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Recipe for creating microstrip filters?

Discussion in 'Electronic Design' started by Nico Coesel, Dec 22, 2012.

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  1. Nico Coesel

    Nico Coesel Guest

    I'm trying to create a microstrip filter from an elliptic filter
    schematic made out of inductors and capacitors. The problem is that I
    can't really find a description of a method on how to do this. What
    I've found so far is using the Kuroda identities but those lead to
    unfeasable thin traces. Another way I've seen is using thin traces
    where the inductance is dominant or wide traces where the capacitance
    is dominant to form the inductors and capacitors.

    I have been trying to get a 3rd order filter to simulate properly
    using Sonnet lite but so far no luck. I think I'm still missing a
    step. Does anyone know a book or a paper which has a clear recipe? Do
    it this way and it will be right (after a few tries)?
  2. Guest

    Does it really make any sense to try such transformation ? After all,
    there are different loss mechanisms in lumped LCR circuits and at
    least with low cost PCB materials when used for microstrip filters.

    Why not start from scratch and design a low pass filter followed by a
    band stop filter, possibly with an isolation stage (amplifier) between
    the sections ?
  3. Nico Coesel

    Nico Coesel Guest

    What I want to build is a 1.5GHz low pass filter with a reasonable
    sharp cut-off.
  4. Baron

    Baron Guest

    Nico Coesel Inscribed thus:
    Try "Puff" ! I haven't used mine for a few years but I seem to recall
    that it was ideal for this sort of thing. The manual had an example
    design for a filter.
  5. Baron

    Baron Guest

    Baron Inscribed thus:
  6. Tim Williams

    Tim Williams Guest

    Preface: I haven't designed a microstrip (or whatever) filter yet, myself.

    My impression of such filters is like this:

    Suppose you want a, say, 6 pole bandpass filter, very narrow. You need 3
    L's and 3 C's. The general design of such a filter is a parallel resonator,
    coupled to another parallel resonator, using a series resonator between (for
    a Pi design). A very sharp bandpass means the impedance of each resonator
    must be very different from the transmission line impedance, while the poles
    are kind of on top of each other (give or take pulling interactions). So
    the parallel resonators need a very low impedance to successfully shunt the
    line, while the series resonator needs a very high impedance to keep
    coupling to a minimum, except in the narrow frequency band where it's

    But with microstrip or what have you, it's very difficult to get such a
    large impedance ratio, so your filter Q (sharpness) is way down and you need
    more stages instead. This is not done with discrete components, because you
    can wind an arbitrarily good inductor, and one expensive inductor is better
    than matching three, smaller, custom inductors.

    As I'm sure you're already familiar with, the basic idea of microstrip (or
    whatever) is to alternate between high and low impedance segments, where the
    low impedance segments look like low-Z parallel resonators and the high
    impedance segments look like high-Z series resonators. Or vice versa.
    Using the impedance of a resonator as the corresponding quantity, it should
    be very easy to calculate a simple bandpass by trace widths, of course you'd
    need to model it to verify dimensions are correct and the poles are in the
    right place.

    A lowpass filter doesn't need large impedance ratios (high Q resonators), at
    least until the higher order poles. Getting a sharp corner could be
    challenging in that case, but using more stages always works.

    You can save on trace width by giving it some height over the ground
    plane -- you can cut out a hole to give the field some room, but I don't
    know how to calculate the cutout required. Would also kill EMC.

  7. Nico Coesel

    Nico Coesel Guest

    I figured out myself! I found another paper which had an example of
    using wide traces for capacitors and thin traces for inductors:

    I decided to follow the same path as the authors and see where that
    would get me.

    The next big problem was finding a piece of software which could
    calculate the inductance and capacitance of a copper strip to the same
    results as in the paper. That turned out to be harder than one would
    expect. Even the tool you can download from Rogers doesn't work
    properly! The problem is that there are two formulas and which one is
    right depends on the ratio between the width of the track and the
    height of the substrate. Most tools only work when the trace width is
    less than the height of the substrate.

    I was just about to give up when I found this web page. The microstrip
    calculator gives the same results as in the paper:

    Now I could start to translate the lumped element diagram I got from a
    program called SVCfilter ( )
    into a distributed one. I first calculated the track widths for a 3rd
    order elliptic low pass filter and simulated that with Sonnet Lite.
    The results where quite good so I decided to go ahead with the 7th
    order elliptic filter I actually want to build. I had to tweak the
    sizes a bit to get the resonating frequencies the same as given by
    SVCfilter. After that I used the sizes from the simulated layout for a
    PCB layout and tested it for real and it actually seems to work
    reasonably close to the simulation. I used 0.2mm lines for the
    inductors. That is about the limit of what I can etch myself so there
    is quite some tolerance on the final width of the tracks which
    probably contributes to the error.

    It looks kinda cool though:
  8. Nico Coesel

    Nico Coesel Guest

    Thanks :)
    I gave up on running Windows software in Wine. Too much doesn't work
    and the Wine crowd seems to focus on games and office. Instead I
    installed virtualbox and installed Windows in a virtual machine. There
    is a Windows license sticker on the Linux box so why not :)
  9. Guest

    Google for KK7B

    One of his design is at
    A bandpass filter section seems to have a Q of about 5.
  10. Nico Coesel

    Nico Coesel Guest

    I recall doing some simulations on those hairpin filters as an
    experiment a few years ago. It turned out that they won't work at all
    at 300MHz. Since then I acquired some equipment to be able to measure
    in the GHz region.
  11. Guest

    I am a bit confused, since I have always assumed that loaded Q,
    unloaded Q and insertion loss are all related.

    Googling for various kinds of resonators, there appears to be a huge
    number of papers how to make microwave resonators with Q _less_ than 5
    (apparently for some wide band services). Traditionally, the unloaded
    Q has been quite low due to the PCB material losses.
  12. Tim Williams

    Tim Williams Guest

    Right -- there are [at least] two "Q"s one could define in a filter
    circuit. The one that causes power loss* is the Q of the components
    alone -- simple resistive (or equivalent) losses. The other is the
    impedance of any given component in relation to the circuit impedance at
    that point -- typically, the line impedance. An inductor with a Q of 100
    and a reactance of 50 ohms at some frequency, connected to a transmission
    line of 50 ohms, has an overall Q of about 0.99 (i.e., about 1/100th less
    than 1.0).

    *It's total power loss, not insertion loss necessarily. When the
    insertion loss is high, reflected power is usually also high, so that
    power is (mostly) conserved. Consider a coupled resonator type bandpass
    filter: if the coupling is very low, bandwidth will be minimal, and
    insertion loss will be high. But power needn't be lost; the first
    resonator reflects the excess back. The total power reflected and
    transmitted is always less than incident, and this loss is due to
    component Q.

  13. Nico Coesel

    Nico Coesel Guest

    It was a long time ago but I recall the resonators started resonating
    at a multitude of 300MHz. Something like 1.2GHz (which makes sense). I
    used Sonnet for simulation. If the simulation results wouldn't be
    close to real life they probably would be out of business real quick.
    I'm not specifically working in a range :) At that time I choose
    300MHz because that was in the range of what my equipment could
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