[SI-LIST] Re: symmetry vs length matching for diff signals
- From: "Cortex Chen" <Cortex.Chen@xxxxxxxxxxxx>
- To: "Kai Keskinen" <kalevi@xxxxxxxxxx>, <ricbrook@xxxxxxxxx>
- Date: Wed, 1 Dec 2004 11:30:53 +0800
Dear Kai,
For some aspect, I agree with your speaking.
In detail, there are some unknown for me as below:
As your said that If the differential impedance is much less than twice =
the single ended impedance for the same track width, (i.e. 50 single, 90 =
differential)
It means as the equation: Zdiff =3D 2 * Zodd =3D 2 * (Z11- Z12)
However, even if impedance occurs (50 Ohms single, 97-100 differential), =
I think that the min (intra-pair) spacing may be the critical value not =
loosely coupled.
If the spacing is more than the critical value, then the it becomes =
loosely coupled. =20
You could observe that differential impedance will be in saturation =
region if the spacing is significantly more than the critical value as =
below example.
I like to seek how to get the critical value and the relationship =
between the critical value and tightly coupled.
For example (reference only):
<Zo> 55 ohms (W: 5 mils)
<Zdiff> 100 ohms (target)
Trace Width Spacing Trace Width Zdiff
5 7 5 94.xx
5 9 5 98.xx =20
5 11 5 101.xx
5 13 5 103.xx=20
5 15 5 104.xx=20
5 20 5 106.xx
5 30 5 108.xx=20
... ... ... ...
... ... ... ...=20
Regards,
Cortex
=20
-----Original Message-----
From: Kai Keskinen [mailto:kalevi@xxxxxxxxxx]
Sent: Wednesday, December 01, 2004 8:06 AM
To: ricbrook@xxxxxxxxx; Cortex Chen
Cc: si-list@xxxxxxxxxxxxx
Subject: RE: [SI-LIST] Re: symmetry vs length matching for diff signals
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@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Rick Brooks
Sent: Tuesday, November 30, 2004 6:34 PM
To: Cortex.Chen@xxxxxxxxxxxx
Cc: si-list@xxxxxxxxxxxxx
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 =3D Lm/(SQRT(L1*L2))
kc =3D 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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