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Designing LC filters for amplifiers?

Discussion in 'Electronic Basics' started by billcalley, Jul 29, 2007.

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  1. billcalley

    billcalley Guest

    Hi All,

    Something has me stumped, and it's pretty fundamental: If I'm
    designing an LC bandpass filter for the output of my narrow-band
    amplifier, how could I possibly do this with any accuracy whatsoever,
    considering that when I design each separate stage I must initially
    "assume" that each will see an infinitely wideband and perfect 50 ohm
    match at both their input and output ports. But this is certainly
    *not* what the amp and the filter will see when they are cascaded
    together, since the stopbands of the filter (and the amplifier) will
    be anything *but* 50 ohms. This would dramatically (I would think!)
    change the response of the filter near and in the stopbands, as well
    as affect the amplifier's stability and gain.
    How could anyone even remotely be able to take this into account
    in the initial design stage? Perhaps I'm completely wrong about this
    being an issue at all? I realize we can cascade and then tweak the
    stages after designing them, but I would like to know how to take this
    into account before that -- or if I even have to?

    Thanks!

    -Bill
     
  2. Leon

    Leon Guest

    It's much easier to use a filter synthesis program. I use ELSIE:

    http://tonnesoftware.com/elsie.html

    Leon
     
  3. billcalley

    billcalley Guest


    But Leon, that doesn't negate the problem at all: The filter's
    stopbands will still interact with the amplifier's own limited
    bandwidth. I.E: Neither stage will see 50 ohms at ALL frequencies, as
    they were designed to see (in ELSIE), and as they were individually
    simulated with in a standard linear simulator (with the simulator's
    own infinitely wide 50 ohm in/out ports)...

    -Bill
     
  4. A passive filter cannot shape response and also provide
    impedance matching. It provides a frequency response by
    producing impedance mismatch. This mismatch prevents the
    passage of power at some frequencies.
     
  5. A passive filter can be equipped with hybrids and the input and output. So
    it will look like more or less nominal impedance in the range of
    frequencies.


    Vladimir Vassilevsky
    DSP and Mixed Signal Consultant
    www.abvolt.com
     
  6. John Larkin

    John Larkin Guest

    There are passive filters that present a nearly-constant input
    impedance across a wide, or theoretically infinite, frequency span.

    Jeroen Belleman has done some nice work on constant-resistance lowpass
    filters, and posted about it here some time ago. His CERN papers are
    probably still available on the web.

    John
     
  7. Do those constant resistance filters include a dummy load
    where all the rejected signal energy goes?
     
  8. Jamie

    Jamie Guest

    Wouldn't a unity gain buffer amp solve that issue for the input impedance?
    just guessing.
     
  9. John Larkin

    John Larkin Guest

    Certainly; they have to. The trick is to get the proper filter
    response and a constant input resistance without a bazillion parts.

    Picosecond Pulse Labs and some others make constant-R lowpass filters,
    so you can stick a filter in a coaxial line and not generate awful
    reflections.

    John
     
  10. John Larkin

    John Larkin Guest

    Sure. Usually when an amp drives a filter, the filter's impedance
    variation isn't an issue. But sometimes you need a passive,
    non-reflective filter, especially when you get into the GHz range
    where the buffer amp starts being a problem.

    John
     
  11. John Larkin

    John Larkin Guest

    This was the problem with the classic image-parameter filter design
    methods, assuming that each section was uniformly 50 ohms. "Modern"
    filter design is done by computers, makes no such assumptions, and
    works in ways I don't understand but appears to involve a lot of
    fiddling.

    A good filter book, Williams maybe, has tables of computer-generated
    normalized filters and procedures for bandpass transformation and
    scaling. That seems to work.

    John
     
  12. billcalley

    billcalley Guest


    But if you design a single 50 ohm bandpass LC filter (ideal) in
    a modern filter synthesis program -- and then duplicate it to make two
    filters -- and then cascade these two duplicate filters together in
    series, the graphed frequency characteristic will now be very FUBARD
    as compared to the single filter. Could someone try this also and see
    what they get? The filter stopband's high VSWR characteristic really
    interacts with the next stage, since both filters "expect" to see only
    50 ohms from DC to infinity. Hasn't anyone else noticed this effect?

    -Bill
     
  13. John Larkin

    John Larkin Guest

    Yup. That's why a higher-order filter doesn't look like a string of
    low-order filters. So don't do that.
    I haven't noticed it, because when I want an Nth order filter, I look
    it up in a book.

    John
     
  14. Can the same thing be done for the filter output, so that it
    has a constant resistance, also? That would make possible
    cascadable stages, though I am sure the combinations would
    not be minimal.
     
  15. John Larkin

    John Larkin Guest

    Seems like it should. Just apply the same techniques to both ends.

    John
     
  16. Tom Bruhns

    Tom Bruhns Guest

    So, simulate it in Spice with some good approximation of the
    impedances it WILL see, and see how bad things are. Or, perhaps how
    good. It may surprise you.

    For the work I do, op amp amplifiers have pretty well characterized
    output impedances, and I can design the following stage to have the
    desired input impedance, at least over a wide enough frequency range
    that it's not a problem. What I generally find is that even with
    mismatch from the design impedance, things don't go too far out of
    whack.

    Someone noted that filters have poor return loss outside their
    passband. That's not necessarily a bad thing, depending on the design
    goals, but if you don't like it, implement a "diplexing" filter that
    passes the signal from one port to another in its passband, and
    outside the passband, it passes the signal to a load resistance. In
    that way, a filter terminated in 50 ohms (for example) can look like
    50 ohms at its input over a much wider range than just its passband.

    Cheers,
    Tom
     
  17. Guest

    There are tables for passive LC filters where the source and load
    resistance are not equal. Thus I don't understand your statement.
     
  18. Guest

    Hi,
    You can try with active filters using Op Amps with Op-Amps
    separating the stages
     
  19. Mark

    Mark Guest



    They are also called DIPLEX filters, they have complimentary filters
    where the undersired power is routed to a dummy load instead of being
    reflected back to the source.
     
  20. John Larkin

    John Larkin Guest

    The discussion is about out-of-passband (broadband) impedance
    matching, not about the filter's characteristic impedance. Standard LC
    filters *work* when connected to the appropriate resistive sources and
    load, but themselves don't appear as pure resistors to those sources
    and loads. With more work, they can be designed to be nearly pure
    resistances.

    John
     
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