S-Parameters Part 2. Why Proper 50Ω Termination is CRUCIAL for Scattering Parameters Measurement.

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   • S-Parameters Part 1. What Is Scatteri...  

S-Parameters Part 2. Why Proper Termination is CRUCIAL for Accurate Measurements for S-Matrix.

Scattering Parameters (S-parameters) describe the input-output relationships between ports in an electrical system. At high frequencies, it becomes essential to characterize a given network in terms of waves rather than voltage or current. Therefore, S-parameters utilize waves for analysis.
In RF design, traditional parameters like Z, Y, or H-parameters are impractical because short-circuit and open-circuit analyses are not feasible at these frequencies.

If either port is terminated with the characteristic impedance, the reflection term for a signal sent to that port becomes zero. Thus, if port 2 is terminated in Zo and an incident signal applied to port 1:

By definition, an S-Parameters is the ratio of the voltage, both magnitude and phase, of a signal “coming out” of a network port to that of the signal “going into” the port. Source & load impedances are 50 ohms.
The S-Parameters of a device can be plotted on a Smith Chart or Polar Plot to show changes with respect to frequency.
it varies with biasing conditions
it varies with frequency

S11= input reflection coefficient
S12= reverse transmission coefficient
S21= forward transmission coefficient
S22= output reflection coefficient

The Scattering matrix is a mathematical construct that quantifies how RF energy propagates through a multi-port network. The S-Parameters is what allows us to accurately describe the properties of complicated networks as simple "black boxes". For an RF signal incident on one port, some fraction of that signal gets reflected back out of the incident port, some of it enters into the incident port and then exits at (or scatters to) some or all of the other ports (perhaps being amplified or attenuated).