Passive Intermodulation in a 7/16 connector
- Passive Intermodulation PIM is a non-linearity phenomenon that
manifests itself in, amongsts other locations, the 7/16 connector if
undertorqued
- PIM can be a serious source of internally generated spurii found
blocking a common antenna duplexed receiver band channel in a
multicarrier/wideband modulation system
- PIM is easily measureable with a special high power RF test
system, the principle of which is shown in this block
diagram
- actually non-linearity can be also measured using a single source
and measuring the harmonic generation that will give further detail on
the source quality
- PIM and reflected power characteristics are related and here you
can see the difference in Return Loss of a hand tightened and torqued
7/16 connector
- the 7/16 connector torque is manufacturer specific, making life
just a tad more difficult, but generally the required torque between 15
- 30 Nm, typically 25 - 30 Nm
- here you can see the PIM intermodulation of a 7/16 connector
using 900 MHz band signals with 25 MHz separation and 10 W carrier
levels (each):
- hand-tightened PIM with the connectors mangled as tight as
possible for a gentleman of robust physical stature (result: -115.3
dB low side IM3)
- torque-tightened PIM with the connectors tightened to 25 Nm as
per manufacturer specification (result: -173.1
dB low side IM3)
- generally speaking, at these frequencies and power levels the
7/16 connector is the only sensible connector to use if you need a
robust connector
- N type connectors are absolutely disastrous in a multicarrier
system at these power levels and higher frequencies - even if you are
lucky enough to find torqueable types
- there do exist N type connectors with excellent PIM
characteristics - but I personally know only one and it is propably not
the brand you are thinking of ;-)
- worth pointing out here too that arcing and corona are also
extremely non-linear phenomena and can generate impressive levels of IM
and harmonics and of course AM detection...
Updated 17.06.2008/OH2AUE
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