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

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From: Raymond.Leung@qsa.idt.com
Date: Sun Jan 16 2000 - 16:21:22 PST


While I have no comment about the coplannar waveguide as
well as the orginal connector discussions, I would like to say
something about the LVDS receiver. As mentioned before, the
CMRR can never be ideal in high freq. D.C. has already complained
in this forum about the output signal edge jitter caused by the wide
common mode variation at the differential inputs, and I have a
similar headache. Supressing the high freq. ac common mode
is definitely a merit for reliable data recovery.

Regards,
Raymond

---------------------- Forwarded by Raymond Leung/QSA/AU on 17/01/2000 10:03
---------------------------

"Bruce W. Marler" <BMarler@WooshCom.com> on 16/01/2000 03:47:35

Please respond to si-list@silab.eng.sun.com

To: si-list@silab.eng.sun.com
cc: (bcc: Raymond Leung/QSA/AU)

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

Ron,

Yes, I understand that there is no necessity but I am wondering what advantages
there might be?
Bruce
  ----- Original Message -----
  From: Ron Miller
  To: si-list@silab.eng.sun.com
  Sent: Friday, January 14, 2000 5:40 PM
  Subject: Re: [SI-LIST] : High Speed Backplane Connector Recommendations

  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.

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