LC filter design

Discussion in 'General Electronics Chat' started by feilip, Dec 22, 2009.

  1. feilip

    Thread Starter New Member

    Dec 22, 2009
    I was reading

    Under "Unexpected response of L-C low-pass filter" section I don't understand why the input and output impedance for the filter is z=sqrt(L/C). Can someone help me out?

    Also, it seems that for the "Parallel resonant band-stop filter", we dont need to worry about input/output impedance matching since we just connect the parallel LC filter to the voltage source and 1kohm resistor. Why we don't need to match it with sqrt(L/C)=100ohm resistor?

    Many thanks for your help! Sorry for the stupid question. I am new in LC filter design.
  2. Wendy


    Mar 24, 2008
    Something that I learned the hard way, the termination you feed into (output impedance) is a major part of the filter. It will affect the shape of the frequency response as well as the rolloff (or center frequency). It is important enough many designs will use an attenuator to improve this characteristics.

    You have to remember that reactance is a variable, determined by the frequency and the part. Since you have two parts their variable will interact.

    Are you familiar with the concept of return loss (or SWR)?
  3. feilip

    Thread Starter New Member

    Dec 22, 2009
    Thanks for the reply Bill!

    Yes, I am familiar with return loss or SWR. I agree with you, output matching is very important. However, in my design, I cannot include even 3dB attenuator to improve the return loss since the signal is not strong enough.

    I just want to find a way to properly determine the output/input impedance of the filter since the information from the website (the link that I included before) wasn't clear to me.