Building Coaxial transmission line on PCB?

Discussion in 'VHDL' 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
    Geronimo Stempovski, Feb 12, 2007
    #1
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  2. Geronimo Stempovski

    John Fields Guest

    On Mon, 12 Feb 2007 12:10:43 +0100, "Geronimo Stempovski"
    <> wrote:

    >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.


    ---
    http://en.wikipedia.org/wiki/Microstrip


    --
    JF
    John Fields, Feb 12, 2007
    #2
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  3. Geronimo Stempovski, Feb 12, 2007
    #3
  4. "Geronimo Stempovski" <> wrote in message
    news:45d06e12$0$30327$-online.net...
    >
    > "John Fields" <> schrieb im Newsbeitrag
    > news:...
    > >
    > > http://en.wikipedia.org/wiki/Microstrip
    > >

    >
    > Microstrip has absolutely nothing to do with the coaxial structure I had

    in
    > mind.


    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
    Meindert Sprang, Feb 12, 2007
    #4
  5. Geronimo Stempovski

    Fred Bloggs Guest


    > 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?


    Nope- in all the decades of high speed PC circuit design, you are the
    first to think of it!

    >Is it possible with regular FR4, anyway?


    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.
    Fred Bloggs, Feb 12, 2007
    #5
  6. Geronimo Stempovski

    Henning Paul Guest

    Henning Paul, Feb 12, 2007
    #6
  7. Geronimo Stempovski

    John Larkin Guest

    On Mon, 12 Feb 2007 12:10:43 +0100, "Geronimo Stempovski"
    <> wrote:

    >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
    >


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

    John
    John Larkin, Feb 12, 2007
    #7
  8. Geronimo Stempovski

    John Fields Guest

    On Mon, 12 Feb 2007 14:39:30 +0100, "Geronimo Stempovski"
    <> wrote:

    >
    >"John Fields" <> schrieb im Newsbeitrag
    >news:...
    >>
    >> http://en.wikipedia.org/wiki/Microstrip
    >>

    >
    >Microstrip has absolutely nothing to do with the coaxial structure I had in
    >mind.


    ---
    Well, Mister Nasty-Ass, what exactly did you have in mind, then?


    --
    JF
    John Fields, Feb 12, 2007
    #8
  9. On 2007-02-12, Geronimo Stempovski <> wrote:
    >
    > "John Fields" <> schrieb im Newsbeitrag
    > news:...
    >>
    >> http://en.wikipedia.org/wiki/Microstrip

    >
    > Microstrip has absolutely nothing to do with the coaxial
    > structure I had in mind.


    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.


    --
    Grant Edwards grante Yow! NANCY!! Why is
    at everything RED?!
    visi.com
    Grant Edwards, Feb 12, 2007
    #9
  10. Geronimo Stempovski

    Joel Kolstad Guest

    "Geronimo Stempovski" <> wrote in message
    news:45d04b34$0$27624$-online.net...
    >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?


    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
    Joel Kolstad, Feb 12, 2007
    #10
  11. Geronimo Stempovski

    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
    Austin Lesea, Feb 12, 2007
    #11
  12. Geronimo Stempovski

    john jardine Guest

    "Geronimo Stempovski" <> wrote in message
    news:45d04b34$0$27624$-online.net...
    > 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
    >

    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.



    --
    Posted via a free Usenet account from http://www.teranews.com
    john jardine, Feb 12, 2007
    #12
  13. Geronimo Stempovski

    Robert Baer Guest

    Geronimo Stempovski wrote:
    > 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
    >
    >

    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.
    Robert Baer, Feb 13, 2007
    #13
  14. Geronimo Stempovski

    Robert Baer Guest

    John Fields wrote:

    > On Mon, 12 Feb 2007 12:10:43 +0100, "Geronimo Stempovski"
    > <> wrote:
    >
    >
    >>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.

    >
    >
    > ---
    > http://en.wikipedia.org/wiki/Microstrip
    >
    >

    He said "coax"...
    Robert Baer, Feb 13, 2007
    #14
  15. Geronimo Stempovski

    Robert Baer Guest

    Fred Bloggs wrote:

    >
    >> 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?

    >
    >
    > Nope- in all the decades of high speed PC circuit design, you are the
    > first to think of it!
    >
    >> Is it possible with regular FR4, anyway?

    >
    >
    > 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.
    >
    >

    Gee, coax cables, even those that use spiral teflon seperators, are
    like that...
    Robert Baer, Feb 13, 2007
    #15
  16. Geronimo Stempovski

    CBFalconer Guest

    Robert Baer wrote:
    > John Fields wrote:
    >> <> wrote:
    >>
    >>> 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?

    >>
    >> http://en.wikipedia.org/wiki/Microstrip
    >>

    > He said "coax"...


    Which is just one more way of implementing a line. As is a piece
    of wire in some medium.

    Ridiculous cross-post level reduced by setting follow-ups.

    --
    <http://www.cs.auckland.ac.nz/~pgut001/pubs/vista_cost.txt>
    <http://www.securityfocus.com/columnists/423>

    "A man who is right every time is not likely to do very much."
    -- Francis Crick, co-discover of DNA
    "There is nothing more amazing than stupidity in action."
    -- Thomas Matthews
    CBFalconer, Feb 13, 2007
    #16
  17. Geronimo Stempovski

    Bob Guest

    "John Fields" <> wrote in message
    news:...
    > On Mon, 12 Feb 2007 14:39:30 +0100, "Geronimo Stempovski"
    > <> wrote:
    >
    >>
    >>"John Fields" <> schrieb im Newsbeitrag
    >>news:...
    >>>
    >>> http://en.wikipedia.org/wiki/Microstrip
    >>>

    >>
    >>Microstrip has absolutely nothing to do with the coaxial structure I had
    >>in
    >>mind.

    >
    > ---
    > Well, Mister Nasty-Ass, what exactly did you have in mind, then?
    >
    >
    > --
    > JF


    How DARE you not know what he had in mind!

    Bob
    Bob, Feb 13, 2007
    #17
  18. Geronimo Stempovski

    John Fields Guest

    On Tue, 13 Feb 2007 04:55:03 GMT, Robert Baer
    <> wrote:

    >John Fields wrote:
    >
    >> On Mon, 12 Feb 2007 12:10:43 +0100, "Geronimo Stempovski"
    >> <> wrote:
    >>
    >>
    >>>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.

    >>
    >>
    >> ---
    >> http://en.wikipedia.org/wiki/Microstrip
    >>
    >>

    > He said "coax"...


    ---
    No, he said: ...:"as much like a coaxial cable as possible.", which
    I took to mean electrically. Otherwise, why not just use a piece of
    micro-coax and treat it like just another component on the PCB?


    --
    JF
    John Fields, Feb 13, 2007
    #18
  19. Geronimo Stempovski

    Uwe Hercksen Guest

    On Mon, 12 Feb 2007 12:10:43 +0100, Geronimo Stempovski
    <> wrote:

    > 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?


    Hello,

    the technology used to build multilayer PCB does not allow to build a
    round coaxial transmission line.
    You may have structured planes of copper foil separated by insulation
    material, but nothing like the shield of a coaxial cable. Even a
    rectangular shield around a center conductor is not possible with the
    existing methods to build multilayer PCB.

    bye
    Uwe Hercksen, Feb 13, 2007
    #19
  20. Geronimo Stempovski

    Uwe Hercksen Guest

    On Mon, 12 Feb 2007 22:30:39 +0100, john jardine
    <> wrote:

    > 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.


    Hello,

    but how about a real closed square shield around the center conductor?

    Bye
    Uwe Hercksen, Feb 13, 2007
    #20
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