FCC Measurement Uncertainty & Calibration 2 — Questions and Answers
Question 1: What is the typical calibration interval for antennas used in FCC radiated emissions testing?
- Annually (every 12 months) unless use or damage warrants more frequent calibration (Correct answer)
- Every 5 years
- Only when the antenna is visibly damaged
- Every 6 months without exception
Correct answer: Annually (every 12 months) unless use or damage warrants more frequent calibration
Most accreditation bodies and laboratory best practices require antenna calibration on an annual basis, with additional calibration required if the antenna is dropped, repaired, or shows signs of damage.
Question 2: What is 'antenna factor' (AF) and how is it used in radiated emissions measurements?
- A calibration factor that converts the voltage at the antenna terminal to the incident electric field strength at the antenna (Correct answer)
- The gain of the antenna relative to a dipole
- The impedance mismatch correction factor for the antenna
- The ratio of radiated power to conducted power
Correct answer: A calibration factor that converts the voltage at the antenna terminal to the incident electric field strength at the antenna
Antenna factor (dB/m) is added to the measured receiver voltage (dBμV) to obtain the electric field strength (dBμV/m): E = V + AF, and it accounts for the antenna's frequency-dependent sensitivity.
Question 3: What is the purpose of an 'uncertainty budget' in a laboratory's test report?
- To document all identified uncertainty sources, their magnitudes, and their combined contribution to the total measurement uncertainty (Correct answer)
- To estimate the cost of performing the test
- To list all equipment used in the measurement
- To record the ambient conditions during testing
Correct answer: To document all identified uncertainty sources, their magnitudes, and their combined contribution to the total measurement uncertainty
An uncertainty budget is a structured table that identifies each source of measurement uncertainty, quantifies its contribution, and combines them (using root-sum-of-squares for uncorrelated components) to give the total combined uncertainty.
Question 4: What does ISO/IEC 17025 require regarding equipment used for testing and calibration?
- All equipment must be calibrated and have calibration records maintained, with calibration traceable to SI units (Correct answer)
- Equipment only needs to be calibrated if it costs more than a defined threshold
- Calibration is only required for equipment that directly measures the parameter being tested
- Equipment must be replaced every three years regardless of calibration status
Correct answer: All equipment must be calibrated and have calibration records maintained, with calibration traceable to SI units
ISO/IEC 17025 clause 6.4 requires that laboratories maintain and calibrate all equipment that has a significant effect on the validity of results, with records demonstrating traceability to SI units.
Question 5: What is 'measurement repeatability' in the context of EMC testing?
- The closeness of agreement between successive measurements of the same measurand under the same conditions (Correct answer)
- The ability to reproduce results across different laboratories
- The stability of a test instrument over its calibration interval
- The degree to which a test site matches an OATS
Correct answer: The closeness of agreement between successive measurements of the same measurand under the same conditions
Repeatability refers to measurement results obtained by one operator, with one instrument, in one location, over a short time period, showing how consistent the measurement process is under unchanged conditions.
Question 6: What is 'reproducibility' in measurement and how does it differ from repeatability in FCC lab contexts?
- Reproducibility is agreement of results under changed measurement conditions (different labs, operators, or equipment); repeatability is agreement under unchanged conditions (Correct answer)
- Reproducibility applies only to calibration labs; repeatability applies to test labs
- Reproducibility measures long-term drift; repeatability measures short-term noise
- They are synonymous terms in FCC testing contexts
Correct answer: Reproducibility is agreement of results under changed measurement conditions (different labs, operators, or equipment); repeatability is agreement under unchanged conditions
Reproducibility characterizes how well results agree when conditions are deliberately varied—such as between different FCC-certified labs or different operators—while repeatability measures consistency under identical conditions.
FCC Measurement Uncertainty & Calibration 2
What is the typical calibration interval for antennas used in FCC radiated emissions testing?