About Touchstone file format

Brief compendium about Touchstone files

touchstone

simulation

History

Touchstone files have been developed by EEsof before being acquired from HP. These ASCII files express in a very simple way the frequency behaviour of an n-port network; in every line, you can find the value (magnitude, phase, real, imaginary, depending on the chosen format) of each S-parameter at a defined frequency. The frequency points are listed in ascending order.

This format has become the de-facto standard for many tools and software used in RF, EMC compliance, design and verification, and simulation. For some simulation software, Touchstone files are used to represent Y and Z parameters, too.

Format

The Touchstone format contains four types of data: comments, option lines, data lines and noise data lines. Let's see how they are defined and

Comments

Comments are lines preceded by the symbol '!'. A comment can be added at the end of a data line, adding a ! at its beginning. There is no symbol for multi-line comments.

0
1

! This is a comment
data data data data data data data    ! This is a comment at the end of a data line

Option line

The option line is used to provide the format of the data in the next sections. The option lines start with the symbol '#' and have the following format

# <frequency unit> <type> <format> <Rn>

The possible values for each field are

# [Hz, kHz, MHz, GHz] [S, Y, Z, G, H] [MA, DB, RI] [Rn]

frequency unit Unit of the datum in the frequency column
type Type of datum (scattering matrix, impedance, admittance)
format DB: dB/angle, MA: magnitude/angle, RI: real/imaginary
Rn Reference resistance to whom the S parameters are normalized to. It is expressed in Ohm

Data

Data is formatted in the following way for a 2-port network.

0
1

<frequency1> |S11| <S11 |S12| <S12 |S21| <S21 |S22| <S22
<frequency2> |S11| <S11 |S12| <S12 |S21| <S21 |S22| <S22

If the network has n-ports, the Touchstone file becomes like this

<frequency1> |S11| <S11 |S12| <S12 |S13| <S13 |S14| <S14 ... |S1n| <S1n
             |S21| <S21 |S22| <S22 |S23| <S23 |S24| <S24 ... |S2n| <S2n
             |S31| <S31 |S32| <S32 |S33| <S33 |S34| <S34 ... |S3n| <S3n
             ...
             |Sn1| <Sn1 |Sn2| <Sn2 |Sn3| <Sn3 |Sn4| <Sn4 ... |Snn| <Snn
             
<frequency2> |S11| <S11 |S12| <S12 |S13| <S13 |S14| <S14 ... |S1n| <S1n
             |S21| <S21 |S22| <S22 |S23| <S23 |S24| <S24 ... |S2n| <S2n
             |S31| <S31 |S32| <S32 |S33| <S33 |S34| <S34 ... |S3n| <S3n
             ...
             |Sn1| <Sn1 |Sn2| <Sn2 |Sn3| <Sn3 |Sn4| <Sn4 ... |Snn| <Snn

Noise parameters

Noise parameters can be included in a Touchstone file too. typically, they are appended to the end of the file. The format used for each line (one line is one frequency point) is the following one:

<freq> <nf [dB]> <|Gamma|> <<Gamma [°]> <Rn>

frequency This is the frequency step
nf [dB] Minimum noise figure expressed in dB
|Gamma| Magnitude of the source gamma (reflection coefficient) at the minimum noise figure
<Gamma [°] Phase of the source gamma (reflection coefficient) at the minimum noise figure
Rn Effective noise resistance normalized to the system impedance defined in the option line

Remember that the noise figure is defined as $$nf = 10 log_{10} \left( \frac{SNR_i}{SNR_o} \right) = SNR_{i,dB} - SNR_{o,dB} $$