The tiny ones and the giants
Even if you have especially good eyes—and I do not—it can be difficult to identify the various connector types found on the bench of the typical microwave/millimeter engineer. This is because the frequencies are very high the the dimensions are very small! Nonetheless, accuracy and repeatability are expensive and hard-won at these extreme frequencies, and so it’s worth it to get interconnections right.
Getting things right also helps avoid the cost and inconvenience of connector damage. Connectors are designed with mechanical characteristics to avoid mating operations that would cause gross connector damage, but these measures sometimes fail, subjecting you to hazards such as loose nut danger.
The vast majority of intermating possibilities can be summarized in two sentences:
- SMA, 3.5 mm and 2.92 mm (“K”) connectors are mechanically compatible for connections.
- 2.4 mm and 1.85 mm connectors are compatible with each other, but not with the SMA/3.5 mm/2.92 mm.
A good single-page visual summary is available from Keysight. Here’s a portion of it.
Avoiding outright damage is important; however, performance-wise, it’s a pretty low bar for the RF engineer. Our goal is to optimize performance where it counts, and microwave and millimeter frequencies demand particular care.
For example, intermating different connector types, even when they’re physically compatible, has a real cost in impedance match (return loss) and impedance consistency. This has implications for amplitude accuracy and repeatability, with examples described in the March 2007 Microwave Journal article Intermateability of SMA, 3.5 mm and 2.92 mm connectors.
And it isn’t just mating different connector types that will give you fits. Like teenagers, it seems you can’t send millimeter signals anywhere without them suffering some sorts of issues. All kinds of connectors, adapters and even continuous cabling will affect signals to some degree, and suboptimal connection performance can be a hard problem to isolate.
Even connector savers, a good practice recommended here, add the effects of one more electrical and mechanical interface. As always, it’s a matter of optimizing tradeoffs, though of course that’s job security for RF engineers.
One approach to mastering the connection tradeoffs is to eliminate some adapters by using cables different from the usual male-at-each-end custom. Cables can take the place of connector savers and streamline test connections, especially when you’ll be removing them infrequently.
While you’re at it, consider cable length and quality carefully. Good cables can be expensive but may be the most cost effective way to improve accuracy and repeatability.
Finally, what about those huge connectors you see on some network analyzers and oscilloscopes? These are the ones that require a 20 mm wrench or a special spanner or both. The threads on some connector parts appear to be missing, though there’s a heck of a lot of metal otherwise. Here are two examples:
The large connectors are actually NMD or ruggedized versions of 2.4 mm and 1.8 mm connectors, providing increased mechanical robustness and stability. They’re designed to mate with regular connectors of the same type, or as a mount for connector savers, typically female-to-female. Test port extension and other cables are also available with these connectors.
I’ve previously discussed the role of torque in these connections. If you’d like something to post near your test equipment, a good summary of the torque values and wrench sizes is available from Keysight.