[SI-LIST] Re: Why shielded twisted-pair is still twisted?

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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