FCC RF Emissions & Electromagnetic Compatibility 2 — Questions and Answers
Question 1: What test site is considered the reference standard for FCC radiated emissions measurements?
- Open Area Test Site (OATS) (Correct answer)
- Fully anechoic room
- Reverberation chamber
- Near-field scanner
Correct answer: Open Area Test Site (OATS)
The Open Area Test Site (OATS) is the reference test environment specified in FCC rules and ANSI C63.4, against which all alternative test sites must validate their performance.
Question 2: What is 'site attenuation' and why is it important in EMC testing?
- A measure of how well a test site replicates free-space propagation, used for site validation (Correct answer)
- The amount of shielding provided by a chamber's walls
- The signal loss in a coaxial cable between the antenna and receiver
- The calibration factor applied to spectrum analyzer readings
Correct answer: A measure of how well a test site replicates free-space propagation, used for site validation
Site attenuation is the insertion loss between a transmit and receive antenna at a test site, measured to validate that the site performs within 4 dB of theoretical free-space values per ANSI C63.4.
Question 3: Which standard does FCC reference for measuring radiated and conducted emissions from digital devices?
- ANSI C63.4 (Correct answer)
- IEC 61000-4-3
- MIL-STD-461
- CISPR 32
Correct answer: ANSI C63.4
FCC references ANSI C63.4 as the primary measurement standard for radiated and conducted emissions from unlicensed digital devices under Part 15 subpart B.
Question 4: What is the purpose of a LISN (Line Impedance Stabilization Network) in conducted emissions testing?
- To provide a defined impedance on the power line and couple emissions to the measurement receiver (Correct answer)
- To filter out low-frequency noise from the power supply
- To boost the signal level for accurate measurement
- To isolate the device under test from ground plane effects
Correct answer: To provide a defined impedance on the power line and couple emissions to the measurement receiver
A LISN provides a standardized 50-ohm impedance at the power input of the device under test and couples the conducted RF emissions from the power line to the measurement receiver.
Question 5: What does 'quasi-peak' detector mean in the context of FCC emissions measurements?
- A detector whose output rises quickly with pulses but decays slowly, weighted by pulse repetition rate (Correct answer)
- A detector that measures the absolute peak value of a signal
- A detector that averages the signal over a defined time window
- A detector that only responds to continuous wave signals
Correct answer: A detector whose output rises quickly with pulses but decays slowly, weighted by pulse repetition rate
A quasi-peak detector charges quickly on signal peaks but discharges slowly, producing a reading that is weighted by the repetition rate of impulsive interference, as specified in CISPR publications.
Question 6: What is the maximum conducted emission limit for a Class B digital device on a power line between 450 kHz and 30 MHz under FCC Part 15?
- 48 to 60 dBμV depending on frequency (Correct answer)
- 80 dBμV flat across the range
- 30 dBμV flat across the range
- 70 dBμV flat across the range
Correct answer: 48 to 60 dBμV depending on frequency
FCC Part 15.107 specifies Class B conducted limits that vary from 48 dBμV (quasi-peak) at 450–500 kHz down to 60 dBμV at frequencies approaching 30 MHz, with specific breakpoints defined in the rule.
FCC RF Emissions & Electromagnetic Compatibility 2
What test site is considered the reference standard for FCC radiated emissions measurements?