Reading S4P Files: Differential Loss, Return Loss and Mode Conversion

A four-port file needs a port map before its differential response is meaningful. The extension .s4p tells you that there are four ports. It does not tell you which two belong to the input pair, which two belong to the output pair, or which conductor is positive. ^{[1, 2]}

Start with the single-ended port numbering

Use the model supplier’s diagram. For the worked example here, input pair 1 is ports 1 (+) and 2 (−), and output pair 2 is ports 3 (+) and 4 (−). The two conductors run 1→3 and 2→4. Select “1–2 in, 3–4 out” in the viewer. A different common convention runs 1→2 and 3→4, with input pair (1,3) and output pair (2,4). The viewer supports both.

Single-ended S31 describes response at port 3 to stimulus at port 1, with the other ports terminated in their reference impedances. Differential excitation drives both input conductors with opposite phase. Combining the appropriate responses produces the mixed-mode result. ^[2]

Read the two letters as output mode, then input mode

The first number is the responding pair, and the second is the stimulated pair. SCD21 and SDC21 therefore describe different excitations; their labels are not interchangeable. ^[2]

Convert complex waves, not dB magnitudes

For the example port mapping and equal real references within each pair, the transformation gives:

These additions operate on complex numbers. Combine real/imaginary values, or convert magnitude/phase to complex form first; only then calculate 20 log₁₀|S|. Averaging two plotted dB traces loses the phase information needed for cancellation. ^[1]

A downloadable example with known results

The synthetic differential-channel S4P file contains two uncoupled paths with transmission magnitudes 0.8 and 0.6, each delayed by 100 ps. Every single-ended port has a reflection magnitude of 0.1; cross-path terms are zero. This is an illustrative reciprocal, passive network for testing the mathematics, not a measured connector or a compliance model.

Since the paths have the same phase, differential transmission is (0.8 + 0.6)/2 = 0.7, while differential-to-common conversion is (0.8 − 0.6)/2 = 0.1. At 1 GHz, both transmitted modes have −36° phase from the entered delay:

Open the viewer, choose the downloaded file and inspect the mixed-mode results →

The unequal paths create mode conversion even though there is no coupling between them. An otherwise balanced pair with unequal amplitude or delay can also convert modes. Reversing one pair’s polarity flips the corresponding differential phase; choosing the wrong input/output pairs changes the network you are analysing.

Check references, signs and the operating band

For two equal single-ended references R, the standard power-wave transformation uses differential reference 2R and common-mode reference R/2. This example uses 50 Ω single-ended ports, giving 100 Ω differential and 25 Ω common-mode references. These are normalisation impedances, not a measurement proving that the board has those physical impedances. ^[1]

The viewer converts single-ended four-port data after loading. Files already declaring a Touchstone [Mixed-Mode Order] are currently unsupported. A two-port differential export can instead contain SDD data under ordinary S11/S21 labels; inspect its documentation and reference impedance before interpreting it. ^[1]

For a real channel, inspect the full required band, port references and measurement planes. Plotted SDD21 is usually negative dB; insertion loss is its positive negative-log counterpart. Low mode conversion is useful, but does not by itself establish an EMI result or interface compliance. Use the actual interface’s channel requirements and the rest of the system model.

Tools used in this guide

• S-Parameter Viewer (.s2p, .s4p) – Open .s1p to .s16p Touchstone files in the browser: insertion and return loss, phase, group delay, mixed-mode SDD21/SCD21 for 4-ports, TDR impedance and interface loss budgets.
• Interface Design Rules – Impedance, skew, loss budget and length limits of PCIe, USB, Ethernet, HDMI, DDR and other interfaces, applied to a layer of your own stackup with the field solver.
• Trace Loss (Insertion Loss) Calculator – Insertion loss in dB/in and over the full length: conductor loss with copper roughness and dielectric loss from the field solver, with laminate Dk/Df across frequency.
• VSWR, Return Loss & Mismatch Loss Calculator – Convert between VSWR, |Γ|, return loss, mismatch loss and reflected power, find Γ of a complex load, and bound the mismatch uncertainty and cascaded VSWR of two ports.

Sources

1. IBIS Open Forum, Touchstone File Format Specification, version 2.1: four-port matrix order, reference impedances, mixed-mode waves and Appendix A.
2. Keysight, Balanced Measurements: response/stimulus notation, differential and common modes, and port mapping.

Related guides

• S11 and S21 Explained: Reading Touchstone S-Parameter Files
• VSWR, Return Loss and Reflected Power: What the Numbers Mean
• PCIe PCB Routing Guidelines: Impedance, Return Paths, Vias, Loss and Skew

Browse all guides

Guides explain published methods and data with their sources; check the values for your own design. Terms · Privacy

Back to contents