Re: [SI-LIST] : Parallel single-ended traces routed to achieve 35Ohm controlledimpedance

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From: Brad Underwood (Brad.Underwood@metera.com)
Date: Tue Oct 31 2000 - 13:11:57 PST


The answer to your original question is yes. Two 70 ohm lines in
parallel will look like 35 ohms for the length that they are in parallel
(and further if terminated properly). 'Parallel' meaning from the
driver, two different traces routed together or seperately.

I have implemented similar structures in the past where a 50 ohm bus has
multiple drivers at various points on the bus. The 50 ohms bus is then
terminated with 25 ohms on each end. The transmission line as seen from
the driver looks like a 25 ohm impedance.

This can be verified with TDR measurements. Launch a pulse into a 'T'
with two cables out of the coupler. The impedance seen will be the
parallel impedance of the two cables as for the length of the shortest
cable.

Regards,
Brad

....................................................
Brad Underwood
Hardware Engineer phone: (469) 330-1765
Metera Networks, Inc. cell: (214) 642-0769
1202 Richardson Dr, Suit 100 fax: (469) 330-1762
Richardson, Tx 75080

Salvador_Aguinaga@3com.com wrote:
>
> Hello Ken,
>
> We have developed clock-driver circuit that benefits from low impedance lines.
> We have tested it in our older system and works well. We decided to add a test
> trace to our new backplane and test the circuit on the new system down the line.
> The backplane's primary design constraints (no. of layers, dielectric material &
> thickness, trace geometry, etc.) were dictated by other application requirements
> and performance objectives. So, now we want to fit this trace (if possible) and
> our board house is recommending we run two 70 Ohm traces in parallel. I just
> want to make sure we do this right --once.
>
> You are incorrect in quickly assuming there was not enough upfront design to do
> this right. Remember that every time you design a PCB one places priority on
> different parts or aspects of your design and normally other constraints come
> into play (over-all thickness, SI of other more important nets, pcb cost,
> fabrication issues, etc., etc.). You know where I'm going with this....
>
> All I ask for is: Would some one like to share their knowledge (and results) in
> regards to routing a 70 ohm trace on one set of layers and another of the same
> type on a different set of layers so tha when connected at the ends (and in
> between at even intervals) so that their parrallel impedance is 35 Ohms?
>
> Picture this (SIDE VIEW):
> _________________________________________ GND Layer
> dielectric material
> +=========+=========+===========+ <-----Signal layer 70 ohm impdance trace
> | | | |
> | | | | dielectric material
> _ | _______ | _______ | _________ | _____ GND Layer
> | | | |
> +=========+=========+===========+ <-----Signal layer 70 ohm impdance trace
> \
> vias dielectric material
> _________________________________________ GND Layer
>
> "Ken Cantrell" <Ken.Cantrell@srccomp.com> on 10/31/2000 10:18:06 AM
>
> Sent by: "Ken Cantrell" <Ken.Cantrell@srccomp.com>
>
> To: Salvador Aguinaga/MW/US/3Com, "Michael Nudelman" <mnudelman@tellium.com>
> cc: si-list@silab.eng.sun.com
> Subject: RE: [SI-LIST] : Parallel single-ended traces routed to achieve 35 Ohm
> controlledimpedance
>
> Sounds like the cart is before the horse. I would ask why 35 ohms in the
> first place. Then if that is a valid requirement, you need to design your
> stack up so that you will get 35 ohms across all layers of the board. If
> other signals require a Zo of 70 ohms, and these particular traces require
> 35 ohms, it sounds like there was not enough upfront design going on during
> layout. Going back to the drawing board may be the fastest way to resolve
> your problem.
> -----Original Message-----
> From: owner-si-list@silab.eng.sun.com
> [mailto:owner-si-list@silab.eng.sun.com]On Behalf Of
> Salvador_Aguinaga@3com.com
> Sent: Friday, October 27, 2000 2:28 PM
> To: Michael Nudelman
> Cc: si-list@silab.eng.sun.com
> Subject: Re: [SI-LIST] : Parallel single-ended traces routed to achieve
> 35 Ohm controlledimpedance
>
> I need to run the trace through the a pin field. The room between pins is
> 2mm
> and because of the dielectric thickness constraint I can only run a single
> 12
> mil wide line through the pin field. If I use a single 35 ohm line, the
> trace
> width is too large to fit through the pin field.
>
> Thanks,
> -- Sal
>
> Michael Nudelman <mnudelman@tellium.com> on 10/27/2000 03:10:31 PM
>
> Sent by: Michael Nudelman <mnudelman@tellium.com>
>
> To: Salvador Aguinaga/MW/US/3Com
> cc: si-list@silab.eng.sun.com
> Subject: Re: [SI-LIST] : Parallel single-ended traces routed to achieve 35
> Ohm
> controlledimpedance
>
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