[SI-LIST] Re: Guard traces for differential pairs

Can you explain what does the presence of guard trace for differential pairs
has any thing to do with driver impedance and load impedance ? You are
mixing up a non-homogeneous transmission line model with SSO noise and
Lij/Lii ne Cij/Cii has nothing to do with inductive or capacitive xtalk but
due to the non-homogeneous dielectric boundary. If the driver is
differential, the differential switching noise cancels each other out. Don't
keep throwing terms out just to confuse people.

-----Original Message-----
From: Duane Takahashi [mailto:duanet@xxxxxxxxxxxxxxxxxxxxxx]
Sent: Tuesday, September 30, 2003 9:25 AM
To: damonjbowser@xxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Guard traces for differential pairs


Hi Damon:

I don't think you can look at the L and C matrices, and determine if the 
xtalk will be capacitive or inductive.  As Scott points out, they move 
together.

IMHO, you need to take into account the driver's impedance and the 
load's impedance.  To me, packages look inductive: bond wires with 
traces over perforated reference planes.  Flip chip helps a lot, but you 
still have the perforated planes.

When the driver fires, the inductance forces the voltage wave to lead 
the current wave.  This phase relationship travels down the line.  To 
the load, the line's impedance is no longer purely real, and has an 
imaginary component to it.

In this case, the capacitive and inductive forward xtalk components do 
not cancel, and the inductance xtalk dominates.

Duane

 > For TEM and quasi-TEM transmission line systems, the Inductance and the
 > Capacitance matrices are related.  For lossless and low loss systems,
 > the inductance matrix is:
 > L = mu * epsilon * (C inverse)
 >
 > They are linked as long as the magnetic and electric fields remain
 > orthogonal.  Thus, whenever you change the capacitance of a system of
 > traces, you also change their inductance.
 >
 > A good quasi-static field solver will compute the correct L and C
 > matrices for any given trace configuration.  In fact, many of the
 > currently available field solvers assume that the conductors are perfect
 > and therefore only solve for the capacitance matrix.  The inductance
 > matrix is computed using the expression above. Clayton Paul has a good
 > discussion of this in his book, "Analysis of Multiconductor Transmission
 > Lines."
 >
 > As you change capacitive coupling, you also change inductinve coupling.
 >
 > regards,
 >
 > scott
 >
> Hi Duane,
> 
> So how does one determine if it is magnetic or
> capacitive crosstalk that is dominating in a
> particular configuration?
> 
> Let's say I get two matrices from a field solver that
> look like this:
> 
> C11 C12
> C21 C22
> 
> and
> 
> L11 L12
> L21 L22
> 
> Do I just look at how C12/C11 compares to L12/L11 and
> how C21/C22 compares to L21/L22 ?
> 
> Thanks
> Damon
> 
> 
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>>Subject: [SI-LIST] Re: Guard traces for differential
>>pairs
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>>
>>It also depends on the type of crosstalk: electric
>>or magnetic (
>>capacitive or inductive ).  If it's magnetic, guard
>>traces won't help.
>>If it's capacitive, guards will help but will leave
>>you with the
>>'magnetic remainder' of the xtalk.
>>
>>I've seen high impedance lines, more than 60 ohms,
>>exhibit inductive
>>xtalk.  My guess is guard traces would not have
>>helped in this case.
>>
>>Regards,
>>Duane
>>
>>
>>>Sogo,
>>>
>>>It is true that guard traces can INCREASE
>>
>>crosstalk, but
>>
>>>this can be avoided (and crosstalk in fact can be
>>
>>REDUCED)
>>
>>>if the via stitching is frequent enough.  Between
>>
>>each
>>
>>>stitching, there is a half-wavelength resonator,
>>
>>with a
>>
>>>lowest resonance frequency of fres=1/(2*tpd),
>>
>>where
>>
>>>tpd is the propagation delay between two stitching
>>
>>vias.
>>
>>>As long as the bandwidth of the signal on the
>>
>>structure
>>
>>>is safely below this resonance frequency, there
>>
>>should be no
>>
>>>problem with signal integrity.
>>>
>>>Regards,
>>>
>>>Istvan Novak
>>>SUN Microsystems
> 
> 
> 
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