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

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From: Muhammad S. Sagarwala (msagarwa@san-jose.tt.slb.com)
Date: Tue Oct 31 2000 - 12:38:46 PST


Salvador,

I have done something similar and have had no problems. I have routed two
52 ohms traces
-but on the same layer - to hit the target impedence of 26 ohms.

Muhammad

-----Original Message-----
From: Salvador_Aguinaga@3com.com <Salvador_Aguinaga@3com.com>
To: Ken Cantrell <Ken.Cantrell@srccomp.com>
Cc: si-list@silab.eng.sun.com <si-list@silab.eng.sun.com>
Date: Tuesday, October 31, 2000 10:54 AM
Subject: RE: [SI-LIST] : Parallel single-ended traces routed to achieve 35
Ohm controlledimpedance

>
>
>
>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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