FreeLists / si-list / [SI-LIST] Re: Diff Tight vs Loosely coupled

["David Banas" <dbanas@xxxxxxxxxx>]

Please, see below.

-db


> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of Rajan HS
> Sent: Wednesday, March 14, 2007 3:08 AM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Diff Tight vs Loosely coupled
>=20
> Hi experts,
>=20
>   Could any one throw some light on when would be a differential
signal
> needs to be routed as tightly coupled or loosely coupled ?
[David Banas] Tightly coupled lines will, for instance, transmit more of
the signal's high frequency energy across a break in their associated
ground plane than will loosely coupled lines, which helps preserve
edges.

also why does
> some of the differential signals needs to meet both single ended and
> differential impedance requirements ?
[David Banas] Because most real-World pairs have some skew, which
converts some of the differential energy into common mode energy. Proper
termination of this common mode energy requires that the single-ended
impedance be matched. If you doubt this, consider the extreme example: a
differential pair with 180 deg. of skew causes all of the differential
mode energy introduced into the pair at the driver to be converted to
common mode energy at the receiver. So, now, you have the same voltage
appearing at either terminal of your differential terminator, which
means no current flows through it. Well, if no current flows through it,
then it is effectively an infinite impedance and we have a completely
unterminated line!

>=20
>   thanks in advance
>=20
>   Rajan
>=20
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