CCT CCT Electrical & Electronic Calibration 2 — Questions and Answers
Question 1: What is the primary use of a Josephson junction array in electrical metrology?
- Measuring high-frequency RF signals
- Realizing the SI unit of voltage with quantum-level accuracy (Correct answer)
- Testing insulation resistance
- Generating AC waveforms for oscilloscope calibration
Correct answer: Realizing the SI unit of voltage with quantum-level accuracy
A Josephson junction array provides a quantum-based, highly accurate realization of the volt, serving as the primary voltage standard at national metrology institutes like NIST.
Question 2: When calibrating an insulation resistance tester (megohmmeter), what type of reference standard is used?
- A precision resistor with a known high-resistance value (Correct answer)
- A standard battery
- A calibrated oscilloscope
- A low-resistance shunt
Correct answer: A precision resistor with a known high-resistance value
High-value precision resistors with known, stable resistance values (often in the megaohm to gigaohm range) are used as reference standards when calibrating megohmmeters.
Question 3: What effect does temperature have on precision resistors used as calibration standards?
- No effect — resistors are temperature independent
- Temperature causes resistance values to change, described by the temperature coefficient of resistance (TCR) (Correct answer)
- Temperature only affects capacitors, not resistors
- High temperature increases accuracy
Correct answer: Temperature causes resistance values to change, described by the temperature coefficient of resistance (TCR)
All resistors exhibit a temperature coefficient of resistance (TCR), meaning their resistance value changes with temperature; precision standards have very low TCR and require temperature-controlled environments.
Question 4: What is the purpose of a 'guard' terminal on a precision resistance standard?
- To protect the resistor from physical damage
- To eliminate leakage currents from affecting the measurement (Correct answer)
- To connect the resistor to a safety ground
- To provide a voltage reference point
Correct answer: To eliminate leakage currents from affecting the measurement
The guard terminal on a precision resistance standard creates a low-impedance path for leakage currents, preventing them from flowing through the measurement circuit and causing errors in high-resistance measurements.
Question 5: Which parameter is calibrated when verifying an oscilloscope's timebase accuracy?
- Voltage amplitude
- Horizontal sweep speed (time per division) (Correct answer)
- Input impedance
- Trigger sensitivity
Correct answer: Horizontal sweep speed (time per division)
Oscilloscope timebase calibration verifies the accuracy of the horizontal sweep speed (time/division), typically using a precision frequency reference or time interval standard.
Question 6: What is 'self-heating error' in precision electrical measurements?
- Errors caused by warm-up time of the instrument
- Resistance change in a resistive element caused by the power dissipated by the measurement current flowing through it (Correct answer)
- The effect of room temperature on a voltage calibrator
- Error introduced by a faulty thermocouple in the instrument
Correct answer: Resistance change in a resistive element caused by the power dissipated by the measurement current flowing through it
Self-heating error occurs when the measurement current flowing through a resistor dissipates power (I²R), raising the resistor's temperature and changing its resistance value during measurement.
CCT CCT Electrical & Electronic Calibration 2
What is the primary use of a Josephson junction array in electrical metrology?