Vector Network Analyzer Measurements
- Updated2023-02-21
- 2 minute(s) read
Reflection coefficient (Γ) is the ratio of reflected voltage divided by the incident voltage, and transmission coefficient (T) is the ratio of transmitted voltage divided by the incident voltage. Modern network analysis provides more information about these ratios with scattering parameters, or S-parameters.
S-parameters are complex vector quantities that represent the ratio of two RF voltages. S-parameters have a magnitude and phase or, in Cartesian form, real and imaginary terms. S-parameters are expressed as Sxy, where x represents the DUT output port being measured and y denotes the DUT input port stimulated by the incident RF signal. The following figure shows a simple two-port device, such as an RF filter, attenuator, or amplifier, and how its S-parameters would be derived.
Both quantities, S11 and S21, are referred to as forward S-parameters because the incident signal originates from the RF source on port one. With the incident source on port two, S22 and S12 are called the reverse S-parameters. The following table defines the S-parameters that correspond with your NI vector network analyzer.
| S-Parameter | Direction | Description |
|---|---|---|
| S11 | Reflected | The ratio of the voltage reflected at port one to the incident voltage placed on port one. |
| S21 | Transmitted | The ratio of the voltage transmitted through the DUT present at port two to the incident voltage placed on port one. |
| S22 | Reflected | The ratio of the voltage reflected at port two to the incident voltage placed on port two. |
| S12 | Transmitted | The ratio of the voltage transmitted through the DUT present at port one to the incident voltage placed on port two. |
As shown in the table above, S11 and S22 (S-parameters with matching numbers) describe reflected signals. S12 and S21 (S-parameters with non-matching numbers) describe transmitted signals. Furthermore, the total number of S-parameters required to fully describe the RF characteristics of a device is given by the number of device ports squared.
S-parameters that describe transmission, such as S21, are analogous to other familiar terms including gain, insertion loss, or attenuation. S-parameters that describe reflection, such as S11, correspond to voltage standing wave ratio (VSWR), return loss, or reflection coefficient.
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Note A spectrum analyzer combined with a tracking generator can perform similar measurements to a vector network analyzer, but the tracking generator system cannot perform phase measurements, and you cannot perform the same type of error correction and calibration that you can do with a vector network analyzer. |