Re: [SI-LIST] : High Speed Backplane Connector Recommendations

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From: Ron Miller (rmiller@Brocade.COM)
Date: Thu Jan 13 2000 - 17:12:46 PST


Right Bruce

This is called co-planar waveguide. It can be broadside coupled or
narrowside coupled with no ground plane required.

ron miller

"Bruce W. Marler" wrote:

> Steve,
>
> What if you put no ground plane, just two traces on a PCB by themselves?
> Can we not look at these by themselves as a transmission line? If driven
> differentially and with a well balanced source impedance will not the
> current flowing down one trace then return down the other assuming they are
> terminated in their characteristic impedance?
>
> You may ask what advantage this would have. It might allow one to create
> higher impedance PCB transmission lines.
>
> It might have the disadvantage of worse EM radiation. But SI should be
> good, shouldn't it?
>
> Bruce
>
> ----- Original Message -----
> From: sweir <weirsp@a.crl.com>
> To: Bruce W. Marler <BMarler@WooshCom.com>
> Sent: Thursday, January 13, 2000 4:54 PM
> Subject: Re: [SI-LIST] : High Speed Backplane Connector Recommendations
>
> > Bruce,
> >
> > When we place two conductors in a diff pair over a ground plane, 85-90% of
> > the energy in each lead couples to the ground plane, while the remaining
> > 10-15% couples to the other trace in the pair. This is true whether we
> > edge couple, or broadside couple.
> > What this means is that if your ground plane does not provide a good
> return
> > path, then you will be subject both to SI and EMC problems you don't
> > want. The bottom line is that even though you are using diff signals, as
> > long as they are DC coupled, ( LVDS, BLVDS ), then you need to assign a
> > good deal of your connector real-estate to signal ground, so that you can
> > maintain a high frequency return path both for EMC and SI. If you miss
> > this point, your timing will degrade, and you may end up with unmanageable
> EMI.
> >
> > The AMP stuff is "Z-pack HS3", pricey, but very good. The data sheets are
> > on their www site. If you can eat the power, you can drive those guys
> with
> > Vitesse 870/880's which i/f 2Gbps to FPGA's nicely with only one pair in
> > the bp. If you are hell-bent on LVDS, there are several solutions out
> > there to reliably move data at higher rates. Lucent has a four pair
> ganged
> > TX/RX module available in their ASIC's which moves 2.4Gbps as a single
> > stream. You could also consider the channel link parts that clock line
> > data reliably between 350 and 750 Mbps / pair, 1.4 - 3+Gbps / 5 pair.
> >
> > Regards,
> >
> >
> > Steve.
> > At 04:19 PM 1/13/00 -0700, you wrote:
> > >Steve,
> > >What do you mean when you say that "most of the energy still couples
> between
> > >the returns and the individual signals in each pair"?
> > >
> > >Also what is the more expensive AMP connector that you refered to which
> can
> > >transport 2.4 Bbps signals?
> > >Thanks
> > >Bruce
> > >----- Original Message -----
> > >From: sweir <weirsp@a.crl.com>
> > >To: Bruce W. Marler <BMarler@WooshCom.com>
> > >Sent: Thursday, January 13, 2000 12:07 PM
> > >Subject: Re: [SI-LIST] : High Speed Backplane Connector Recommendations
> > >
> > >
> > > > Bruce,
> > > >
> > > > I think your plan has some scalability problems. Multidrop backplanes
> are
> > > > generally not a very good thing to use these days as the parasitics
> place
> > > > an upper limit which is very limiting, even using a fairly good
> technology
> > > > such as LVDS. It sounds like you are planning to use BLVDS on a
> > > > non-redundant backplane.
> > > >
> > > > For connectors, the older and less expensive AMP 2mm hard metric,
> which
> > >has
> > > > several alternate sources is not bad, provided you do your
> > > > homework. Please remember that most of the energy still couples
> between
> > > > the returns and the individual signals in each pair, sic, make sure
> you
> > > > have adequate ground density. AMP has a much more expensive connector
> > >line
> > > > intended to reliably transport 2.4Gbps across backplanes than the HM
> > > > stuff. The HM connectors are a better choice if your plan is to
> remain at
> > > > a fairly low performance of 100MHz on each wire pair. A single
> connector
> > > > provides 125 signal pins in 50MM. Realistically, you can get 48 good
> > >pairs
> > > > out of one such connector.
> > > >
> > > > I strongly recommend that you get a good SI consultant to review your
> > > > backplane plan before you commit to your architecture. My concern
> arises
> > > > from the fact that the rise time on LVDS is between 100 and 300pS,
> which
> > > > makes even 0.1" of stub significant. You are more likely to see about
> > > > 1". Also, the parasitic capacitance of many connectors could be a
> serious
> > > > problem. Most folks who want to handle a lot of bandwidth have long
> since
> > > > gone to point to point configurations for these reasons.
> > > >
> > > > Good luck with your MPEG2 endeavor.
> > > >
> > > > Regards,
> > > >
> > > > Steve.
> > > > At 10:40 AM 1/13/00 -0700, you wrote:
> > > > >"Bruce W. Marler" <BMarler@WooshCom.com>
> > > >
> > > >
> >
> >
>
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--
Ronald B. Miller  _\\|//_  Signal Integrity Engineer
(408)487-8017    (' 0-0 ') fax(408)487-8017
     ==========0000-(_)0000===========
Brocade Communications Systems, 1901 Guadalupe Parkway, San Jose, CA  95131
rmiller@brocade.com,  rbmiller@sjm.infi.net

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