Straight talking on torque - this article looks at the issues

23-01-2015 | Aspen Electronics | Design Applications

Howard Venning, of Aspen Electronics, explores the topic of connector
coupling or mating torque.

Anyone who has worked in an electronics measurement lab and used coaxial
connectors will know the importance of mating torque and will typically have
a range of torque spanners complimenting their coax connectors, adaptors and
test cables. Valid, repeatable test results rely on a constant mating
torque. The correct torque also ensures the longest life of your expensive
measurement standard connectors, cables and adaptors, especially considering
the repeated connect - disconnect cycles many connectors go through during
their life, plus the test equipment itself.

RF & Microwave equipment typically relies upon an RF connector pair when it
comes to connecting one part of the system to the other. This might be via a
coax cable, a pair of back to back connectors or when you have different
connector types via a coaxial adaptor. The system designer, and those
responsible for system installation and maintenance, has to consider an
increasing array of differing connector types.

Over the past few years we have seen an increase in the types of connectors
used, plus the range of uses. Industry standards such as the BNC, TNC,
N-type and SMA have seen numerous alternatives, each offering some technical
advantage.

In wireless applications the N-type was the 'industry standard', but we have
seen the 7/16 DIN become the preferred connector, especially where low PIM
need to be achieved. However, the 7/16 DIN is now seeing the 4.1-9.5 DIN
being proposed due to its smaller size, increased frequency performance
whilst maintaining its good PIM performance. In high frequency applications
the SMA has been complimented by the 3.5mm, 2.92mm, 2.4mm, 1.85mm and 1.00mm
connectors as microwave systems get ever closer to 110GHz.

A similar situation applies on the production line, where certain tests need
a specified coupling torque to be used during the measurement process. A
good example is when using a PIM Tester, as the handbook will always specify
a torque setting for the connectors used in the measurement system.
However, the correct mating torque is also just as important in the field as
it is in the lab, but for slightly different reasons. Whilst any wireless
system that is designed to ensure a specified PIM performance will need to
follow the same guidelines as used in production test, one might think that
because the connection is only being made once it is less important. This is
not the case.

Under-tighten a connector and you may get a high loss or intermittent
contact which might not reveal itself for some time. Worse still is over
tightening a connector pair. To understand why we need to look at the way
coax connectors actually meet. Typically the male/female centre contacts
have a mating plane that is specified in the connector design reference
drawing. During connector production and/or assembly the centre contact is
positioned with reference to the reference plane to within a set dimension,
typically a few 10ths of a millimetre (or 100ths of an inch). This ensures
that when connectors are correctly mated the centre pins mate without
damage.

Over tightening coax connectors has the effect of compressing the internal
components and in severe cases causing pressure on the mating centre pins,
which in turn can lead to failure of the joint between the centre pin and
whatever it is connected to. In the case of cables, this might be the cables
centre conductor, although this is relatively rare given the flexibility of
coax cables. This problem can be mitigated by using a connector with a
'captive centre contact' design.

In our experience the real problem comes when you are connecting to a system
component such as a load, attenuator or coupler. These components typically
have a connector centre contact soldered directly to a PCB or internal
component such as a resistor. Compression on this joint will probably lead
to failure. The solder joint between centre contact and internal component
will have the minimum of solder used to ensure as good an RF performance as
possible, as 'over engineering', with excessive use of solder might result
in poor VSWR.

Howard Venning, Aspen Electronics.

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