EMC/EMI

Conducted Susceptibility

Q: What is the difference between conducted susceptibility and radiated susceptibility?

Conducted susceptibility evaluates interference that reaches a device through its cables, where energy is injected as a voltage or current directly onto power and signal leads. Radiated susceptibility evaluates interference that arrives as a field illuminating the whole enclosure. The two regimes overlap in frequency: MIL-STD-461 CS114 bulk cable injection runs from 10 kHz to 200 MHz, while radiated test RS103 picks up the field-coupling regime from 2 MHz to 18 GHz, extendable to 40 GHz when the procurement requires it. Below roughly 30 MHz cables are short compared with a wavelength, so the dominant coupling path is conduction; above that, cables behave as antennas and field illumination matters more. A robust design needs both: cable filtering and bonding address conducted immunity, while shielding and aperture control address radiated immunity.

Q: How does CS114 bulk current injection work and what levels are typical?

CS114 uses a current injection probe clamped around a cable bundle to induce a calibrated RF current without breaking the harness. The probe is first calibrated in a 50 ohm jig to establish the forward power that produces the required current limit, then that power is applied around the equipment cable. Typical limits range from about 77 dBuA for benign commercial platforms up to 109 dBuA (roughly 0.28 A) for severe environments such as flight-line or external aircraft cables, with a 6 dB margin commonly added during qualification. The sweep covers 10 kHz to 200 MHz. Because the probe couples common-mode current, CS114 is sensitive to cable shield termination quality and to the common-mode rejection of the victim circuit.

Q: What design techniques improve conducted susceptibility immunity?

Effective fixes attack the coupling path before interference reaches sensitive circuits. Power and signal feedthrough filters, common-mode chokes, and ferrite beads roll off the injected energy; a pi filter providing 40 to 60 dB of insertion loss across the CS test band is common on power leads. Solid 360 degree shield termination at the connector backshell keeps shield current off the conductors, and a low-impedance chassis bond keeps the reference plane quiet. Differential signaling with good common-mode rejection, transient protection diodes, and adequate decoupling at the IC pins raise the upset threshold. The goal is to keep the injected level below the circuit upset voltage by a comfortable margin, typically 6 dB or more.

/kən-DUK-təd sə-sep-tə-BIL-i-tee/

Often shortened to CS, this property describes how well a device withstands electromagnetic interference that arrives through its cables rather than through the air. Verification is done by deliberately injecting a calibrated RF voltage or current onto power and signal leads and watching for malfunction, the inverse of conducted emissions testing. Military qualification uses MIL-STD-461 CS101 (30 Hz to 150 kHz on power leads) and CS114 (10 kHz to 200 MHz via bulk current injection), while commercial equipment follows IEC 61000-4-6 from 150 kHz to 80 MHz. Because the energy travels as common-mode current, immunity is dominated by cable filtering, shield termination, and bonding rather than by enclosure shielding. The lowest interference level that causes an observable upset is the susceptibility threshold, and a passing design keeps that threshold above the required test limit with margin.

Category: EMC/EMI

CS114 Band: 10 kHz to 200 MHz

Typical Margin: 6 dB

How Interference Couples Onto Cables

Conducted susceptibility addresses the reality that most real-world interference never has to penetrate a metal enclosure. It rides in on the wires. Power feeds, control harnesses, and antenna runs act as efficient conduits that carry external RF energy straight to printed circuit boards and integrated circuits inside. Because cables behave electrically short relative to a wavelength below roughly 30 MHz, conduction is the dominant entry mechanism across the low and mid HF range, which is exactly why the standardized CS tests concentrate their energy there.

The injected energy is almost always common-mode, meaning equal current flows on every conductor in a bundle and returns through the chassis or ground reference. A victim circuit only upsets when that common-mode current converts to a differential voltage across a sensitive node, through finite common-mode rejection, ground impedance, or unbalanced loading. This is the key insight for designers: raising the conducted susceptibility threshold is mostly about preventing common-mode to differential conversion, not about brute-force attenuation alone.

The pass or fail criterion is the susceptibility threshold, the lowest interference level that produces a defined indication of upset such as a bit error, a relay chatter, or a measurement excursion beyond tolerance. Qualification compares that threshold against the platform limit curve. Aircraft and shipboard equipment see the harshest limits, with CS114 currents reaching 109 dBuA, while a benign commercial bench instrument may only need to survive a few volts of injected RF under IEC 61000-4-6.

Governing Relationships

Susceptibility margin:
Margin (dB) = V_threshold (dB) − V_limit (dB) → pass when ≥ 0 (typically ≥ 6 dB)

CS114 injected current from probe power:
I_cm ≈ √(P_fwd / 50) (probe calibrated in a 50 Ω jig)

Current limit in dBµA:
I_dBµA = 20 × log₁₀(I_µA) → 109 dBµA ≈ 0.28 A

Common-mode to differential conversion:
V_diff ≈ I_cm × Z_imbalance / CMRR

Where P_fwd = forward power into the probe, I_cm = common-mode cable current, Z_imbalance = unbalanced source/load impedance, and CMRR = circuit common-mode rejection ratio. Example: 1 W into a CS114 probe produces ≈ 141 mA (≈ 103 dBµA).

Standard Test Methods Compared

Standard / Test	Coupling Method	Frequency Range	Typical Level	Primary Use
MIL-STD-461 CS101	Series voltage on power leads	30 Hz to 150 kHz	1 to 5 V rms	Power-line ripple immunity
MIL-STD-461 CS114	Bulk current injection probe	10 kHz to 200 MHz	77 to 109 dBµA	Cable-bundle RF immunity
MIL-STD-461 CS115	Cable impulse (BCI)	30 ns pulse, 30 pps	5 A peak (default)	Fast transient / ESD-class
MIL-STD-461 CS116	Damped sinusoid	10 kHz to 100 MHz	5 to 10 A peak	Ringing transient immunity
IEC 61000-4-6	CDN / EM clamp	150 kHz to 80 MHz	1, 3, or 10 V rms	Commercial conducted RF
ISO 11452-4	BCI (substitution)	1 MHz to 400 MHz	30 to 200 mA	Automotive harness immunity

Common Questions

Frequently Asked Questions

What is the difference between conducted susceptibility and radiated susceptibility?

Conducted susceptibility tests interference that reaches a device through its cables, injected as a voltage or current on the leads, while radiated susceptibility tests interference arriving as a field that illuminates the enclosure. They overlap in frequency: CS114 runs 10 kHz to 200 MHz, and RS103 covers 2 MHz to 18 GHz (extendable to 40 GHz by procurement). Below about 30 MHz cables are electrically short and conduction dominates; above that, cables act as antennas. A complete design needs both cable filtering and enclosure shielding.

How does CS114 bulk current injection work and what levels are typical?

CS114 clamps a current probe around a cable bundle to induce a calibrated RF current without cutting the harness. The probe is first calibrated in a 50 Ω jig to find the forward power for the target current, then that power drives the equipment cable. Limits run from about 77 dBµA on benign platforms to 109 dBµA (≈ 0.28 A) on severe ones, often with 6 dB of qualification margin, swept from 10 kHz to 200 MHz. Because it couples common-mode current, results depend heavily on shield termination quality.

What design techniques improve conducted susceptibility immunity?

Attack the coupling path first. Feedthrough filters, common-mode chokes, and ferrite beads roll off injected energy, with pi filters giving 40 to 60 dB of power-lead attenuation across the band. A 360 degree shield termination at the connector backshell keeps shield current off the conductors, and a low-impedance chassis bond holds the reference quiet. Differential signaling with strong common-mode rejection, transient protection, and good IC decoupling raise the upset threshold above the limit by 6 dB or more.

Related Terms

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