RE: [SI-LIST] : Broadside v edge coupled striplines

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From: Larry Miller (ldmiller@rhapsodynetworks.com)
Date: Thu Jan 18 2001 - 07:40:53 PST


RE: [SI-LIST] : Broadside v edge coupled striplinesYes, yes, I did mean
EDGE-coupled, Rick. Thank you for the correction.

Larry
  -----Original Message-----
  From: Rick Brooks [mailto:ribrooks@nortelnetworks.com]
  Sent: Thursday, January 18, 2001 7:28 AM
  To: Larry Miller
  Subject: RE: [SI-LIST] : Broadside v edge coupled striplines

  Larry,
  don't most people, including the AMP paper you refer to, recommend
  edge coupled diff pairs, instead of broadside coupled?

  Just curious.
  You really are having fun, aren't you?
  - Rick

    -----Original Message-----
    From: Larry Miller [SMTP:ldmiller@rhapsodynetworks.com]
    Sent: Thursday, January 18, 2001 6:02 AM
    To: 'David Instone'; SI-LIST
    Subject: RE: [SI-LIST] : Broadside v edge coupled striplines

    There have been some pointers to good papers here (look in the archives)
    that indicate that from a signal integrity standpoint (i.e., rise time,
    etc.) there is NO difference between edge-coupled and broadside-coupled
    pairs IF you can meet the geometry requirements for each.

    However, several studies have shown that broadside-coupled pairs are
more
    noise-sensitive due to the propagation mode (even vs odd) differences.
Also,
    in general, broadside-coupled pairs require propagation mode conversion
    going through connectors.

    There is considerable (not always agreeing) literature at the web sites
of
    AMP, Vitesse and Teradyne on the topic, as well as those of some of the
    "regulars" here such as Doug Brooks (UltraCAD). Searching on DesignCon
2000
    will also bring up a lot of papers and studies.

    One of the clearest expositions I have seen is AMP 98060A_5.pdf, though
you
    do have to carefully watch their sometimes non-conventional terminology.
    This paper was done for a large Telco equipment manufacturer and the
results
    turned out to be very accurate in practice.

    I think that at bottom most purveyors of high speed connector systems
and
    backplane/midplane board manufacturers recommend broadside-coupled
    differential pairs.

    Larry Miller

    -----Original Message-----
    From: owner-si-list@silab.eng.sun.com
    [mailto:owner-si-list@silab.eng.sun.com]On Behalf Of David Instone
    Sent: Thursday, January 18, 2001 4:48 AM
    To: SI-LIST@silab.eng.sun.com
    Subject: [SI-LIST] : Broadside v edge coupled striplines

    All,
    I would like to know what the advantages and disadvatages are of
    differential lines laid out as broadside coupled striplines versus edge
    coupled striplines, assuming of course that both types are designed to
    have the same differential impedance.

    One obvious advantage of broadside is be the increased routing density
    possible, and the minimising of trace length differencies when routing.
    It also seems intuitive that adjacent pairs of broadside coupled lines
    will have a net differential cross talk that is less than that of edge
    coupled, as the geometry can be arranged such that difference in the
    distance between one line of one pair to BOTH lines of the other pair
    will less than if they were edge coupled pairs.

    However, I have recently read the following about broadside coupled
    lines:
    '...the RF characteristics of this design suffer greatly....should only
    be used for high density or tight areas'
     and
    'One must be careful that the lines are not too capacitive. This could
    destroy the rise and fall times at 2Ghz or higher frequencies.'

    The latter statement seems strange, Zo is root(L/C) and is frequency
    independent. Now granted that in the real world we have skin losses and
    dielectric losses, but for the same Zo do broadside lines have more
    frequency dependent loss than edge coupled lines?

    Regards

    Dave Instone. Compliance Engineer
     Storage Systems Development, MP24/22
     Xyratex, Langstone Rd., Havant, Hampshire, P09 1SA, UK.
    Tel: +44 (0)23-92-496862 (direct line)
    Fax: +44 (0)23-92-496014
    http://www.xyratex.com Tel: +44 (0)23-92-496000

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