ISCET Test Equipment Operation & Troubleshooting 2 — Questions and Answers
Question 1: When using an oscilloscope to measure a signal, the waveform appears distorted and noisy. What is the MOST likely first corrective action?
- Increase the timebase setting
- Adjust the trigger level and source (Correct answer)
- Replace the probe
- Switch to AC coupling mode
Correct answer: Adjust the trigger level and source
Improper trigger settings are the most common cause of an unstable or distorted waveform display on an oscilloscope.
Question 2: A frequency counter displays an erratic reading when measuring a low-frequency signal. What is the most probable cause?
- The signal amplitude is too high
- The input impedance is mismatched
- The gate time is set too short (Correct answer)
- The counter is in ratio mode
Correct answer: The gate time is set too short
A gate time that is too short provides insufficient sample time for accurate low-frequency measurements, resulting in erratic readings.
Question 3: Which oscilloscope control is used to shift the waveform vertically without changing the amplitude scale?
- Volts/Div
- Position (vertical) (Correct answer)
- Trigger holdoff
- Coupling switch
Correct answer: Position (vertical)
The vertical position control moves the waveform up or down on the display without altering the amplitude scale.
Question 4: A technician measures 12 VDC with a DMM, but the circuit is not functioning. The NEXT best diagnostic step is to:
- Check for voltage drop across series components under load (Correct answer)
- Replace the power supply
- Measure resistance across the load
- Switch the DMM to AC mode
Correct answer: Check for voltage drop across series components under load
Measuring voltage drop under load reveals resistive losses or failed components that are not apparent from an open-circuit voltage reading.
Question 5: What does a 'leading edge' trigger on an oscilloscope mean?
- The scope triggers on the falling edge of the waveform
- The scope triggers on the rising edge of the waveform (Correct answer)
- The scope triggers on the peak amplitude
- The scope triggers continuously regardless of signal
Correct answer: The scope triggers on the rising edge of the waveform
A leading (positive or rising) edge trigger causes the oscilloscope to begin its sweep when the signal transitions from low to high.
Question 6: When probing a high-frequency circuit with an oscilloscope, adding a 10X probe instead of a 1X probe primarily:
- Increases the displayed amplitude by 10 times
- Reduces loading effect and extends bandwidth (Correct answer)
- Improves low-frequency response
- Decreases the input resistance of the probe
Correct answer: Reduces loading effect and extends bandwidth
A 10X probe attenuates the signal by 10 but reduces capacitive loading on the circuit, significantly improving bandwidth and measurement accuracy at high frequencies.
Question 7: A logic analyzer shows all data lines staying LOW during a microcontroller bus cycle. The MOST likely cause is:
- Incorrect timebase setting on the analyzer
- The microcontroller is in a low-power sleep mode
- A shorted data bus or failed bus driver (Correct answer)
- The logic analyzer probe threshold is set too high
Correct answer: A shorted data bus or failed bus driver
All data lines stuck LOW typically indicates a shorted bus or a failed bus driver pulling the lines to ground.
ISCET Test Equipment Operation & Troubleshooting 2
When using an oscilloscope to measure a signal, the waveform appears distorted and noisy.
What is the MOST likely first corrective action?