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

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From: sweir (weirsp@a.crl.com)
Date: Thu Jan 13 2000 - 19:23:23 PST


Ron,

Agreed, but then there is the practical matter of the far-end, and
detecting what has been sent.

Regards,

Steve.
At 05:12 PM 1/13/00 -0800, you wrote:
>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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