Saturday, April 17, 2010

N2PK VNA - SMA calibration standards

For my home-brewed SMA calibration standards I used the construction method, previously used for my BNC standards.

The Open, Short and Load standard are made of Male SMAs for PCB. The back side of the center pin of all three connectors is trimmed down to ~0.5 mm - the height of a 0603 SMD component. In addition, the SMA test fixture has its center pin also trimmed at 0.5 mm for improved solderability to the DUTs.

For the SHORT standard, I used a small teflon washer (cut from the center dielectric of a MIL grade coax) placed on the center pin. The thickness of the washer is ~0.3-0.4mm. This washer serves as an insulator in order to raise the actual "short" to the very tip of the 0.5 mm center pin stub, (the back plane is covered with silver bearing solder to short the pin). For the LOAD standard I used the same Vishay FC series 50.0 ohm (0.1%) high-frequency precision resistor as the one in my BNC standards set (Digikey p/n FC0603-50BWCT-ND). This type of resistor is the preferred component for the LOAD - it is designed with maximum precision and minimal reactance at high frequencies.

For mechanical protection, RF shielding and easy handling, I used sleeves made of brass tubing (Stock #135, K&S Engineering, 3/8 x .014") to cover the back side of each connector. Each sleeve is 1 cm long and it is soldered on the inside wall to the 4 ground pins of the SMA connector. The end cap is made of closed-cell foam and it is held in place by the heat-shrink tubing. The OPEN standard also has an additional rear RF shield made of a tin-plated brass disc, soldered to the inside wall of the sleeve. The OPEN is more sensitive to stray RF than the others and must be fully shielded. Such shield could be installed on the LOAD as well. The SHORT does not need one as it is self-shielded.

In addition, I made a set of female standards in order to cover DUTs with any port gender. I used PCB female SMA connector (round base) and the brass sleeves were made of Stock #134, K&S Engineering, 11/32 x .014"

The complete SMA calibration kit. Heat-shrink tubing is used for color-coding and it gives a nice and smooth finished surface, protecting the brass from tarnishing.


Anonymous said...

Thanks for your info. One quick question. How did you trim SMA connector center pin? Nathan

Andrey E. Stoev said...

I used diagonal flush cutters to remove most of the pin. I used a very small file to precisely trim it to length.

Anonymous said...

Hello Andrey,

thank you for your valuable informations about the construction of coaxial calibration standards.

What capacitance do you use for the OPEN standard definition?
Did you calculate or estimate the fringing capacitance or did you have the opportunity to actually measure it?

Thank you for your assistance

Andrey E. Stoev said...

I have not measured the capacitance of the OPEN. This will just introduce another error (caused by the accuracy of my LCR bridge). If I can put my hands on a commercial VNA with the proper cal kit, it is a different story. Instead I use the default value in MyVNA - i think it is based on a SMA connector.

Anonymous said...

Dear Mr Stoev
I will build up SMA OSL following your instruction.
After that I need to characterize them to input data
to my VNA HP8753B.
I am a little confused about how to practically do it.
I read carefully the article on chemandy site but I
did not understand enough.
Can you give some help please ?