ISCET Test Equipment and Measurement 3 — Questions and Answers
Question 1: A technician connects a voltmeter with an input impedance of 10 MΩ across a 10 MΩ resistor in a circuit. What is the effective resistance seen by the circuit?
- 10 MΩ
- 5 MΩ (Correct answer)
- 20 MΩ
- 1 MΩ
Correct answer: 5 MΩ
Two equal resistances in parallel yield half the value: 10 MΩ ∥ 10 MΩ = 5 MΩ, significantly loading the circuit.
Question 2: On an analog VOM, the ohmmeter scale reads from right to left with zero on the right. Why is this scale non-linear?
- The battery voltage changes with use
- Ohm's law causes a non-linear relationship between current and resistance (Correct answer)
- The meter movement is logarithmic by design
- AC signals are rectified before measurement
Correct answer: Ohm's law causes a non-linear relationship between current and resistance
Resistance is inversely proportional to current (R = V/I), which produces a compressed, non-linear scale crowded at the high-resistance end.
Question 3: Which oscilloscope trigger mode causes the sweep to run continuously even when no trigger event occurs?
- Normal
- Single
- Auto (Correct answer)
- Hold-off
Correct answer: Auto
Auto mode allows the sweep to free-run when no trigger is detected, ensuring a trace is always visible.
Question 4: A logic analyzer differs from an oscilloscope primarily because it:
- Can display analog voltage levels more accurately
- Captures and displays many digital channels simultaneously with time correlation (Correct answer)
- Has a much wider analog bandwidth
- Measures resistance and capacitance values
Correct answer: Captures and displays many digital channels simultaneously with time correlation
A logic analyzer captures multiple digital lines simultaneously and allows protocol decoding and timing analysis across all channels.
Question 5: When calibrating a DMM, the technician applies a precise 10.0000 V reference and the meter reads 9.9850 V. What is the meter's absolute error?
- +0.0150 V
- −0.0150 V (Correct answer)
- +0.150 V
- −0.150 V
Correct answer: −0.0150 V
Absolute error = measured value − true value = 9.9850 − 10.0000 = −0.0150 V.
Question 6: An LCR meter is used to measure a capacitor at 1 kHz and again at 100 kHz. Why might the two readings differ?
- LCR meters are only accurate at 60 Hz
- Capacitor parasitics (ESR and lead inductance) affect measured values at different frequencies (Correct answer)
- The test voltage changes with frequency
- Only the first reading is valid; subsequent readings drift
Correct answer: Capacitor parasitics (ESR and lead inductance) affect measured values at different frequencies
Real capacitors have equivalent series resistance and inductance that become significant at higher frequencies, changing the apparent capacitance.
Question 7: A technician uses a frequency counter to measure an unknown signal and reads 455.000 kHz. The counter's timebase accuracy is ±1 ppm. What is the maximum frequency error?
- ±0.455 Hz (Correct answer)
- ±4.55 Hz
- ±45.5 Hz
- ±455 Hz
Correct answer: ±0.455 Hz
Error = 455,000 Hz × 1×10⁻⁶ = 0.455 Hz maximum deviation.
ISCET Test Equipment and Measurement 3
A technician connects a voltmeter with an input impedance of 10 MΩ across a 10 MΩ resistor in a circuit.
What is the effective resistance seen by the circuit?