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R-L-C structures on PCB

Discussion in 'CAD' started by Melanie Nasic, Dec 20, 2005.

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  1. Hi,

    recently I was assigned with the job to design a Printed Circuit Board (PCB)
    which does a kind of analog signal formatting through on-board designed
    capacitances, inductances and resistances. These components must be designed
    not as discrete components (SMD, etc.) but deliberately as stray
    capacitances in the form of specially layouted transmission line segments
    and 3D arrangements. The material of the PCB is predefined to be FR4.
    Surely I can design the circuit with tools like PSpice etc. but when it
    comes to design the calculated R, L, C values on the PCB I'm out of luck
    with my tools. So my question is: Is there a tool (Mentor?) which contains
    both circuit simulation AND PCB layout where I can design my defined
    capacitances, inductances and resistances in the form of specially routed
    transmission lines? What would be the best way to perform that task, anyway?
    In my opinion it must be an iterative process. Has anyone experience in this
    particular application field? Any help, suggestions and examples (maybe)
    would be appreciated.

    Many thanks in advance and best regards,

    Melanie
     
  2. Melanie,
    I am assuming that you are dealing with microwave frequencies, right?
    If so, what you need is an RF design tool, not just a basic analog
    simulator. For my money, take a look at Microwave Office
    (www.mwoffice.com) and get the free demo for a month. It is a lot more
    intuitive and usable than many more expensive packages, and a few
    friends of mine work there!

    Charlie
     
  3. Leo Baumann

    Leo Baumann Guest

    Hello Melanie,

    up to 2 or 3 GHz it's possible to compute by hand. It works good if i use a
    thickness of PCB of 1 mm.

    Computation-formulas i've found in ISBN 3-723-6545-0, but only in german
    language.

    regards

    baumann engineering
     
  4. Hi Melanie,
    Many years ago I designed a few CAT V components with such printed
    capacitors. I tested two types:

    1. Two copper planes (SMD-pads from stack;-) on each side of the board (we
    used only doublesided FR4 1.6 mm)
    2. traces with vias in a matrix

    ---O O O O O
    X X X X ...
    ---O O O O O
    X X X X ...
    O O O O O
     
  5. Joerg

    Joerg Guest

    Hello Melanie,

    Just FWIW: R will be a problem here. For controlled resistor values you
    would have to deposit carbon and laser trim. I have never done that on
    FR4 but a lot on alumina (thick film hybrid).


    I still do that by hand. Sometimes I even use the old slide rule calculator.

    Regards, Joerg
     
  6. linnix

    linnix Guest

    Just wondering why? SMD R & C are so cheap and light weight. PCB area
    is more expensive than the parts.
     
  7. Leon

    Leon Guest

  8. qrk

    qrk Guest

    You might find something of interest on this site:
    http://www.rfcascade.com/

    HP (Agilent) has a suite for RF design. I don't know if this will
    cover your needs.
    http://www.hp.woodshot.com/
     
  9. Joel Kolstad

    Joel Kolstad Guest

    These components must be designed
    Pretty much all the "big boys" of RF design software do this: They contain a
    frequency domain simulator that has some reasonably good models of
    microstrips, spiral inductors, etc. and easily link up with a field solver for
    when you need a more detailed answer. I.e., Agilent's ADS does this, as does
    Eagleware Genesys, Sonnet, Ansoft's RF Designer, etc.

    Since you're just starting out, I'd suggest you go over to Sonnet and download
    a copy of Sonnet Lite for free (http://www.sonnetsoftware.com/products/lite/).
    I believe they're the only one of the companies listed above that has ANY free
    offerings. Agilent and Ansoft, however, will hook you up for next to nothing
    if you're part of a Real University and either are or can get a professor to
    contact the appropriate people. Eagleware isn't quite as generous in my
    (limited) experience; I don't know about the others.

    If you're buying any of these packages for "professional" usage they all come
    in a dizzying number of different configurations and start with 4 digit price
    tags that can rapidly creep up to 5.

    One final note: Although all these RF simulators will let you do "PCB layout,"
    they're really not optimized for the generic layout task. What most people do
    is use the RF simulators to design just the, uh, critical RF portions and then
    export the copper layers into a more conventional PCB package such as PADS,
    Protel, etc. Most RF things these days hit a bunch of digital circuitry
    sooner or latter, and you really don't want to use ADS to layout some 676 ball
    FPGA with hundreds of digital I/O lines coming off of it. :)

    ---Joel Kolstad
     
  10. John  Larkin

    John Larkin Guest

    Puff is an old, free (not sure if it's still free) program that lets
    you simulate planar transmission-line-based filters

    http://www.its.caltech.edu/~mmic/puff.html

    And there's also Sonnet Lite, the free version of the Sonnet e-m
    simulator.

    A bit of googling should turn up the basics of planar
    transmission-line-based filters.

    John
     
  11. Hi Marte,

    thanks for your response. Some questions still:
    How did you do this? I mean, if I calculated the needed capacitance, how
    would I get the corresponding PCB layout for that?
    Again, what was your approach to model the layout in maths? Sorry, if I bore
    you but I never did it before and thus I have no experience on that topic.
    For a seminar work at university I have to suggest a compensation network on
    PCB for a certain case. I don't want any of you to do my work, I just want
    to comprehend how to get there. :)

    Bye Mel
     
  12. I thought about Mentor Hyperlynx since the university has some Montor
    licenses but I am not quite sure if this tool is exactly what I need? Any
    suggestions / experiences?

    Bye Mel
     
  13. John  Larkin

    John Larkin Guest

    No, I can't afford Mentor products. They did buy out PADS, which we
    use, and I'm wondering what they'll do with that.

    John
     
  14. Jim Thompson

    Jim Thompson Guest

    They'll give it the same treatment that Cadence gave PSpice ;-)

    ...Jim Thompson
     
  15. John  Larkin

    John Larkin Guest

    You mean the Autocad/GenericCad maneuver?

    John
     

  16. Hello Melanie,

    I am sure that nobody wants to make R with the PCB-Traces.
    It may be Z(transmission line impedance) but never R.
    L and C can be stubs for example or other odd shapes
    along the "main" trace.

    Please give a clear example(with numbers) what you have to design.
    All the postings so far are pure speculations about your real task.
    It's your turn to make the picture more clear.

    Best regard,
    Helmut
     
  17. Ken Smith

    Ken Smith Guest

    I'm sure that isn't true.
     
  18. Reg Edwards keeps some useful utilities for this kind of stuff on his
    website. Check out this page and see if it's covered...

    http://www.btinternet.com/~g4fgq.regp
     
  19. It sounds totally impractical to me, at any rate, to try to make R
    from copper cladding.
    Anyway, I don't think the OP has thus far actually even told us the
    frequency of interest!
     
  20. Hello Ken,

    I am interested if there are other applications.
    On what application do you think except current sensing?

    Best regards,
    Helmut
     
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