A Touchstone file was originally a proprietary file format for the eponymous frequency-domain linear circuit simulator^[1] from EEsof, launched in 1984 and acquired by HP. The simulator has been called HP/EEsof, then its engine has been successively included in the jOmega and ADS software suites and it is now owned by Keysight.

The Touchstone simulator has long since been superseded,^[2] but its file format lives on.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behavior of the component have been developed under the P2D and S2D pair of formats,^[7] but these two have been superseded by the X-parameters functionality.