To: File: wpfiles\techniques\netwkan.wp6

From: Jon Hagen

Date: 4-11-00

Subject: Network analyzer calibration for S11 measurements

For S11 measurements, a vector network analyzer is provided with a calibration kit made up of three standards for the type of line being used, i.e. waveguide, coax, etc. The cal kit allows one to establish a reference plane that a. lies in the type of line being used and b. is at a convenient point away from the front panel connector of the anlyzer. The purpose of this memo is to show how the analyzer uses the cal kit, why three standards are necessary and sufficient, and why the standards can be any mixure loads, opens, shorts, or other arbitrary (but known) impedances.

In the figure below, the 2-port device with labelled S-parameters represents the adapter between the analyzer and the device to be tested. If we are working in waveguide, this 2-port consists of a coaxial cable and a coax-to-waveguide transistion. Neither the cable nor the transition need to be perfect, nor do they need to have been previously characterized. The analyzer outputs a wave of unit amplitude so the overall reflection coefficient, , is the signal reflected back into the analyzer. The device under test has reflection coefficient . Let us find as a function of :

By inspection we see that the forward wave into the DUT, V1 , can be expressed as

It follows that

Solving for , we find

where A, B and C are three constants. By measuring for each of the three known values of provided in the cal kit, three simultaneous linear equations determine the values of A,B, and C. This is done automatically by the analyzer when one performs a full S11 calibration. Moreover, it is done for every discrete frequency contained between the start and stop frequencies.

Part II "Response" Calibration using a single standard

It is more convenient to do a quick calibration with a single standard. This procedure is listed on the analyzer menu as "Response" as opposed to "Full 1-Port Calibration". Note that the standard used will be a short or an open; a load would give the analyzer little or no clue as to the length of the cable/adapter between the the instrument and the DUT. In this case, the analyzer can do nothing but assume that the constants A and C are zero, i.e. that S11 and S22 are zero. This is equivalent to assuming that the connecting device has an electrical length and loss, but causes no reflections.

Part III Measurement of S21