I investigated those 10b-t isolation modules a while back, and have
saved every module from every network card and router/hub/switch that
I have junked out. The very old 10b-t stuff is the best for getting
LPFs and individual per-channel (port) type parts. When they started
making 10/100 Mb/sec, the 17 MHz filters were eliminated, and the
parts got integrated to ever-higher levels, with multiple channels in
each module.
You have to be able to find the data sheets to be sure of what's in
them - some are transformers only, and some also have LPFs and
common-mode chokes in various combinations. The filter sections can
also be cascaded for even sharper cutoff, but there's quite a bit of
crosstalk, so a lot of higher frequency stuff gets through,
especially above 100 MHz, so it's mostly effective from around 20-100
MHz. It has been mentioned before that very sharp filters will tend
to have more phase noise (phase shift with temperature/component
variations), but the negative effects depend on the application - I
only care about frequency reference distribution to SAs and
synthesizers, for example, so I don't worry about exact phase and
timing between equipment.
There is a nice variety of magnetic parts from all types of network
devices, including DSL an ISDN.
Ed
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