[SI-LIST] Re: Advice
- From: steve weir <weirsi@xxxxxxxxxx>
- To: Michael Rose <mrose@xxxxxxxxxxxx>
- Date: Mon, 20 Jul 2009 15:05:34 -0700
A degraded signal that depends on capacitor placement implies that:
1. The capacitor / mount / transition does not look resistive and
matched to the channel. AND
2. The placement is close to another discontinuity and the two are
interacting.
Often it's the latter that creates the headaches. If the resulting
performance is an issue, a 3D solver and some sweat equity should solve
the problem.
Steve.
Michael Rose wrote:
> Generally, I found that having the caps close to the receiver (or
> transmitter in this case) allows the fewest vias and the shortest via
> stubs. The pin escape from from the IC can go microstrip to the caps and
> then via down to a stripline layer close to the bottom of the PCB and on
> to the connector pin via. At this data rate you may want to tune the cap
> pads in a full wave simulator to minimize the cap discontinuity.
>
> I think it's more important to minimize the magnitude of the resonances
> from the discontinuities than to worry about how/where they're lumped
> together. I've read that the biggest impact with having the caps near
> the receiver is that it can slow the edge rate slightly. Having them on
> the transmitter side offers better protection to shorts/faults.
>
> Mike
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of Smith, Justin D.
> Sent: Monday, July 20, 2009 2:20 PM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Advice
>
> I'm working on a 6.25Gbps channel SI issue with a constraint that is not
> ideal. We have a legacy issue which requires that we put decoupling caps
> on the "transmit" side of a transmission line instead of the receive
> side (where we'd like them). This must be done to maintain "backwards
> compatibility" with MODULE 2 that does not have decoupling on the
> receiver side.
>
> That said, we have this type of setup:
>
> ==================================================================
> # MODULE 1 BACKPLANE MODULE 2 #
> # XMT DCPL BP. BP. RCVR #
> # |\ CAP Conn Trace Conn |\ #
> # | \_____||__________[ ]________________[ ]______________| \ #
> # | / || [ ] [ ] | / #
> # |/ |/ #
> =============================Fig 1================================
> (hope that cheesy ascii graphic comes through)
>
> Some questions arise that I would value the SI-LIST opinion on:
>
> 1)Where to put the decoupling cap with the constraint that we have to
> put it on MODULE 1, which is the XMT side of the overall channel?
>
> a. Near to the transmitter (possibly greater reflected power
> into the transmitter?)
> b. Near the BP connector (as close to the RCV as we can? makes
> the discontinuity more capacitive - but "lumps" issue with connector
> discontinuity).
> c. In the middle (distribute the discontinuities along the path
> - possibly results in multiple reflections?).
>
> 2) Knowing that the backplane trace is very short (~1.5") does that
> effect how you'd advise?
>
> 3) Parameters that I can work with are the length of the trace from FPGA
> to connector on the module, location of the cap, and cap value/size.
>
> We have Hyperlynx as a SI tool. Thoughts? Anybody ever dealt with an
> issue like this before?
>
> -justin
> ============================================================
> The information contained in this message may be privileged
> and confidential and protected from disclosure. If the reader
> of this message is not the intended recipient, or an employee
> or agent responsible for delivering this message to the
> intended recipient, you are hereby notified that any reproduction,
> dissemination or distribution of this communication is strictly
> prohibited. If you have received this communication in error,
> please notify us immediately by replying to the message and
> deleting it from your computer. Thank you. Tellabs
> ============================================================
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
> or to administer your membership from a web page, go to:
> //www.freelists.org/webpage/si-list
>
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>
> List technical documents are available at:
> http://www.si-list.net
>
> List archives are viewable at:
> //www.freelists.org/archives/si-list
> or at our remote archives:
> http://groups.yahoo.com/group/si-list/messages
> Old (prior to June 6, 2001) list archives are viewable at:
> http://www.qsl.net/wb6tpu
>
>
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
> or to administer your membership from a web page, go to:
> //www.freelists.org/webpage/si-list
>
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>
> List technical documents are available at:
> http://www.si-list.net
>
> List archives are viewable at:
> //www.freelists.org/archives/si-list
> or at our remote archives:
> http://groups.yahoo.com/group/si-list/messages
> Old (prior to June 6, 2001) list archives are viewable at:
> http://www.qsl.net/wb6tpu
>
>
>
>
--
Steve Weir
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
California office
(866) 675-4630 Business
(707) 780-1951 Fax
Main office
(401) 284-1827 Business
(401) 284-1840 Fax
Oregon office
(503) 430-1065 Business
(503) 430-1285 Fax
http://www.teraspeed.com
This e-mail contains proprietary and confidential intellectual property of
Teraspeed Consulting Group LLC
------------------------------------------------------------------------------------------------------
Teraspeed(R) is the registered service mark of Teraspeed Consulting Group LLC
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List technical documents are available at:
http://www.si-list.net
List archives are viewable at:
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
