Are Touchstone comments compatible across all RF simulation tools?

The exclamation mark comment syntax is universally supported by all major RF simulation and EDA tools including Keysight ADS, Cadence AWR Microwave Office, Ansys HFSS, CST Studio, Sonnet, Qucs, and open-source tools like scikit-rf. However, some older tools have specific limitations: certain parsers require comments to be on separate lines (not inline after data), some tools limit comment line length to 80 or 256 characters, and a few legacy parsers may not handle comments between data blocks in multi-port files. Touchstone 2.0 (IEEE 370-2020) formalized the comment syntax and added structured keywords like [Version], [Number of Ports], [Two-Port Data Order], and [Number of Frequencies] that provide machine-readable metadata alongside human-readable comments.

Data Formats

Comments (Touchstone)

Q: What is the comment syntax in Touchstone files?

In both Touchstone 1.0 and 2.0 formats, any line beginning with an exclamation mark (!) is treated as a comment and ignored by parsers. Comments can also appear after data on the same line in some implementations, though placing comments on their own lines ensures maximum compatibility. In Touchstone 2.0, comments can additionally appear between the [Version] and [Number of Ports] keywords. The option line (beginning with #) is NOT a comment; it defines the frequency unit, parameter type, format, and reference impedance. Common comment conventions include documenting the device under test, measurement date, VNA model, calibration type, port assignments, temperature, and bias conditions.

Q: What information should be included in Touchstone comments?

Best practice documentation includes device identification (part number, serial number, lot code), measurement setup (VNA model, firmware version, calibration kit, calibration type such as SOLT or TRL), frequency range and number of points, port assignments (which physical connector maps to which port number), DC bias conditions (voltage and current at each port), temperature during measurement, date and operator, and any de-embedding or port extension applied. For transistor S-parameters, comments should include the bias point (Vds, Ids or Vce, Ic) since different bias conditions produce completely different S-parameter datasets. Component manufacturers typically include absolute maximum ratings and recommended operating conditions in the comment header.

/kom-ents tuch-stohn/

Human-readable annotations in Touchstone S-parameter files (.s1p, .s2p, .snp) preceded by an exclamation mark (!) that document measurement conditions, device information, calibration details, and port definitions without affecting the numerical S-parameter data parsed by simulation tools. Comments appear on any line starting with ! and are ignored by all compliant parsers. Best practice includes documenting the device under test, VNA model and calibration method, frequency range, port assignments, DC bias conditions, temperature, and measurement date. Touchstone 2.0 (IEEE 370-2020) formalized comment syntax and added structured keywords for machine-readable metadata alongside human-readable comments.

Category: Data Formats

Syntax: ! (exclamation mark)

Standard: IEEE 370-2020

Understanding Touchstone Comments

Touchstone files are the universal exchange format for measured and simulated S-parameter data in RF engineering. Every vector network analyzer exports Touchstone, every simulation tool imports it, and component manufacturers distribute device models as .s2p files. The comment system provides a standardized way to embed documentation directly in the data file, ensuring that critical measurement metadata travels with the data rather than being lost in separate documentation.

A well-commented Touchstone file is self-documenting: an engineer receiving the file years later can immediately understand what was measured, how it was measured, and under what conditions. This is particularly important for transistor S-parameters, where the data is completely different at different bias points, and for connector/cable measurements where the calibration reference plane determines what the data represents. Without comments, an unlabeled .s2p file with 201 frequency points is essentially meaningless, as there is no way to determine whether it represents a filter, an amplifier, a transmission line, or noise.

Touchstone File Structure

Comment Lines:
! Any text after exclamation mark is ignored by parsers
! Device: BFP740 SiGe HBT, Lot 2024-Q3
! Bias: Vce=2.0V, Ic=10mA

Option Line (NOT a comment):
# GHz S MA R 50
(frequency unit, parameter type, format, reference Z)

Data Lines:
freq |S11| ∠S11 |S21| ∠S21 |S12| ∠S12 |S22| ∠S22

The # option line defines: frequency unit (Hz/kHz/MHz/GHz), parameter type (S/Y/Z/H/G), format (MA=mag-angle, DB=dB-angle, RI=real-imaginary), and reference impedance. All data after the option line is numerical S-parameter data. Comments can appear before, between, or after data lines.

Touchstone Version Comparison

Feature	Touchstone 1.0	Touchstone 2.0 (IEEE 370)
Comment character	! (exclamation)	! (exclamation)
Structured keywords	None	[Version], [Number of Ports], etc.
Max ports	Unlimited (convention)	Formally defined for N ports
Mixed-mode S-params	Not supported	[Mixed-Mode Order] keyword
Noise data	After S-data block	[Noise Data] keyword
File extension	.s1p, .s2p, .snp	.ts (recommended), .snp (compatible)
Reference impedance	Single value (R 50)	Per-port [Reference] keyword

Common Questions

Frequently Asked Questions

What is the comment syntax in Touchstone files?

Any line beginning with ! is a comment, ignored by parsers. In Touchstone 2.0, comments can appear between structured keywords. The # option line is NOT a comment; it defines frequency unit, parameter type, format, and reference impedance. Place comments on separate lines for maximum tool compatibility.

What information should be included in Touchstone comments?

Best practice: device ID (part number, serial, lot), VNA model and firmware, calibration type (SOLT/TRL), frequency range and point count, port assignments, DC bias (Vds/Ids or Vce/Ic for active devices), temperature, date, operator, and any de-embedding or port extension applied. Different bias conditions produce entirely different datasets.

Are Touchstone comments compatible across all RF tools?

The ! syntax is universally supported by ADS, AWR, HFSS, CST, Sonnet, Qucs, and scikit-rf. Limitations: some older parsers require comments on separate lines, some limit line length to 80 to 256 characters, and legacy tools may not handle inline comments after data. Touchstone 2.0 adds structured keywords for machine-readable metadata.

Related Terms

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