Notch Filter @ 20Mhz

Discussion in 'Wireless & RF Design' started by kevinraphael, Aug 26, 2014.

  1. kevinraphael

    Thread Starter New Member

    Apr 28, 2014
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    Hi all,

    I would like to build a notch filter at 20MHz with a -6db bandwidth of about 200khz and a centre frequency depth of about 40db. I have tried with the conventional LC topology but the results is not good. Mainly due to the inductors' low Q.

    I did came across some other topology such as crystal notch filter, waveguide & cavity filters. However, I do not have much knowledge about them and having trouble gathering information from websites. I would really appreciate it if anyone can help shed some light regarding this issue.

    If I am to pursue with this project, which topology should I choose? Thanks!
     
  2. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    BR-549 likes this.
  3. kevinraphael

    Thread Starter New Member

    Apr 28, 2014
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    Hi Eric,

    I am currently using ADS by Agilent Technologies(Not sure whether you are familiar with it or not). I have ran quite a few simulation using Coilcraft inductors. Currently I would like to pursue with a different topology, do you have any idea regarding the other topologies that I mentioned above?
     
  4. alfacliff

    Well-Known Member

    Dec 13, 2013
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    a series LC filter would work, but there is another if you are looking for higher power, an open 1/4 wave coaxial trap. center to what you want trapped, and shield grounded at the same place. use the formula for 1/4 wave and decrease length by the velocity factor of the coax. leave it a little long, and trim the open end for maximun effect.
     
  5. kevinraphael

    Thread Starter New Member

    Apr 28, 2014
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    Hi Alfacliff,

    I tried with LC, but the notch is only a mere depth of 8db, a far cry from 40db. Since the centre frequency is 20Mhz, will the wavelength be too long?
     
  6. alfacliff

    Well-Known Member

    Dec 13, 2013
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    the wavelengh at 20 mhx isnt very long, and a quarter wavelength is even shorter. ifusing coax, look up the specs for the coax used, the "velocity factor" is the amount of shortening even more. the quarter wavelength is aproximatly 11 ft. I dont know the velocity factor, but it is probably near .85 a piece of rg144 coax of that length would take up very little room. for a trap, the unused end of the coax is left open, making a low impedance at the resonant length.
     
  7. kevinraphael

    Thread Starter New Member

    Apr 28, 2014
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    sorry man. I wish I could understand what you are trying to say. My knowledge regarding ifusing coax is zero. Is there any good reference that can help me to understand u better? Btw, last I count, the quarter wavelength would be 3.75m. Correct? Based on what you say, if I am to build this trap thing, I will be able to get a Notch filter @ 20MHz? The depth at the rejection band is what I am concerned about.
     
  8. Lestraveled

    Well-Known Member

    May 19, 2014
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    How stable do you need this filter to be? A quarter wavelength notch filter at that frequency would be pretty temperature sensitive. (Copper likes to expand and contract with temperature.)

    May I suggest that you look at microcircuits.com for filters in this range.

    There are a lot of good RF filter houses out there.

    Oh yea, I forgot to mention, a filter that has a 10% bandwidth (of the center frequency) is easy to build/buy. You are asking for a 1% BW. It will not be an easy task nor will it be cheap.
     
    Last edited: Aug 27, 2014
  9. kevinraphael

    Thread Starter New Member

    Apr 28, 2014
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    Hi Lestraveled, I would like it to be as stable as possible, for testing purposes. Yes I understand that it would be quite expensive. Heard that a notch filter @ 500MHz cost about 1.5k USD. Since we are dealing with low frequency, I thought it would have have been cheaper.

    Any suggestion if I would like to build instead of purchasing one? LC topology is definitely out already. Thanks!
     
  10. The Electrician

    AAC Fanatic!

    Oct 9, 2007
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    http://www.hb9amo.net/hfcoaxialfilter.php

    http://www.hb9amo.net/fmcoaxialfilter.php
     
  11. Lestraveled

    Well-Known Member

    May 19, 2014
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    Your requirement of a 1% bandwidth at 20MHz is a tough spec to meet. There is a filter house I used to use, KRfilters.com. They make a 23.75 MHz lumped element notch filter (PN KR2409-3). This filter, at 6dB, has a 5 MHz BW and costs between 100 and 200 dollars. You are looking for a filter that is over ten times as tight as this one.
    I would suggest that you look at crystal filters. 20MHz crystals are available and cheap. The crystal filter circuits are on the net. There is probably a book on crystal filters that would help you a lot.
     
    Last edited: Aug 29, 2014
  12. alfacliff

    Well-Known Member

    Dec 13, 2013
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    for a crystal filter you would need several crystals staggared in frequency to get the mentioned 200 khz bandpas.
     
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