Measurements, Testing, and Calibration Additional Practical Test Questions Informational

How do I perform a radiated emissions scan of an RF device for pre-compliance testing?

Performing a radiated emissions scan of an RF device for pre-compliance testing measures the unintentional electromagnetic radiation from the device to predict whether it will pass formal regulatory testing (FCC Part 15, CISPR 32/EN 55032). Pre-compliance testing identifies problems early, before the expensive formal test. The setup: a spectrum analyzer or EMI receiver (a spectrum analyzer with appropriate RBW settings can be used for pre-compliance; a proper EMI receiver (R&S ESR, Keysight N9048B) with quasi-peak and average detectors is required for formal testing; for pre-compliance: a spectrum analyzer with peak detection provides a conservative (worst-case) estimate), a broadband antenna (a biconical antenna covers 30-300 MHz, and a log-periodic or horn antenna covers 300 MHz-6 GHz; both are needed to cover the full CISPR 32 frequency range; calibrated antenna factors are required to convert the received voltage to field strength), a measurement distance (formal testing: 3 m or 10 m in an anechoic chamber or OATS (Open Area Test Site); pre-compliance: any distance in a reasonable environment; correct to 3 m or 10 m using the 1/R relationship: E_3m = E_d × (d/3)), and measurement procedure (place the DUT on a non-conductive table at 0.8 m height; configure the DUT to generate maximum emissions (worst-case operating mode); scan the spectrum from 30 MHz to 6 GHz (or as required by the standard); rotate the antenna between horizontal and vertical polarization; rotate the DUT or scan from multiple angles; record the peak emissions and compare to the applicable limit (FCC Part 15 Class B or CISPR 32 Class B for consumer devices).

Category: Measurements, Testing, and Calibration

Updated: April 2026

Product Tie-In: VNAs, Signal Generators, Power Meters

Radiated Emissions Pre-Compliance

Pre-compliance testing saves time and money by finding emissions problems before formal testing at an accredited lab ($3,000-10,000+ per test session). Fixing a problem after formal testing fails can cost weeks of delay and engineering time.

Test Environment

Ideal: An anechoic chamber (absorber-lined room that eliminates reflections). Pre-compliance chambers are available for $10,000-50,000
Acceptable: A shielded room (Faraday cage) that blocks ambient interference. Reflections inside the room add uncertainty but: the test identifies major emissions problems
Minimum: An open area (parking lot, rooftop) away from other electronics. Subject to ambient interference (cell towers, broadcast signals), which may mask DUT emissions. Useful only for identifying very strong emissions

EMI Test Parameters

Field strength: E(dBμV/m) = V_rx(dBμV) + AF(dB/m) + CL(dB)
Where AF = antenna factor, CL = cable loss
Distance correction: E_3m = E_d + 20×log₁₀(d/3) [for d > 3m]
FCC Part 15B Class B limits (at 3m):
30-88 MHz: 40 dBμV/m, 88-216: 43.5, 216-960: 46, >960: 54

Common Questions

Frequently Asked Questions

What equipment do I need?

Minimum pre-compliance setup: spectrum analyzer: any spectrum analyzer covering 30 MHz-6 GHz. Cost: $3,000-20,000 (used instruments available for less). Set RBW to 120 kHz (for CISPR Band C/D) and 9 kHz (for Band A/B). Antennas: biconical (30-300 MHz, $200-500) + log-periodic (300 MHz-3 GHz, $300-700) or hybrid bilog ($500-1000). Antenna factors: calibration data from the antenna manufacturer (converts received voltage to field strength). Tripod and non-conductive table. Total cost: $5,000-25,000 for a basic pre-compliance setup.

How do I interpret the results?

Compare the measured emissions (in dBuV/m) to the regulatory limits: if all emissions are more than 6 dB below the limit (adequate margin): the device is likely to pass formal testing. If emissions are within 6 dB of the limit: the device may pass or fail depending on measurement uncertainty and the formal test environment. Take corrective action to add margin. If emissions exceed the limit: the device will likely fail formal testing. Identify the emission frequency and determine the source (clock harmonic, switching regulator, data bus, RF oscillator). Apply EMI mitigation: shielding, filtering, grounding, PCB layout changes. Common emission sources: clock harmonics (integer multiples of the system clock), switching regulator harmonics (at the switching frequency and harmonics), and unshielded cables acting as antennas.

What about conducted emissions?

Conducted emissions: measure the RF noise conducted from the device onto the AC power cord or other cables. These emissions must also comply with regulatory limits (CISPR 32, FCC Part 15). Measurement: use a LISN (Line Impedance Stabilization Network) between the AC power source and the DUT. The LISN provides a standardized impedance and routes the conducted noise to the spectrum analyzer. Frequency range: 150 kHz-30 MHz. Limits: FCC Part 15B Class B specifies maximum conducted emission levels. Conducted emissions testing is often done alongside radiated emissions for complete pre-compliance verification.

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