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CISPR

EMC margin to CISPR / FCC limit calculator

Enter the raw analyzer reading, antenna factor, cable loss and preamp gain. The calculator corrects the emission, picks the CISPR or FCC limit by standard, class, frequency and detector, then shows the margin in dB (positive = under the limit = pass).

Inputs
Measurement type

Radiated: E-field at the antenna. Conducted: voltage on the mains port via LISN.

The standard and class set the limit applied at your frequency.

Class

Class B (residential) is stricter than Class A (industrial).

Radiated: 30 to 1000 MHz. Conducted: 0.15 to 30 MHz. Selects the limit sub-band.

Raw level shown by the spectrum analyzer or EMI receiver.

Antenna factor (AF) from calibration, added to the reading to get field strength.

Coaxial cable loss (positive value), added to the reading.

Preamplifier gain (positive value), subtracted from the reading.

Detector

Conducted limits publish both a quasi-peak and an average value.

Measurement distance

If different from the limit distance, converted by 20 log10(d_test / d_limit).

Reading + AF + cable - preamp Corrected Limit margin under = pass

Correction chain to signed margin (schematic)

Result

-9 dB

Breakdown

FAIL : 49 dBuV/m vs 40 dBuV/m · 30-230 MHz · limit @ 3 m

Pre-compliance estimate against the standard limit. A full-compliance measurement (OATS or chamber, calibrated antennas) is the authority for certification.

Frequently asked questions

FAQ

How is the corrected emission computed from the raw reading?
Radiated: corrected = reading + antenna factor + cable loss - preamp gain (in dBuV/m). Conducted: corrected = reading + cable loss - preamp gain (in dBuV), with no antenna factor. Antenna factor and cable loss add, preamp gain subtracts. You enter exactly what the instrument shows, not a pre-corrected number.
What is the sign convention for the margin?
Margin = limit - corrected emission. A positive (green) margin means the emission is under the limit, so it passes. A negative (red) margin means it exceeds the limit, so it fails. Example: +6.5 dB passes, -3.2 dB fails. A 6 dB margin is a common design target before testing at an accredited lab.
How is the measurement distance handled?
In the far field the field strength scales inversely with distance. If your test distance differs from the limit table distance, the calculator normalizes the corrected emission by 20 log10(test distance / limit distance) before comparing. This lets a 3 m measurement be compared to a limit published at 10 m on the same basis.
Why do quasi-peak and average detectors give different limits?
CISPR 22 and CISPR 32 conducted limits publish separate quasi-peak (QP) and average (AVG) values, with the average being lower. The QP detector weights impulsive emissions by their repetition rate. You must meet both limits with the matching detector; the calculator applies the limit for the detector you select.
Which limits are used and where do the values come from?
Radiated CISPR 32 / EN 55032 Class B at 3 m: 40.0 dBuV/m from 30 to 230 MHz, 47.0 dBuV/m from 230 to 1000 MHz (QP). FCC Part 15B Class B at 3 m derives from published uV/m: 40.0, 43.5, 46.0, 49.5 dBuV/m per sub-band. Values come from each standard table and are cited on the page for audit.
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