ISCET Test Equipment and Measurement 5

Question 1

A technician suspects a transistor amplifier is oscillating at high frequency. Which instrument quickly confirms oscillation and shows its frequency?

Accepted Answer

Oscilloscope or spectrum analyzer

Answer

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:

Accepted Answer

Loads the node and can reduce the signal's rise time or cause oscillations to stop

Answer

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:

Accepted Answer

Greater than 1 MΩ

Answer

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:

Accepted Answer

The AC function is capacitively coupled and rejects the DC component

Answer

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:

Accepted Answer

Noise causing the trigger threshold to be crossed multiple times per cycle

Answer

If noise on the signal crosses the trigger threshold more than once per cycle, the counter registers extra counts, producing erratic readings.

(ISCET) International Society of Certified Electronics Technicians Practice Test