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SOLT calibration for measuring impedance in a circuit

Question asked by ivanferez86 on Mar 21, 2016
Latest reply on Apr 11, 2016 by Ryoji
SOLT or SOLR calibration for measuring impedance in a circuit
Dear experts,
I would like to measure the impedance from 10 MHz to 1GHz in a circuit with a different geometry in comparison with a SMT part characterization or similar. I have a 5071C network analyzer. Each line impedance has around 10 Ohms, and the circuit is offering an impedance around 2 Ohms for each Z1 and Z2. The cirtuit is like the circuit_2.jpg.

So, to eliminate the influence of the probe test (a 50 Ohm coaxial to a contact microstrip transition ) I did a full two port SOLR calibration with a non-coaxial calibration kit following the guidelines of the book Advanced measurement techinques, PCB example (SMT part characterization), but adapting it to my problem.

After evaluating the performance of the standards ( short-load-thru.png) and the calibration I noticed a not very good performance from 300 MHz, that they make me wonder some questions.

The coaxial to microstrip transition, has an equivalent shunt capacitance of 5 pF. Measuring the response of the standards and adjusting values in a simplified transition model.

Should I change the coaxial to microstrip transition to a tapered microstrip line and then a coaxial to reduce mismatching or that sunt capacitances?

How can I evaluate if I have a good transmission line and transitions to my load standard or if does not matter much because calibration can overcome that mismatches? 

Which standard put the reference plane in a SOLT calibration? In the book advanced measurement techniques, the standard Load puts the reference plane but I have seen in few white papers that the standard short also puts the reference plane. 

It would be a good idea to use an arbitrary standard instead of load standar with an impedance similar with the impedance I want to measure in the circuit? For example 10 Ohm or less?
Thanks in advanced.

Edited by: ivanferez86 on Mar 21, 2016 5:35 AM