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:

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.

Forward Reflection represents the measurement in which the signal leaves port 1 and is reflected back to port 1.

Forward Transmission represents the measurement in which the signal leaves port 1 and is transmitted to port 2.

Reverse Transmission represents the measurement in which the signal leaves port 2 and is transmitted to port 1.

Reverse Reflection represents the measurement in which the signal leaves port 2 and is reflected back to port 2.

 

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).

 

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.