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400HZ Notch Filter how to.

Discussion in 'Electronic Components' started by Joop van der Velden, Sep 29, 2003.

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  1. There is a chance that filtering the 400Hz tone will not be sufficient,
    and that there are several harmonics present in the audio signal (800,
    1200Hz etc).

    Find this out first, by comparing the hum you hear with a clean sine
    wave of 400Hz.
  2. Dana

    Dana Guest

    Looking to notch out a 400HZ whine from some audio circuits connecting
    remote manned sites to a central location.
    The audio is microwaved from remote location to the central location.
    Would like to know the formula for determing the resistor and capacitor
    The overall bandpass is 300HZ to 3KHZ.
    Or would it be better to just use an High Pass filter.
    Problem with that is getting the inductors locally.
    Up here In Fairbanks our choices are very limited.

  3. John Fields

    John Fields Guest

    You might want to try this:
  4. Al Clark

    Al Clark Guest

    This is an easy application for one of our small DSP boards. Our DSP-8300
    is flash programmable, and has a stereo 16 bit audio codec. You could
    program the board with a 400 Hz notch and also a very good bandpass
    filter if you want. We can help you with the code.

    Details are on our web site.
  5. John Larkin

    John Larkin Guest

    Or use three capacitors and three resistors in a twin-tee notch. The
    software is somewhat easier.

  6. Dana,

    We have two products that have built-in notch filters - the DSP-599zx and the
    DSP-8100c Noise Filters. Both units have an automatic notch filter and a
    manual tunable notch filter. No programming is necessary. Just push a button
    and/or dial in the notch frequency you want. They also have tunable highpass
    and lowpass filters to tailor your overall bandpass to meet your specifc
    requirements. Check them out on our website -

    Both units are in stock for immediate shipment anywhere in the world.

    Thank you,

    Randy Gawtry
    Timewave Technology Inc.
    St. Paul, Minnesota
  7. John Fortier

    John Fortier Guest

    If the tone is pure 400 Hz, you should be able to use a twin T filter.

    By themselves Twin Ts have a very low Q, but if you "activate" them they
    can be made to provide a pretty sharp filter.

    The advantage is that no inductors are required.

    I won't try to draw one using ASCII which would probably be very confusing,
    but if you need a circuit diagram, e-mail me and I can send you one.

    On the other hand, if you use a high pass filter, the problem is going to be
    building a filter which has a sharp enough cut off to eliminate the 400Hz
    while leaving the rest of the signal relatively unattenuated. You can use a
    multi-stage Sallen and Key filter, but the complexity and the tight
    component values required make this a difficult solution to achieve in

    All in all, the Twin T seems your best bet.

  8. ASCII dwgs are ok if viewed in a fixed width font.

    C C
    | | |
    | \ |
    | /R/2 | Fo= 1/2.pi.R.C.
    | \ |
    | | |
    | --+--0v |
    In --+ +--Out.
    | R R |

    That's the basic Twin-T, suffers from a not very sharp notch.

    Bootstrapping can steepen the sides of the notch and gain
    a flatter response in the passband.

    +-------+ ______
    | | | |
    +-|-\ +--|Twin-T|----|+\
    | >--+ |__ ___| | >--+----Out
    In---|+/ | +-|-/ |
    | | |
    | +-------+
    | |
    | \
    +----->/ <--Bootstrapping adjust
  9. default

    default Guest

    I don't know how useful this reply will be . . . forgot where the
    schematic got to. The basic circuit is a single transistor amp common
    emitter with collector load and emitter resistors the same value.

    The input has a twin T filter that attenuates all but the notch
    frequency, so you accentuate the frequency you want to notch out.
    Emitter and collector are sampled via individual caps and combined.
    At the twin T frequency, the emitter and collector are 180 out of
    phase and cancel providing a very sharp notch.

    I saw the circuit some 30 years ago and built it and fooled with it.
    The notch attenuation was impressive and sharp. Perhaps someone
    following one of your cross posts has the schematic.
  10. Al Clark

    Al Clark Guest

    An analog notch filter may be fine for this application if the following
    conditions are actually true:

    1. Temperature is stable, so that the filter will not drift.
    2. The interfering signal is actually 400 Hz. My guess is that it has a
    fundamental at 400 Hz and also some significant harmonics. The harmonics
    may be a bigger problem than the fundamental, since hearing is more
    sensitive at higher frequencies (1-2kHz).

    I would look at the signal with an FFT analyzer (You can probably find a
    PC program on the web). This information will suggest an appropriate

    These might be:

    1. A simple notch filter as discussed.
    2. Multiple notch filters.
    3. Automatic notch filters using adaptive filtering.
    4. Comb filters.
    5. Bandpass Filters.

    All of these solutions are easily addressed with a DSP and I have used
    each of these methods for similar noise problems. The best solution
    depends on the specifics.
  11. Baphomet

    Baphomet Guest . This is a bandpass
    filter so you can either use the twin tee feedback circuit as a stand alone,
    or you can add an additional op amp to invert the signal. This circuit was
    liberally filched from "Network Synthesis" by Charles A. Vergers
  12. Dana

    Dana Guest

    That is more like what I am looking for, as I would prefer a passive device
    over an active. I originally came up with an LC notch filter, but cannot
    obtain any inducters locally, so I would like to look at just using an rc
    filter. But am having problems remembering from school the calculations
    needed. That is the problem with technology going to board level and even
    box level repair, tend to forget the basics.
    So does anyone remember how to determine component values for an rc notch
  13. Dana

    Dana Guest

    Yes I tried that, and I found that to reduce the 400hz I needed to have my
    cutoff at around 650hz and higher to obtain enough attenuation at 400hz.
    The 400hz is the whine of the 400hz power that supplies the remote location
    electronics, and as the equipment technology is old, the audio of the
    operator is neing swamped out by this noise. Why thye did not go with some
    kind of vox circuit like is used in aircraft I do not know.
  14. John Fields

    John Fields Guest

  15. Dana

    Dana Guest

    Once the site is turned up, the temp remains fairly constant.
    Big fear here is latter on this winter when outside temps are -20 and lower,
    may take a bit longer to stabilize.

    It is 400hz power that is making the whine. We use 400hz power on the
    electronic equipment at the site, except of course the comm gear which is
    your basic 120.

    I would agree with this outlook in most other situations. But being that
    this is the noise of the 400hz power, these harmonics are not so bad.
    Looked at with a Spec A, as well as one of those level meters that gives
    freq's and levels that are used to find rf hotspots to comply with OSHA
    For the remote locations these would be best option, as I have restrictions
    on electrical supply, being these are very remote locations.

    Active devices are kind of limited due to the scaricty of electricity at

    Cutoff is to high to properly knock down the 400. This was also my first
    choice of trying to find a solution for this problem, which has existed
    since before I came here.
    I agree, active would be nice, but electrical supply is very limited.
  16. Dana

    Dana Guest

    Just wanted to say thanks for all the helpful replies.
  17. Dana

    Dana Guest

    Domo arigato
  18. Jim Thompson

    Jim Thompson Guest

    See the gyrator-based filters shown on the S.E.D/Schematics page of my
    website. A notch is just another variation since the gyrator creates
    an inductor to ground.

    These filters are *very* stable compared to Sallen-Key... component
    sensitivities are unity. (In an S-K realization the sensitivities are
    a function of Q.)

    ...Jim Thompson
  19. John Fields

    John Fields Guest

    If the offending signal is being coupled into the microphone
    acoustically, then VOX won't help at all, since the 400Hz will still be
    there when speech turns on the transmitter.

    The solution is to notch out the 400Hz at the transmitter (at the mic or
    downstream from the mic) and keep it from being transmitted in the first
    place. A passive RC notch will do it, but the price you'll pay will be
    quite a bit of attenuation of the audio you _want_ to send. Use an
    active solution like the one described in the URL I posted for you
    earlier and you can adjust the Q of the filter to pretty much leave most
    of the audio alone except for the offending signal. Cheap, too. A dual
    opamp and a handful of discretes and you're there...

    If you'd rather buy than build, I can put something together for you for
    about $20.00. Email me if you're interested.
  20. Al Clark

    Al Clark Guest


    I tried to send you an email reply but the address bounced. Call me if
    you like. Our phone number is on our web site.
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