[SI-LIST] Re: symmetry vs length matching for diff signals

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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