Thursday, April 30, 2015

Using the Chinese SNA

First thing I had to do after I received the Chinese SNA was its internal calibration.
To calibrate the system you will need an in-line attenuator, 6db. if calibrating linear or 40db. for log.  Using Eagle cad software I did a quick layout for a pi-attenuator.  Since the layout for the attenuator was so small I had room to put 7 attenuators on the  blank along with a Return Loss Bridge.  I have a fairly large selection of 1206 size resistors, but still need to parallel some to get the correct values for the different attenuators.  I did the layout to allow space for 4 resistors for the shunt resistance , and 3 for the series resistance.   After etching a set of boards, I computed the required values using the pi-attenuator calculator at 
Return Loss Bridge and 40db attenuator finished

I built up the required 40db. attenuator and a couple of other values. Looking at the software, there is a function to measure impedance of the device under test.  Using one of attenuator boards I fabricated the inline 50 ohm resistor required for this. I also built up the small Return Loss Bridge I had etched.

Calibration is very easy, go into the calibrate mode and follow the instructions on screen.  Place the 40db. attenuator between the input and output connector and run one sweep,  then remove the attenuator and connect the output directly to the input and run another sweep. After this is finished the calibration data is saved to a file.

I have the Eagle files and a PDF of the boards ready for toner transfer in the Drop box at

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