[SI-LIST] Re: diff pair signal
- From: steve weir <weirsp@xxxxxxxxxx>
- To: "Landrum, Chris" <chris_landrum@xxxxxxxxxx>,<si-list@xxxxxxxxxxxxx>
- Date: Wed, 30 Jun 2004 09:20:28 -0700
Chris, that is pretty much correct. The primary goal is to tightly match
Tflight. Termination is simplest with loosely coupled pairs, at the cost
of routing density. The catch with loosely coupled pairs is that there is
loose, and then there is too loose. If you run disparate routes, or even
orientations, then at very high speeds, things like variations in material
thickness, water density and even the weave orientation can cause
differences in the effective Er and consequently skew the electrical length
versus perfectly matched physical length.
There was some spirited discussion about diff. pair routing practice
between Chris and Lee awhile back with regard to some of these
issues. Scott McMorrow has done extensive study and characterization of
these subtle effects for customers pushing the bleeding edge.
Regards,
Steve.
At 12:07 PM 6/30/2004 -0400, Landrum, Chris wrote:
>Thanks Steve...
>
>With that said, routing the pairs together with some minimum spacing is
>not a requirement to allow the subtraction of this CM noise... The only
>requirement is that they are matched in time?
>
>Chris Landrum
>PCBA Design Engineer
>Maxtor Corporation
>333 South Street
>Shrewsbury, MA 01545
>508-770-2282 (d)
>
>-----Original Message-----
>From: steve weir [mailto:weirsp@xxxxxxxxxx]
>Sent: Wednesday, June 30, 2004 12:03 PM
>To: Landrum, Chris; mibrown@xxxxxx; Aubrey_Sparkman@xxxxxxxx;
>gopalak@xxxxxxxxxxx; si-list@xxxxxxxxxxxxx
>Subject: RE: [SI-LIST] Re: diff pair signal
>
>Chris, the common mode rejection is not against aggressor fields as on a
>
>PCB with most trace configurations the geometry is far from the
>isosceles
>triangle needed between the aggressor and each of the diff signals. Lee
>
>correctly emphasizes that point repeatedly.
>
>The common mode rejection is of bias point movement due to SSO inside
>both
>the transmitting and receiving devices. In a single ended
>configuration,
>Vtt moves with the internal ground network of the receiver against its
>output SSO noise, and fails to track the transmitter. With diff
>signaling,
>as long as we stay within the linear common mode range of the receiver,
>the
>receiver diff amp subtracts out most of both the transmitter and
>receiver
>SSO induced noise, which is CM.
>
>Regards,
>
>
>Steve.
>At 11:50 AM 6/30/2004 -0400, Landrum, Chris wrote:
> >How would you get common-mode noise rejection by using differential
> >routing on a PCB with GND planes? One requirement for common mode
>noise
> >rejection is equal and opposite EM fields. This is impossible in a
> >common PCB structure where the GND plane is beneath or above AND
>beneath
> >the differential signals.
> >
> >I do understand how you can obtain common-mode noise rejection in a
> >cable...but in a PCB, I'm not so sure.
> >
> >Chris Landrum
> >PCBA Design Engineer
> >Maxtor Corporation
> >
> >-----Original Message-----
> >From: si-list-bounce@xxxxxxxxxxxxx
>[mailto:si-list-bounce@xxxxxxxxxxxxx]
> >On Behalf Of Brown, Mike (Austin, TX)
> >Sent: Wednesday, June 30, 2004 11:42 AM
> >To: Aubrey_Sparkman@xxxxxxxx; weirsp@xxxxxxxxxx; gopalak@xxxxxxxxxxx;
> >si-list@xxxxxxxxxxxxx
> >Subject: [SI-LIST] Re: diff pair signal
> >
> >In many board geometries, the coupling between "differential pairs"
> >approaches zero. You are really dealing with two single-ended signals
> >that happen to be complements of each other. It is possible to use one
> >side of such a "differential pair" as a single-ended signal source.
> >Assuming, for the moment, that you don't care about common-mode noise
> >rejection at the receiver, and that you can generate a reference
>voltage
> >that does not introduce too much duty-cycle distortion in the recovered
> >signal. It's generally not a good practice, IMO.
> >
> >Mike
> >
> >
> >-----Original Message-----
> >From: si-list-bounce@xxxxxxxxxxxxx
>[mailto:si-list-bounce@xxxxxxxxxxxxx]
> >On Behalf Of Aubrey_Sparkman@xxxxxxxx
> >Sent: Wednesday, June 30, 2004 10:26 AM
> >To: weirsp@xxxxxxxxxx; gopalak@xxxxxxxxxxx; si-list@xxxxxxxxxxxxx
> >Subject: [SI-LIST] Re: diff pair signal=20
> >
> >
> >I don't know of an actual instance where anyone has done this...
> >
> >BUT, for "other logical interfaces", one might attach a single ended
> >sampler or detection circuit off one side IF (and big IF) acceptably
> >isolated with high impedance resistor or whatever. Of course, SI work
> >would need to be done to insure the loading would be acceptable to the
> >diff pair interface.
> >
> >Aubrey Sparkman=3D20
> >Enterprise Engineering Signal Integrity Team
> >Dell, Inc.=3D20
> >Aubrey_Sparkman@xxxxxxxx=3D20
> >(512) 723-3592
> >
> >
> >-----Original Message-----
> >From: si-list-bounce@xxxxxxxxxxxxx
>[mailto:si-list-bounce@xxxxxxxxxxxxx]
> >On Behalf Of steve weir
> >Sent: Wednesday, June 30, 2004 10:12 AM
> >To: gopalak@xxxxxxxxxxx; si-list@xxxxxxxxxxxxx
> >Subject: [SI-LIST] Re: diff pair signal=3D20
> >
> >
> >In such a situation where do you think the diff amplifier will obtain
> >its=3D20 reference?
> >
> >At 07:38 PM 6/30/2004 +0530, Gopalakrishnan Sethuraj wrote:
> > >Hello
> > >
> > >
> > >Is there any scenario where one (positive) of the differential=20
> > >pair=3D20 signal output will be connected to single pin in LVDS or =
> >other
> >
> > >logical
> >
> > >interfaces ?
> > >
> > >
> > >
> > >
> > >
> > >Thanks&Regards
> > >
> > >Gopalakrishnan
> > >
> > >
> > >
> > >
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