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LC notch filter not working!

Discussion in 'Electronic Basics' started by [email protected], Apr 26, 2007.

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  1. Guest

    Hi,

    I am trying to filter out a 13.56 MHz signal (and if possible I would
    want to filter some
    of its harmonics in succeeding circuits).

    I have tried a LC parallel resonance circuit put in series with the
    load. In theory
    the impedance of the LC parallel circuit becomes infinite at resonance
    frequency, i.e.
    the circuit becomes open.

    I used a fixed inductance L=10uH and a variable C, i.e. a trimmer to
    get the product
    L*C = 1 / (2*pi*13.56MHz)^2 right. C should be approx 14 pF, however,
    due to +/-20%
    tolerances in L, I use a trimmer.

    However, I can turn the trimmer (in the range from 10 to 20pF) as much
    as I want and
    I don't see ANY effect at all on my scope.

    Any hints ? What am I missing ?

    I have also looked at active notch filters, but this seems to be
    rather difficult at these
    high frequencies (see http://focus.ti.com/lit/an/slyt235/slyt235.pdf).

    Thank you!!
     
  2. Please learn to cross-post, it makes everyone's life easier when you want to
    cover the topic in more than one group.

    What kind of termination do you have on the filter?
     
  3. Well no.

    Learn to figure out the most appropriate newsgroup, and post there.
    There is rarely any good reason to cross-post, it is usually the sign
    of someone too lazy to find the best place, or too clueless.

    Just because cross-posting is better than multiple posting does not
    make cross-posting a good thing. It's just better than multiple posting.

    And multiple posting is just as much a mark of laziness or cluelessness
    as cross-posting.

    This is a .basic question, not a .design question (I assume that's where
    else the guy posted to, you and the rest have made it such a cesspool
    with your cross-posting and non-beginner questions in there that I have
    stopped reading it).

    ANd once again, Mark Zenier's guide to the sci.electronics.* hierarchy
    explains, including a bit about how cross-posting is better than
    multiple-posting, but not by much:
    ftp://ftp.eskimo.com/u/m/mzenier/seguide9706.txt

    Michael
     
  4. Oh gawd here we go again. :-( Are you a person or a bot? Look, I don't
    care what your opinion on this matter is. I don't know how to make it any
    more clear Michael, but GFY comes to mind.

    Do you have anything TECHNICAL *OR* ON TOPIC to add to this thread?
    I really hadn't noticed you missing, but then you don't offer much in the
    way of technical stuff anyway. The cesspool that exists there now is
    primarily a result of trolls and netkops like yourself, not from people
    x-posting electronics questions.

    So you consider LC filters and reactive impedance a .basics thing huh? If
    so, why don't you add something since we're here?
    Have you actually bothered to read it yet? I certainly have.
    Building/debugging LC filters *do* constitute design AND x-posting is ok in
    this context, should I post the appropriate excerpts?

    After seeing MZ himself tell you to give it a rest, I figured you would stop
    with this. I guess I figured wrong. You're not going to change my beliefs
    on this matter, so can't you just PLEASE give it a rest? OTOH if I'm still
    not making myself clear, I'll be happy to choose verbiage more to the point.
     
  5. John Fields

    John Fields Guest


    Try this: (View in Courier)

    VIN>-----+---->VOUT
    |
    [L]
    |
    [C]
    |
    GND>-----+---->GND
     
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