[SI-LIST] Re: symmetry vs length matching for diff signals
- From: "handw0522" <handw0522@xxxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Wed, 01 Dec 2004 01:43:26 -0000
The loosely coupled pair is easier to route but seems to lower noise
immunity. Is there any mechanism to balance loosely coupled and noise
immunity ?
Thanks,
Jeff
--- In si-list@xxxxxxxxxxxxxxx, "Kai Keskinen" <kalevi@xxxx> wrote:
> One quick way to determine loose or close coupling is to look at
the single
> ended and differential impedances. If the differential impedance is
much
> less than twice the single ended impedance for the same track
width, (i.e.
> 50 single, 90 differential), you have closely coupled lines. If the
> impedance is slightly less (i.e. 50 Ohms single, 97-100
differential, you
> are loosely coupled). The loosely coupled pair is easier to route
with
> minimal differential impedance discontinuities if you have to go
around vias
> or other obstructions by breaking up the diff pair.
>
>
>
> -----Original Message-----
> From: si-list-bounce@xxxx
> [mailto:si-list-bounce@xxxx]On Behalf Of Rick Brooks
> Sent: Tuesday, November 30, 2004 6:34 PM
> To: Cortex.Chen@xxxx
> Cc: si-list@xxxx
> Subject: [SI-LIST] Re: symmetry vs length matching for diff signals
>
>
> I'm not sure this is what you are looking for or not...
>
> There is a description in Ron Poon's book: Computer Circuits
Electrical
> Design, pg 198.
> He defines a magnetic coupling coefficient km and a capacitive
coupling
> coefficient kc
> with regard to coupled transmission lines
>
> km = Lm/(SQRT(L1*L2))
> kc = Cm/(SQRT(C1*C2))
>
> Lm and Cm are mutual inductance and mutual capacitance per unit
length
> L1 and L2 are self inductance and capacitance per unit length.
>
> As a rough guideline: if km and kc are less than 0.2, the lines are
said to
> have "loose" coupling.
>
>
> I hope it helps,
> - Rick
>
>
>
>
>
>
> At 07:30 PM 11/30/2004 +0800, Cortex Chen wrote:
> >Dear Abe Riazi,
> >How to determine the tightly and loosely coupled pairs?
> >
> >Regards,
> >
> >Cortex
> >
> >
> >
> >
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