to electronic abstractions when first learning oscilloscope operation.

An important detail to note in this scenario is that the strobe will flash four times per fan rotation!
Question 3 of 15

The only way to consistently guarantee a repetitive waveform will appear “still” on an analog oscilloscope screen is for each left-to-right sweep of the CRT’s electron beam to begin at the same point on the waveform. Explain how the “trigger” system on an oscilloscope works to accomplish this.

Reveal answer

The “trigger” circuitry on an oscilloscope initiates each left-to-right sweep of the electron beam only when certain conditions are met. Usually, these conditions are that the input signal being measured attains a specified voltage level (set by the technician), in a specified direction (either increasing or decreasing). Other conditions for triggering are possible, though.

Notes:
Triggering is a complex feature for students to grasp in even simple analog oscilloscopes. Spend as much time with students as you must to give them understanding in this area, as it will be very useful in their labwork and eventually in their careers.