[SI-LIST] Re: Why shielded twisted-pair is still twisted?
- From: Neo <neoflash2008@xxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Thu, 13 May 2010 13:30:02 -0700 (PDT)
Dear Colin,
Is this statement applicable to twin-ax cable? For twin-ax case, the outer
shield thickness is also just a few mils. Can I say twin-ax also suffer AC
power interference?
Thanks,Neo
--- On Thu, 5/13/10, colin_warwick@xxxxxxxxxxx <colin_warwick@xxxxxxxxxxx>
wrote:
From: colin_warwick@xxxxxxxxxxx <colin_warwick@xxxxxxxxxxx>
Subject: [SI-LIST] Re: Why shielded twisted-pair is still twisted?
To: neoflash2008@xxxxxxxxx, si-list@xxxxxxxxxxxxx
Date: Thursday, May 13, 2010, 6:40 PM
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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