[SI-LIST] Re: Why shielded twisted-pair is still twisted?
- From: "Jim Nadolny" <jim.nadolny@xxxxxxxxxx>
- To: "Neo" <neoflash2008@xxxxxxxxx>
- Date: Thu, 13 May 2010 11:06:34 -0400
The STP version is 30 AWG, the twinax is 26 AWG. I'm not sure on the
dielectric constants.
STP cable has been tuned/tweaked and studied extensively for Ethernet
applications. The limitation tends to be in the connector and
termination areas. Twinax construction has a shield/reference and you
have better impedance and crosstalk control in the termination area.
Plus it is a lot easier to keep the transition region short with a
twinax.
From a physics point of view, STP has periodic discontinuities which
result in higher structural return loss than a twinax.
you might find some of the work by Solarflare interesting
http://www.solarflare.com/technology/10GBASE-T_Solutions.php
jn
________________________________
From: Neo [mailto:neoflash2008@xxxxxxxxx]
Sent: Wednesday, May 12, 2010 8:14 PM
To: Jim Nadolny
Cc: List Si
Subject: Re: [SI-LIST] Re: Why shielded twisted-pair is still twisted?
Jim,
I realized that I'm a bit jump to the conclusion.
Are these two cables you listed using the same wire gauge? ( I assume
their insulators' dielectric constants are different.)
Thanks,
Neo
--- On Wed, 5/12/10, Neo <neoflash2008@xxxxxxxxx> wrote:
From: Neo <neoflash2008@xxxxxxxxx>
Subject: [SI-LIST] Re: Why shielded twisted-pair is still
twisted?
To: "Jim Nadolny" <jim.nadolny@xxxxxxxxxx>
Cc: "List Si" <si-list@xxxxxxxxxxxxx>
Date: Wednesday, May 12, 2010, 11:41 PM
Jim,
I do appreciate your data based answer and comments. That's
really helpful.
I'm wondering these days that how well can STP perform. As long
as it can be manufactured to be uniform along the cable, it should
perform well.
However, STP should be more lossy than twin-ax since the twisted
wire increases the total copper length that the signal need to travel
through. Your data confirmed this hypothesis.
Thanks,Neo
--- On Wed, 5/12/10, Jim Nadolny <jim.nadolny@xxxxxxxxxx> wrote:
From: Jim Nadolny <jim.nadolny@xxxxxxxxxx>
Subject: [SI-LIST] Re: Why shielded twisted-pair is still
twisted?
To: "Neo" <neoflash2008@xxxxxxxxx>
Cc: "List Si" <si-list@xxxxxxxxxxxxx>, "steve weir"
<weirsi@xxxxxxxxxx>
Date: Wednesday, May 12, 2010, 7:46 PM
We make some HDMI cables with different cable types (STP and
twinax).
It's a little surprising (to me anyway) how well the STP
assemblies
perform. Sure, there is more loss (return loss and insertion
loss) with
the STP cable, and when the performance goes bad, it really goes
bad.
But crosstalk is about the same and the STP price is lower than
twinax
as it's a lot easier to manufacture.
If you dig into the raw cable performance you see some marked
differences, but at the assembly level some of those differences
fade
due to the effects of the connector and termination.
You can check out some cable assembly level test data on our
website if
you are into it
http://www.samtec.com/Documents/WebFiles/testrpt/hsc-report_hplsp_web.pd
f
http://www.samtec.com/Documents/WebFiles/testrpt/hsc-report_HPSTP_web.pd
f
Jim N
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of steve weir
Sent: Monday, May 10, 2010 8:56 PM
To: Neo
Cc: List Si
Subject: [SI-LIST] Re: Why shielded twisted-pair is still
twisted?
Twin ax is not just two insulated wires inside a common shield.
Proper
twin ax tightly controls the dimensions preferably by suspending
both
conductors in one homogenous jacket that is then shielded. An
alternate
approximation is two coaxial leads each with their own
unjacketed
shield, where that shield is then in contact with an outer
shield. In
both cases the dielectric properties between each conductor and
the
outer shield are well controlled as are the properties between
the
conductors.
A shielded pair is just two jacketed conductors inside a common
shield.
The dimensions and physical consistency are mediocre at best.
Twisting
the leads makes the combination closer to isotropic but is
nowhere near
as good as proper twin ax.
Steve.
Neo wrote:
> Steve,
> I agree that twisting will make the cable more balance if the
shielding is ideal. However, I think twist will make the
shielding more
difficult to be uniform along the cable. Therefore, impact
impedance's
consistency.
> Is it true?
> Neo
> --- On Mon, 5/10/10, steve weir <weirsi@xxxxxxxxxx> wrote:
>
> From: steve weir <weirsi@xxxxxxxxxx>
> Subject: [SI-LIST] Re: Why shielded twisted-pair is still
twisted?
> To: "Neo" <neoflash2008@xxxxxxxxx>
> Cc: "List Si" <si-list@xxxxxxxxxxxxx>
> Date: Monday, May 10, 2010, 10:53 PM
>
> STP is not twin ax. STP do not have individual shields.
Twisting
> makes the parasitics more isotropic.
>
> Steve
> Neo wrote:
>
>> 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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>
--
Steve Weir
IPBLOX, LLC
150 N. Center St. #211
Reno, NV 89501
www.ipblox.com
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