ISCET Test Equipment and Measurement 4 — Questions and Answers
Question 1: Which measurement technique is used to eliminate the resistance of test leads when making a precise 4-wire (Kelvin) resistance measurement?
- Current is forced through two leads while voltage is sensed on two separate leads (Correct answer)
- All four leads carry current simultaneously and readings are averaged
- The two voltage leads carry most of the current to reduce lead drop
- Lead resistance is manually subtracted using a reference table
Correct answer: Current is forced through two leads while voltage is sensed on two separate leads
In Kelvin sensing, current flows through the outer pair of leads while the voltmeter's high-impedance inner leads sense voltage with negligible error from their own resistance.
Question 2: A power supply's output has 50 mV of ripple superimposed on 12 V DC. To best display the ripple on an oscilloscope without saturating the input, the technician should:
- Use DC coupling and set sensitivity to 5 V/div
- Use AC coupling and set sensitivity to 20 mV/div (Correct answer)
- Use DC coupling and set sensitivity to 20 mV/div
- Use DC coupling and increase the timebase to 1 s/div
Correct answer: Use AC coupling and set sensitivity to 20 mV/div
AC coupling blocks the 12 V DC component so the oscilloscope can be set to a sensitive range that reveals the small ripple without overdrive.
Question 3: What does the term 'true RMS' mean in the context of a digital multimeter?
- The meter measures only pure sine waves accurately
- The meter computes the actual RMS value of non-sinusoidal AC waveforms (Correct answer)
- The meter applies a calibration factor for square waves only
- The meter doubles the peak reading to estimate RMS
Correct answer: The meter computes the actual RMS value of non-sinusoidal AC waveforms
True-RMS meters calculate the root-mean-square mathematically, yielding accurate readings for distorted, non-sinusoidal waveforms.
Question 4: When measuring small resistances (less than 1 Ω) with an ohmmeter, which factor is most likely to cause significant measurement error?
- The battery voltage being too high
- Test lead and contact resistance comparable to the measured value (Correct answer)
- The meter's internal shunt resistance
- Stray magnetic fields from nearby equipment
Correct answer: Test lead and contact resistance comparable to the measured value
Lead and contact resistance can easily be a significant fraction of 1 Ω or less, introducing large percentage errors in the measurement.
Question 5: A network analyzer measures the S11 (return loss) of an antenna and displays −20 dB. This means the antenna is reflecting what percentage of the incident power?
- 1% (Correct answer)
- 10%
- 20%
- 50%
Correct answer: 1%
−20 dB return loss means reflected power = 10^(−20/10) = 0.01 = 1% of incident power.
Question 6: An oscilloscope's bandwidth specification of 100 MHz means that a 100 MHz sine wave input will be displayed at what amplitude compared to its true value?
- Exactly the same (100%)
- Approximately 70.7% (−3 dB point) (Correct answer)
- 50% of true amplitude
- Double the true amplitude
Correct answer: Approximately 70.7% (−3 dB point)
Bandwidth is defined at the −3 dB frequency, where amplitude drops to approximately 70.7% (0.707) of the true value.
Question 7: Which type of probe is specifically designed to measure high-frequency current waveforms without breaking the circuit?
- High-voltage differential probe
- Current transformer (CT) clamp probe (Correct answer)
- Passive 10× voltage divider probe
- Capacitive coupling probe
Correct answer: Current transformer (CT) clamp probe
A current transformer clamp probe clamps around the conductor and induces a proportional voltage in the secondary winding without interrupting the circuit.
ISCET Test Equipment and Measurement 4
Which measurement technique is used to eliminate the resistance of test leads when making a precise 4-wire (Kelvin) resistance measurement?