What are mixed-mode S-parameters?

From RF Design Wiki · By the RF Design Wiki editorial team · Published October 21, 2026 · Last reviewed October 21, 2026 · How we research

Mixed-mode S-parameters describe balanced (differential) networks with differential-mode and common-mode waves instead of single-ended ones. A four-port single-ended matrix converts to a 2×2 block matrix [S_dd S_dc; S_cd S_cc]. S_dd21 is differential insertion loss, while S_cd21 measures mode conversion from differential to common mode, a key EMI and imbalance metric.

How are differential and common modes defined?

For a pair of lines with voltages V₁ and V₂: differential voltage V_d = V₁ − V₂; common-mode voltage V_c = (V₁ + V₂)/2. The differential reference impedance is twice the single-ended (100 Ω for a 50 Ω-per-line system) and the common-mode is half (25 Ω).

Why does mode conversion matter?

Imbalance converts differential signals to common mode, which radiates efficiently from cables and causes EMI, and converts common-mode noise into differential noise that receivers cannot reject. Baluns, differential amplifiers and differential antenna feeds are characterized by S_cd and S_dc. See baluns.^[1]

How are they measured?

A four-port VNA measures the single-ended matrix and converts mathematically, or true-mode stimulus drives both ports with controlled phase. For linear devices the results agree.^[2]

Frequently asked questions

What is Sdd11?

Differential return loss at port 1.

Is 100 Ω the standard differential impedance?

It is common for many interfaces; RF differential impedances vary by IC.

What does Scd21 mean?

Common-mode output at port 2 caused by a differential input at port 1.

References

Online sources accessed October 21, 2026; printed works cited by edition. See all sources.

“Electromagnetic Compatibility Engineering”. H. W. Ott, Wiley. 2009.
“Understanding the Fundamental Principles of Vector Network Analysis (Application Note 1287-1)”. Keysight Technologies. application note.

Categories: Matching and S-parameters

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