[SI-LIST] Re: Why shielded twisted-pair is still twisted?
- From: steve weir <weirsi@xxxxxxxxxx>
- To: Neo <neoflash2008@xxxxxxxxx>
- Date: Mon, 10 May 2010 17:55:42 -0700
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
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