Free tool

Radiated Emissions Pre-Compliance Calculator

Estimate radiated emissions from cable common-mode current and board loop area, plotted against the CISPR or FCC limit line across 30 MHz to 1 GHz. It tells you which of the two mechanisms is the problem — and how few microamps of common-mode current you can actually afford.

Common-mode current on cables — µA

Cable length — m, the radiating element

Differential current in the loop — mA

Loop area — cm²

Limit

Measurement distance — m

Frequency of interest — MHz

Design margin wanted — dB below the limit

Common mode

–

Differential mode

–

Limit here

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I_CM you can afford

–

µA at this frequency

Runs entirely in your browser. Nothing is uploaded, stored, or sent anywhere.

Microamps, not milliamps

The number that shocks people the first time they meet it: to pass CISPR 32 Class B at 3 m, the common-mode current on an attached cable has to stay below roughly 2 µA at 100 MHz. Not milliamps. Microamps.

That is why EMC failures feel arbitrary. A current far too small to measure with ordinary instruments, on a cable you were not thinking about, radiates enough to fail the test — while the signal you were worried about, confined to a small loop on the board, contributes much less.

E_CM = 1.257×10⁻⁶ · f · I_CM · L / d — cable as a radiating element
E_DM = 1.316×10⁻¹⁴ · f² · I_DM · A / d — current loop on the board

The two mechanisms scale differently, so they swap places

Common-mode radiation grows linearly with frequency. Differential-mode grows with the square. At 30 MHz a typical cable common-mode current dominates comfortably; by a few hundred megahertz a large loop can overtake it.

The plot draws both against the limit line, so the shape tells you which mechanism you are fighting. That matters because the fixes have nothing in common: common mode is filtering, shield termination, connector grounding and where the cable attaches. Differential is loop area, return path continuity and stackup. Chasing one when the other is the problem is the standard way a pre-compliance week gets wasted.

Where common-mode current comes from

Nobody designs a common-mode current. It arrives by accident:

Ground bounce driving a cable shield relative to chassis — the dominant mechanism on most products.
Imbalance in a differential pair, so the two halves no longer cancel. The skew calculator covers the conversion mechanism.
Return-path discontinuities, where current forced to detour creates a voltage between board ground and the outside world.
Heatsinks and mechanical parts capacitively coupled to switching nodes, acting as unintended antennas.

This is a first-order estimate, not a chamber. It treats the cable as an ideal radiating element and the loop as electrically small, ignores enclosure resonances, ground-plane images, antenna factor detail, and the fact that a real cable's efficiency varies enormously with length relative to wavelength. Use it to see which mechanism dominates and roughly how much margin you need to find — not to predict a pass.

Limits

CISPR 32 (and its predecessor CISPR 22) sets radiated limits at 10 m; the 3 m figures shown here are distance-corrected by the conventional 20 dB per decade. FCC Part 15 quotes at 3 m. Class B is residential and roughly 10 dB tighter than Class A industrial — a factor of about three in field strength, which is usually the difference between shipping and redesigning.

Design to a margin, not to the limit. Six to ten decibels is normal, because the chamber will not agree with your estimate, production units vary, and cable orientation during the scan is chosen to maximise what you emit.

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