Possible signal interference with audio circuit

Discussion in 'Analog & Mixed-Signal Design' started by Steven Woody Wood, Aug 18, 2016.

  1. Steven Woody Wood

    Thread Starter New Member

    Mar 16, 2016
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    I've designed a 5th order Sallen Key High Pass Filter (HPF) (Passband>=1KHz) that feeds into a LM386 amplifier into an 8 ohm speaker. The input signal to the filter is a probe tip which picks up stray frequencies inside walls and other nearby metallic objects or antennas and sends that signal to the amplifier to generate a tone so the user can detect if something is present. The main board and filter board is powered by a 9 volt battery.

    I tested the HPF board independent of the main circuit board and all works to spec. I then integrated the HPF into the main board and used a 1KHz tone generator connected between the tip and GND of the circuit and I can hear the tones perfectly without any distortion.

    I then connected one end of the tone generator to a metal plate so it can radiate that same frequency so I can test if the probe works as intended to be used. This is where the probe circuit has failed to work and I can't hear anything at all.

    Any input to this issue is appreciated as its the first time I've ever encountered this 10 years as an engineer.
     
  2. Papabravo

    Expert

    Feb 24, 2006
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    Why do you think the tone generator will do anything to the plate? Why do you think the high pass filter with a probe tip will pick up any vibrations at all? What kind of an engineer are you?
     
  3. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    "10 years as an engineer" = SCHEMATIC

    ak
     
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  4. OBW0549

    Well-Known Member

    Mar 2, 2015
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    At this point, my input would be that you need to post a schematic diagram showing exactly what you've built. Without that, any advice we could give you would be based purely on wild-assed guesses and would waste both your time and ours.
     
  5. Papabravo

    Expert

    Feb 24, 2006
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    That certainly cuts to the chase in a most emphatic way.
     
  6. ci139

    Member

    Jul 11, 2016
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    [​IMG]
    probe sensor sensor probe maybe?
     
  7. BobTPH

    Active Member

    Jun 5, 2013
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    Stray frequencies in walls? Please explain what that means.

    Bob
     
  8. Papabravo

    Expert

    Feb 24, 2006
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    Maybe crickets rubbing their legs together to make a "cricket" sound?
     
  9. #12

    Expert

    Nov 30, 2010
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    I think this one is not coming back. Probably already found out a metal plate isn't a speaker (at 1KHz) and a metal probe isn't a microphone.
     
  10. Papabravo

    Expert

    Feb 24, 2006
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    I was actually hoping to learn some new physics here. Oh well...one can but hope for better outcomes.
     
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  11. ci139

    Member

    Jul 11, 2016
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    this thing alone is worth to see - it's extremely difficult to find any example diagram of it in web
    random test
    stray frequency паразитная частота
    stray frequency 雜散頻率
    雜散頻率 疑似周波数
    疑似周波数 Spurious taajuus
    Spurious taajuus spurious frequency
    spurious frequency паразитная частота
    - if this thing occurs in building and construction - may have importance ???

    also the search of [human body as a signal ground] returned a Figure 5
     
    Last edited: Aug 24, 2016
  12. OBW0549

    Well-Known Member

    Mar 2, 2015
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    Yeah. He hasn't logged onto AAC since Friday night, after several of us called for schematics so we could see what the heck he's doing.

    My own wild-ass guess as to what's going on is that he is trying to construct some sort of "E-field sniffer" to probe for mysterious emanations from god-knows-what, and doesn't understand that he can't simply hang a wire probe onto the input of his filter circuit without going through some sort of high input impedance buffer amplifier, such as a bootstrapped FET voltage follower, and expect to pick up much of anything.

    Who knows what he was expecting to find, other than mains hum coming from power wiring or the high-frequency crap that's spewed out by switching regulators and CFL lighting. Secret signals from space aliens?
     
  13. #12

    Expert

    Nov 30, 2010
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    Termites is a possibility. I have heard people say they could, "hear the termites munching during the night".
    An amplified version of that sound might be a diagnostic tool and I think it would have a content at 1KHz.
     
  14. Nykolas

    Member

    Aug 27, 2013
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    Singing termites? Anyone up to writing a new gaudy Xmas tune? E
     
  15. OBW0549

    Well-Known Member

    Mar 2, 2015
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    Three Termite Sounds Every Homeowner Should Know
     
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