Conducted Emissions
How Conducted Noise Couples Onto Cables
Every switching converter, rectifier, and high-speed digital interface produces sharp current edges, and those edges contain harmonic energy reaching tens of megahertz. When that energy finds a path onto the mains cord or an interconnect cable, the cable acts as both a transmission line and an antenna feed. Conducted emissions are the portion of that energy measured directly on the wires, before it has a chance to radiate. Because the mains cable is shared with every other device on the branch circuit, regulators cap conducted disturbance tightly: an unfiltered offline supply can exceed the CISPR 32 Class B limit by 30 to 40 dB at the switching fundamental.
Compliance testing forces repeatability by inserting a line impedance stabilization network between the equipment under test and the mains. The LISN isolates utility noise, presents a stable RF impedance, and routes the device disturbance to a 9 kHz-resolution-bandwidth EMC receiver. The receiver sweeps 150 kHz to 30 MHz and reports amplitude in dBμV using both a quasi-peak detector and an average detector. A scan passes only if it stays below both limit lines at every frequency, which is why a spectrum that clears the quasi-peak limit can still fail on the tighter average limit for a narrowband switching tone.
Diagnosing a failure starts with sorting the disturbance into its two physical modes. Differential-mode current flows out on line and back on neutral, dominating below roughly 1 MHz where it is set by the converter's input ripple. Common-mode current flows out on both conductors together and returns through the chassis and safety ground via parasitic capacitance, dominating above a few megahertz. Mistaking one for the other wastes board space: a common-mode choke does little for a differential-mode problem, and an X capacitor does little for a common-mode problem.
Quantifying the Disturbance
VdBμV = 20 × log10(V / 1 μV) (0 dBμV ≈ -107 dBm in 50 Ω)
Common-mode and differential-mode split:
VCM = (VL + VN) / 2 VDM = (VL − VN) / 2
LISN reference network (CISPR 16-1-2):
ZLISN = 50 μH ‖ (50 Ω + 5 Ω) per line, ≈ 50 Ω above 150 kHz
Where VL and VN are the noise voltages on line and neutral referenced to the ground plane. Quasi-peak and average detectors both use a 9 kHz resolution bandwidth across the 150 kHz to 30 MHz band.
Conducted Emissions Limits by Standard
| Standard | Band | Quasi-Peak Limit | Average Limit | Detector / RBW | Typical Use |
|---|---|---|---|---|---|
| CISPR 32 Class B | 0.15 to 30 MHz | 66 to 56 dBμV | 56 to 46 dBμV | QP + AVG / 9 kHz | Residential IT |
| CISPR 32 Class A | 0.15 to 30 MHz | 79 to 73 dBμV | 66 to 60 dBμV | QP + AVG / 9 kHz | Industrial IT |
| FCC Part 15B | 0.15 to 30 MHz | 66 to 60 dBμV | not specified | QP / 9 kHz | US unintentional radiators |
| MIL-STD-461 CE102 | 0.01 to 10 MHz | 94 to 60 dBμV (peak) | not specified | Peak / 1 kHz, 10 kHz | Military platforms |
| CISPR 25 | 0.15 to 108 MHz | Class 1 to 5 lines | Class 1 to 5 lines | QP + AVG + Peak | Automotive components |
Frequently Asked Questions
How do you separate common-mode from differential-mode conducted emissions?
A LISN measures the noise voltage on line and neutral against the ground plane. Common-mode is the average, VCM = (VL + VN) / 2, returning through chassis and safety ground; differential-mode is half the difference, VDM = (VL − VN) / 2, circulating in the line-neutral loop. The fixes differ: DM needs X capacitors and series inductance, CM needs Y capacitors and a common-mode choke. Below about 1 MHz DM usually dominates; above a few MHz CM takes over through parasitic capacitance to ground.
Why does a LISN use 50 μH and a 50 Ω impedance?
The network presents a defined, repeatable RF impedance to the device under test so results match between labs regardless of building wiring. The 50 μH / 50 Ω LISN of CISPR 16-1-2 asymptotes near 50 Ω above 150 kHz, matching the receiver input. The inductor blocks device noise from flowing back into the mains and forces it through the measurement port, while bypass capacitors couple RF to the receiver but block mains-frequency current. Without it, the source impedance seen by the noise could vary by 20 dB or more between sites.
What detectors and bandwidth are used for conducted emissions limits?
CISPR and FCC limits in the 150 kHz to 30 MHz band use a quasi-peak detector and, separately, an average detector, both at 9 kHz resolution bandwidth per CISPR 16. The quasi-peak detector weights amplitude by repetition rate, correlating with broadcast-reception annoyance, while the average detector is the tighter limit for narrowband switching tones. A scan passes only if it meets both limit lines everywhere, with the average limit typically 10 to 13 dB below quasi-peak. MIL-STD-461 CE102 instead covers 10 kHz to 10 MHz with a peak detector and a band-dependent measurement bandwidth (1 kHz below 150 kHz, 10 kHz above).