Designing Notch filters

Discussion in 'General Electronics Chat' started by cx1111, Nov 18, 2013.

  1. cx1111

    Thread Starter New Member

    Nov 4, 2013
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    Hello all

    I'm trying to obtain a narrow notch (stopband) filter to get rid of a very large source of interference. This frequency f0 is in the order of 0.5-5MHz, but I have not been able to find any manufactured notch filters at this frequency.

    I have a few questions.

    -Why not? Is this due to difficulty or just a lack of demand?
    -If it is difficult, what are the main sources of trouble?
    -Is there a way I should approach this? All I know about what I want is the gain, bandwidth, and that it needs to have BNC leads.

    Thank you very much for any help.
     
  2. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Notch filters have a very narrow band of frequencies that they filter out. Hence the name notch. Your range is very large (4.5 Mhz.). What is the total bandwidth you need?
     
  3. cx1111

    Thread Starter New Member

    Nov 4, 2013
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    Sorry I should've worded it better. I meant I need to filter out bands of about 50kHZ centered around 0.5MHz, 1MHz,.....5MHz.
     
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Your frequency range is not covered by OpAmp Active Filters so you will have to go LC.
    A series L-C will can be used as a notch filter. It can have Qs of up to several hundred, and be tuneable over a 10:1 range.
    What kind of Power Levels, and operating voltages are we talking about?
     
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  5. cx1111

    Thread Starter New Member

    Nov 4, 2013
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    Thanks for your help. I did a few calculations with the series LC. So at resonance, w=1/sqrt(LC) the impedance becomes 0.

    The aim of my project is to measure signals in the 0.25-5MHz range, and there is a huge source of 'noise' I need to filter out at 0.5MHz. Do I put LC 'load' in parallel with the oscilloscope 'load', and the whole thing in series with another resistor or something, to create a voltage divider which doesn't read when w=resonance?

    Also, how would I calculate the gain/DB?
     
  6. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Is the measurement to be made using a high input impedance (1megΩ or 10megΩ) device like an oscilloscope? Or a 50Ω measurement system like a spectrum analyzer?
     
  7. cx1111

    Thread Starter New Member

    Nov 4, 2013
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    It is to be made using a 1 mega ohm scope, but we're using a 50 ohm terminator on the scope to avoid reflections from the 50 ohm BNC cable.
     
  8. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Here is what you can expect with a 50Ω source impedance, a 50Ω termination, a 100uH inductor with a series resistance of 50mΩ as a function the tuning capacitor "tune".
     
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