Electronic Design Automation

Touchstone Data Format

Q: What does the option line in a Touchstone file mean?

The option line begins with a hash symbol and declares four global settings that apply to the whole file, in fixed order with defaults filling any omitted fields. The fields are the frequency unit (Hz, kHz, MHz, or GHz), the parameter type (S, Y, Z, H, or G), the data format (MA for magnitude-angle, DB for decibel-angle, or RI for real-imaginary), and the keyword R followed by the system reference impedance in ohms. A typical line, hash space MHz S MA R 50, says frequencies are in MHz, the data are S-parameters in linear magnitude and degrees, normalized to 50 ohms. If the option line is missing, parsers assume the default GHz S MA R 50.

Q: How are the columns ordered in a .s2p versus a .s3p file?

For a 2-port (.s2p) file the order is special: each data line is frequency, then S11, S21, S12, S22, as four magnitude-angle or real-imaginary pairs on a single row. Note S21 comes before S12, which trips up many parsers. For three ports and above the data follow strict row-major order, S11 S12 S13 then S21 S22 S23 then S31 S32 S33, and a single frequency point spans multiple physical text lines (one matrix row per line by convention). Touchstone 1.1 has no explicit matrix-dimension marker, so the reader must know the port count from the file extension; Touchstone 2.0 adds a Number of Ports keyword and a Two-Port Data Order keyword to remove this ambiguity.

Q: What is the difference between Touchstone 1.1 and Touchstone 2.0?

Touchstone 1.1 is the legacy de facto format with the simple hash option line and no version marker; it relies on the .sNp extension to convey port count and assumes a uniform 50 ohm reference. Touchstone 2.0, standardized by the IBIS Open Forum in 2009, adds explicit keywords delimited by square brackets: Version 2.0, Number of Ports, Number of Frequencies, Reference (allowing a different real reference impedance per port), Matrix Format (full, lower, or upper triangular), and an end marker. It also formally supports noise parameters and mixed-mode data. Version 2.0 files keep a .ts extension or the legacy .sNp extension, and a compliant 2.0 reader still parses 1.1 files.

/ˈtuhch-stohn ˈday-tuh ˈfor-mat/

Originally introduced by EEsof for its CAD tools, this plain ASCII file standard stores the frequency-dependent network parameters of an n-port device, most commonly the S-parameters measured on a vector network analyzer. Files carry a .sNp extension where N is the port count, so a one-port reflection sweep is a .s1p file and a two-port device is a .s2p file. A single leading option line (beginning with #) declares the frequency unit, parameter type, the magnitude-angle, decibel-angle, or real-imaginary data convention, and the reference impedance; every subsequent row lists a frequency point followed by the parameter pairs. The format is the universal interchange medium between VNAs, EM solvers, and circuit simulators, and the 2009 Touchstone 2.0 revision added explicit bracketed keywords for port count, per-port reference impedance, and noise data.

Category: Electronic Design Automation

Extension: .s1p, .s2p, .sNp, .ts

Default reference: 50 Ω

How a Touchstone File Encodes Network Data

A Touchstone file is deliberately human-readable: it is whitespace-delimited ASCII with optional comment lines starting with an exclamation mark. Three line types make up the structure. Comment lines carry header text such as the instrument name, calibration date, and DUT description. The single option line, beginning with a hash, defines how the numbers should be interpreted. Data lines then list one frequency per record, followed by the network-parameter pairs in either magnitude/angle (MA), decibel/angle (DB), or real/imaginary (RI) form. Because the convention is global rather than per-column, mixing MA and RI within one file is not allowed; a converter must rewrite the entire data block to change format.

The most error-prone detail is two-port column ordering. A .s2p row lists S₁₁, then S₂₁, then S₁₂, then S₂₂, so the forward transmission term S₂₁ appears before the reverse term S₁₂. This single anomaly exists only for two-port files; three-port and larger files revert to strict row-major matrix order (S₁₁ S₁₂ S₁₃, then the second row, and so on), with one matrix row per physical text line. Touchstone 1.1 carries no internal record of the port count, so a parser must infer it from the .sNp extension. The data file formats used across the industry, including CITIfile and the newer Touchstone 2.0, were partly motivated by removing this ambiguity.

Option Line and Data Conversion

The option line takes the form: # [freq unit] [parameter] [format] R [Z_ref]. Any omitted field falls back to its default, but the fields must appear in the fixed order unit, parameter, format, reference, so a parser keys on position rather than on the value it encounters. Angle data are always in degrees, never radians. When data are stored as decibels and angle, the recorded value is 20·log₁₀ of the linear magnitude, so a reflection coefficient with a return loss of 15 dB is stored as a magnitude entry of -15 dB (S₁₁ in dB is negative for a passive port), while the phase travels in the paired angle column.

Option line syntax:
# <freq_unit> <param> <format> R <Z_ref> e.g. # GHz S RI R 50

Decibel-angle format (DB):
mag_dB = 20 × log₁₀|S_ij|, θ in degrees

Real-imaginary ↔ magnitude-angle:
S_ij = a + jb → |S_ij| = √(a² + b²), θ = atan2(b, a) × 180/π

.s2p row order (fixed):
f S₁₁ S₂₁ S₁₂ S₂₂

Defaults when the option line is absent: GHz, S, MA, R 50. For an n-port (n ≥ 3) file the matrix is written row-major, one row per text line, n² pairs per frequency.

Touchstone 1.1 vs. 2.0 and Related Formats

Attribute	Touchstone 1.1	Touchstone 2.0	CITIfile
File extension	.sNp	.ts (or .sNp)	.cti / .citi
Port count source	Inferred from extension	[Number of Ports] keyword	NAME / VAR blocks
Reference impedance	Single R value, all ports	Per-port [Reference] list	Constant per package
Data conventions	MA, DB, RI	MA, DB, RI	MAG/ANG, RE/IM, dB
Noise parameters	2-port only, appended block	Formal [Noise Data] section	Supported via data blocks
Matrix layout marker	None	[Matrix Format] full/lower/upper	Implicit by segment
Standardized by	EEsof (de facto)	IBIS Open Forum, 2009	HP / Agilent

Common Questions

Frequently Asked Questions

What does the option line in a Touchstone file mean?

The line begins with # and declares four global settings: frequency unit (Hz, kHz, MHz, GHz), parameter type (S, Y, Z, H, G), data format (MA, DB, or RI), and the keyword R with the reference impedance in ohms. For example, # MHz S MA R 50 means S-parameters in linear magnitude and degrees, in MHz, normalized to 50 Ω. If no option line is present, parsers assume # GHz S MA R 50.

How are the columns ordered in a .s2p versus a .s3p file?

A .s2p row is f, S₁₁, S₂₁, S₁₂, S₂₂, so S₂₁ precedes S₁₂, a frequent parsing pitfall. For three ports and up the data are row-major (S₁₁ S₁₂ S₁₃, then row two, and so on) with one matrix row per text line. Touchstone 1.1 has no port-count marker, so the reader must trust the .sNp extension; Touchstone 2.0 adds a [Number of Ports] keyword.

What is the difference between Touchstone 1.1 and Touchstone 2.0?

Version 1.1 is the legacy format: simple # option line, no version tag, port count taken from the extension, single 50 Ω reference. Version 2.0 (IBIS Open Forum, 2009) adds bracketed keywords such as [Version] 2.0, [Number of Ports], [Reference] for per-port impedance, and [Matrix Format], plus formal noise-parameter support. A compliant 2.0 reader still parses 1.1 files.

Can a Touchstone file hold Y- or Z-parameters instead of S-parameters?

Yes. The second field of the option line selects the parameter type: S for scattering, Y for admittance, Z for impedance, H for hybrid, or G for inverse hybrid. The most common choice for high-frequency measured data is S, because scattering parameters are what a VNA measures directly and they remain well-defined at frequencies where open and short terminations are impractical.

Related Terms

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