@KSKelvin

I’m confused by your slides on simulating S parameters in QSpice.

“In this circuit, no matter what loading condition is, if we couple forward wave and terminate into 50 ohms”… is very confusing.

Do you mean to emphasize that the [S] or DUT block can be arbitrary, and the load can be arbitrary, but the combination, or termination of the DUT with the load, must be 50 ohms?

I just revised my slide as below, will update the pdf in Github.

Forward and reverse voltage are basically voltage wave travel in a transmission line in forward and reverse direction. If you use a voltage probe to measure a voltage, you are measuring their resultant, i.e. resultant voltage = forward voltage + reverse voltage. My understanding is that, you cannot directly “measure” forward or reverse voltage; a direct measurement is only resultant voltage and current.

There are at least two method to get forward and reverse voltage

- Calculate forward and reverse voltage from resultant voltage and current with their magnitude and phase information. (e.g. RF VI probe)
- Couple the forward wave and reverse wave and terminate the coupled signal with characteristic impedance Zo. (e.g. directional coupler)

Source voltage in series a 50ohms resistor can force forward voltage at port 1 equal to source voltage / 2 is because of the concept of coupling.

** the beauty is that, the reverse voltage is terminated by series 50-ohms resistor (=Zo) for the wave into source direction, and therefore, reverse wave cannot contribute to source voltage magnitude, and the source voltage magnitude can only contribute into forward.