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 Network Analyzers using S4VNA software

There are two signal flow graphs considered for two-port measurements. One of the graphs describes the case where Port 1 is the stimulus source, the other graph describes the case where Port 2 is the stimulus source.

The signal flow graphs of error effects in a two-port system are represented in the figure below.

Two-port error model

a1, a2 — incident waves, b1, b2 — reflected waves

S11a, S21a, S12a, S22a — actual value of DUT parameters

S11m, S21m, S12m, S22m — measured DUT parameters values

Two-port error model

For normalization the stimulus value is taken equal to 1. All the values used in the model are complex. The measurement result in a two-port system is affected by twelve systematic error terms.

These terms are also described in the table below.

Description

Stimulus Source

Port 1

Port 2

Directivity

Ed1

Ed2

Source match

Es1

Es2

Reflection tracking

Er1

Er2

Transmission tracking

Et1

Et2

Load match

El1

El2

Isolation

Ex1

Ex2

After determining all twelve error terms for each measurement frequency by means of a two-port calibration, it is possible to calculate the actual value of the S-parameters: S11a, S21a, S12a, S22a.

There are simplified methods, which eliminate the effect of only one or several of the twelve systematic error terms.


note

When using a two-port calibration, all four measurements S11m, S21m, S12m, S22m need to be known to determine any S-parameters. That is why updating one or all S-parameters necessitates two sweeps: first with Port 1 as a signal source, and then with Port 2 as a signal source.


For a detailed description of calibration methods, see Calibration Methods and Procedures.

 

Rev.:  22.4