RF Notch Filter

Discussion in 'The Projects Forum' started by lew247, Feb 15, 2013.

  1. lew247

    Thread Starter Member

    Apr 23, 2010
    15
    0
    Can anyone help with a circuiot please?
    I'm trying to recieve the weather satellites around 137Mhz, but VERY close to me are pagers on both 138Mhz and 153Mhz and they totally swamp the wanted signals out

    I want to build a notch filter that will notch the following frequencies (make them inaudable)
    137.175Mhz with a 200Khz bandwidth
    138.850Mhz with a 200Khz bandwidth

    and also one for 153.275Mhz with a 300Khz bandwidth

    Ideally I;d like to make them all as one unit, but it I have to make several filters I wont mind.

    Can anyone help with circuits please?

    I did think about coax stubs but it wouldn't "notch" the signal enough

    Lewis
     
  2. Papabravo

    Expert

    Feb 24, 2006
    10,138
    1,789
    You can do this yourself
    1. Design Butterworth low pass filters from tables for 1 rad/sec corner frequency
    2. Transform to the band reject topology
    3. Frequency scale the components
    4. Figure out how to fabricate the components

    Your big problem will be how to make inductors at that frequency without parasitic capacitance. Your final problem will be layout and adjustment. VHF filters are tough to do on a breadboard. If this works -- then great. If it doesn't then you might need some very expensive test equipment to diagnose and debug what might go wrong.
     
  3. crutschow

    Expert

    Mar 14, 2008
    13,001
    3,229
    Perhaps you could do a simple LC series resonant circuit to ground to generate the desired notch frequencies (low impedance at the resonant frequency). You could use a trimmer to tune the notch frequency.
     
  4. w2aew

    Member

    Jan 3, 2012
    219
    64
    Rather than design two notch filters, why not design a single band pass filter for your desired frequency?
     
  5. lew247

    Thread Starter Member

    Apr 23, 2010
    15
    0
    Because there are frequencies around 145Mhz I want to receive as well
    and the bandpass filter wouldnt have steep enough slopes
     
  6. lew247

    Thread Starter Member

    Apr 23, 2010
    15
    0
    If I knew what any of that meant I might
    Fabricating it shouldn't be a problem it will be built in a tin box
     
  7. lew247

    Thread Starter Member

    Apr 23, 2010
    15
    0
    I like this idea, especially using the trimmer to tune
    Any idea for circuits?
    or how to calculate the values?
     
  8. crutschow

    Expert

    Mar 14, 2008
    13,001
    3,229
    Look at the Series RLC Circuit in this reference. You select the L and C to be practical values. For example a a 22nH inductor would require a trimmer capacitor set to about 50pF for a center frequency of 150MHz.
     
  9. Papabravo

    Expert

    Feb 24, 2006
    10,138
    1,789
    Well the longest journey begins with a single step. Google and Wikipedia are your online friends. If you do not understand what I was referring to you are also unlikely to be able to precisely specify your requirements and will also be disappointed with the results of circuit suggestions given by others. In order to get where you want you're going to have to make some effort. We're here to help but we can't do it for you.

    You can find online a set of tables for a multiple section Butterworth (maximally flat) filter made of capacitors and inductors. The values in the table give you the values for a low pass filter with a cutoff (corner) frequency of 1 radian per second. There are two things you can do with this prototype design. You can scale the components to any frequency of your choosing. The other thing you can do is apply a simple transformation to turn the low pass filter into a high pass filter, or a bandpass filter, or a NOTCH filter. These processes are commutative. That means you can do them in either order.

    Whatever you decide to do you will have a problem finding or fabricating suitable inductors for your application. Using a tin box is completely irrelevant if the inductors have too much parasitic capacitance to function as inductors at your frequencies of interest. If you construct something, and it does not work as you expect, you will have a very hard time figuring out what is wrong and how to fix it without a substantial investment in equipment and knowledge.
     
    Last edited: Feb 16, 2013
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