Hi,
If you are driving lines with fast rise times then you will still only
get external currents if the shield of the coax is leaky or if the
termination is not properly matched over the entire spectrum of the data
bandwidth. If the ferrite is lossy and spaced closely enough compared
to the shortest wavelength of concern then you break up the long outer
conductor and make it an very inefficient antenna. For the case of a
test site, the ferrites are spaced close enough so that while there will
be resonant lengths between, them the frequency of resonance will be
well out of the range of concern. Once installed the only real
nuisance of ferrites is their weight if you use a large number of them.
As for the heat, fortunately none of my products has tried to radiate
that much energy unintentionally :-), but you are right there should be
heat generated though we are talking picowatts at best I suspect.
If there was no loss the resonance could be a problem for product EMI,
but they would still work for test sites where preventing current flow
is really all that is needed.
Regards,
Colin Brench
Compaq,
Maynard, MA
-----Original Message-----
From: Chris Dupres
[mailto:[email protected]]
Sent: Saturday, November 14, 1998 3:03 AM
To: Colin Brench
Cc: emc-pstc
Subject: Ferrites
Hi Colin and the group.
You mentioned:
<The ferrites will greatly minimize this induced current
and so practically
eliminate this effect.>
I find ferrite sleeves, snap-on and otherwise,
incredibly useful for
reduction of emissins from cables, but for HF feeder and
antenna drive
purposes I find them a bit iof a nuisance.
I haven't studied these especially, apart from the
manufacturers data etc.,
but I find these ferrites to be very lossy. This
'lossiness' is invaluable
for providing an HF dumpng ground for any loose HF
resonant energy, or HF
ringing energy on high speed digital lines for instance.
If you drive coax
lines with high speed data, all those 0/1 transitions
are very fast, and
quite capable of kicking the coax screens into a
resonant condition
sufficient to provide a very significant peak somewhere,
particularly
noticeable on video outputs from PC's etc. Especially
bad if cables are
incorrectly terminated of course.
These ferrites provide some impedance to act as a low
pass filter, and also
provide enough losses for the ferrite to absorb that
blocked energy. If
there was no losses I reckon the resonance would simply
move somewhere else
in the spectrum.
I suppose that if there was sufficient HF activity in
these cables with
ferrites fitted, one could expect the ferrites to get
hot! I haven't
noticed this particular phenomenon yet....
Has anyone tried the trick of running a 3mm copper wire
through the Ferrite
with the suspect cable, and soldering it into a 'shorted
turn'
configuration. Got some interesting results, not
spectacular, just
interesting
Just a twopence worth for the opinion bucket.
Chris Dupres
Surrey, UK.
---------
This message is coming from the emc-pstc discussion list.
To cancel your subscription, send mail to [email protected]
with the single line: "unsubscribe emc-pstc" (without the
quotes). For help, send mail to [email protected],
[email protected], [email protected], or
[email protected] (the list administrators).
