What causes common-mode noise?

Ground loops: when two circuits share a common ground conductor with finite impedance, current from one circuit creates a voltage drop that appears as common-mode noise on the other circuit. At RF, even milliohms of ground impedance can create significant noise. Electromagnetic coupling: external fields (radiated EMI, nearby switching circuits) couple equally to both conductors of a differential pair, appearing as common mode. This is the primary advantage of differential signaling: the common-mode noise is rejected by the differential receiver. Asymmetric differential pairs: if the two conductors of a differential pair have different lengths, impedances, or coupling to ground, some of the differential signal converts to common mode (mode conversion). This is characterized by the mixed-mode S-parameter Scd (differential to common conversion). PCB routing: even small asymmetries (different via counts, trace bends, reference plane gaps) cause mode conversion. A 10 percent length mismatch in a differential pair can produce 20 dB of mode conversion, turning a clean differential signal into a significant common-mode EMI source. Switching power supplies: the high dv/dt of switching waveforms couples through parasitic capacitances to create common-mode currents on all cables connected to the system.

How is common mode rejected?

Common-mode chokes: a pair of coupled inductors wound on a common core. The differential signal produces opposing magnetic fields that cancel in the core, so the choke presents low impedance to differential signals. The common-mode signal produces additive fields, so the choke presents high impedance (100 to 10000 ohms) to common mode. Used on: data cables (USB, HDMI, Ethernet), power lines, and RF differential pairs. CMRR of differential amplifiers: CMRR = 20 log(A_diff / A_cm). Ideal: infinite CMRR (perfect balance). Practical: 40 to 80 dB for discrete differential pairs, 60 to 100 dB for monolithic IC differential amplifiers. CMRR degrades with frequency due to parasitic mismatch. Balanced-to-unbalanced (balun) transformers: convert between balanced (differential) and unbalanced (single-ended) interfaces while providing common-mode rejection. At RF: Marchand balun, Guanella balun, coupled-line balun. Board-level: ground plane integrity is the first line of defense. Continuous ground planes under differential pairs minimize common-mode generation. Ground plane gaps or splits create common-mode currents that radiate.

What are mixed-mode S-parameters?

Mixed-mode S-parameters describe differential and common-mode behavior of multi-conductor structures. For a 2-port differential pair (4 single-ended ports), the mixed-mode S-matrix has four 2x2 sub-matrices: Sdd: differential-mode response (differential in, differential out). This is what you want: the intended signal path. Scc: common-mode response (common in, common out). Describes how common-mode signals propagate. Should be low (good CM rejection). Sdc: mode conversion, differential to common. Measures how much differential signal converts to common mode due to asymmetry. This is the primary EMI mechanism. Target: less than minus 30 dB. Scd: mode conversion, common to differential. Measures how much common-mode noise converts to differential signal (contaminates the desired signal). Target: less than minus 30 dB. These are measured using a 4-port VNA and calculated from the single-ended S-parameters using a mathematical transformation. Modern VNAs compute mixed-mode S-parameters directly. The key design rule: maintain symmetry in differential pairs to minimize Scd and Sdc. Asymmetry of any kind creates mode conversion.

Signal Integrity

Common Mode

/kom-un mohd/

Equal signals on both conductors (same direction vs ground). V_cm=(V₁+V₂)/2. Differential: V_diff=V₁−V₂. CMRR=20log(A_diff/A_cm): 60-100 dB (IC). CM choke: high Z to CM, low Z to diff. Mode conversion (Scd/Sdc): asymmetry creates CM ↔ diff conversion. Target: <−30 dB. Sources: ground loops, EMI coupling, routing asymmetry.

CMRR: 60-100 dB

Choke Z: 100-10kΩ

Scd: <−30 dB

Understanding Common Mode

Common-mode signals are the enemy of clean RF design. They cause EMI emissions (FCC/CISPR failures), corrupt differential signal quality, and create ground bounce that degrades system performance. Every unintended asymmetry in a differential circuit converts desired differential signal into unwanted common mode.

Understanding common mode is essential for both analog RF design (differential amplifiers, baluns) and high-speed digital design (USB, PCIe, HDMI), where mode conversion from routing asymmetry is the primary EMI mechanism.

Common Mode Equations

Signal decomposition:
V_cm = (V₁+V₂)/2 (common mode)
V_diff = V₁−V₂ (differential)

CMRR:
CMRR = 20log(A_diff/A_cm) dB
Ideal: ∞. IC diff amp: 60-100 dB

Mode conversion:
Scd = differential → common
Sdc = common → differential
10% length mismatch: ~20 dB Scd

CM Rejection Methods

Method	Z_CM	Frequency	CMRR	Application
CM choke	100-10kΩ	1M-1G	20-40 dB	Cables, power
Diff amplifier	N/A	DC-10G	60-100 dB	Receiver front
Balun	N/A	10M-100G	20-40 dB	Bal/unbal conv
Sym routing	N/A	All	Prevention	PCB design
Ground plane	N/A	All	Prevention	Reference cont

Common Questions

Frequently Asked Questions

Sources?

Ground loops: finite ground Z creates CM voltage. EMI: external fields couple equally to both conductors. Asymmetry: different lengths/impedances = mode conversion (Scd). Switching: dv/dt through parasitic C. 10% length mismatch: ~20 dB mode conversion. Any routing asymmetry = CM generation = EMI.

Rejection?

CM choke: coupled inductors, high Z to CM (100-10kΩ), low Z to diff. CMRR: A_diff/A_cm, IC: 60-100 dB. Balun: balanced/unbalanced conversion. Board: continuous ground plane, symmetric routing, matched vias. Prevention (symmetry) beats correction every time.

Mixed-mode S?

Sdd: diff→diff (signal). Scc: CM→CM (propagation). Sdc: diff→CM (EMI source). Scd: CM→diff (noise coupling). Target Scd/Sdc <−30 dB. Measured with 4-port VNA. Key: maintain symmetry to minimize mode conversion. Asymmetry is the root cause of all mode conversion.

Related Terms

← Coaxial Cable Compression →

EMC Design

Request a Quote

Need CM rejection design, EMI troubleshooting, or differential pair optimization? Contact our team.

Get in Touch