A Touchstone file was originally a proprietary file format for the eponymous frequency-domain linear circuit simulator[1] from EEsof, launched in 1984 and now owned by Agilent Technologies.
The Touchstone simulator has long since been superseded[2], but its file format lives on.
A Touchstone file (also known as an SnP file after its set of file extensions[3]) is an ASCII text file used for documenting the n-port network parameter data and noise data of linear active devices, passive filters, passive devices, or interconnect networks. An example of the format of the S-parameter section is given in the article about S-parameters. In addition to S-parameters, other representations such as Y-parameters and Z-parameters can be recorded.
It later became a de facto industry-standard file format not only for circuit simulators but also for measurement equipment (e.g. vector network analyzers, or VNAs), then later still an EIA standard as part of the Input/output Buffer Information Specification (IBIS) project[4]. On April 24, 2009, the IBIS Open Forum ratified version 2.0[5], superseding Version 1.1[6]. Version 2.0 adds IBIS-style keywords such as [Reference], which permits per-port definition of the reference environment.
Several further enhancements to the file format that allow description of the non-linear behavior of the component have been developed under the P2D and S2D pair of formats[7], but these too have been superseded by the X-parameters functionality.