The first step to get started with vector network analyzer - what are S parameters?

In microwave measurements, power is an important parameter. Engineers all hope to transmit power more efficiently in the design of microwave circuits, and power is usually conducted using transmission lines. Ideally, we hope that in this transmission line, the power from the input end to the receiving end will be the same. Yes, but the actual transmission process will inevitably lead to three situations:

1. Part of the signal will be reflected back from the input end. Just like the refraction of light, certain reflections will occur due to different media.

refraction of light

2. Part of the signal is transmitted to other ports

3. The remaining part is dissipated through thermal radiation or electromagnetic radiation during the transmission process.

Therefore, the signal undergoes certain changes during the transmission process. The S parameter is used to describe the transmission and reflection characteristics of radio frequency energy in a multi-port network. The first number in the S parameter subscript represents the response end (receiving end), and the second number represents the excitation end (incidence end). For example, S11 represents the response of port 1 relative to the port 1 input signal, and S21 represents the response of port 2 relative to the port. 1 response to the input signal.

We can briefly understand the definition of dual-port S parameters through the following picture:

Dual-port S-parameter network diagram

Assume that port 1 is the signal input end and port 2 is the signal receiving end, then a1 is the incident signal at port 1, b1 is the signal reflected back by the DUT at port 1, and b2 is the signal received by port 2. At this time, a2 is zero.

Then S11=b1/a1, S21=b2/a1

In the same way, when we use port 2 as the signal input terminal and port 1 as the signal receiving terminal, a2 is the incident signal, b2 is the signal reflected back by the DUT at port 2, and b1 is the signal received by port 1. At this time, a1 is zero.

Then S22=b2/a2, S12=b1/a2

From the above, we can know that when we measure the transmission and reflection characteristics of single-port devices, we can measure S11. When measuring dual-port devices, we can measure S11, S21, S22, and S12. The S parameters of other multi-port devices are based on the incident There are more S parameters that need to be measured in the transmission relationship with reflection to verify the changes in the microwave circuit during the transmission process.