RE: [SI-LIST] : LVDS Skew

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From: Ricchiuti Vittorio ([email protected])
Date: Fri Aug 11 2000 - 00:55:54 PDT

Scott,
I don't understand why the differential crossing ceases to exist when the
skew between signals is 200ps. Why not 400ps?

Regards
Vittorio

ing. Vittorio Ricchiuti
CAD support and SI engineer
Siemens ICN
Loc. Boschetto
67100-L'Aquila
ITALY

-----Original Message-----
From: Scott McMorrow [mailto:[email protected]]
Sent: Thursday, August 10, 2000 1:50 AM
To: Vinu Arumugham; [email protected]
Subject: Re: [SI-LIST] : LVDS Skew

Vinu,

Take two sides of a differential transition that goes from
low to high in 400ps with a perfectly balanced transition.

Call the differential crossing point 0ns. Thus the complete
differential transition (irrespective of the bit rate) occurs
in +/-200 ps.

Now skew one side of the pair thus translating one of the
waveforms. As the skew increases, the differential crossing
point slides up (or down, depending on the edge) in voltage.
Eventually, in this case at 200 ps of skew, in a noiseless
perfect system the differential crossing ceases to exist.

This is true no matter what the bit period is and is dependent
only on the edge transition time of the differential signals. You
can prove this to yourself by taking two pieces of paper.
On one draw a rising edge. On the other draw a falling edge.
both should have the same edge transition time and same
low to high signaling level.

Now, assuming you can see through the paper, align the
edges so that they cross exactly in the center. Then, slide
the falling edge to the right. The crossing point moves up
in voltage and out in time across the rising edge. This is
what differential skew does to the signal seen at the
receiver.

After 1/2 the edge transition time the crossing point is at
the high signaling level, where a differential crossing no
longer exists.

Now increase the edge transition times by a factor
of 2:1. There is now twice as much latitude for differential
skew.

The moral of this story is to use the slowest edge rate necessary
to sustain a particular operating bit period. It is more tolerant of
differential skew.

regards,

scott 

--
Scott McMorrow
Principal Engineer
SiQual, Signal Quality Engineering
18735 SW Boones Ferry Road
Tualatin, OR  97062-3090
(503) 885-1231
http://www.siqual.com

Vinu Arumugham wrote:

> Scott McMorrow wrote:
>
> > David,
> >
> > It depends on the edge rate of LVDS signal.  Differential skew, besides
> > causing common mode currents, will cause a translation in time and
voltage
> > of the differential crossing point.  This causes the received Eye to
close
> > down and may be perceived as timing jitter.  If the skew is greater than
> > 50% of the edge transition time, then the eye becomes totally closed.
> > (i.e. A 50%  edge transition time skew causes the differential crossing
to
> > never be seen at the receiver.)
> >
>
> This may be true if the period = 2 x transition time. But at 200MHz, this
> does not look like a possibility.
>
> >
> > Some LVDS drivers have edge rates in the 400ps range.  200ps of skew
> > would totally close the eye.  50ps of skew would reduce the eye opening
> > by 25%, a significant reduction in noise margin.
> >
> > With 1ns edge rate drivers, like some of the "nice" LVDS devices, 50ps
> > of skew is not a big deal.  This translates to only a 10% noise margin
> > reduction.
> >
> > regards,
> >
> > scott
> >
> > --
> > Scott McMorrow
> > Principal Engineer
> > SiQual, Signal Quality Engineering
> > 18735 SW Boones Ferry Road
> > Tualatin, OR  97062-3090
> > (503) 885-1231
> > http://www.siqual.com
> >
> > David Haedge wrote:
> >
> > > Dear SIer's:
> > >
> > > I have a person concerned about 55ps of skew between the two traces
> > > on an LVDS differential pair.  The LVDS  bus is running at 200MHz, a
> > > 5ns period.  What negative effect on the system would occur if say,
> > > there was a 100ps or even 200ps mismatch?  Timing margins are
> > > still within spec with even a 1ns mismatch.  Some common mode
> > > currents may be launched, but I think they would cause minimal
> > > noise and not cause any circuit upsets.  Has anybody seen problems
> > > with LVDS signaling with >55ps diff pair line skew?
> > >
> > > David Haedge
> > > Raytheon
> > >
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