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Building Coaxial transmission line on PCB?

Discussion in 'Electronic Design' started by Geronimo Stempovski, Feb 12, 2007.

  1. I think transmitting high-speed signals is very easy when you have a
    360-degree ground reference, round conductors,
    and no other nearby signals like in coaxial cables. My aim is to design PCB
    tracks as much like a coaxial cable as
    possible. Anyone tried this before? Is it possible with regular FR4, anyway?
    Thanks for your help.

    Gero
     
  2. John Fields

    John Fields Guest

     
  3. But a microstrip will achieve the same results without the technical
    difficulty of a real coaxial line. It is even easier to use
    balanced/differential connections.

    Meindert
     
  4. Fred Bloggs

    Fred Bloggs Guest

    Nope- in all the decades of high speed PC circuit design, you are the
    first to think of it!
    Not even close, the "phase velocity," or speed to you, will be less than
    that of free space by a factor of 1/sqrt(epsilon-sub-r), so go figure.
     
  5. Henning Paul

    Henning Paul Guest

    http://en.wikipedia.org/wiki/Stripline

    You won't get any more TEM-like on a PCB.

    regards
    Henning

    F'up2 d.s.e. (Da darfst Du dann auch wieder deutsch sprechen.)
     
  6. Guest

    You can build a wave guide out of a multi-layer board with lots of
    micro vias. The performance gain was non-existent vs traditional micro/
    strip/line, and cost a lot of money. These days you can satisfy
    yourself of this reality with a 3D field solver. Back when we tried
    this, workstations were as slow as building it "for real", and more
    expensive.
    Just break out to a connector, use a coax assembly, and connect back
    in. Easy peasy.
     
  7. Fred Bloggs

    Fred Bloggs Guest

    There is such a thing as microcoax, so he can chisel out a little groove
    in a thick pc and stuff that in there.
     
  8. MassiveProng

    MassiveProng Guest


    Semi-rigid is the term, and if it is in coax, it doesn't need to be
    IN the PCB from point to point.

    I wish you guys would stop cross posting all over the place. Many
    ISPs have group inclusion limits.

    It is also not considered proper Usenet practice.
     
  9. John  Larkin

    John Larkin Guest

    Sure. Microstrip, stripline, coplanar waveguide, or even the very
    strange slotline.

    John
     
  10. John Fields

    John Fields Guest

     
  11. If you're such an expert, why are you asking here?

    In my experience working with stuff in the low end of the
    microwave region (~ 1GHz), microstrip is pretty much what
    everybody uses as a PC board alternative to coax.
     
  12. Tom Bruhns

    Tom Bruhns Guest

    (Trimming the number of groups to one...no need to post it so
    broadly.)

    Have a look at some PCBs that route critical analog signals around.
    You'll likely find stripline, which is indeed TEM, that's guarded by
    closely-spaced vias on either side. The stripline can be single-ended
    or balanced. This is done much more for signal integrity (extremely
    low cross-coupling to other signals) than for speed.

    Transmitting high-speed signals doesn't necessarily need such fancy
    lines, but it does need low loss, and that is a problem with FR4 as
    the frequency goes up. Thus, people use Teflon, Duroid and the like.
    Have a look at PCB designs that deal with 10GHz and above.

    Cheers,
    Tom
     
  13. Ecnerwal

    Ecnerwal Guest

    And then there's the crude method - put in a coax jumper wire. Worth a
    thought, depending on the application.
     
  14. Joel Kolstad

    Joel Kolstad Guest

    I haven't tried it myself, but it's not entirely uncommon in microwave designs
    to do something like this -- it's essentially a stripline design with a
    "picket fence" worth of vias to serve as the sidewalls. I suspect the reason
    it isn't particular popular is that the performance isn't that much better
    than a stripline, the models for it aren't found in ADS/Microwave
    Office/Ansoft Designer, and the manufacturing costs may be higher. ("Maybe"
    because I've seen a lot of people who've started transitioning from microstrip
    to a co-planar waveguide, which requires a bazillion drill hits as well. You
    trade off the number of drill hits for isolation...)

    ---Joel
     
  15. Austin Lesea

    Austin Lesea Guest

    Gero,

    Why?

    Seems others have already progressed to the "abuse" phase, however I am
    curious.

    If microstrip, or stripline constrains the electric fields such that for
    all practical purposes the matched line does not radiate, why is a
    coaxial line any better? The only benefit of a coaxial line is that
    unmatched, it can not radiate (all the RF energy has to come out of the
    ends).

    Is this a concern for radiated emissions?

    Again, unmatched microstrip or stripline structures can be engineered
    with adjacent shielding such that EMI/RFI should not be an issue.

    Trying to create a coaxial guide by placing many vias and metal lines is
    just too ugly to even think about without a valid reason. Once the
    reason is known, the first or second approximation to the structure is
    probably completely adequate.

    Austin
     
  16. john jardine

    john jardine Guest

    Had trouble with crosstalk on a mass of video signals. Cured with a
    multilayer board where each signal was 'boxed in' by ground plane to the
    sides, above and below. Sort of square coax.
     
  17. Robert Baer

    Robert Baer Guest

    I would think that would be difficult and/or costly.
    Working from bottom to top:
    Start with (say) 6 mil laminate 0.5/0.5 at bottom; bottom layer has a
    narrow stripe to emulate the lowest part of a coax, and top part is a
    wider stripe: both ground (shield).
    Next layer (#2) is (say) 6 mil laminate 0/0.5 (ie bottom has no
    copper and top is 0.5 ounce); stripe is wider and is ground.
    "Middle" or next layer is (say) 6 mil laminate 0/0.5 (ie bottom has
    no copper and top is 0.5 ounce); 3 stripes: ground / center conductor /
    ground.
    Next layer is (say) 6 mil laminate 0/0.5 (ie bottom has no copper and
    top is 0.5 ounce); stripe is as wide as layer #2.
    Then use (say) 6 mil laminate 0/0.5 at top; where the top copper has
    a "wide" stripe same as first laminate top stripe as ground.
    Finish with (say) 6 mil laminate 0/0.5 at top; where the top copper
    has a "narrow" stripe same as first laminate bottom stripe as ground.

    Use more layers if they are thinner.
    Use vias liberally for tying the ground stripes together.
    Note the 6 mils is a wild guess.
     
  18. Robert Baer

    Robert Baer Guest

    He said "coax"...
     
  19. Robert Baer

    Robert Baer Guest

    Gee, coax cables, even those that use spiral teflon seperators, are
    like that...
     
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