[Coco] [off-topic] cabling problems

[Mike Pepe]

Gene - on this topic, I was under the impression that twisted pair cable
only really worked right if the signal is balanced. If it's a single-ended
signal, the twisting does nothing, since one side of the pair is grounded.
Is that right?

-Mike

> -----Original Message-----
> From: coco-bounces at maltedmedia.com [mailto:coco-
> bounces at maltedmedia.com] On Behalf Of gene heskett
> Sent: Thursday, April 19, 2012 9:26 PM
> To: coco at maltedmedia.com
> Subject: Re: [Coco] [off-topic] cabling problems
> 
> On Thursday, April 19, 2012 11:44:17 PM Patrick Wilson did opine:
> 
> > From: William Astle <lost at l-w.ca>
> > To: coco at maltedmedia.com
> > Sent: Wednesday, April 18, 2012 5:48 PM
> > Subject: Re: [Coco] [off-topic] cabling problems
> >
> > >I thought the twisted part was to keep the transmission line effect
> > >going  and to keep the wires together rather than shielding. The
> >
> > Actually, the function of the twist is to provide a means for
> > cancellation of spurious interference.  The more twists per a given
> > length, the better.
> 
> You are doing fine so far.  But...
> 
> > It uses the effects of the magnetic fields created by current flow to
> > couple back in such a way that it counters the
> > spurious waveform.   The twist also has another interesting effect in
> > that the inductive effect of the signals traversing the medium give
> > the cable its effective impedance.  (electrical trivia, enjoy)
> 
> The better explanation is that the twist of the pair causes the net effect
of
> the interfering magnetic field to be balanced in a supposedly balanced
> circuit, so the net effect of the interference is a longitudinal signal on
both
> wires that matches, so a balanced load is effectively seeing the same
> interference on each wire, but they are in perfect timing, so the balanced
> load, which is supposed to be sensitive only to the difference on the two
> wires can, if properly designed, ignore the in-phase component by 90 to
120
> db.
> 
> Capacitive coupling can also inject a signal, but with the twist being
short
> compared to the wavelength of the interference, the injected signal is
again
> very close to being perfectly in phase, so the load can ignore the noise.
> 
> You threw in impedance, which the twist has only a very very minor effect
> on, impedance is for all practical purposes, a product of the capacitance
> between the wires and the inductance of that strand of wire.  Unless
> dealing with frequencies high enough for skin effect to become a
> measurable item.
> 
> It (the skin effect) doesn't add much inductance, but because all the
> currents are forced to flow only in the skin of the conductor, the
effective
> cross section of the conductor is reduced, raising the conductors
resistance.
> 
> The impedance itself is a product of the conductors diameter and the
> separation of the conductors.  The formula is in both the ARRL Hambook,
> and in the ITT Reference Data for Radio Engineers.
> 
> This twisted pair concept has taken over the long microphone cable scene,
> where the mic might be making a 2 millivolt signal, and we can and have
> shoved that 300+ feet to get to the uplink truck when doing remote
> broadcasts.  The secret that is that Belden, and Clark, and probably
others,
> are now making a star-quad cable, which is 4 wires laid up in a square lay
in
> the cable, with about a 1.5" length of a full turn of twist and its wired
to the
> two wire mic's and audio boards such that the opposite corner wires are
> paralleled.  With a 2 millivolt signal from the mic, the only thing you
hear in
> the monitors is what was said within 50 feet of the mic.  That extreme
> balanced cable layup, coupled with a 98% coverage braided shield, seems to
> be better than I have the equipment to measure.
> The shield is good for 60 db all by itself.  The 4 way twist adds at least
> another 60db of protection.  Want to get super ultra picky, use a cable
with a
> layer of conductive mylar film under the braid, that adds another 50 to 60
db
> of noise attenuation.  I believe Alpha wire makes such a beast, but its 2x
the
> Clark version per foot and not near as flexible, shortening its working
life.  I
> personally have never used it, Clark Wire's nice flexible stuff has always
> worked for me.
> 
> Cheers, Gene
> --
> "There are four boxes to be used in defense of liberty:
>  soap, ballot, jury, and ammo. Please use in that order."
> -Ed Howdershelt (Author)
> My web page: <http://coyoteden.dyndns-free.com:85/gene>
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> 
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