Vector Network Analyzer Measurement Guide : VNA Fundamentals : S‑Parameters
 
S‑Parameters
To simplify the description of the types of measurements a VNA can make, the reflection and transmission measurements are defined in terms of scattering parameters, or S‑parameters. For a 2-port network, four fundamental S-parameters can be measured, and they are defined as SXY. For a 2‑port VNA, measurements of signals leaving Port 1 are called forward measurements, and those leaving Port 2 are called reverse measurements. Signals that leave and return to the same port are designated reflection measurements, and those that leave one port and return to another port are designated transmission measurements. S‑parameters are an abbreviated designation for these measurements, and are used as shown in the following list:
S11: Forward Reflection
S21: Forward Transmission
S12: Reverse Transmission
S22: Reverse Reflection
The first number (X) in SXY is the port number into which the signal is being injected, and the second number (Y) is the port number from which the signal is leaving. The S-parameter is a ratio of these two signals.
Additional Examples:
S11
Forward Reflection represents the measurement in which the signal leaves port 1 and is reflected back to port 1.
S21
Forward Transmission represents the measurement in which the signal leaves port 1 and is transmitted to port 2.
S12
Reverse Transmission represents the measurement in which the signal leaves port 2 and is transmitted to port 1.
S22
Reverse Reflection represents the measurement in which the signal leaves port 2 and is reflected back to port 2.
 
VNA Master Architecture
A VNA can have 1-port only, in which case it measures only reflection signals. A VNA can have 2-ports, in which case it can measure both reflection and transmission. A 2-port VNA can also have two different capabilities: 1-path 2-port, or full-reversing. The 1-path 2-port design allows reflection measurements only at one of the two ports and allows transmission measurements only in one direction. The MS20xxB VNA Master and the S412E LMR Master are two‑port, 1‑path VNA instruments that allow S11 and S21 measurements with a single connection. The MS20xxC VNA Master is a full-reversing VNA that allows reflection measurements from both ports and allows transmission measurements in both directions (S11, S21, S22, and S12 measurements with a single connection).
The MS20xxB compact VNA Master and the S412E LMR Master have an architecture that automatically measures two S-parameters (S11 and S21) with a single connection. Three receivers are used, so the forward sweep from Port 1 simultaneously yields S11 and S21.
The MS20xxC VNA Master has an architecture that automatically measures the four S-parameters with a single connection. Three receivers are used, so the forward sweep from Port 1 simultaneously yields S11 and S21, and the reverse sweep from Port 2 simultaneously yields S22 and S12. Thus, measurement of the four S-parameters for a two-port DUT requires only two sweeps, the forward and reverse transmission.
Figure: MS20xxC VNA Master Block Diagram During Forward Sweep and Figure: MS20xxC VNA Master Block Diagram During Reverse Sweep show a general block diagram of the three-receiver architecture that is used in the MS20xxC VNA Master and show how the S-parameters are related to the signals that are being transmitted and received by the ports. From Figure: MS20xxC VNA Master Block Diagram During Forward Sweep, you can see how S11 and S21 are generated by a forward sweep (signal directed from Port 1). Figure: MS20xxC VNA Master Block Diagram During Reverse Sweep shows how S22 and S12 are generated by a reverse sweep (signal directed from Port 2).
MS20xxC VNA Master Block Diagram During Forward Sweep
MS20xxC VNA Master Block Diagram During Reverse Sweep
Note
The MS20xxC VNA Master, when equipped with Option 77, can calculate the balanced differential, common, and mixed mode S‑parameters (Sd1d1, Sc1c1, Sc1d1, Sd1c1) using the 4 measured S‑parameters S11, S21, S12, and S22).
These additional S-parameters can be used to measure reflections from differential cables when the two ends of the cable are connected to Port 1 and Port 2 of the MS20xxC VNA Master. Because these S‑parameters are a function of all 4 S-parameters, it requires both a forward and reverse sweep to complete the calculation.
For more information about these additional S-parameters, refer to Balanced Ports, Option 77.
 
Figure: MS20xxB VNA Master and S412E LMR Master Block Diagram shows a general block diagram of the three-receiver architecture that is used in the MS20xxB VNA Master and the S412E LMR Master and shows how the S-parameters are related to the signals that are being transmitted and received by the ports.
MS20xxB VNA Master and S412E LMR Master Block Diagram