Vector Network Analyzer Measurement Guide : VNA Fundamentals : Calculating and Displaying S-Parameters
 
Calculating and Displaying S-Parameters
S-parameters are a measure of the ratio of two complex voltage levels, one measured by the port receiver, and one measured by the reference receiver. S-parameters therefore consist of unitless complex numbers.
Depending on the application, S‑parameters can be displayed in many ways and can be used to calculate other parameters. S-parameters consist of real and imaginary numbers. More typically, however, they are represented as magnitude and phase. In most cases, the magnitude is displayed in dB (this term is often called log magnitude). We can display phase as “linear phase”. With phase, we cannot tell the difference between one cycle and the next. After going through 360 degrees, we are back to where we began. We can display the measurement from – 180 degrees to + 180 degrees, which keeps the display discontinuity removed from the important 0 degrees area used as the phase reference.
The VNA Master supports the following display types. Each type is associated with a particular S-parameter, Sxy = SReal + jSImaginary (where j is the square root of – 1).
Log Magnitude
Application Notes
To measure return loss at Port 1 (or Port 2), use the Log Mag display with S11 (or S22).
To measure the gain or loss in a DUT that is connected between Port 1 and Port 2, use the Log Mag display with S21 or S12.
Log Magnitude / 2
Application Note
For measuring 1‑port cable loss, use S11 or S22 with the Log Mag / 2 display type to account for the round trip signal path through the cable. When using reflection data to measure cable loss, the end of the cable must be shorted or must be a perfect open.
Real and Imaginary
SReal = Real S‑parameter
SImaginary = Imaginary S‑parameter
SWR
Application Note
SWR, or Standing Wave Ratio, is a measure of the reflection from the DUT input port or output port, and it must be used, therefore, with S11 or S22.
Group Delay
Group Delay (sec) = rate of change of phase over a specified frequency aperture
Application Note
Group Delay is a measure of the time delay of the signals that are propagating through the DUT versus frequency (using S21 or S12). Group delay is a good measure of phase distortion through the DUT.
Smith Chart
Smith Chart = graphical tool for plotting impedance or admittance data versus frequency
Application Note
Use Smith Chart with S11 or S22 to plot the input or output impedance of the DUT.
Use the Inverted Smith Chart to plot admittance data.
 
Note: The Inverted Smith Chart is available only on VNA Master models MS20xxC.