What is a Touchstone (.s2p) file?

A Touchstone file (extension .s1p, .s2p … .sNp) is the de facto text format for exchanging network parameter data between instruments and simulators. An option line such as # GHz S MA R 50 declares the frequency unit, parameter type (S, Y, Z, H or G), data format (MA magnitude-angle, DB dB-angle, RI real-imaginary) and reference resistance. Version 2.0 adds keywords such as [Version], [Number of Ports] and per-port reference impedances.^[1]

How is a two-port file laid out?

Each data line holds frequency followed by S11, S21, S12, S22 as pairs (note the unusual S21-before-S12 order for two-ports only). Comment lines start with “!”. Lines after the S data whose frequencies restart may hold noise parameters: frequency, NF_min (dB), |Γ_opt|, ∠Γ_opt and normalized R_n.^[1]

What pitfalls occur?

• Port ordering for files with 3+ ports is row-major and can wrap lines.
• Defaults (if option line absent) are GHz, S, MA, R 50.
• Datasheet S-parameters are measured at a specific bias and reference plane; check the de-embedding description.
• Extrapolation to DC or beyond the data range by simulators can be non-physical; causality and passivity checks help.

What changed in Touchstone 2.0?

Keywords for port count, two-port data order, frequency count, per-port reference impedance, matrix format (full, lower, upper) and mixed-mode data; version 2.1 added further extensions. Many tools still emit 1.x files.^[1]

Frequently asked questions

See also

• S-parameters
• Vector network analyzer
• Simulation overview
• De-embedding

References

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

1. “Touchstone File Format Specification, Version 2.0”. IBIS Open Forum. 2009.