## S parameter question

• 0
Hi

I am trying to understand S parameters and how they are measured. If I have a 2 port network and want to measure S11, I need to excite port 1 and measure the reflected voltage at port 1. If I set V to be the excitation voltage and I measure the value V1 at port 1, how do I know the reflected signal? Does the generator match to the load and the chracteristic impedance of the line which allows me to say if there was no reflection I should measure V/2, but now I measure V1, so the difference is the reflected voltage?
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Roader, the network analyzer har directional couplers on the ports, making it possible for the analyzer to separate forward and reflected power when measuring s11 and s22.. For a scalar analyzer (that only can measure magnitude); for instance, if forward power out on port 1 is 0 dBm, and the reflected wave is -10 dBm, the return loss can then be calculated to 10 dB, or, in linear fashion, gamma is 0.1.
For a network analyzer, also the phase between the stimuli ( forward power) and the recieved (reflected power) can be measured. Then, not only the magnitud can be measured but the reflection in complex form, as magnitud + angle, or real+im component.
The same goes of course for transmission measurements, s21 and 12, but that is easier to understand.
There are a lot of good tutorials on network analyzers, explaining there way of working.
With some math skills in complex numbers the secrets will be revealed for you in no time :)
• 0
Hi

I still dont understand how the reflected wave is measured with a coupler. Is there some pictures or documents you could link to?
• 0
OK, Roader, take a look at this: https://www.microwaves101.com/encyclopedias/directional-couplers
A directional coupler, think of it in this way; think of a road, going north-south, with traffic in both directions. That road is the main path of the coupler, the signal source is in the north and the load in in south.
Then, add a coupled path; a measurement device that registrate the traffic only in north-south direction. Thats your forward coupled path. You can also add a coupled path registrating the traffic south - north. That would be the reverse coupled path.
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