Millimeter Measurements: Beware of Changes in Connector Scale

Blog Post created by benz on Jul 20, 2017

  Taking extra care in the lands of the large and the small

Recently, I found myself peering at a dial indicator while checking the blade runout on my shiny new 12-inch miter saw. I’m putting up new trim in my house, and the big blade allows me to make some cuts directly on larger assemblies. However, I’m no professional woodworker, so my motto is when in doubt, measure... and measure again.

Given the size of the blade, details such as its flatness and mounting are especially important to making good cuts and tight joints. These factors got me thinking of recent developments at the other end of the scale in our RF work, namely the small physical geometry of the millimeter-frequency hardware we’re increasingly using to send information or sense things.

The new N9041B UXA X-Series signal analyzer and its two different input connectors are good examples of what happens when you scale frequencies up and geometries down.

Comparing the two RF/millimeter frequency inputs of the Keysight UXA 110 GHz signal analyzer.  The full frequency range is available from the 1 mm connector of RF input 2, while the 2.4 mm connector of RF input 1 provides a more rugged connection when frequency coverage to 50 GHz is sufficient

The two coaxial input connectors on the UXA signal analyzer have different characteristics and capabilities. The 2.4 mm connector of input 1 (left) covers frequencies to 50 GHz with a power limit of 1W. The 1 mm connector (right) covers frequencies to 110 GHz and power levels to 1.8 mW.

RF Input 1 is a normal 2.4 mm front-panel connector and, as is common with test equipment, the gender is male to reduce the chance of damage and encourage the use of adapters as connector savers. This separate input provides several benefits for users of the UXA when measuring signals below 50 GHz. It’s more mechanically robust than 1 mm or 1.85 mm connectors. It can also handle much more power without damage: 1 W vs. 0.0018 W for RF Input 2.

RF input 2 is also male, and has the more complicated and challenging job: covering higher frequencies with its smaller and more delicate geometry.

In addition to its conductor size, two mechanical differences are apparent. First, the connector body has an additional, larger outer thread ring to mate with test-port adapters rather than standard 1 mm adapters. These adapters are mechanically stronger and less susceptible to damage, and are the best way to connect to the 1 mm input (if they’re available).

The second difference is the pair of threaded bosses, one on either side of the connector. These bosses are used to mount an input-connector vise assembly, perhaps the smallest vise you’ll ever use.

The 1 mm 110 GHz RF input 2 of the UXA signal analyzer can be fitted with a vise or clamp assembly to isolate the connector from the higher torque that may be needed for other connectors, cables or adapters.

A small vise or clamp assembly is attached around the 1 mm, 110 GHz input of the UXA signal analyzer, isolating the mounting torque for the adapter from the torque needed for connecting the adapter to cables, waveguide adapters, etc.

The small size of the 1 mm connectors mean that they don’t need—and probably won’t withstand—the torque that’s appropriate for larger connectors. The torque for the 1 mm connector is 3 or 4 inch-pounds, while the torque for 1.85 mm and larger microwave connectors is 8 inch-pounds.

This is a formula for very expensive damage! To prevent it, the vise holds the flats (part of the body) of the 1 mm end of a standard adapter after it has been tightened to the analyzer’s front panel connector, avoiding the transfer of torque used to connect cables or other adapters to the adapter mounted to the instrument.

As discussed here before, torque is important at microwave and millimeter frequencies. DUT connections are difficult enough, and this simple little clamp can neutralize an important source of problems. You can learn more in the connector kit overview.

As for me, if I had just purchased one of these expensive 110 GHz analyzers, I’d be tempted to quickly stencil a warning around the 1 mm connector in fluorescent green: maybe “Are you sure?” or “Are you authorized to use this port?” You can never be too careful in the land of the very small.