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Building Coaxial transmission line on PCB?

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

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  1. CBFalconer

    CBFalconer Guest

  2. werty

    werty Guest

    ----------------------------------------------------------

    Boxed ! the wavelength is far greater than
    your dimensions , thus higher modes can not
    exist , thus you do NOT need sides .
    When you reach 10 Ghz , then maybe
    you need sides in ur boxed "coax" .

    But the big joke , is in the real world ,
    they use cheap PCB to xmit 2.5 Ghz .
    No strip line , no microstrip , nada ..
    It works well , so quit arguing reality .

    BTW , i saw some novice , trying to
    use juice cans to launch WiFi .
    He figured the more cans , the more
    gain . He had 3 cans , T'd .
    to divide the power .
    Gain is not in cans , its in size of
    the dish .

    Another book worm said all i needed
    was $26 for 100 meters of blah blah
    coax at 2.5 Ghz ..

    10 times that price !
    and 1.8" dia hard line !

    At these wavelengths , its lower loss
    to send it TEM and thru the air ,
    not thru a coax .

    This is goin to FPGA ? Do those relics
    still exist ?! Oh well , i supose ya gotta
    try to "protect" your firmware by reinventing
    the CPU !
     
  3. Bob

    Bob Guest

    How DARE you not know what he had in mind!

    Bob
     
  4. Tom Bruhns

    Tom Bruhns Guest

    Ah, yes, reality. The reality is that we do need the vias for our
    work. I can imagine that John J. may well also. When you're aiming
    at 100+dB isolation among traces, you do have to be pretty careful,
    even at "low" frequencies.

    Cheers,
    Tom
     
  5. <snip>

    Seems video distribution is not one of your areas of expertise ?

    Note that John was talking about crosstalk, in the analog domain,
    and yes, what he did certainly will have a measurable improvement, and
    is somewhat 'industry practise' when minimising crosstalk.
    Note he says 'cured', that means he is on both sides of the problem,
    and it is a brave (or something else?) person that counters such direct
    experience

    I imagine in extreme digital cases, such as where you are worried not
    only about sending the clock, but also about the ps/fs of jitter, then
    this design approach would also help : it's not hard to do.

    -jg
     
  6. John Fields

    John Fields Guest

     
  7. Uwe Hercksen

    Uwe Hercksen Guest

    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
     
  8. Uwe Hercksen

    Uwe Hercksen Guest

    Hello,

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

    Bye
     
  9. John  Larkin

    John Larkin Guest

    When you need, say, 40x the crunch power of a decent DSP processor,
    those relics come in handy.

    John
     
  10. John  Larkin

    John Larkin Guest

    I do stuff down to a few ps RMS jitter on a regular 6 or 8-layer
    board, microstrip traces, with switching supplies and uPs and display
    drivers on the same board. Picoseconds aren't tough these days.

    John
     
  11. Thanks for your help, so far. Really appreciate it. I'm already a step
    further in my considerations.

    Now I'm looking for a diagram like frequency (some MHz to 10 GHz for
    example) versus loss tangent and / or epsilon R for FR4. I only found a poor
    black-and-white copy from 1991 in a paper which I searched with Google. I
    wouldn't have thought it to be so hard to find a graph... Does anybody know
    where I can find that?
     
  12. john jardine

    john jardine Guest

    Would have been ideal. At the time was thinking about a method to do this
    and sorted a setup that may have been worth talking to the PCB people about
    but a customer was paying to clear down his urgent problem and not to start
    up a research project :).
    No ... I don't remember what I figured out. Ideas are easy, it's the
    implementation that's a problem :).
    No doubt it'll surface again if I'm I'm under pressure.
    john
     
  13. To do that would need plated slots, and slots have the PCB fabs
    luke-warm at best : hard to do cleanly, they also weaken the PCB if
    long, and also have minimum router sizes, plus machine time......

    A better direction would be thinner laminates, and using the space you
    would have lost to the slot anyway, as wider GND webs on the same plane,
    coupled with stitching vias (which can be smaller dia than slots)

    -jg
     
  14. john jardine

    john jardine Guest

    [...]
    Que?.
    Don't listen to the worms.
    Video comes in forms other than digital.
     
  15. Uwe Hercksen

    Uwe Hercksen Guest

    Hello,

    I am sorry to tell that, but for frequencies of 1 to 10 GHz, FR4 ist not
    the right material, there are other PCB materials which are better for
    high frequencies, take a look here
    http://www.andus.de/Leiterplatten/Impedanz/hfmat.htm
    They write there that FR4 may be used up to 4 GHz.

    Bye
     
  16. John  Larkin

    John Larkin Guest


    FR4 can be used at 20 GHz, depending on what you're trying to do.

    John
     
  17. Del Cecchi

    Del Cecchi Guest

    Also you need to narrow down which variety of FR4 you are interested in.
     
  18. John  Larkin

    John Larkin Guest

    FR4 is a commodity that's not well controlled. There really aren't
    controlled varieties, and most pcb houses don't guarantee they'll
    always use the same stuff. If you want loss or Er consistancy, you
    usually have to call out something more specific. Or design so that it
    doesn't matter.

    John
     
  19. Again, I'm looking for a diagram like frequency (some MHz to 10 GHz for
    example) versus loss tangent and / or epsilon R for FR4 or other usual PCB
    material. I only found a poor black-and-white copy from 1991 in a paper
    which I searched with Google. I wouldn't have thought it to be so hard to
    find a graph but as noone replied to my previous question so far it does
    seem to be hard! :)

    Does anybody know where I can find that?

    Regards Gero
     
  20. John  Larkin

    John Larkin Guest

    Google gives lots of hits on stuff like "loss tangent fr4 frequency"

    But FR-4 varies a lot, so there's no definitive data.

    What are you trying to do?

    John
     
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