Notch Filter

Discussion in 'Homework Help' started by ELCE-N, Dec 22, 2012.

  1. ELCE-N

    Thread Starter New Member

    Dec 22, 2012
    6
    0
    Hello,

    I've designed a Notch Filter with cut off frequencies from 100Hz to 20kHz.
    When I simulated the design using AC analysis, I found out that I've got a problem with high frequencies. The problem is that there is a decrease in the output. The following picture will explain my problem.

    http://forum.allaboutcircuits.com/attachment.php?attachmentid=50010&stc=1&d=1356210439

    PS: I used active filters.

    I'd appreciate some explanation of why do I have such problem. :confused:
     
  2. WBahn

    Moderator

    Mar 31, 2012
    17,788
    4,807
    There is no valid attachment.
     
  3. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
    1,159
    Besides the lack of a diagram of your circuit, can you tell me what you expect from a notch filter.
     
  4. ELCE-N

    Thread Starter New Member

    Dec 22, 2012
    6
    0
    I'm sorry I thought it was already attached.

    Here is my output:

    [​IMG]


    About the design, I used LPF and HPF in parallel.

    [​IMG]
     
    Last edited: Dec 23, 2012
  5. ELCE-N

    Thread Starter New Member

    Dec 22, 2012
    6
    0
    I expect it to reject everything in the range of 100Hz to 20kHz.
     
  6. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
    1,159
    Two things ....

    (1) Most designs have a unity gain mixer.
    (2) Did you notice the gain of the amplifiers were different?
     
  7. The Electrician

    AAC Fanatic!

    Oct 9, 2007
    2,286
    331
    You can't connect the outputs of opamps together like that. What would happen if you connected two voltage sources in parallel, such as connecting a 12 volt source in parallel with a 10 volt source?
     
  8. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
    197
    Google 'Summing Amplifier"
     
  9. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
    1,159
  10. ELCE-N

    Thread Starter New Member

    Dec 22, 2012
    6
    0
    Woops! I'm actually aiming for gain of 5.. do I make design both of them to give me gain of 5?
     
  11. ELCE-N

    Thread Starter New Member

    Dec 22, 2012
    6
    0
    Summing amplifier! That's slipped out of my mind somehow! Thank you :)
     
  12. ELCE-N

    Thread Starter New Member

    Dec 22, 2012
    6
    0
    Oh!! Yes that was a mistake.. summing amplifier slipped out of my mind somehow.
    Thanks for pointing that out :)
     
  13. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    LM741 does not have enough bandwidth or slew rate for your notch filter. LM741 is a dinosaur.
    What is the highest frequency you want to pass?
    What is the amplitude of your signal?
    What are your supply voltages?
     
  14. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Your opamps are not powered so they will do NOTHING.
    The lousy old 741 opamp has trouble above 9kHz.

    Your filters are very simple with only one capacitor and only one resistor so their cutoff slopes are very gradual.

    If you use a mixer then the lowpass filter cuts 100Hz slightly (-3dB), cuts 200Hz a little more (-6dB) and has reduced but still has plenty of output at 1600Hz.
    The very simple highpass filter reduces 20kHz slightly (-3dB), cuts 10kHz a little more (-6dB) and has reduced but still has plenty of output at 1250Hz.

    If the gains are the same then the phase shifts of the filters might produce a notch at about 1400Hz.
     
  15. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    I simulated your extremely simple filter with good quality opamps.
    10Hz and 100kHz have a gain of 5 (+14dB).
    100Hz and 20kHz are down -3dB to +11dB.
    200Hz and 10kHz are down a little more to +7dB.
    Most frequencies will be at a lower level but are still very audible because the filters are so simple.
     
  16. vk6zgo

    Active Member

    Jul 21, 2012
    677
    85
    "I've designed a Notch Filter with cut off frequencies from 100Hz to 20kHz."

    That is one hell of a wide "Notch"!

    The term "Notch" filter is usually reserved for one which has a bandwidth appreciably smaller than its centre frequency.

    Maybe a "Bandstop" filter would be a better name for what you are attempting.

    As the others have said,it will have a very gentle slope.
    It will,in fact be useless!
    Maybe,you could use two separate channels with a high degree of isolation,one with a fairly savage LPF having a cutoff at 100Hz,& another,with a HPF cutting off below 20kHz,then add the two channels to get the required result.
    I wish you all the best!:D
     
Loading...