[RFI] 160M/Modem problem
jim at audiosystemsgroup.com
Wed Dec 19 12:11:50 EST 2012
On 12/19/2012 8:17 AM, Missouri Guy wrote:
>> CAT5, CAT6, and all variations thereof consists of four very good
>> twisted pairs, each twisted at a different rate to minimize
>> from one pair to another.
> Hi Jim, (This is off list because of a fork in the subject matter) :D)
> I can understand the need/use of twisted pair. But,
> are the pairs designed for different frequencies and/or different
> impedences?....and that's why the different twist rates?
Good question. CAT5/6/7 has a nominal Zo of 100 ohms, and is designed
for wide bandwidth (so that it can transport digital pulses that make up
the Ethernet data). The twist rate does NOT affect the Zo, but making
the twist rates DIFFERENT minimizes crosstalk between pairs, and that is
quite important in Ethernet. It is equally important if that same cable
might be used to carry multiple telephone conversations, or DSL
circuits, or audio signals.
CAT5/6/7 is very good cable, and experience in the pro audio world have
shown it to be as effective as shielded twisted pair in resisting the
coupling of RF into problematic audio circuits.
It should be remembered that since the earliest days of telephony, the
sole means of rejecting crosstalk between the telephone system and the
60 Hz power system that ran parallel to it for hundreds of miles was the
twisting of the telephone pair. Thanks to the long wavelength of noise
source was 60 Hz, twisting was effectively accomplished by crossing over
at alternate support poles. With the advent of broadcasting and more
broadband noise sources, twisted pair cables in balanced circuits became
the standard means of noise rejection in audio systems, and shielding
was added for microphone circuits.
In general, twisting is FAR more important (and effective) than the use
of a cable shield in rejecting most forms of interference.
73, Jim K9YC
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