Why do different frequency bands use different QP time constants?

CISPR 16-1-1 defines different charge and discharge time constants for different frequency bands to match the characteristics of the radio services being protected. Band A (9 kHz to 150 kHz): charge = 45 ms, discharge = 500 ms. Band B (150 kHz to 30 MHz): charge = 1 ms, discharge = 160 ms. Band C/D (30 MHz to 1 GHz): charge = 1 ms, discharge = 550 ms. The shorter charge time in Band B allows the detector to respond to fast transients common in power electronics, while the longer discharge in Band C provides more averaging for the VHF/UHF broadcast bands where amplitude fluctuations are less perceptible.

When is the average detector used instead of quasi-peak?

The average detector is increasingly used as a supplementary or primary measurement above 1 GHz (CISPR 32) and for certain conducted emission limits. It measures the linear mean of the detected signal envelope, providing a direct indication of the total energy content. Average readings are always less than or equal to QP readings. For continuous wave (CW) signals, average and QP give identical results. For pulsed signals, the difference can be 10 to 20 dB depending on duty cycle and repetition rate. Some CISPR standards require both QP and average compliance (whichever is stricter for the signal type), while above 1 GHz, average with 1 MHz RBW is standard.

EMC Measurement & Instrumentation

CISPR Detector

Q: How does the quasi-peak detector work?

The QP detector uses an asymmetric charge-discharge circuit: the charge time constant is much shorter than the discharge time constant. When a pulse arrives, the detector charges quickly to near the peak value. Between pulses, it discharges slowly. For high repetition rate signals (like continuous clock harmonics), the detector stays near the peak value because it barely discharges between pulses. For low repetition rate signals (like occasional switching transients), the detector discharges significantly between pulses, reading much lower than the peak. This weighting was designed in the 1930s to correlate with the subjective annoyance of radio interference to human listeners: frequent clicks are more annoying than occasional ones.

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A standardized signal weighting function in an EMI receiver defined by CISPR 16-1-1. The quasi-peak (QP) detector uses asymmetric charge/discharge time constants to weight signals by repetition rate, correlating with perceived radio interference annoyance. The peak detector captures maximum instantaneous amplitude. The average detector measures linear mean value. Different CISPR emission standards specify which detector(s) apply at each frequency band, and compliance requires meeting limits with all specified detectors simultaneously.

Category: EMC Measurement

Standard: CISPR 16-1-1

Types: QP, Peak, Average

Understanding CISPR Detectors

Electromagnetic interference signals are rarely continuous sine waves; they are typically pulsed, modulated, or broadband noise with varying amplitude, duty cycle, and repetition rate. Different detector types weight these signals differently, producing different numerical readings from the same physical emission. The choice of detector fundamentally affects whether a product passes or fails EMC compliance. A pulsed emission might read 60 dBμV on the peak detector but only 40 dBμV on the average detector, with the QP reading somewhere between.

The quasi-peak detector was originally designed in the 1930s to simulate the annoyance level of radio interference to human ears. Its asymmetric time constants (fast charge, slow discharge) mean that frequently repeated pulses produce readings close to the peak value (high annoyance), while infrequent pulses produce readings much lower than peak (low annoyance). For a continuous wave (CW) signal, QP, peak, and average all read the same. For a pulsed signal with 1% duty cycle and 100 Hz repetition rate, peak might read 20 dB higher than average, with QP about 10 dB above average. Modern EMI receivers implement all three detectors digitally using FFT-based processing, enabling simultaneous QP, peak, and average measurements in a single frequency sweep, dramatically reducing test time compared to analog detector implementations.

CISPR Detector Time Constants

Band B (150 kHz to 30 MHz):
QP charge: 1 ms ; QP discharge: 160 ms ; RBW: 9 kHz

Band C/D (30 MHz to 1 GHz):
QP charge: 1 ms ; QP discharge: 550 ms ; RBW: 120 kHz

QP vs Peak for Pulsed Signal:
V_QP / V_peak ≈ f_rep · τ_discharge [for low duty cycle]

Where f_rep = pulse repetition frequency, τ_discharge = QP discharge time constant. For CW: V_QP = V_peak = V_avg. Above 1 GHz: average detector with 1 MHz RBW replaces QP.

Detector Type Comparison

Detector	Measures	CW Reading	Pulsed Reading	Primary Use
Peak	Maximum amplitude	= signal level	Highest	Pre-scan, MIL-STD-461
Quasi-Peak	Rep-rate weighted	= signal level	Between peak & avg	CISPR emissions < 1 GHz
Average	Linear mean	= signal level	Lowest	CISPR > 1 GHz, conducted
RMS	Root-mean-square	= signal level	Between avg & QP	Military, some IEC

Common Questions

Frequently Asked Questions

How does the quasi-peak detector work?

QP uses fast charge (1 ms) and slow discharge (160 to 550 ms). High-rep-rate signals stay near peak (barely discharge between pulses). Low-rep-rate signals discharge significantly, reading much lower. This weights signals by perceived annoyance: frequent interference is more annoying than occasional clicks. For CW, QP = peak = average.

Why do different bands use different time constants?

Band A (9 to 150 kHz): charge 45 ms, discharge 500 ms. Band B (150 kHz to 30 MHz): 1 ms / 160 ms. Band C/D (30 MHz to 1 GHz): 1 ms / 550 ms. Constants are tuned to the radio services being protected and the characteristics of typical interference sources in each band.

When is average used instead of QP?

Average is primary above 1 GHz (CISPR 32) and supplementary for conducted emissions. For CW, average = QP. For pulsed signals, the difference can be 10 to 20 dB. Some standards require both QP and average compliance. Above 1 GHz, average with 1 MHz RBW is standard because QP weighting is less meaningful at those frequencies.

Related Terms

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