50Hz notch filter

Discussion in 'The Projects Forum' started by Asdis, Nov 30, 2010.

  1. Asdis

    Thread Starter New Member

    Nov 30, 2010
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    Hi

    I need to design a notch filter that filters out 50Hz noise. I don't know how to start. Any ideas??
     
  2. bertus

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  3. beenthere

    Retired Moderator

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  4. tyblu

    Member

    Nov 29, 2010
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    Start with your requirements such as power (voltage, current), Q-factor, maximum attenuation, cost, and manufacturability (one-off or mass-production?).
     
  5. tom66

    Senior Member

    May 9, 2009
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    Paste this code into Falstad circuit sim: http://www.falstad.com/circuit/

    Code ( (Unknown Language)):
    1.  
    2. $ 1 5.0E-6 5 50 5.0 40
    3. % 0 28853.998118144256
    4. l 352 32 352 128 0 0.0159154943 0.0
    5. c 352 128 352 224 0 6.366197723699999E-4 0.0
    6. r 240 32 352 32 0 100.0
    7. O 352 32 416 32 0
    8. g 352 224 352 256 0
    9. 170 240 32 208 32 3 20.0 60.0 5.0 0.5
    10. o 5 64 0 34 5.0 9.765625E-5 0 -1 in
    11. o 3 64 0 34 5.0 9.765625E-5 1 -1 out
    12. o 0 64 0 34 10.0 0.025 2 -1 inductor
    13. o 1 64 0 34 10.0 0.025 2 -1 cap
    14.  
    It's approximately 50.1Hz, but it wouldn't be manufacturerable with standard values.
     
  6. Audioguru

    New Member

    Dec 20, 2007
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    The power mains is very noisy. If you filter out the 50Hz fundamental frequency then you still have plenty of 100Hz and 150Hz harmonics plus dimmer buzz and other interference.

    In some countries (I am not allowed to say where) the mains frequency varies all over the place so a high-Q notch filter won't work.
     
  7. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Why is that?
     
  8. Audioguru

    New Member

    Dec 20, 2007
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    "I am not allowed to say where"
    A moderator here does not like when I comment about the extremely poor quality of products (and people) of 3rd-world countries.
     
  9. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    You don't need to be in a third world country to have harmonic rich power.

    I do know that JA Pierce, who selected 10.2 kHz as the Omega Navigation System's frequency say he wished he didn't choose that frequency as it is the 170th Harmonic of 60 Hz. Obviously there were some interference problems.

    Also PLC's, Power Line Carrier, has been known to interfer with systems in the 100 kHz band ... Loran-C ... but now that the government shut down all the Loran-C stations, that is no longer an issue. The one I observed caused a minor 1788 miles error in the distance between me and home. I was less than 10 miles from home at the time.

    Here's the spectral view I did in Feb 1999.
     
  10. Audioguru

    New Member

    Dec 20, 2007
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    Its odd-order harmonics are pretty high so the sine-wave is "compressed" on its peaks.
     
  11. Asdis

    Thread Starter New Member

    Nov 30, 2010
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    I have an EMG signal, I have made a Butterworth filter to filter out some other unwanted frequencies but now I just want to get rid off the 50Hz :)

    Thanks for all of your advice :)
     
  12. Audioguru

    New Member

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    An ECG circuit usually uses an instrumentation amplifier that has very good common-mode rejection and "right leg" cancellation of common-mode AC and DC signals.
     
  13. JDT

    Well-Known Member

    Feb 12, 2009
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    One way to do this is to use a Twin-Tee Notch filter like this:-
    http://cnyack.homestead.com/files/afiltimp/notch1.htm
    Not too difficult to build. Google for other information.

    To get a very sharp notch, the component values need to be accurate. The frequency and sharpness of the notch will drift with temperature. As someone pointed out, the mains frequency may vary and you will not be filtering out the harmonics.

    A better solution is to use a comb filter:-
    http://en.wikipedia.org/wiki/Comb_filter

    As these are usually built using switched capacitor or digital techniques the filter can be made very accurate and can track the mains frequency. This is quite advanced stuff- not easy for a beginner!
     
  14. Asdis

    Thread Starter New Member

    Nov 30, 2010
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    Where do I find the formula to calculate the order of a Notch filter? :confused:
     
  15. Asdis

    Thread Starter New Member

    Nov 30, 2010
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    how do I find the filters orden?
     
  16. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    On Notch filters you do not use the therm order.
    A Q-factor is used on Notch filters for the quality of the filter.
    Take a look at section 5 of the attached PDF for more info.
    The other PDF is to look at the quality of the filter versus component tolletance.

    Bertus
     
  17. tyblu

    Member

    Nov 29, 2010
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    There are many types of notch filters.
     
  18. Asdis

    Thread Starter New Member

    Nov 30, 2010
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    Ok, if I use a Twin-T filter. Do you know how I find the order for that? or maybe where I can find some examples?
    I´m new to this circuit building stuff and so not getting this.... hope I will see the light soon :p
     
  19. beenthere

    Retired Moderator

    Apr 20, 2004
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  20. tyblu

    Member

    Nov 29, 2010
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    I hadn't heard that term before. Plugging it into Google yields dozens of examples. I didn't see a multi-stage version in the 2 I clicked, but it is basically a combination of two 1st order filters -- a high pass and a low pass. This means you will be -20dB/dec per 'order'. Given a minimum attenuation at a certain frequency and a symmetric circuit, you have a solution.
     
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