What does a Touchstone file contain?

A Touchstone file is a plain text file with a header line and data rows. The option line (starts with #) specifies: frequency unit (Hz, kHz, MHz, GHz), parameter type (S, Y, Z, H, G), format (MA = magnitude/angle, DB = dB/angle, RI = real/imaginary), and reference impedance (R 50 for 50 ohms). Example option line: # GHz S DB R 50 (frequency in GHz, S-parameters in dB/angle, 50 ohm reference). For a 2-port .s2p file, each data row contains: freq S11_mag S11_ang S21_mag S21_ang S12_mag S12_ang S22_mag S22_ang. The order is important: S11, S21, S12, S22 (not S11, S12, S21, S22). This column ordering trips up many engineers. Comments start with an exclamation mark. Noise parameters can be appended after the S-parameter data, each line containing: freq Fmin Gamma_opt_mag Gamma_opt_ang Rn/Z0 (noise figure, optimal source impedance, normalized noise resistance). These are essential for LNA design: they define the noise circles on the Smith chart.

What are the Touchstone file extensions?

.s1p: 1-port device (antenna, load, open, short). One S-parameter per frequency point (S11 only). Used for: impedance measurements, calibration standards, antenna return loss. .s2p: 2-port device (amplifier, filter, attenuator, cable). Four S-parameters per frequency point (S11, S21, S12, S22). The most common Touchstone format. Every component datasheet with S-parameter data provides .s2p files. .s3p: 3-port device (circulator, power divider). Nine S-parameters per point. Less common. .s4p: 4-port device (directional coupler, differential pair). Sixteen S-parameters per point. Increasingly important for differential signaling (USB, PCIe, HDMI) where the 4-port data is converted to mixed-mode (Sdd, Scc, Sdc, Scd). .snp: general n-port device. Used for multi-port structures like power dividers, switch matrices, and antenna arrays. Touchstone 2.0 (.ts) supports unlimited ports and removes formatting limitations of the original spec. Touchstone 1.0 had a practical limit of about 99 ports due to line-length constraints.

How are Touchstone files used in design?

Component models: every RF transistor, amplifier, filter, and passive component vendor provides .s2p files measured at specific bias conditions. These files are imported into circuit simulators (ADS, AWR, Microwave Office) as black-box models. The simulator uses the S-parameter data to predict circuit performance when the component is embedded in a matching network or signal chain. Accuracy depends on the measurement quality and the similarity between the measurement fixture and the actual PCB environment. EM simulation validation: electromagnetic simulators (HFSS, CST, Sonnet) export results as Touchstone files. These are compared against VNA measurements to validate the simulation. Good correlation (within 1 to 2 dB on S21, within 2 to 3 dB on S11) confirms the simulation model is accurate. System simulation: cascading .s2p files using matrix multiplication (converting S to T-parameters, multiplying, converting back) predicts the performance of a multi-stage signal chain: LNA plus filter plus mixer plus IF amplifier. This is the foundation of system-level budgeting. Behavioral models: Touchstone files can be used directly in time-domain simulators (SPICE) via inverse FFT, enabling mixed signal-integrity and RF simulation. This is increasingly important for high-speed serial links.

Data Format

Touchstone

/tuch-stohn/ — .s2p, .snp

S-parameter file standard. # GHz S DB R 50 (option line). .s1p: 1-port (S11). .s2p: 2-port (S11, S21, S12, S22). .s4p: 4-port (diff pairs). Formats: MA (mag/angle), DB (dB/angle), RI (real/imag). Noise params: Fmin, Γ_opt, Rn. Touchstone 2.0 (.ts): unlimited ports, mixed-mode. Every VNA, every EDA tool, every component datasheet.

.s2p: most common

v2.0: unlimited ports

Z₀: 50Ω

Understanding Touchstone

Touchstone files are the universal data exchange format for RF/microwave engineering. Every S-parameter measurement from a VNA, every EM simulation result, and every component model from a vendor is stored as a Touchstone file. Mastering this format is as fundamental as knowing Ohm's law.

The format is deliberately simple: plain text, one frequency point per row, with S-parameters in the engineer's choice of representation (magnitude/angle, dB/angle, or real/imaginary). This simplicity has ensured universal adoption across every EDA tool and measurement platform.

Touchstone File Structure

Option line:
# [freq_unit] [param] [format] R [Z0]
# GHz S DB R 50

.s2p data row (DB format):
freq |S11|dB ∠S11 |S21|dB ∠S21 |S12|dB ∠S12 |S22|dB ∠S22

Noise parameters (after S-data):
freq F_min(dB) |Γ_opt| ∠Γ_opt R_n/Z₀

Touchstone File Types

Extension	Ports	S-params	Typical Use	Example
.s1p	1	1 (S11)	Antenna, load	Return loss
.s2p	2	4	Amp, filter, cable	Gain + match
.s3p	3	9	Circulator, splitter	Isolation
.s4p	4	16	Diff pair, coupler	Mixed-mode
.ts (v2.0)	N	N²	Array, multi-port	Phased array

Common Questions

Frequently Asked Questions

Contents?

Option line: # GHz S DB R 50. Data rows: freq + S-params. .s2p order: S11, S21, S12, S22 (NOT S11, S12, S21, S22). Comments: ! prefix. Noise params: Fmin, Γ_opt, Rn after S-data. Formats: MA, DB, RI. Plain text, universally readable.

Extensions?

.s1p: 1-port (antenna RL). .s2p: 2-port (amp/filter, most common). .s4p: 4-port (diff pair, mixed-mode). .snp: n-port. Touchstone 2.0 (.ts): unlimited ports, removes line-length limits, supports mixed-mode natively. Every EDA tool reads all formats.

Usage?

Component models: vendor .s2p in circuit sim (ADS/AWR). EM validation: HFSS/CST export vs VNA measurement. System cascade: S→T multiply→S for multi-stage budget. Time-domain: inverse FFT for SPICE. Noise circles: Fmin + Γ_opt from noise params for LNA design. Foundation of RF design flow.

Related Terms

← Thermal Noise Transceiver →

RF Data

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