Originally posted Sept 17, 2013
As your parents used to say, “It’s for your own good”
Most engineers enjoy a good puzzle and we’ve all encountered a device or technique that makes us scratch our heads. We spot something peculiar, we don’t see any obvious reasons for it and, in a Pavlovian moment, our speculative juices overflow. We wonder what’s behind this puzzle—maybe it has multiple explanations— and we suspect that it’s all due to some hard-won experience that came at great expense to someone else.
One such nice, little puzzle is found on the front panel of many millimeter-frequency signal generators and analyzers. Instead of the usual female connector—Type-N, 3.5 mm, 2.4 mm, etc.—you’ll often find a male connector with a large-diameter knurled flange that enables finger-tightened connections (see below).
While the typical front-panel connector is female, the gender is often reversed for equipment covering millimeter frequencies (30 GHz to 300 GHz). The male connector offers some degree of protection from several types of damage.
Because most cable assemblies use a male connector at each end, there must be at least one good reason for this departure from the norm. Indeed that’s the case, and the reasons can be summarized with the words “connector saver.” Two examples and some common part numbers are shown below.
“Connector savers” are coaxial adapters placed between instrument front-panel connectors and cables or DUTs. They can be easily replaced if they are damaged or become worn.
Two more key reasons are captured with the words “expendable” and “replaceable.” Millimeter connectors are inevitably small and somewhat delicate, and replacing an instrument’s front-panel connector is an expensive operation. What’s more, recalibration is often required, and that can take an instrument out of service for a day, maybe longer.
In situations that require frequent connection changes, it makes sense to keep a sacrificial adapter semi-permanently attached to the front panel of the instrument. When the adapter gets damaged or worn, it can be easily replaced at modest expense, relatively speaking: even the most expensive metrology-grade adapter costs less than an instrument repair and recalibration.
Back to damage and wear. Front-panel connectors are subject to several kinds and the use of the male connector and a connector saver helps in several ways. First, the male connector tends to encourage the use of a connector saver. An operator not trained in the proper use and care of connectors cannot connect most cables (again, male at both ends) directly to the front panel and is therefore less likely to cause accidental damage.
Second, the male connector is generally more physically robust than the female. The most common type of damage is destruction of the female collet by a center conductor that is either misaligned or the wrong size (see the post “Loose nut danger”). A male front-panel connector focuses the damage on the less-expensive part.
As a final point, this gender choice tends to dramatically reduce the number of connection cycles for the instrument connector itself. Even with careful operation, wear and damage are inevitable and it’s better to focus the wear and risk on expendable parts.
Bonus trivia #1: How do you determine the gender of connectors with varying configurations of outer shells? Remember that gender is determined by the center-most conductor of a connector, regardless of the position of the outer nuts, threaded barrels or bayonet hardware.
Bonus trivia #2: The flats machined into the barrels of the connector savers pictured above are not just for torque wrenches. They also allow use of a common open-ended wrench to avoid connector rotation when nuts are tightened. Connector rotation should be avoided because of the wear it causes, especially with delicate millimeter hardware.
Bonus trivia #3: Some instruments use custom adapters that can function as connector savers rather than the standard adapters described above. These custom adapters are a user-replaceable part of a front-panel connector set and may offer a choice of connector types at the user end. This approach may provide a shorter physical connection and some improvement in mechanical robustness due to the compact configuration. However, the special adapter may be a disadvantage if it is damaged in its connector-saver role and a replacement is not readily available.