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

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From: Ron Miller (rmiller@Brocade.COM)
Date: Fri Jan 14 2000 - 16:40:46 PST


Hi Bruce

I agree that there is no necessity to go to coplanar. I was just answering his
question
about the possibility of doing so.

Ron

"Bruce W. Marler" wrote:

> Ron, This seems reasonable, especially given the high common mode range of
> LVDS. I think of the high common mode tolerance of LVDS and the common
> chassis ground as getting the signal in-the-ball-park so that the receiver
> input stage is biased correctly, then the differential natura of the receiver
> can do its thing by discriminating the differential signal. However, I think
> we always need to remember that the CMRR of any device decreases extremely
> quickly at high frequency and so we need to make sure that high frequency junk
> is kept off the common mode as much as possible. What then would be the
> advantage of a coplanar waveguide? Bruce
>
> ----- Original Message -----
> From:Ron Miller
> To: si-list@silab.eng.sun.com
> Sent: Friday, January 14, 2000 11:34 AM
> Subject: Re: [SI-LIST] : High Speed Backplane Connector
> Recommendations
> Steve
>
> I disagreee with your conclusion. See below.
>
> sweir wrote:
>
> > Bruce,
> >
> > No, both the EMI and the SI will be worse. To understand this,
> > think about
> > how each of the LVDS drivers develops its signal:
> >
> > The LVDS "high" line sources from a current source in the driver,
> > while the
> > "low" line sinks to the driver ground. The driver current source
> > has a
> > limited common mode range, as does the sink. At the far end we
> > place a 100
> > ohm resistor between the lines. So, ok current flow in one
> > direction or
> > another creates a relative voltage difference across the
> > resistor. BUT,
> > the common mode voltage is just the average voltage of the two.
> > Our
> > receiver must be able to:
> >
> > 1) Survive the common mode voltage, and
>
> Yes, but anyone with an ounce of sense will still use the same
> supply voltages
> so there should be no problem
>
>
> >
> > 2) Register the intended results
> >
> > For a demonstration of 1) you can destroy a perfectly good PC and
> > printer
> > by lifting the grounds on both and plugging each into extension
> > cords at
> > opposite ends of your house, and then connecting the PC to the
> > printer. When your refrigerator turns on and off watch as one or
> > the other
> > lets all the smoke out.
>
> Again I say this is a fools scenario.
>
> Incidentally, all equipment uses the green wire for chassis/safety
> ground and the
> green is connected at the power box, circuit breaker panel to the
> low side of the power.
> Because of inductances it is possible to get an AC difference up to
> a couple volts due to
> unequal loads on the AC phases at a wall socket. DC is not
> effected.
>
> Within a system as in a PCB as was the question set up, the ground
> voltages are very
> close to equal at the receiver and driver.
>
> >
> >
> > This is one of the reasons that box to box transports use
> > transformers or
> > optics. If you operate within a box, it is generally much more
> > economical
> > to deal with a common ground.
>
> :Yes these work well to eliminate any offset where long cables are
> used.
>
> >
>
> Conclusion: within a chassis all grounds should be connected
> together keeping
> any common mode voltages low. Then coplanar waveguide works nicely
> without
> a ground plane.
>
> Ron Miller
>
>
> >
> >
> > Regards,
> >
> > Steve.
> >
> > At 05:43 PM 1/13/00 -0700, you 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
>
>
>

--
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

**** To unsubscribe from si-list: send e-mail to majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE si-list, for more help, put HELP. si-list archives are accessible at http://www.qsl.net/wb6tpu ****


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