How does the CISPR average detector differ from a simple RMS measurement?

The CISPR average detector operates on the envelope of the IF-filtered signal, not on the RF waveform directly. It measures the linear average of the detected envelope: V_avg = (1/T)∫|envelope(t)|dt. A true RMS measurement computes the square root of the mean of the squared signal: V_rms = √((1/T)∫v²(t)dt). For a sinusoidal envelope, average = 0.637 × peak while RMS = 0.707 × peak. The CISPR average detector specifically uses a video bandwidth filter with a time constant defined in CISPR 16-1-1 (typically 100-160 ms) to determine the averaging period, ensuring consistent results across different test receivers.

EMC Testing

Average Detector (EMC)

Q: What is the difference between peak, quasi-peak, and average detectors in EMC testing?

Peak detector captures the maximum instantaneous envelope level within the measurement time, providing the worst-case reading regardless of signal duty cycle. Quasi-peak (QP) detector applies a charge/discharge weighting that gives higher readings for signals with higher repetition rates, correlating with subjective annoyance to radio reception. Average detector provides the true time-averaged envelope level. For a continuous CW signal, all three detectors read the same value. For pulsed signals, peak > quasi-peak > average. The difference between peak and average readings indicates the signal's duty cycle: a 10% duty cycle pulse has approximately 10 dB difference between peak and average.

Q: When is the average detector required in EMC testing?

CISPR standards require average detector measurements in addition to quasi-peak for several product categories. FCC Part 15 Subpart B uses average limits for radiated emissions above 1 GHz. CISPR 11 (industrial equipment) and CISPR 32 (multimedia equipment) specify both QP and average limits for conducted emissions, with the average limit typically 10-13 dB below the QP limit. The average detector is faster than the quasi-peak detector (which requires long dwell times for its charge/discharge dynamics), so many test labs use a peak detector for initial scanning and only switch to quasi-peak and average for final compliance measurements at frequencies where the peak reading exceeds the limit.

/AV-rij dee-TEK-tor/

A detector function in EMC testing that measures the weighted time-average of a signal's envelope after IF bandwidth filtering, per CISPR 16-1-1. Unlike the peak detector (maximum instantaneous level) or quasi-peak detector (repetition-weighted charge/discharge), the average detector provides a true time-averaged measurement that correlates with the interference potential of continuous narrowband emissions. Required alongside quasi-peak for final compliance testing under CISPR 11, CISPR 32, and FCC Part 15.

Category: EMC Testing

Standard: CISPR 16-1-1

Relationship: Peak > QP > Average

Understanding the Average Detector in EMC

EMC emission testing measures the electromagnetic energy a device unintentionally radiates or conducts onto power lines. The measured level depends on the detector function used, and standards specify which detector(s) must be used for compliance assessment. The three primary detectors (peak, quasi-peak, average) respond differently to pulsed and modulated signals, providing complementary information about the emission characteristics.

The average detector is particularly useful for distinguishing between continuous narrowband emissions (clock harmonics, oscillator leakage) and broadband pulsed emissions (switching transients, motor commutation). A continuous emission reads the same on all three detectors. A low-duty-cycle pulsed emission reads much lower on the average detector than on peak, because the time-averaging process dilutes the brief high-amplitude pulses over the measurement interval. Standards exploit this difference by setting average limits lower than QP limits, ensuring that even low-duty-cycle emissions do not produce excessive average interference power.

Detector Response Comparison

For CW Signal (continuous):
Peak = QP = Average (all equal)

For Pulsed Signal (PRF >> 1/BW):
Average ≈ Peak + 20 log(duty cycle) dB
10% duty cycle: Average ≈ Peak − 20 dB
1% duty cycle: Average ≈ Peak − 40 dB

CISPR IF Bandwidths:
Band B (150 kHz – 30 MHz): 9 kHz
Band C/D (30 MHz – 1 GHz): 120 kHz
Band E (1 – 18 GHz): 1 MHz

Average Detector Video BW:
Time constant: 100 – 160 ms (CISPR 16-1-1)

EMC Detector Comparison

Detector	Response	Speed	Use Case	CW vs Pulsed
Peak	Maximum envelope	Fast (μs)	Pre-scan, worst case	Same for CW; highest for pulsed
Quasi-Peak	Weighted by PRF	Slow (1-10 s dwell)	Compliance (CISPR B/C/D)	Same for CW; intermediate for pulsed
Average	Time-averaged envelope	Medium (100 ms)	Compliance (conducted + >1 GHz)	Same for CW; lowest for pulsed
RMS-Average	RMS of envelope	Medium	CISPR 16-1-1 amendment	Same for CW; between QP and avg

Common Questions

Frequently Asked Questions

What is the difference between peak, quasi-peak, and average detectors?

Peak captures maximum instantaneous level (worst case). Quasi-peak applies charge/discharge weighting correlating with subjective radio interference. Average provides true time-averaged level. For CW, all read the same. For pulsed signals, peak > QP > average. The difference between peak and average indicates duty cycle: 10% duty cycle = ~20 dB difference.

When is the average detector required in EMC testing?

Required alongside QP for CISPR 11 (industrial), CISPR 32 (multimedia), and FCC Part 15 Subpart B above 1 GHz. Average limits are typically 10 to 13 dB below QP limits. Labs often use peak for initial scanning and switch to QP + average only at frequencies near limits for final compliance determination.

How does the CISPR average detector differ from simple RMS?

CISPR average measures the linear average of the detected envelope: V_avg = (1/T)∫|envelope| dt. True RMS computes √((1/T)∫v² dt). For sine envelope: average = 0.637×peak, RMS = 0.707×peak. The CISPR detector uses a specified video bandwidth (100 to 160 ms time constant) for consistent results across test receivers.

Related Terms

← Available Time Average Power Sensor →

EMC Test Components

Waveguide Loads for EMC Measurement Systems

RF Essentials precision waveguide terminations serve as reference loads and calibration standards for EMC test receivers and pre-amplifier chains used in conducted and radiated emission measurements.

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