S parameter question
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?
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
I still dont understand how the reflected wave is measured with a coupler. Is there some pictures or documents you could link to?
OK, Roader, take a look at this:
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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