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EMI Radiated Emissions Estimator

Estimate radiated emissions from PCB loops and cables using Paul's analytical DM/CM models. Monte Carlo confidence intervals vs. FCC Part 15 and CISPR 32 limits.

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Inputs

FCC Class B: 3 m. FCC Class A / CISPR: 10 m. Limits scale automatically.

Peak current circulating in the PCB loop (e.g., power rail decoupling current).

Area of the current loop on the PCB. Smaller = less DM radiation.

Common-mode current on attached cables. Even µA levels can dominate emissions.

Length of the longest unshielded cable attached to the PCB.

Fundamental clock frequency. Harmonics are computed up to 1 GHz.

50% duty cycle suppresses even harmonics. Other values can excite them.

Signal 10%–90% rise time. Longer rise times reduce high-frequency harmonics.

More trials = tighter confidence intervals. 100K is a good balance.

How It Works

Radiated emissions from PCBs arise from two sources: differential-mode (DM) radiation from current loops, and common-mode (CM) radiation from cables acting as antennas.

DM radiation (Paul, 1992): E = (π·µ₀/c) × f² × I × A / d, with π·µ₀/c ≈ 1.3169×10⁻¹⁴ — a small magnetic loop scales with frequency squared and loop area.

CM radiation: E = µ₀ × f × I_CM × l / d, with µ₀ ≈ 1.2566×10⁻⁶ — a short monopole scales linearly with frequency. Even 1 µA of CM current on a 0.5 m cable can approach the FCC Class B limit.

Clock harmonics use the one-sided Fourier amplitude of a trapezoidal waveform, 2·I·D·|sinc(kD)|·|sinc(k·f·tr)|, so the fundamental of a 50% clock has an amplitude of 2I/π.

Limits are the published values: FCC Part 15 Class B at 3 m and Class A at 10 m, and CISPR 32 at 10 m — Class B 30 dBµV/m (30–230 MHz) and 37 dBµV/m (230–1000 MHz), Class A 40 and 47 dBµV/m. Other distances are scaled 20·log₁₀ of the distance ratio.

The Monte Carlo varies current magnitude (±6 dB log-normal), cable orientation (uniform [0°, 360°]), and ground-plane shielding effectiveness to produce 5th/50th/95th percentile E-field confidence bounds.

Related Calculators

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Read the Guide

Predicting Radiated Emissions Before FCC Testing

An SBC fails FCC Part 15 Class B pre-compliance. The EMI Radiated Emissions Estimator separates the power loop from the USB cable and shows which fix matters.

FAQ

What is the difference between DM and CM radiation?+

DM radiation comes from current flowing in a loop on the PCB — it scales with f² and loop area. CM radiation comes from cables acting as antennas — it scales with f and cable length. CM usually dominates at higher frequencies.

Why do even harmonics disappear at 50% duty cycle?+

For a 50% duty cycle square wave, sin(kπ×0.5) = 0 for even k, so even harmonics have zero amplitude in the trapezoidal model.

What does the 95th percentile confidence band mean?+

The Monte Carlo models uncertainty in current magnitude, cable orientation, and ground-plane effectiveness. The 95th percentile is the worst-case E-field across 95% of simulation trials — useful for a conservative compliance estimate.

Is this a substitute for a pre-compliance scan?+

No. These are analytical estimates using simplified models. A real pre-compliance scan with an antenna, spectrum analyzer, and LISN on a test bench is required before formal certification. This tool helps identify the most likely problem frequencies before testing.