For most ATE applications, independent, 8 term, one port calibrations are sufficient to measure S-parameters accurately. This is because the S12 and S21 terms of the Device under test are typically not close to 1. For instance, in an amplifier, the output and input tend to be reasonably well isolated from each other. As such, mismatch effects at the input are a secondary effect at the output.
However for some classes of device, such as a low loss switch, the test system attributes at the input and output at the device, can interact through the device and cause errors significant enough to be an issue.
Here is an example of an s21 measurement of a MEMS switch.
In this case there is a residual ripple of as much as 0.3 dB, even with the input and output part both having an 8 term, 1-port cal.
For this case of device, a more accurate approach is to implement the 12 term error model, with the addition of a thru measurement, will remove the ripple effects of transmission tracking.
The process involves adding a new path object to the collection, that represents the thru calibration terms.
In the Ri Fixture environment, fixture paths define connections from the test head all the way to the device.
PathRf3DutRf28is a path that connects Testhead port RF3 to the Dut interface port DutRf28.
PathRf7DutRf29 is a path that connects Testhead port RF7 to the Dut interface port DutRf29
In order to hold the cal data (the thru), a path needs to be created between DutRf28 and DutRf29.
Following the convention it must be: DutRf28DutRf29. (The capitalization is critical)
To add this to a either fixture or a dut-interface file. Open up the cal table for the object.
Do this by activating the fixture, RMBC and select calibration inspect.
(If it is a DIX, it must be edited and then the cal can be inspected)
Now select, Calibration Entry> Add Item
Input the through path name. In this case we are connecting DutRf28 to DutRf29. It must be typed precisely, including the capitalization.
Now the path object has been created. Now a calibration plan must be written. It takes the form of all the RiApps fixture calibration, containing the three elements of a cal:
1.) The "reset calibration" of the path
2.) The measurement of the data
3.) Writing the calibration data into the path.
Since this cal is looking for the maximum accuracy available, the measurement of s-parameters is suggested.
This panel includes the reset cal and the write cal.
Note the special Calc: as2PortEA.
That is a text block cal that uses the text in the attached txt file.
the text is: [ :a | a as2portThruEA ]
This uses a special method in the Ri Software to take the s-parameter measurements of the thru and create the cal values that contain the transmission tracking information.