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What is microstrip?

Discussion in 'Electronic Basics' started by Dummy, Sep 12, 2003.

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  1. Dummy

    Dummy Guest

    What is microstrip? What does it do.
    I bet I've never seen one in my life before.
     
  2. John Larkin

    John Larkin Guest

    Bet? How much?

    A microstrip is a planar conductor on the surface of a dielectric,
    usually but not always with a big conductive ground plane on the other
    side, or embedded part-way in the dielectric. So every PC board
    surface trace is a microstrip, pretty much.

    Fast boards usually have an interior ground plane, and there are
    equations that let you figure out the impedance of microstrip traces,
    given the geometry and material properties involved; some of those
    equations are reasonably accurate, sometimes. Agilent's free Appcad
    program does this pretty well.

    Try something adventurous, like googling "microstrip." Do "stripline"
    and "embedded microstrip" while you're at it.

    John
     
  3. Bob Penoyer

    Bob Penoyer Guest

    Microstrip is a lot like a regular, run of the mill PC board. On a PC
    board, traces are typically laid out in whatever manner gets things
    connected as the designer might like. Things aren't so simple using
    microstrip.

    Microstrip typically uses a solid ground plane on one side of the
    board material with traces only on the other side. What makes
    microstrip distinctive is that the board material is very consistent
    so that its dielectric constant (in a simplistic sense, its insulating
    characteristic) is very predictable. On such a board, traces are
    constructed with particular widths so that they have a controlled
    characteristic impedance. That way, traces behave as transmission
    lines, something like coax cable.

    Clever things can be done with microstrip. It can be used as a
    transmission line as just described. By simply controlling its
    geometry, microstrip can be used to create filters, power splitters,
    power combiners, impedance transformers, couplers, planar antennas,
    etc. The dimensions of microstrip are dependent on the substrate's
    dielectric constant and the frequency of operation. The higher the
    frequency, the smaller the dimensions. For this reason, microstrip
    tends to be used at frequencies roughly from 1 GHz on up.

    If you've looked at an RF module with something that looked like a PC
    board but with some strange looking geometric traces, you've probably
    seen microstrip. To the uninitiated, micrstrip is a lot like black
    magic.

    Stripline is like microstrip except that stripline traces are
    completely contained inside the board (substrate) with ground planes
    on both outer layers.
     
  4. Chris Carlen

    Chris Carlen Guest


    How old are you? Might you consider (if of appropriate age) a college
    degree in EE? If you do that, or even if you take just the single
    relevant course that explains all this, then you will understand deeply.

    That course is one in electromagnetic field theory, based on calculus,
    which implies that you need a certain degree of calculus competence in
    hand as well before you take the course. In it you will be exposed to a
    thorough derivation of the wave equations that describe EM wave
    propagation on a transmission line, and you will learn through Maxwell's
    equations how to calculate the per unit length conductance, shunt
    conductance, inductance, and capacitance of typical transmission line
    geometries such as microstrip, and thereby calculate characteristic
    impedances, as well as get into all the fun of reflection diagrams,
    impedance matching, etc.

    Study it well, not just to get the answers but rather go exploring the
    topics in depth. Ie., get yourself a TI-92 calculator, or get on a PC
    with Mathematica or MatLab or something like that, so you can develop
    visualizations of the concepts, and you will not only ace the course but
    really get a "feeling" for what it all means. This understanding will
    revolutionalize your understanding of all aspects of electronics design,
    such as cabling signals, PCB design, and why wires aren't always just wires.

    If possible take an EM course that emphasized transmission lines, and
    treats it in the first part of the course, rather than at the end.

    Good luck!


    --
    _______________________________________________________________________
    Christopher R. Carlen
    Principal Laser/Optical Technologist
    Sandia National Laboratories CA USA
    -- NOTE: Remove "BOGUS" from email address to reply.
     
  5. John Larkin

    John Larkin Guest

    Do you use Maxwell's equations to calculate microstrip impedance?

    The last time I used calculus was in estimating a mosfet's switching
    power dissipation, maybe 5 years ago. It came out close to a quickie
    graphical estimate, so wasn't actually necessary at all.

    I did take a year's worth of field theory in college, and we finally
    got up to the full expression of Maxwell's equations (divergences,
    gradients, curls, all that nasty vector field stuff.) I remembered it
    just long enough to struggle through the final exam.

    John
     
  6. Do you have a link for it or maybe you could post it to A.B.S.E. ?

    I tried a search for it and all I could find was PDF's with reference to it.

    thanks!
     
  7. Chris Carlen

    Chris Carlen Guest

    Maxwell's equations are needed to derive the formaulas to determine the
    incremental (per unit length) R', G', C', and L' for a particular
    geometry. Once those formulas are in hand, then Maxwell's equations
    aren't needed anymore, unless you encounter a new geometry (which
    wouldn't be called microstrip of course, which is just one particular
    geometry.)

    The values of the incremental parameters plug into the characteristic
    impedance formula for a TEM transmission line to give the impedance.
    That formula can be derived from circuit theory applied to an
    incremental length of transmission line, which leads to the pair of PDEs
    that are the wave equations. Solution of the wave equations leads to
    the characteristic impedance relation.

    In practice, one needs only the formulas. But I consider all formulas
    suspect unless I work through the derivations myself. Also, if you are
    the sort of person that gets deep satisfaction out of the way
    mathematics so precisely and eloquently desribes physical phenomena,
    then the experience of going through the math is very fulfilling.
    That is often the case, for this sort of thing.
    What really impressed me was the transmission line theory and the plane
    wave propagation theory. I wish I had the time and patience to continue
    on with the plane wave stuff, because we stopped just before getting
    into the derivations of the Fresnel equations, and other optical issues
    which are of great interest to my laser work. Ultimately, I'd like to
    be able to understand plane waves in anisotropic media, which would
    reveal the inner workings of frequency doubling crystals and optical
    parametric oscillator crystals, and other non-linear optical and
    electro-optical stuff. But I don't know if one can get that far without
    tensors.

    I had to learn the vector calc. on the fly as I took the course, because
    I didn't have a formal class in it. But it wasn't difficult.

    Trouble with all this stuff is you do indeed loose fluency in the
    subject quickly without practice. But the overall impression and the
    fundamental understandings remain, and are far deeper than anything I
    could have gotten without the math treatment.

    Good day!



    --
    _______________________________________________________________________
    Christopher R. Carlen
    Principal Laser/Optical Technologist
    Sandia National Laboratories CA USA
    -- NOTE: Remove "BOGUS" from email address to reply.
     
  8. John Larkin

    John Larkin Guest

  9. Check It Out

    Check It Out Guest

    Tiny planes land there.
     
  10. John Larkin

    John Larkin Guest

    Pygmy exotic dancers.

    John
     
  11. When you can't change gears in your processor. ;-)
     
  12. John Larkin

    John Larkin Guest

    Small alley in Las Vegas.

    John
     
  13. My crow strip? Sort of like flypaper but larger.

    Cheers!

    Chip Shults
    My robotics, space and CGI web page - http://home.cfl.rr.com/aichip
     
  14. Thanks for the link! Google ... I should of known! I went to Agilent and searched there and I left a
    message for some support. I got this email back four times! (for a single inquire...)
    ***********************************************
    Hi ånønÿmøu§,
    Thanks for contacting us at Agilent Technologies.
    You can contact the following number to confirm it and the number is as follows:
    Product Purchase
    Hours:6:00am - 6:00pm MT, Mon - Fri
    Check prices or request a formal quotation, place an equipment, upgrade or retrofit order, verify
    delivery availability, check order status or expedite an order.

    Thanks and Regards,
    (***Name removed***)
    Phone: +1 (800) 829-4444
    FAX: +1 (800) 829-4433
    **************************************************
    Oh well! Thanks again!
     
  15. John Larkin

    John Larkin Guest

    Google is amazing. When I want a datasheet, instead of going to the
    mfr's web site, I usually just punch the part number into google and
    hop directly to the right page. I can't understand how this much data
    could be indexed as fast as it does... googling "lm2991" got the
    National product folder first hit, in about 2 seconds. I suppose it
    did about a million other lookups at the same time.

    I think google is like ebay: it only makes sense to have one auction
    site (why would you want to list on #2?) and I think the dynamics is
    that one search engine ultimately dominates.

    John
     
  16. Oof. I took "Fields, Matter, and Waves" as an undergrad. The main thing I
    remember was that it was a large classroom on the other end of campus from
    the class I had just before it, so I was always stuck somewhere way in the
    back of the room, panting from the across-campus sprint; and the professor
    had an almost incomprehensibly thick German accent, and a habit of drawing
    vector diagrams with colored chalk so that he could get a massive amount of
    information into each drawing, information which I could barely read from my
    distance and which I anyway had no good way of capturing in my notes unless
    I remembered to bring colored pens. So each class was just a blur for me.
    I got a "C", if I remember, which was uncharacteristically poor; and I've
    always wished since then that I understood the topic better. I keep
    thinking of trying to take it again, but of course now the calculus has
    faded too.
     
  17. Chris Carlen

    Chris Carlen Guest

    It is at this point that one would be wise to begin learning how to
    learn on one's own. There have been a few lecturers in my academic
    experiences who really had something to offer that went beyond the text.
    But most of the time I could ignore the lectures, and just study the
    text. Ultimately, at the graduate level, this is an essential skill.

    I am fortunate to work with folks who can pick up advanced texts outside
    their fields, and ina few weeks start functioning at a respectable level
    in the new material. Such as a bunch of mechanical engineers doing some
    pretty sophisticated spectroscopy. They are very impressive.


    Good day!
     
  18. Wow! I find just the opposite. I pick up far more from a
    lecture than by reading. In fact I do *very* well listening to a
    good lecturer, enough that I can aid the instructor in explaining
    the material. I get virtually nothing from a text alone. A good
    lecture will get me over the hump such that the text makes sense.

    Perhaps we're on opposite ends of the learning stick. ;-)
    Certainly. Thet would impress me! ..and I've known several like
    this. Sadly, I'm not one. However, a simple lecture does
    wonders.
    Indeed it was! ;-)
     
  19. gwhite

    gwhite Guest


    I find I do grounded co-planar wave guide (GCWG) more
    often these days, although as you know they are "close."
    GCWG seems convenient because using a standard ground fill
    gap creates it automatically. Microstrip requires special
    keep-outs or design rules if the top is ground filled.
     
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