Oscilloscope Trigger Controls
AC Electric Circuits
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.
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.
Suppose an oscilloscope has been set up to display a triangle wave:
The horizontal position knob is then turned clockwise until the left-hand edge of the waveform is visible:
Now, the point at which the waveform triggers is clearly visible, no longer hidden from view past the left-hand side of the screen:
What will happen now if the trigger level knob is turned clockwise? How will this affect the positioning of the waveform on the oscilloscope screen?
Suppose an oscilloscope has been set up to display a triangle wave, with the horizontal position control turned clockwise until the left-hand edge of the waveform is visible:
Then, the technician changes the slope control, changing it from “increasing” to “decreasing”:
Draw the waveform’s new appearance on the oscilloscope screen, with the slope control reversed.
A student is experimenting with an oscilloscope, learning how to use the triggering control. While turning the trigger level knob clockwise, the student sees the effect it has on the waveform’s position on the screen. Then, with an additional twist of the level knob, the waveform completely disappears. Now there is absolutely nothing shown on the screen! Turning the level knob the other way (counter-clockwise), the waveform suddenly appears on the screen again.
Based on the described behavior, does this student have the oscilloscope trigger control set on Auto mode, or on Norm mode? What would the oscilloscope do if the other triggering mode were set?
Large electric motors and other pieces of rotating machinery are often equipped with vibration sensors to detect imbalances. These sensors are typically linked to an automatic shutdown system so that the machine will turn itself off it the sensors detect excessive vibration.
Some of the more popular industrial-grade sensors generate a DC voltage proportional to the physical distance between the end of the sensor and the nearest metallic surface. A typical sensor installation might look like this:
If the machine is running smoothly (or if it is shut down and not turning at all), the output voltage from the sensor will be pure DC, indicating a constant distance between the sensor end and the shaft surface. On the other hand, if the shaft becomes imbalanced it will bend ever so slightly, causing the distance to the sensor tip to periodically fluctuate as it rotates beneath the sensor. The result will be a sensor output signal that is an AC “ripple” superimposed on a DC bias, the frequency of that ripple voltage being equal to the frequency of the shaft’s rotation:
The vibration sensing circuitry measures the amplitude of this ripple and initiates a shutdown if it exceeds a pre-determined value.
An additional sensor often provided on large rotating machines is a sync pulse sensor. This sensor works just like the other vibration sensors, except that it is intentionally placed in such a position that it “sees” a keyway or other irregularity on the rotating shaft surface. Consequently, the “sync” sensor outputs a square-wave “notch” pulse, once per shaft revolution:
The purpose of this “sync” pulse is to provide an angular reference point, so any vibration peaks seen on any of the other sensor signals may be located relative to the sync pulse. This allows a technician or engineer to determine where in the shaft’s rotation any peaks are originating.
Your question is this: explain how you would use the sync pulse output to trigger an oscilloscope, so that every sweep of the electron beam across the oscilloscope’s screen begins at that point in time.
A student is trying to measure an AC waveform superimposed on a DC voltage, output by the following circuit:
The problem is, every time the student moves the circuit’s DC bias adjustment knob, the oscilloscope loses its triggering and the waveform begins to wildly scroll across the width of the screen. In order to get the oscilloscope to trigger on the AC signal again, the student must likewise move the trigger level knob on the oscilloscope panel. Inspect the settings on the student’s oscilloscope (shown here) and determine what could be configured differently to achieve consistent triggering so the student won’t have to re-adjust the trigger level every time she re-adjusts the circuit’s DC bias voltage:
A student wants to measure the “ripple” voltage from an AC-DC power supply. This is the small AC voltage superimposed on the DC output of the power supply, that is a natural consequence of AC-to-DC conversion. In a well-designed power supply, this “ripple” voltage is minimal, usually in the range of millivolts peak-to-peak even if the DC voltage is 20 volts or more. Displaying this “ripple” voltage on an oscilloscope can be quite a challenge to the new student.
This particular student already knows about the AC/DC coupling controls on the oscilloscope’s input. Set to the “DC” coupling mode, the ripple is a barely-visible squiggle on an otherwise straight line:
After switching the input channel’s coupling control to “AC”, the student increases the vertical sensitivity (fewer volts per division) to magnify the ripple voltage. The problem is, the ripple waveform is not engaging the oscilloscope’s triggering. Instead, all the student sees is a blur as the waveform quickly scrolls horizontally on the screen:
Explain what setting(s) the student can change on the oscilloscope to properly trigger this waveform so it will “hold still” on the screen.
All electric motors exhibit a large “inrush” current when initially started, due to the complete lack of counter-EMF when the rotor has not yet begun to turn. In some applications it is very important to know how large this transient current is. Shown here is a measurement setup for an oscilloscope to graph the inrush current to a DC motor:
Explain how this circuit configuration enables the oscilloscope to measure motor current, when it plainly is a voltage-measuring instrument.
Also, explain how the oscilloscope may be set up to display only one “sweep” across the screen when the motor is started, and where the vertical and horizontal sensitivity knobs ought to be set to properly read the inrush current.
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.
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