LC filter can by as 10KOhm resistance?

Discussion in 'General Electronics Chat' started by levnu, Oct 16, 2011.

  1. levnu

    Thread Starter New Member

    Mar 30, 2011
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    Hi to all

    is it possible to replace 10KOhm resistor w LC parallel or serial filter or any filter
    that will be the same as if this 10 KOhm resistor?

    i'm using 10KOhmic res and i would like to achive the same effect w/o using resistance that brings me loses!!!
    but using filter at 15MHz freq

    Thanks a lot,
    Arye
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,449
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    You can calculate the reactance of L and C as follows:

     X_L= 2\pi fL

     X_C= \frac{-1}{2\pi fC}


    For a tuned parallel LC circuit, the resonant frequency f is given by:

     f= \frac{1}{2\pi sqrt{LC}}
     
    Last edited: Oct 16, 2011
  3. levnu

    Thread Starter New Member

    Mar 30, 2011
    14
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    Ok this one i know
    but when i connecting it instaed of 10KOhm resistance it doesn't have the same stoping effect.. why ??
    when i'm in resonance what impedance is equal in such case ?
    and is it the same effect as 10KOhm?

    Thanks
     
  4. MrChips

    Moderator

    Oct 2, 2009
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    Looks like you will need 0.56pF and 200uH.
     
  5. Adjuster

    Well-Known Member

    Dec 26, 2010
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    The resonant impedance would depend on the reactance of the components, and the Q-factor: information which we do not have.

    For instance if C=50pF, at 15MHz Xc = 212Ω, and for a Q of 50 you would expect a parallel equivalent resistance at resonance of 10.6kΩ

    But we don't know your circuit values.
     
  6. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Is that meant to be a joke? The OP may take it literally and go looking for a 560fF capacitor. He may be unaware that a real-world 200μH inductor could have parasitic capacitances greater than this value.

    Edit: A few μH would be more realistic, the optimum value depending somewhat on how good a Q could be obtained, and how much parasitic capacitance you have to contend with.

    At 15Mhz, don't expect miracles on the Q front. You might get past 10kΩ, just have to try it and see.
     
    Last edited: Oct 16, 2011
  7. levnu

    Thread Starter New Member

    Mar 30, 2011
    14
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    i'm using 0.7uH and 200pF what do you think about that,
    it's important for me to use low inductance..

    Thanks again,
    Arye
     
  8. Adjuster

    Well-Known Member

    Dec 26, 2010
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    I think that with an inductance as low as this, the resonant impedance may be fairly low. At 15mHz XL = 2πfL = 66Ω.

    Achieving a resonant parallel resistance of 10kΩ would require a Q of 152. I'm not sure if you can get that at 15MHz.
     
  9. MrChips

    Moderator

    Oct 2, 2009
    12,449
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    Just checking to see how many folks pay attention to details or if they're even awake when they surf this forum.
     
  10. levnu

    Thread Starter New Member

    Mar 30, 2011
    14
    0
    does it means there is no way or other filter w higher order that i could achive this impedance? or ohmic resistance that i still dono if its the same in this situation..... 10Kohm resistance or 10 Kohm impedance does it mean the same in my situation?

    Arye
     
  11. Papabravo

    Expert

    Feb 24, 2006
    10,163
    1,796
    You could only achieve the same impedance as the resistance at a single frequency. The 10K resistor is not dependent on frequency until you get to VHF and above where parasitics start to become significant.
     
  12. Adjuster

    Well-Known Member

    Dec 26, 2010
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    I've told you what I can. Others, active radio hams or people who work with RF may have a better idea of what is / is not practicable. In the end it may be as well just to try it and see what you get.

    Obviously the resonant impedance of a tuned circuit is not identical to a resistor. That may or may not make a difference in your application.

    Unfortunately, it's now after midnight my local time, so that's all I have time for. Good luck.
     
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