[SI-LIST] Re: No need of termination within critical length??
- From: wolfgang.maichen@xxxxxxxxxxxx
- To: Praveen Gowda <praveenkumardr@xxxxxxxxx>
- Date: Thu, 17 Sep 2009 10:32:54 -0700
The closer the spacing, the higher the crosstalk. Theoretically there is
no upper boundary (or you could argue, tongue in cheek, once the traces
touch - zero distance - you'll have the maximum crosstalk - 100%).
Actually the crosstalk statement refers to near end crosstalk (NEXT); NEXT
plateaus once the coupled line length equals the signal rise time. As for
far-end crosstalk (FEXT), for normal trace configurations the saturation
length is much larger (several meters), and FEXT only occurs for
microstrips or other configurations with inhomogeneous dielectrics (ideal
striplines do not exhibit FEXT).
Wolfgang
Praveen Gowda <praveenkumardr@xxxxxxxxx>
Sent by: si-list-bounce@xxxxxxxxxxxxx
09/17/2009 10:18 AM
To
Lee Ritchey <leeritchey@xxxxxxxxxxxxx>
cc
"olaney@xxxxxxxx" <olaney@xxxxxxxx>, sonukurian@xxxxxxxxxxxxxxx,
si-list@xxxxxxxxxxxxx, praveen.dr@xxxxxx, "H, Arvind (GE Infra, Energy)"
<arvind.h@xxxxxx>
Subject
[SI-LIST] Re: No need of termination within critical length??
Lee ritchey,
you have mentioned that crosstalk reaches maximum at critical length (
length for which traces run in parallel).
but no where it is mentioned about their spacing.
for what spacing, crosstalk becomes maximum?
regards
Praveen Kumar
Engineer at GE aviation systems
praveen.dr@xxxxxx
On Thu, Sep 17, 2009 at 10:04 PM, Lee Ritchey
<leeritchey@xxxxxxxxxxxxx>wrote:
> Critical length is that length at which backward crosstalk between two
> parallel transmission lines reaches a maximum. It is a crosstalk
> parameter, not a reflection parameter. This is from the RF engineering
> world.
>
> We still need a definition of when a transmission line is long enough at
a
> given rise time to observe reflections large enough to cause
malfunctions.
> I have been using the length of a rise time in the given dielectric as a
> starting point. I call this the Transition Electrical Length or TEL.
>
> Then, in most cases, the length at which reflections become important is
> 1/4 to 1/6 of this length. You can see this for yourself by using any
> good SI tool such as Hyperlynx or Cadence Allegro PCB SI.
>
> Lee Ritchey
>
>
> > [Original Message]
> > From: <olaney@xxxxxxxx>
> > To: <sonukurian@xxxxxxxxxxxxxxx>
> > Cc: <si-list@xxxxxxxxxxxxx>
> > Date: 9/17/2009 1:12:13 AM
> > Subject: [SI-LIST] Re: No need of termination within critical length??
> >
> > Flip the conceptual machinery around; critical length is by definition
> > the boundary between where termination doesn't matter and where it
does.
> > It has no direct relation to backmatching at the driver, which is a
> > popular way of terminating lines longer than critical length. Below
> > critical length, the time delay on the line is short enough compared
to
> > the risetime of the signal that the voltage delta between the ends is
> > small enough to ignore, i.e. the signal is 'local'.
> > It's a fuzzy boundary that depends on lots of things, but another way
to
> > look at it is that if the transmission line is a small fraction of a
> > wavelength, the energy reflected from a mismatched load is so nearly
in
> > phase with the driving end (or 180 degrees out, depending on the sign
of
> > reflection) that the formula that transforms load impedance to what is
> > seen at the driving end never strays very far from unity. Therefore
the
> > transmission line is very nearly transparent and can be ignored (all
else
> > being equal), even to the point of not requiring matching at either
end.
> > A resistor might still be required for pull up, pull down, or voltage
> > divider purposes, but on a short enough line there is no need to tie
the
> > value to trace impedance.
> >
> > Orin Laney
> >
> > On Thu, 17 Sep 2009 12:27:07 +0530 "Sonu Kurian N.D"
> > <sonukurian@xxxxxxxxxxxxxxx> writes:
> > > Hi All,
> > >
> > > I am puzzled about why termination is not required if trace length
> > > is
> > > less than critical length.
> > > I read like if a driver encounters a reflected signal when it is
> > > rising(driving) , its able to absorb reflection.Is it so?
> > > Or is the situation like ..the reflections will be there..but the
> > > Setup
> > > condition can be met..so we need not worry..??
> > > Can anyone please throw some light on this..
> > >
> > > Many thanks,
> > > Sonu
> > >
> > >
> > >
> > > ------------------------------------------------------------------
> > > To unsubscribe from si-list:
> > > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject
> > > field
> > >
> > > or to administer your membership from a web page, go to:
> > > //www.freelists.org/webpage/si-list
> > >
> > > For help:
> > > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
> > >
> > >
> > > List technical documents are available at:
> > > http://www.si-list.net
> > >
> > > List archives are viewable at:
> > > //www.freelists.org/archives/si-list
> > > or at our remote archives:
> > > http://groups.yahoo.com/group/si-list/messages
> > > Old (prior to June 6, 2001) list archives are viewable at:
> > > http://www.qsl.net/wb6tpu
> > >
> > >
> > >
> > ____________________________________________________________
> > Lose up to 20 lbs in one month with a new diet. Click here.
> >
>
>
http://thirdpartyoffers.juno.com/TGL2141/fc/BLSrjpTFoYbnjpxpyPGuHaWex3S8KUp3
> 83121B6bTJ5YEE1gA7TmiP2s1GY/
> >
> > ------------------------------------------------------------------
> > To unsubscribe from si-list:
> > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
> >
> > or to administer your membership from a web page, go to:
> > //www.freelists.org/webpage/si-list
> >
> > For help:
> > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
> >
> >
> > List technical documents are available at:
> > http://www.si-list.net
> >
> > List archives are viewable at:
> > //www.freelists.org/archives/si-list
> > or at our remote archives:
> > http://groups.yahoo.com/group/si-list/messages
> > Old (prior to June 6, 2001) list archives are viewable at:
> > http://www.qsl.net/wb6tpu
> >
>
>
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
> or to administer your membership from a web page, go to:
> //www.freelists.org/webpage/si-list
>
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>
> List technical documents are available at:
> http://www.si-list.net
>
> List archives are viewable at:
> //www.freelists.org/archives/si-list
> or at our remote archives:
> http://groups.yahoo.com/group/si-list/messages
> Old (prior to June 6, 2001) list archives are viewable at:
> http://www.qsl.net/wb6tpu
>
>
>
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List technical documents are available at:
http://www.si-list.net
List archives are viewable at:
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List technical documents are available at:
http://www.si-list.net
List archives are viewable at:
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
