ISCET Test Equipment and Measurement 5 — Questions and Answers
Question 1: A technician suspects a transistor amplifier is oscillating at high frequency. Which instrument quickly confirms oscillation and shows its frequency?
- Capacitance meter
- Oscilloscope or spectrum analyzer (Correct answer)
- Megohmmeter (megger)
- Wheatstone bridge
Correct answer: Oscilloscope or spectrum analyzer
An oscilloscope reveals the oscillation waveform in the time domain, while a spectrum analyzer shows its frequency components clearly.
Question 2: Probing a CMOS logic circuit with a standard 1× passive probe can alter circuit behavior because the probe's capacitance:
- Adds DC leakage current that forward-biases internal protection diodes
- Loads the node and can reduce the signal's rise time or cause oscillations to stop (Correct answer)
- Increases the supply voltage seen by the IC
- Has no effect at frequencies below 1 GHz
Correct answer: Loads the node and can reduce the signal's rise time or cause oscillations to stop
Probe input capacitance (10–100 pF for 1× probes) adds a load that slows signal edges and can de-stabilize high-frequency circuits.
Question 3: A megohmmeter (megger) is used to test insulation resistance of a motor winding. A healthy motor winding should typically read:
- Less than 1 Ω
- Between 1 Ω and 100 Ω
- Greater than 1 MΩ (Correct answer)
- Exactly 0 Ω (short to ground)
Correct answer: Greater than 1 MΩ
Healthy motor insulation should present resistance well into the megohm range; low readings indicate moisture, damage, or deterioration.
Question 4: When measuring AC voltage with a DMM on a circuit that also has a high DC offset, you should use the AC voltage function rather than DC because:
- The AC function measures true peak values
- The AC function is capacitively coupled and rejects the DC component (Correct answer)
- The AC function has a higher input impedance
- The AC function automatically selects the best range
Correct answer: The AC function is capacitively coupled and rejects the DC component
Most DMMs AC-couple their AC voltage input, blocking DC so only the AC component is measured.
Question 5: A frequency counter shows an unstable, constantly changing reading on a low-frequency signal. The most likely cause is:
- The counter's timebase oscillator has failed
- Noise causing the trigger threshold to be crossed multiple times per cycle (Correct answer)
- The counter's input impedance is too high
- The counter needs an external 10 MHz reference
Correct answer: Noise causing the trigger threshold to be crossed multiple times per cycle
If noise on the signal crosses the trigger threshold more than once per cycle, the counter registers extra counts, producing erratic readings.
Question 6: On a dual-trace oscilloscope in CHOP mode versus ALT mode, CHOP mode is preferred when:
- Displaying very high frequency signals above 10 MHz
- Both channels contain fast transient events
- Displaying slow signals where ALT mode would cause visible flicker (Correct answer)
- Measuring phase difference between two identical frequencies
Correct answer: Displaying slow signals where ALT mode would cause visible flicker
CHOP mode rapidly switches between channels many times per sweep, giving smooth display of both traces at low sweep speeds where ALT mode would show annoying flicker.
Question 7: A bench power supply is set to 5 V / 1 A current limiting. When the connected circuit draws 1.5 A, the supply will:
- Increase voltage to maintain constant power
- Reduce output voltage until current equals the 1 A limit (Correct answer)
- Shut down completely and require a manual reset
- Blow an internal fuse and require replacement
Correct answer: Reduce output voltage until current equals the 1 A limit
In current-limit mode, the supply reduces output voltage as needed to keep current at the programmed limit, protecting the circuit.
ISCET Test Equipment and Measurement 5
A technician suspects a transistor amplifier is oscillating at high frequency.
Which instrument quickly confirms oscillation and shows its frequency?