[SI-LIST] Re: No need of termination within critical length??
- From: Praveen Gowda <praveenkumardr@xxxxxxxxx>
- To: Lee Ritchey <leeritchey@xxxxxxxxxxxxx>
- Date: Thu, 17 Sep 2009 22:48:38 +0530
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
> > >
> > >
> > >
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