ISCET Test Equipment Operation & Troubleshooting 3 — Questions and Answers
Question 1: When using a signal generator to inject a test signal into an amplifier, the output is absent. The technician should NEXT:
- Increase the generator frequency
- Verify the generator output with an oscilloscope before injecting (Correct answer)
- Replace the amplifier transistors
- Change the signal generator waveform to square wave
Correct answer: Verify the generator output with an oscilloscope before injecting
Confirming that the signal generator is actually producing an output before injection prevents misdiagnosis caused by a faulty or misconfigured generator.
Question 2: What measurement does an LCR meter make that a standard DMM cannot?
- DC voltage
- AC current
- Inductance and capacitance values (Correct answer)
- Resistance in mega-ohm ranges
Correct answer: Inductance and capacitance values
An LCR meter measures inductance (L), capacitance (C), and resistance (R) at specific test frequencies, which is beyond the capability of a standard DMM.
Question 3: A spectrum analyzer shows unexpected sidebands around a carrier frequency. This MOST likely indicates:
- The carrier is amplitude-modulated or has spurious emissions (Correct answer)
- The spectrum analyzer sweep speed is too fast
- The reference level is set too low
- The carrier frequency is drifting
Correct answer: The carrier is amplitude-modulated or has spurious emissions
Sidebands symmetrically placed around a carrier are characteristic of modulation or harmonic distortion products.
Question 4: Which setting on a DMM should be used when measuring the forward voltage drop of a diode?
- DC voltage range
- Resistance (ohms) range
- Diode test mode (Correct answer)
- Continuity mode
Correct answer: Diode test mode
The diode test mode applies a small forward current and displays the forward voltage drop, providing a definitive diode junction check.
Question 5: When calibrating an oscilloscope probe, a technician observes an over-compensated waveform (peaks on the square wave). The correction is to:
- Increase the probe's trimmer capacitance
- Decrease the probe's trimmer capacitance (Correct answer)
- Adjust the vertical sensitivity
- Replace the probe cable
Correct answer: Decrease the probe's trimmer capacitance
Over-compensation (peaks or 'horns' on the square wave) is corrected by decreasing the probe trimmer capacitance until the waveform has flat tops.
Question 6: A technician needs to measure ripple voltage on a DC power supply output. The BEST oscilloscope coupling mode to use is:
- DC coupling
- AC coupling (Correct answer)
- Ground coupling
- Differential coupling
Correct answer: AC coupling
AC coupling blocks the DC component and allows the small AC ripple superimposed on the DC voltage to be observed at a higher sensitivity.
Question 7: The bandwidth specification of an oscilloscope is defined as the frequency at which the displayed amplitude is:
- Reduced to 50% of the actual signal amplitude
- Reduced to 70.7% (−3 dB) of the actual signal amplitude (Correct answer)
- Exactly equal to the input amplitude
- Reduced to 10% of the actual signal amplitude
Correct answer: Reduced to 70.7% (−3 dB) of the actual signal amplitude
Oscilloscope bandwidth is the −3 dB point, meaning the displayed amplitude is 70.7% of the true amplitude at that frequency.
ISCET Test Equipment Operation & Troubleshooting 3
When using a signal generator to inject a test signal into an amplifier, the output is absent.
The technician should NEXT: