RE: [SI-LIST] : the old high-frequency return current model

Eric Bogatin ([email protected])
Mon, 4 Oct 1999 18:00:20 -0500


you're right- the common mode impedance is not infinite across the slot,
just really high compared to 50 Ohms. Since its near field, it can be much
higher than 377 Ohms- after all, isn't the impedance of the open end of a
transmission line a lot higher than 377 Ohms?.

Interesting comment about the slot wave mode- I hadn't thought through the
details before. You're right, there will be additional effects that a simple
segmented transmission line simulation will not be including, related to the
slot modes. Any feel for how large these effects might be and how to
include them in a circuit simulator?

I don't have a good feel for the magnitude of the slot mode wave that would
be launched. This is where the FDTD tool might provide value. When a single
ended line crosses a slot in the return path, there is a strong launch of
the slot wave mode. I've seen FDTD simulations by Henri Merkelo that show
the slot wave sloshing up and down the gap. The coupling across this gap,
with a discrete cap, for example, will strongly influence the coupling
across it and how much of a discontinuity the gap represents. However, I
would expect that the slot mode is not as strongly excited by the
differential signals. Is it 10% of the single ended line? or even less? I
suspect the amount of slot wave mode excited is dominated by the asymmetries
in the diff signals- due to skew, etc, so it may be in the 20-50% of the
launched magnitude of a single ended line.

Bruce Archambeault had a great paper, Archambeault, Bruce; Proper design of
intentional splits in the ground reference plane of PC Boards to minimize
emissions from I/O wires and cables, Proc. 1998 IEEE conf on EMC, p. 768,
where he calculated the impedance of a slot. It obviously depends on the
length of the slot, and can be close to zero at resonance. His conclusion,
similar, I think, to the UMC folks, is that slots are effective at blocking
common mode signals only below the resonance of the slot wave mode, where
the coupling across the slot can be very high, ie., the impedance of the
slot is low. So, the slot under the diff pair would have to be less than
1/2 inch laterally, in order to be below the resonance for the 70 psec edges
and be able to block common mode signals. Since a typical slot might be much
longer than this, I think you're right, that the slot will create some kind
of ringing in the common mode signals. Near resonance, the slot may have a
low impedance to common mode signals. I don't have any feel for what the
impedance will be for differential mode signals near resonance- any one have
any insight on this? How would we quantifiy this to include it in a circuit
simulation? Anyone have any ideas?

Scott- good comments, thanks for the response- it got me thinking about a
different perspective.


Eric Bogatin
Training for Signal Integrity and Interconnect Design
26235 W. 110th Terr.
Olathe, KS 66061
v: 913-393-1305
f: 913-393-1306
pager: 888-775-1138
e: [email protected]

> -----Original Message-----
> From: Scott McMorrow [mailto:[email protected]]
> Sent: Monday, October 04, 1999 12:37 PM
> To: Eric Bogatin
> Subject: Re: [SI-LIST] : the old high-frequency return current model
> Eric,
> Eric Bogatin wrote:
> > The common mode will see an impedance of
> > 57 Ohms in the microstrip region and infinite in the coplanar region.
> Huh? The common mode impedance might be high, but it
> won't be infinite, will it? Even if the signal is referenced to infinity
> the self impedance will be approximately 377 ohms, the characteristic
> impedance of free space.
> Since Zcommon for a differential pair is (Z11 + Z12) then in even the
> worst case we would see (377 + Z12). Then the question becomes,
> how much mutual coupling does one have across a slot? This will
> ultimately determine the upper bounds on the common mode
> impedance.
> > To first order, this will be the main effect the signal sees in
> crossing the
> > gap. --
> If I remember correctly, we will launch a slot mode wave within the gap
> which will greatly increase mutual coupling between the two lines.
> This coupling will be dependent upon edge rate. Not only will the
> differential and common mode signals couple, but the common mode
> will radiate. With 70ps driver edge rates these phenonema will not
> be negligible. Some of the more recent drivers for Gigabit and above
> telecommunications circuits have edge rates as fast as 70ps.
> Unfortunately, a simple transmission line analysis of the differential
> pair across the slot will not show the slot mode effect.
> scottt
> Scott McMorrow
> Principal Engineer
> SiQual, Signal Quality Engineering
> 18735 SW Boones Ferry Road
> Tualatin, OR 97062-3090
> (503) 885-1231

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