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How to (not) trash a calibration kit!

Updated March 29, 2013

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This content was contributed by Lou, a true microwave muleskinner, who no longer lends his calibration kit to anyone. He's got his reasons! The photos are of a 3.5 mm cal kit, but the lessons here apply to any cal kit; when you damage one standard, the rest of the team will get bad data until Mayor McCheese can be convinced to pony up $10,000 to cover your carelessness.

Here's a view of Maury's "ruggedized" 3.5 mm connector (female). The center conductor internal diameter expands to allow variations in pin diameter. It mates well with SMA connectors that may be slightly out of spec, and it might not get damaged if you rotate it during mating, but don't tempt fate (we've seen these ruggedized connectors get get trashed). This connector is from a Maury calibration kit for a load-pull set-up.

Here's Maury's 3.5 mm pin (male connector).

Agilent (Hewlett Packard) has their own "precision" 3.5 mm connector. The internal diameter of the center conductor is fixed which allows for minimum variation in impedance, which you'll need to get a good calibration on your analyzer. The drawback is that the six spring fingers inside are very delicate and easily destroyed.

Another view of the Agilent 3.5 mm connector.

Agilent 3.5 mm pin. It may be a little more pointed than the Maury pin. If you thought that all 3.5 mm connectors were alike, you were wrong!

The rest of the pictures below are from a trashed Agilent 3.5 mm cal kit. Some advice: never lend out your cal kit to someone that doesn't know what they are doing. It's a wise idea to label the kit with a notice about being drawn and quartered if one rotates the standards while tightening them. And it's best to keep your cal kit hidden.

This is what happens when you mate an Agilent 3.5 mm with an SMA that has a burr on the pin. You can see how the burr caught a finger and bent it in. It was probably rotated during attachment.

Another Agilent 3.5 mm calibration standard. This one is wrecked, it's now a worthless calibration standard.

Here's an Agilent 3.5 mm missing all fingers. It looks OK if you didn't know it was supposed to have six fingers down in there!

Some more advice: don't bid on a 3.5 mm calibration kit on Ebay if the seller's name is Lou!

In summary, here's some rules to follow in order to not trash a connector:

1) Never rotate 3.5 mm connectors when tightening. Use a 8 in/lb torque wrench and a smaller wrench to hold the other connector. A handy RF connector wrench is made by grinding (on a bench grinder) down the jaws of a small crescent wrench thin enough that it can grasp the flat on a SMA barrel. Or you can buy a set of miniature SAE (not metric) open-end wrenches (spanners for you limeys) that will fit all of your connector needs. Look for thin wrenches so you don't have to grind them ("ignition" wrenches are a good bet). The ideal wrench set would be purely open-end, box wrenches serve no purpose for microwave connectors (if you don't get that last point, it's time for a new career!). These pictures were cribbed from the Techni-Tool web site:

Thanks to Steve from White Sands!

2) Dust out the dirt with dry gas. Use 100% isopropyl alcohol (lab grade, not rubbing) to swab the mating surfaces however the "proper" cleaner was banned several years ago and they have not yet updated the cleaning instructions.

3) Buy some 3.5 mm to 3.5 mm connectors and use them as jack savers when measuring SMA devices. Label these as jack savers. SM Electronics actually sells 3.5mm to SMA adapters, which would serve the same purpose and might save you a few bucks.

Never, ever connect a SMA to a 3.5 mm calibration standard, VNA cable, or some other expensive 3.5 mm device. Use the jack savers.


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