Electronic Design Automation

Data File Formats

Q: What does the N in a Touchstone .sNp file extension mean?

The N is the number of ports the device has, and it sets the number of S-parameter columns in the file. A one-port DUT is stored as .s1p with one complex S11 value per frequency, a two-port as .s2p with four parameters (S11, S21, S12, S22), and a four-port as .s4p with sixteen parameters. The data line count per frequency follows N squared complex pairs. A .s2p file fits all four parameters on one row, but Touchstone 1.x wraps files of three or more ports across multiple physical lines so a 4-port row of 16 complex numbers spans four lines. Touchstone 2.0 removed that ambiguity by adding explicit keywords such as [Number of Ports] and [Network Data].

Q: How do I convert a Touchstone file between magnitude-angle and real-imaginary format?

The option line that begins with a hash sign declares the data format: MA is linear magnitude and angle in degrees, DB is decibels and angle in degrees, and RI is real and imaginary parts. To convert from DB/MA you first recover linear magnitude m as 10 raised to the power dB over 20, then compute real = m times cosine of theta and imaginary = m times sine of theta with theta in radians. The reverse conversion uses magnitude as the square root of real squared plus imaginary squared and angle as the arctangent of imaginary over real. Most EDA tools and Python libraries such as scikit-rf read all three notations and convert internally, so the choice mainly affects human readability and file size.

Q: Why does my S-parameter file have the wrong reference impedance?

The reference impedance is set by the R value at the end of the Touchstone option line, for example the trailing R 50 in a hash GHz S MA R 50 header. If that value is missing the standard default is 50 ohms, which causes errors when the data was actually referenced to 75 ohms or to a complex port impedance. S-parameters are defined relative to a specific reference impedance, so re-normalizing to a different value requires the full impedance-renormalization transform, not a simple scaling. CITIfile and MDIF handle non-50-ohm and complex reference impedances more cleanly through explicit metadata, which is one reason they are preferred for transistor and on-wafer data sets.

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Standardized text and binary structures used to store and exchange frequency-dependent network data between measurement instruments and EDA software. The dominant family is Touchstone (file extensions .s1p through .s99p), which captures complex S-parameters in magnitude-angle, decibel-angle, or real-imaginary notation against a stated reference impedance. Richer alternatives such as CITIfile and MDIF (Measurement Data Interchange Format) carry multiple sweeps, parameter-swept device libraries, noise data, and complex port impedances that plain Touchstone cannot represent. Whether the data originates from a vector network analyzer or a circuit simulator, a consistent file format lets a Touchstone .s2p exported in St. Petersburg drop directly into a harmonic-balance simulation without manual re-keying.

Category: Electronic Design Automation

Primary format: Touchstone .sNp

Default Z_ref: 50 Ω

Anatomy of an S-Parameter File

The Touchstone format, originally defined by EEsof and later revised as Touchstone 2.0 (ratified by the IBIS Open Forum in 2009, with a 2.1 clarification in 2024), is the lingua franca of RF data interchange. A file is plain ASCII with three logical parts: optional comment lines that begin with an exclamation mark, a single option line that begins with a hash sign, and the network-data block. The option line declares four things in a fixed order: the frequency unit (Hz, kHz, MHz, or GHz), the parameter type (S, Y, Z, H, or G), the data notation (MA, DB, or RI), and the reference impedance preceded by the letter R. A header reading # GHz S MA R 50 therefore means S-parameters in linear magnitude and degrees, swept in gigahertz, referenced to 50 Ω.

Each subsequent row begins with a frequency point followed by the S-parameter values for that point. The number of complex pairs equals the square of the port count, so a two-port file lists S11, S21, S12, S22 in that specific column order, while a four-port file lists sixteen pairs. This ordering is a frequent source of error: Touchstone 1.x lists S21 before S12 for two-port files but uses row-major order (S11, S12, S13 then S21, S22 and so on) for files of three or more ports, a historical inconsistency that Touchstone 2.0 resolved with explicit keywords and a mandatory port-order block.

When a single Touchstone file is not enough, engineers reach for CITIfile or MDIF. CITIfile (Common Instrumentation Transfer and Interchange File) stores multiple independent data arrays, each with its own independent variable, so a single file can hold an S-parameter sweep alongside the noise figure and source-pull contours it was measured with. MDIF wraps many parameter-indexed sub-files into one library, which is how foundries deliver bias-dependent or geometry-swept transistor models to a simulator.

The Option Line and Data Notation

Touchstone option line syntax:
# [freq-unit] [param] [notation] R [Z_ref]  →  e.g. # GHz S RI R 50

Complex pairs per frequency point:
N_pairs = N_ports²  (2-port → 4, 4-port → 16)

DB notation to linear magnitude / angle:
|S_ij| = 10^{(dB / 20)}, θ = angle in degrees

Magnitude-angle to real / imaginary:
Re = |S| × cosθ, Im = |S| × sinθ  (θ in radians)

Reverse: |S| = √(Re² + Im²), θ = arctan(Im / Re). If R is omitted, Z_ref defaults to 50 Ω.

Format Comparison

Format	Extension	Encoding	Multiple sweeps	Complex Z_ref	Typical use
Touchstone 1.x	.s1p to .s99p	ASCII	No	No	VNA export, passive components
Touchstone 2.0	.ts	ASCII	No	Yes (per-port)	Multi-port, mixed-mode data
CITIfile	.cti / .citi	ASCII	Yes	Yes	Noise, load-pull, multi-array
MDIF	.mdf	ASCII	Yes (indexed)	Yes	Swept transistor model libraries
P2D / S2D	.p2d / .s2d	ASCII	Yes (power-swept)	Yes	Large-signal amplifier models (ADS)

Common Questions

Frequently Asked Questions

What does the N in a Touchstone .sNp file extension mean?

N is the port count, which sets the number of S-parameter columns: a .s1p holds one S11 value per frequency, a .s2p holds four (S11, S21, S12, S22), and a .s4p holds sixteen. Each frequency point carries N² complex pairs. Touchstone 1.x wraps files of three or more ports across multiple lines; Touchstone 2.0 fixed that with explicit [Number of Ports] and [Network Data] keywords.

How do I convert a Touchstone file between magnitude-angle and real-imaginary format?

The notation token on the option line selects MA (linear magnitude, degrees), DB (decibels, degrees), or RI (real, imaginary). From DB, recover linear magnitude as 10^(dB/20), then Re = |S| × cosθ and Im = |S| × sinθ. Reverse with |S| = √(Re² + Im²) and θ = arctan(Im / Re). Tools such as scikit-rf read all three and convert internally, so the choice is mainly about readability.

Why does my S-parameter file have the wrong reference impedance?

Z_ref is set by the trailing R value, as in # GHz S MA R 50. If R is missing the default is 50 Ω, which corrupts data that was actually 75 Ω or complex. Because S-parameters are defined relative to a specific reference impedance, re-normalizing requires the full impedance-renormalization transform, not simple scaling. CITIfile and MDIF carry explicit per-port impedance metadata, which is why they suit transistor and on-wafer data.

Related Terms

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