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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. CISPR 22 / EN 55022 was superseded by CISPR 32 / EN 55032 (in the EU since March 2017): keep it for reading an old report.

Class

Class B (residential) is stricter than Class A (non-residential).

Radiated: 30 to 6000 MHz (above 1 GHz the limits are average and peak, not quasi-peak). 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.

Reading detector

Detector the reading was taken with. Below 1 GHz radiated, the limit is quasi-peak: a peak reading compares conservatively, an average reading does not. Above 1 GHz, pick AVG or PK. Conducted: QP and AVG each have their own limit.

Measurement distance

CISPR 32 publishes separate 3 m and 10 m tables (below 1 GHz): the row for the chosen distance is applied. FCC and CISPR 22 publish one 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 · 30-230 MHz · QP

Corrected level 49 dBuV/m
Limit 40 dBuV/m QP
Limit (linear) 100 µV/m
Limit distance 3 m
Over by 9 dB

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You receive the result with its direct link. Our design house gets a copy and can react to it, simply reply if you want an engineer's eye on it.

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

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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?
Below 1 GHz, CISPR 32 / EN 55032 publishes a 3 m table and a 10 m table, 10 dB apart: the calculator applies the row for your distance, with no conversion. Above 1 GHz, CISPR 32 publishes 3 m only. The FCC (47 CFR 15.31) and CISPR 22 publish a single distance: the corrected emission is then normalized by inverse distance, 20 log10(test distance / limit distance).
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. Radiated limits are QP up to 1 GHz, then average and peak above it (peak = average + 20 dB for the FCC). The calculator applies the limit for the detector you select and flags an average reading compared against a QP limit, which is not conservative.
Which limits are used and where do the values come from?
Radiated CISPR 32 / EN 55032 Class B, QP: 40 dBuV/m from 30 to 230 MHz and 47 dBuV/m from 230 to 1000 MHz at 3 m (30 and 37 at 10 m), then 50 / 54 dBuV/m average from 1 to 3 and 3 to 6 GHz. FCC Part 15B Class B at 3 m derives from the published uV/m (47 CFR 15.109): 40.0, 43.5, 46.0 and 54.0 dBuV/m per sub-band, the last one above 960 MHz. Values come from each standard's tables.
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AESTECHNO is an electronics design office based near Montpellier, France, with 10+ years of experience in design and EMC compliance, and a 100% success rate on CE/FCC certifications, across dozens of campaigns run over more than 10 years of practice.