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Calculating Reflection Coefficient

Question asked by kl83 on Dec 18, 2008
Latest reply on Jan 7, 2009 by kl83
Greetings all,

First, let me say that this is a great forum with lots of information and knowledgeable people.  I am glad that I have found it as I am really stuck on something now that I don't quite understand... so let me get straight to the topic.

Basically I am trying to do a full 2-Port 16 error term calibration.  I have read a lot of Doug Rytting's papers and I am following the steps he described to do my calibration.  My setup is such that I have a RF stimulus unit, and I have two couplers and connectors and such.  I have attached an image from one of Agilent's service manuals, so I basically followed that.

I started out by trying to calibrate 1-port first by calculating the 3 error terms associate with it.  I did this by connecting a load, short, open.  so my RF out port generates the frequency range desired, it goes into a splitter (resistive), one side of the splitter goes into a 10dB attenuator then to my REF receiver port.  The other end of the splitter goes through a 10dB attenuator and goes into a directional coupler.  The output of the coupler would be my PORT 1 for S-Parameter measurement, the coupled port goes into port A (2nd reference port) of my receiver.  REF port is one of the receiver ports, and Port A is the second receiver port that measures the reflected signal.  However, I am stuck on how I can calculate the Gamma from my data.  Basically my receiver spits out I (inphase) and Q (quadrature) data of however many points was put in.  Supposed the magnitude is (square root (I^2 + Q^2))  and the phase is arctan (Q/I) Do I calculate the Gamma by dividing the reflected signal (magnitude) over the incident signal? 

I have calculated the 3 error terms for 1 port, I used a 3dB attenuator for my test case, so I am supposed to see a 6 dB return loss (S11).  But I am unable to get that result.  Am I missing something or doing something something wrong?  I am really stuck here...

Thanks in advance for all your help!