# [SI-LIST] Re: Return Path

• From: Istvan Novak <istvan.novak@xxxxxxx>
• To: darshanmehta2k@xxxxxxxxx
• Date: Fri, 05 Aug 2005 08:38:09 -0400
Darshan,

An excellent book on field solvers is:

Daniel Swanson, Wolfgang Hoefer: Microwave Circuit Modeling using
Electromagnetic Field Simulation. Artech House, 2003.

Regards,

Istvan Novak
SUN Microsystems

Darshan Mehta wrote:

>Steve,
>
>
>
>Now I understood why calculation does not give correct answer and field
>solvers are used for Impedance calculation.
>
>
>
>Could you please tell me which book I should refer to know more about “How the
>field Solver works”, I am really keen to know about field solvers.
>
>
>
>Thanks,
>
>Darshan Mehta
>
>
>steve weir <weirsi@xxxxxxxxxx> wrote:
>
>1. That all closed form formulas for wave guide impedance are approximations
>with limited accuracy over finite ranges.
>2. Line to line coupling for several configurations.
>3. That to get a really accurate answer you need to use a field solver.
>
>Steve.
>At 12:24 AM 8/5/2005 -0700, Darshan Mehta wrote:
>
>Steve and Abhijit,<?xml:namespace prefix = o ns =
>"urn:schemas-microsoft-com:office:office" />
>
>
>
>Once again Thanks for clarifying my doubts.
>
>
>
>The formula for Capacitance calculation for Stripline configuration is as
>follows (pF/inch);
>
>
>
>C = 1.4 * Er / ln [ 2.4 * (b / w)]
>
>
>
>Where b = total height of Dielectric between the Plane Layer and w is the
>width of Trace.
>
>
>Generally we take the height between the Plane layers (return current path) as
>the b in the capacitance calculation. But as per your mail if the nearby
>conductors also act as a return current, what dielectric height I should
>consider while calculating the Capacitance.
>
>Thanks,
>
>Darshan Mehta
>
>
>
>
>steve weir <weirsi@xxxxxxxxxx> wrote:
>   Darshan, Abhijit, I find Dr. Bogatin's book "Signal Integrity Simplified"
>   is an excellent text that presents the material clearly without losing
>   technical accuracy. It belongs right up there with other excellent books
>   such as those by Dr. Johnson, Dally & Poulton, Young, and Hall. And I am
>   not just saying that because it features Teraspeed's development of "Final
>   Inch" for Samtec.
>
>
>   Abhijit is correct. Any conductor is a better wave guide than
>   dielectric. Either provide conductors to supply the return path by
>   intention, or your signals will find other conductors on their own.
>
>
>   Steve.
>   At 11:53 PM 8/4/2005 -0700, Abhijit Mahajan wrote:
>   >Darshan
>   >
>   >Nearby signals traces can also act as return path. One possible way to
>   >improve return path situation (on two layer boards) is to use coplanar
>   >waveguide configuration. It can be useful for two layer boards having =
>   >high
>   >speed traces, and also when the board thickness is on the higher side.=20
>   >
>   >BTW, I haven't read this book but it certainly sounds like a good one =
>   >since
>   >it got you thinking so much on return currents!
>   >
>   >Abhijit.
>   >=20
>   >
>   >-----Original Message-----
>   >From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] =
>   >On
>   >Behalf Of Darshan Mehta
>   >Sent: Friday, August 05, 2005 11:35 AM
>   >To: steve weir; si-list@xxxxxxxxxxxxx
>   >Subject: [SI-LIST] Re: Return Path
>   >
>   >Steve,
>   >
>   >=20
>   >
>   >
>   >=20
>   >
>   >So you mean to say that if we don't have Plane Layer, the Electric and
>   >Magnetic field will be coupled to the nearby conductor. If we have a =
>   >Plane
>   >layer also present on the board, does the nearby conductor act as a =
>   >Return
>   >path? My basic question is, on what basis we should think that this =
>   >conductor
>   >will act as a return path for given signal. If the Plane shape is =
>   >present on
>   >the board, does the Plane as well as the nearby conductors will act as a
>   >return current or only the Plane shape will act like return current =
>   >path.=20
>   >
>   >=20
>   >
>   >Thanks,
>   >
>   >Darshan Mehta
>   >
>   >
>   >steve weir wrote:
>   >Darshan, if you have a board with no dedicated plane, then the fields =
>   >will
>   >just spread out, coupling the signal into multiple conductors. A field =
>   >solver
>   >can determine the coupling on each line for a given configuration.
>   >
>   >Steve.
>   >At 10:31 PM 8/4/2005 -0700, Darshan Mehta wrote:
>   > >Hello Experts,
>   > >
>   > >
>   > >
>   > >I have a question on return current. I was going through the book=20
>   > >"Signal Integrity Simplified" and came to know that return current path
> =
>   >
>   > >can be a Power Plane or Ground Plane. I am still confused about
> return=20
>   > >path. Let me describe what I understood.
>   > >
>   > >
>   > >
>   > >The Signal Path is the active path and the Electric field from Signal=20
>   > >will terminate in the return path. The Magnetic field will form a=20
>   > >circular loop around the signal and it will be coupled with return path
> =
>   >
>   > >so that equal and opposite current will flow in return path. Normally=20
>   > >if we have Power Plane or Ground place, the return path will be easy to
> =
>   >
>   > >find out. Let's assume, if we have a 2 layer board with no copper shape
> =
>   >
>   > >drawn on it, how to find the return path for the signal? Please help me
>   >understanding this.
>   > >
>   > >
>   > >
>   > >Thanks,
>   > >
>   > >Darshan Mehta
>   > >
>
>
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