[SI-LIST] Re: symmetry vs length matching for diff signals
- From: "Kai Keskinen" <kalevi@xxxxxxxxxx>
- To: <lambert@xxxxxxxxxxxxxxxxxx>, <Cortex.Chen@xxxxxxxxxxxx>, <ricbrook@xxxxxxxxx>
- Date: Wed, 1 Dec 2004 18:50:33 -0500
Bert is basically using a field solver to quantitatively define what I said.
The only question is do we define Kc of 2.0% as the threshold of loosely
coupled or something less or higher? For most applications, a coupling
factor of 2% can be deemed loosely coupled. The more important issue is, can
the differential line geometry one chooses be routed through connector pin
fields (i.e. backplanes) or through device pin fields (0.050" pitch or
0.040" pitch)? If it can't, you better chooses a loosely coupled geometry to
minimize impedance discontinuities when you have to break up the pair to go
around vias and pins.
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Bert Simonovich
Sent: Wednesday, December 01, 2004 9:46 AM
To: 'Cortex.Chen@xxxxxxxxxxxx'; Kai Keskinen; ricbrook@xxxxxxxxx
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: symmetry vs length matching for diff signals
Another way to determine coupling is when odd mode impedance (Zodd) is much
less than even mode impedance (Zeven), the pairs are considered close
coupled. The closer Zodd is to Zeven, the looser the coupling.
For a given stripline geometry where trace widths and reference plane
heights are constant, moving the traces closer together reduces the odd mode
impedance while increasing the even mode impedance thereby increasing the
coupling coefficient Kc. The Knee for Kc(min) is when Zodd equals Zeven.
Increasing the separation past the Knee does not significantly decrease Kc,
and each trace can be considered as single ended trace for impedance.
To give you the idea, I have run some quick examples;
5-5-5 geometry: Zodd = 39.8 ohms; Zeven = 48.5 ohms; Kc = 9.8%
5-10-5 geometry: Zodd = 46.1 ohms; Zeven = 48.0 ohms; Kc = 2.0%
5-15-5 geometry: Zodd = 44.4 ohms; Zeven = 44.0 ohms; Kc = 0.5%
5-20-5 geometry: Zodd = 46.5 ohms; Zeven = 46.6 ohms; Kc = 0.1%
Regards,
Bert Simonovich
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Cortex Chen
Sent: Tuesday, November 30, 2004 10:31 PM
To: Kai Keskinen; ricbrook@xxxxxxxxx
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: symmetry vs length matching for diff signals
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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