How do quasi-peak and average detectors affect the limits?

CISPR 25 specifies limits for both peak and average detector modes. The peak detector captures the maximum instantaneous amplitude of the emission, while the average detector integrates over time. For continuous emissions (clock harmonics), peak and average readings are similar. For pulsed or intermittent emissions (CAN bus traffic), the average reading is lower than the peak. Both must be below their respective limits, but the peak limit is usually the harder one to meet for digital bus emissions.

EMC & Compliance

Automotive Emissions Limits

Q: What are the CISPR 25 Class 5 radiated emission limits?

CISPR 25 Class 5 radiated emission limits (peak detector, 1-meter distance) are approximately: 150 kHz to 300 kHz: 34 dBuV/m; 300 kHz to 530 kHz: 34 dBuV/m; 530 kHz to 1.8 MHz (AM band): 24 dBuV/m; 1.8 MHz to 30 MHz (SW): 24 dBuV/m; 30 MHz to 54 MHz: 24 dBuV/m; 54 MHz to 88 MHz: 24 dBuV/m; 88 MHz to 108 MHz (FM band): 18 dBuV/m; 108 MHz to 400 MHz: 24 dBuV/m; 400 MHz to 960 MHz (cellular): 24 dBuV/m; 960 MHz to 2500 MHz: 30 dBuV/m. These limits represent the tightest protection class and are the default requirement for most OEMs.

Q: Do OEMs impose limits stricter than CISPR 25 Class 5?

Yes. Many OEMs add 6 to 10 dB of margin on top of Class 5 in frequency bands that correspond to their specific antenna frequencies. For example, if an OEM uses a GNSS antenna on the roof, they may impose a limit 6 dB below Class 5 at 1.575 GHz (L1 band) for any module within 0.5 meters of the antenna. Some OEMs also extend the frequency range above 2.5 GHz to cover 5G NR bands and 5.9 GHz V2X frequencies.

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The maximum allowable levels of radiated and conducted electromagnetic emissions from vehicle-installed electronic modules, defined by CISPR 25 in five severity classes. Class 5 (the strictest) is the de facto industry standard, with radiated limits as low as 18 dBμV/m in the FM band (88 to 108 MHz) measured at 1 meter distance. Individual OEMs often impose additional margin above Class 5 for frequency bands corresponding to their specific antenna installations.

Category: EMC & Compliance

Standard: CISPR 25 Ed. 5

Strictest Class: Class 5 (FM: 18 dBμV/m)

Understanding Automotive Emissions Limits

CISPR 25 defines emission limits designed to protect the vehicle's own onboard radio receivers. The limits are not arbitrary: they are derived from the sensitivity of each receiver type (AM, FM, GPS, cellular) and the coupling path between the noise source and the victim antenna. The FM band gets the tightest limit (18 dBμV/m) because the FM antenna is typically a wire embedded in the windshield or rear window glass, only centimeters from dashboard-mounted electronics.

CISPR 25 Class 5 Radiated Emission Limits (Peak, 1 m)

Frequency Band	Protected Service	Limit (dBμV/m)	Antenna Type
150 kHz to 300 kHz	LW Radio	34	Rod (1 m)
530 kHz to 1.8 MHz	AM Radio	24	Rod (1 m)
5.9 MHz to 6.2 MHz	SW Radio	24	Rod (1 m)
76 MHz to 108 MHz	FM / DAB Band III	18	Biconical
174 MHz to 230 MHz	DAB Band III	24	Biconical
470 MHz to 770 MHz	DVB-T / TV	24	Log-Periodic
824 MHz to 960 MHz	Cellular (GSM/LTE)	24	Log-Periodic
1.559 GHz to 1.610 GHz	GPS / GNSS	24	Log-Periodic
1.71 GHz to 2.17 GHz	Cellular (PCS/LTE)	30	Log-Periodic
2.4 GHz to 2.5 GHz	Wi-Fi / BT	30	Log-Periodic

Converting Field Strength to Power at the Antenna:
P_ant = (E × λ)² / (480 × π²) × G_ant

Example at FM (100 MHz), Class 5 limit of 18 dBμV/m:
E = 18 dBμV/m = 7.94 μV/m
λ = 3 m, G_ant = 1.64 (dipole)
P_ant ≈ -106 dBm

A sensitive FM receiver has a noise floor of ~-110 dBm (in 200 kHz BW).
So Class 5 provides only 4 dB of margin above the receiver noise floor.
This is why many OEMs demand 6 dB below Class 5.

Common Questions

Frequently Asked Questions

What are the CISPR 25 Class 5 radiated emission limits?

They range from 18 dBμV/m in the FM band (88 to 108 MHz) to 34 dBμV/m in the LW band (150 to 300 kHz), all measured at 1 meter distance with a peak detector. The FM band is the tightest because FM antennas in the windshield are physically closest to dashboard electronics.

Do OEMs impose limits stricter than CISPR 25 Class 5?

Yes. Many OEMs add 6 to 10 dB of margin in bands that correspond to their specific antenna frequencies, especially at 1.575 GHz (GPS) and 5.9 GHz (V2X). Some extend the range above 2.5 GHz to cover 5G NR and C-V2X bands.

How do quasi-peak and average detectors affect limits?

CISPR 25 specifies limits for both peak and average detectors. For continuous emissions (clock harmonics), both readings are similar. For pulsed emissions (CAN bus), the average is lower than the peak. Both must pass, but the peak limit is usually harder for digital bus emissions.

Related Terms

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