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50Hz Filter

Discussion in 'Electronic Basics' started by Ed, Aug 3, 2004.

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  1. Ed

    Ed Guest


    I've made a simple circuit which consists of an ultrasonic transmitter and
    reciever. I send out 40KHz pulses and when they return I amplify the them
    and put the amplified signal into a comparator (this creates a nice square
    wave signal for a microprocessor). The problem is, when I'm not sending out
    pulses the receiver picks up the 50Hz signal from the mains electricity
    supply (I'm based in England, hence the 50Hz). Should I use a notch filter
    to remove the 50Hz signal or should I implement some sort of band-pass
    filter, which only accepts 40KHz? What would you all recommend and which is
    the easiest to implement?

    Thanks for any help.
  2. P

    P Guest

    Hi, I made such things too several years ago.

    Make a simple band-pass filter, it does not have to be very steep, at
    40 kHz you are many decades away from 50Hz, some RC filtering will do.
    Notice that you probably also have some 100, 150 Hz components.

    Pieter Hoeben
  3. Active8

    Active8 Guest

    Screw the BP and notch approach. Usee a HP filter and set the cutoff
    freq well above say, 150 Hx just in case. There's bound to be
    equipment around with rectifiers in it that will produce harmonics
    that will get back into the power lines and radiate. IME, in a 60 Hz
    system, I'll find plenty of 120 Hz noise. I can filter it out and
    still pass a 100 Hz signal, though.
  4. Ed

    Ed Guest

    Does that just consist of a resistor and capacitor? If so, what values do I
    select for the cap and resistor? I've done a google search but can't find
    any specific answers (or rather answers I understand).

  5. Active8

    Active8 Guest

    It could, but it depends on how much noise we're dealing with. You
    might need an active filter like a Sallen-Key or better.

    In either case, the cutoff freq is 1/(2piRC)

    You also want to keep the input conductors to the amplifier as short
    as possible and examine the current flow of the input. Keep the loop
    as small as possible.
  6. Tthat would still leave some rather broad band noise from where the high pass
    begins up through "daylight," yes? How hard is a simple bandpass, anyway?

    But what crosses my mind is that the input picks up 50Hz too easily, perhaps.
    Using a resistor divider (and an 'HP' capacitor) at the input might help.
    Something like:

    / 2M
    1uF \
    || |
    ,-----||-------+-+--> first gain stage
    | || |
    --- \
    | | receiving / 2M
    --- transducer \
    | |
    gnd gnd

    Just a thought.

  7. Jamie

    Jamie Guest

    pass the square wave into a one-shot 555 timer and AND gate.
    if the incoming wave does not expire on the high side before
    the one-shot timer does then using a gated circuit won't
    trigger a pulse.
    this means that time base wider than 40 kHz will not be detected/
  8. Active8

    Active8 Guest

    Sure. Try something simple like this first.
  9. Sounds like a rx shielding problem and the need for a high pass filter.
  10. Neither. The two freqs are so far apart that you should be able to get
    what you want by using a high pass filter. Change the coupling
    capacitor in the preamp to a smaller value, so that the lower audio
    freqs can't get thru. Or else add a high pass filter somewhere near the
  11. Rich Grise

    Rich Grise Guest

    Watson A.Name - "Watt Sun, the Dark Remover" wrote:
    [yet another HPF suggestion]
    Everybody here is kind of barking up a tree, from what I see.

    There is no 40 KHz ultrasonic transducer that will give a perceptible
    signal in response to 50 Hz in the audio band.

    The guy's circuit is picking up hum electronically, from the ambient
    50 HZ EMI.

    The whole circuit is acting as an inductive pickup loop.

    Find the ground loop(s), fix it, and move on. :)

    (admittedly, a filter to get the 50 Hz interference out of a 40 KHz
    signal is kind of a naptime design project. ;-) )

    Good Luck!
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