Frank,
Placing ferrites on the line adds reactance and so changes the
impedance of the line at the ferrite. If they didn't add reactance,
they wouldn't function as filters. The better they work at reducing
interference, the higher the impedance bump they've introduced.
The impedance bump causes reflections, which translate into
bit-errors. So it's good that you're using CAT-5 cable. The spec for
this cable requires that the data and power pairs be twisted and
separated from each other by a spline. This goes a long way toward
controlling the impedance of each pair and so reducing radiation.
CAT-6 cable should be better, but it will still be vulnerable to
impedance perturbations when you apply ferrites.
I've seen ferrites make Ethernet connections completely unusable.
This is the reason I always hope that filters on the DC lead to the
modem will do the trick. Or grounding the coax shield.
73, Brad
KV5V
On Sat, Feb 4, 2012 at 9:57 AM, Frank Davis <fdavis@nfld.net> wrote:
> In reply to Jim:
>
> The CAT5 cable used with the 3800 modem is the one that came in the box with
> it. The cable is very small diameter and very flexible...the modular plugs
> on it are smaller then the other plugs I have here on what i call regular
> CAT5 cable. The regular CAT5 modular plugs will not plug unto the jack on the
> modem.
> When that small flexible cable was wound on the two stacked toroids that I
> used It was twisted in a couple of places so maybe that contributed to the
> FEC events being seen by the telco test equipment. The cable appears to be
> very cheap and a minimal attempt by the manufacturer to provide a cable for
> general use ...certainly not robust.
>
> The toroids are not on this line now and all seems to be working fine. The
> iMAc download speed testing within the BellAliant network is 6.6 mbps. The
> full capacity of the line is supporting 4 IPTV set-top boxes two of which are
> HD.
>
> Fibre Op coming within a few months to my area!
>
> Frank VO1HP
>
>> The BellAliant technician told me that placing the ferrites on the CAT5
>> feeding the modem caused a significant number ( hundreds of '000's on a
>> continual basis.) of FEC (forward error correction) events to begin happening
>> on my line.
>
> That does not make sense unless the CAT5 was mechanically distorted by
> the winding. That would disturb the impedance at bit, but a LOT of
> errors doesn't make sense to me. The ferrites form a common mode choke,
> which the differential circuit should not see.
>
> _______________________________________________
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