[SI-LIST] Re: Why shielded twisted-pair is still twisted?
- From: Richard Feldman <Richard.Feldman@xxxxxxxxxxx>
- To: "colin_warwick@xxxxxxxxxxx" <colin_warwick@xxxxxxxxxxx>, "neoflash2008@xxxxxxxxx" <neoflash2008@xxxxxxxxx>, "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
- Date: Thu, 13 May 2010 15:38:35 -0700
Also importantly:
Since there are multiple pairs within a common shield,
twisting each pair (with a unique pitch) greatly reduces
differential-mode crosstalk between pairs.
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of colin_warwick@xxxxxxxxxxx
Sent: Thursday, May 13, 2010 11:40 AM
To: neoflash2008@xxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Why shielded twisted-pair is still twisted?
Hi Neo,
I think the original motivation for twisting was in audio telephony
applications to cancel low frequency hum (e.g. 60 Hz or 50 Hz) from EM fields
from AC power wires. For a uniform field, the EMF from the flux threading
through alternate half twists cancels out. Without the twist, the EMF from the
flux component perpendicular to the plane of a twin ax would add up.
Shielding (if any) doesn't keep low frequency EM fields out because the skin
depth of copper is ~1 cm, so an economical viable shield is too thin to block
it.
hth
-- Colin Warwick
Signal Integrity Product Marketing Manager, Agilent EEsof EDA
Blog http://signal-integrity.tm.agilent.com
Information Exchange for IBIS Model Users http://www.ibis-user.org
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Neo
Sent: Monday, May 10, 2010 6:41 PM
To: List Si
Subject: [SI-LIST] Why shielded twisted-pair is still twisted?
Hi,
I'm thinking about why shielded twisted-pair is still twisted?
Pair is twisted for controlled distance/impedance (due to twist) and for x-talk
reduction. If a pair is already shielded together, it is not necessary to twist
them any more.
Without twist, it will be equivalent to twin-ax cable, isn't that better?
Thanks,Neo
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