Reading CE Graphs & Detector Types

A compliance plot is only useful if you can read it correctly, and the number it shows depends heavily on which detector produced it. This module covers both: how to interpret a real conducted-emission graph, and how Peak, Quasi-Peak, Average, and RMS detectors report the same signal differently.

Every conducted-emission plot has the same three ingredients: frequency on the X-axis (usually log-scaled), level in dBµV on the Y-axis, and a limit line from the applicable standard. Hover the interactive plot below to read exact values, and see the margin computed automatically at the worst-case point.

Interactive: Compliance Plot with Live Margin

–

Worst-Case Frequency

–

Measured Level

–

Margin to Limit

–

Result

A "pass" with less than about 3-6 dB of margin is still risky in production -- unit-to-unit variation, temperature drift, and different mains impedance at different test sites can easily consume a few dB. Engineers typically design for at least 6 dB of margin before calling a design final.

The same noisy pulse train can produce four very different displayed levels depending on which detector is selected, which is why standards specify the detector to use for compliance rather than leaving it to the tester's choice.

Detector	How It Responds	Purpose
Peak	Captures and holds the single highest instantaneous amplitude.	Fast pre-scan -- always reads at or above the true compliance level.
Quasi-Peak	Weights repetitive pulses by how often they occur using standardized charge/discharge time constants.	CISPR compliance -- the detector named in most commercial limit lines.
Average	Averages the signal envelope over the measurement time.	Continuous / narrowband emissions such as clock harmonics.
RMS	Reads the true root-mean-square power of the signal.	Broadband measurements where total energy content matters most.

Animated: Four Detectors, One Pulse Train

A repetitive noisy pulse scrolls across the top trace. Each colored bar shows what that detector would report in real time -- Peak always sits highest, Average and RMS settle lower and steadier.

Why is Peak > Quasi-Peak > Average, in that order, for the same real-world signal? Peak reports the single worst instant seen. Quasi-Peak weights by repetition rate, so it sits below Peak unless the pulse repeats extremely often. Average smooths everything over time, discarding the pulse's peakiness entirely -- so it reads lowest of the three for a typical repetitive noise source.

KNOWLEDGE CHECK

Module 6 Quiz

Score: 0 / 10

Select the best answer for each question. Instant feedback is provided after each response.

Question 1 of 10

← Previous Lesson Next Lesson →