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So what makes this buzzing-radio sound anyway?

Discussion in 'Electronic Design' started by Tim Williams, Nov 4, 2013.

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  1. Tim Williams

    Tim Williams Guest

    www.seventransistorlabs.com/QRZ.mp3
    (...QRN? I don't remember the Q-codes.)

    Recorded from a Theremin-in-progress, but the same sound shows up all over
    the airwaves, LW, SW, professional receivers or homemade. In this case,
    it shows up when connecting only a foot of wire to a 5MHz oscillator (yes,
    rather high for a Theremin, this is for S&G's at the moment). Said
    oscillator seems to be acting in an autodyne capacity.

    The waveform seems to be a frequency modulation (advancing at least one
    cycle) every 8.3ms. Hence the buzzing sound, and hence the FM synth
    character at different frequencies.

    It must be line driven, somewhere, but it's not my power supply, and
    anyway, that wouldn't explain it being everywhere on the dial. It also
    seems rather dubious (or impressive?) that it's having such a strong
    effect on my oscillator, despite using only a one foot antenna.

    Tim
     
  2. Tauno Voipio

    Tauno Voipio Guest


    QRZ? = who is calling?

    QRN = man-made noise.

    It could be e.g. a Chinese energy-saver lamp switcher fed with raw
    full-wave rectified supply, with the power line filter components
    saved from the BOM.
     
  3. Guest

    QRM, I think.
    From 0 to about 0.7 sec, it sounds like a "carrier", so to speak. From
    about 0.7 sec to about 1.5 sec, it sounds like there is "data" modulated
    on it somehow. Then, the theremin starts up.

    A quick analysis in Audacity shows some peaks at roughly 17.5 kHz, 14
    kHz, 10 kHz, and maybe 6.5 kHz.
    http://birdbird.org/tmp/sed/williams-spectrum-begin.png

    Of course, doing an FFT on an mp3 is somewhat dubious. Record to .wav
    (or other lossless format), grab a copy of Audacity, and see for
    yourself locally. Not real-time but the price is right.

    From about 12.0 sec to the end of recording also seems to be free of
    theremin sounds. I still think it sounds like "data", but the frequency
    is different. The spectrum doesn't have peaks, either.
    http://birdbird.org/tmp/sed/williams-spectrum-end.png
    Hit breakers until it stops. If it never stops, try it in the daytime
    (when streetlights and other lights are off) and and at night. If you
    have any overhead power lines in your area, wander the immediate
    neighborhood and look and listen for small arcs at the poles. Is there
    any (possibly unannounced) broadband over powerline in your area?

    Matt Roberds
     
  4. Martin Brown

    Martin Brown Guest

    I'd hazard a guess that it is a very badly behaved switched mode PSU,
    speed controller, CFL light or dimmer somewhere. To make such a mess it
    seems likely that the outward and return paths are not close together so
    that you have a room bathed in magnetic loop coupled RF noise.

    A bit like the method that the T setting on hearing aids uses - might be
    worth seeing if you can pick it up with a pancake coil and a crystal
    earpiece then you might be able to metal detect to the source.
    Daqarta (sp?) will do it in realtime if you can feed the signal into a
    soundcard. I have used it for public demos of Sound and Music. ISTR You
    have 30 days free use before you have to register and it still works as
    a basic signal source after your evaluation period expires. It isn't
    outrageously priced so if it does what you want worth buying.
     
  5. Tim Williams

    Tim Williams Guest

    The theremin action is continuous of course, the different periods are
    just where I put my hand to give you an idea what it sounds like at
    various places.

    Like I said, the waveform looks like frequency modulation -- no matter
    what the carrier frequency; it seems like constant displacement FSK. I
    don't get what kind of signal is pulling the oscillator like that, nor how
    it can do it so strongly from so little antenna. (Is it really that
    sensitive? Why don't theremins in the MF range do it?)
    Beats me. Unfortunately, all noise abatement advice is N/A -- I'm in an
    apartment, suburban area. The elephant in the room is obviously the
    computer (for which without, how do I record..), but based on experience,
    I don't think that's the source anyway.

    Tim
     
  6. Jasen Betts

    Jasen Betts Guest

    Could your oscilator be squegging ?
    Vackar's topology is particularly prone to this.
     
  7. Tim Williams

    Tim Williams Guest

    Waveform at the osc (before the mixer) is nice and clean. Of course, I
    can't see the FM/noise at frequency.

    Tim
     
  8. Guest

    Since there aren't any liner notes, I can only go by what I hear.
    Like Martin Brown said, maybe whatever it is has a huge loop area for
    some reason.
    Jeff Liebermann gave the link to look up BPL deployments.
    You can, almost for sure, find out if it's coming from something inside
    your apartment. (You might not be able to turn off things like a fire
    alarm system.)

    Apartment buildings tend to have main circuit breakers for each
    apartment at the meter. This may be an unpopular approach, though. :)

    Since you said it shows up in commercial receivers too, maybe grab a
    radio and take a stroll. Or, if your theremin can be made portable,
    take it along with you.
    You could get a piece of plastic 5/32" wide and 5,000 feet long, put
    some iron oxide on it, and drag it at 1.9 inches per second past a coil
    with the signal you want to record on it. Naah, it'd never work.
    The ARRL page that Jeff mentioned is http://www.arrl.org/sounds-of-rfi ,
    but nothing there really sounds like yours. On the other hand, all of
    those recordings were made with things that were designed to be radio
    receivers, so they may sound different than your theremin.

    The thing is, *lots* of equipment can generate radio frequencies. At
    least a handful of people have now listened to your sound sample and
    nobody has yet said "I know exactly what that is", so it seems likely
    that you'll have to do more snooping around to find out.

    Matt Roberds
     
  9. Have yout tried turning the lights off, especially overhead
    fluorescent lights? Even some non-electronic ones can make a buzz on
    the radio (I think the old 8' ones were particularly bad).
     
  10. Tim Williams

    Tim Williams Guest

    Yes -- all I have nearby is a 20W CFL; no effect.

    Update: I've put in proper coils (now operating ca. 2.23MHz) and changed
    around a few things (mixer ports, gain, filtering); I think filtering much
    over ~5kHz between AF stages has helped (including getting rid of the hiss
    and whine from the volume section, which uses a slope detector).

    It still does it, but only when... get this... it's completely
    *un*grounded. I can put ferrite beads on all cables (i.e., power and
    signal out) and it buzzes; ground it with only one (e.g., clip scope probe
    to the chassis) and it goes away. This sounds like a grounding problem,
    but it's build on copper clad, ground plane everywhere, oscillators
    partitioned. Also, ceramic caps on all outside connections. No
    opportunity for RF ground loop.

    Visual aid: the design is very similar to this one I built a while ago.
    http://www.seventransistorlabs.com/Theremin/
    Main difference, I split the pitch mixer so there's a separate VGA so I
    can add effects to the CW pitch signal. (Yes, as a "7 Transistor"
    project, it's all discrete; this one uses 24 transistors in total, only a
    little over.)

    Tim
     

  11. A dimmer would be highly suspect with an 8.3333 millisecond time period.

    --
    Many thanks,

    Don Lancaster voice phone: (928)428-4073
    Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
    rss: http://www.tinaja.com/whtnu.xml email:

    Please visit my GURU's LAIR web site at http://www.tinaja.com
     
  12. Martin Brown

    Martin Brown Guest

    How are you doing the amplitude modulation? Mine was via a slowish LDR
    optoisolator to avoid the two RF signals fighting each other.

    I also had a bit of fun and games tuning it so that it was linear in
    distance for logarithmic changes in pitch - the hardest part of all. It
    drifted like hell depending on room temperature but you could play by
    ear well enough that people could sort of recognise the tune.
    In case it is squegging you might want to try decoupling capacitors at
    strategic points. Mine proved very problematic when I came to putting it
    in a nice screened box which was tedious in the extreme until I managed
    to make it stable again. Something as wild and woolly as a theramin is
    sat on the edge of wild instability so you have to fight pretty hard to
    get both the pitch control and the amplitude modulator to behave
    themselves at the same time.

    It could easily be that it needs to be properly earthed to work and
    protect it form local interference. Mine went through a period where it
    would only work when earthed too. Eventually it did accept living in a
    metal box, but it came very close to being put in a shoe box!
     
  13. Tim Williams

    Tim Williams Guest

    Single balanced mixer. Whereas the schematic at the end of my link shows
    volume (as a control voltage from the slope detector) being fed directly
    into the pitch mixer (and hence performing gain control), this has
    separate mixers for each (pitch alone and volume control alone). Pretty
    basic circuits, seems to work well, and everyone else seems to be doing it
    (a little searching on VCAs turned up a couple standard circuits following
    the Moog synthesizer, which is part of the envelope generator).
    How was that, pitch into CV into nonlinear function into VCO?

    V-to-F's being another synth topic that's notoriously drifty, no surprises
    there. One thing where monolithic is pretty much required.

    Log pitch would be wonderful. These things are so tricky to play because
    the 'width' of a note varies depending on your distance to the antenna,
    plus geometry. It's hyperbolic (roughly; there'll be some (x + d)^-3/2
    sort of action going on with near fields too), then polynomial or power
    series (freq goes as sqrt(LC)), then finally log (or exp) for pitch. It's
    still a one-to-one function, but absorbing all of it analytically (even
    given a suitable approximation for the antenna field and geometry) into
    one invertible function is unlikely. An approximation (PWL or whatever)
    would be possible, but might still require considerable skill to operate
    because even a 1% accurate correction is off by... about a major sixth?
    Getting that as clean as a few cents would be miraculous by any means!
    Strange, I haven't had much trouble with them (or radio stuff in general,
    well, a little, but nothing I couldn't iron out).

    The present model is happy right now -- good clean tone (while grounded,
    at least), and playable, though the volume is a bit too sensitive (i.e.,
    you have to move your hand rather far to go from 'on' to 'off'!), and I
    need to install fine tuning adjustments (the trimmer caps are tunable, but
    not very user-friendly).
    Obviously it's being more sensitive to noise somehow -- but it doesn't
    make sense that it's more sensitive when the chassis is less grounded.
    The only obvious 'mode' would be dipole action across the width, but at
    2ish MHz and a couple feet across, it's hardly a resonant length. And
    being a balanced mode, that wouldn't change with grounding (much). So,
    clearly it's an unbalanced mode. Dunno.

    Tim
     
  14. josephkk

    josephkk Guest

    I am afraid i am forced to split this hair:

    QRN: Noise created by inexperienced (novice) operators
    QRM: Other man made noise.

    Upon checking is got this wildly wrong. See:

    http://en.wikipedia.org/wiki/Q_code

    Oh well.

    ?-)
     
  15. josephkk

    josephkk Guest

    All said and done it still sounds like a hacked arc welder to me.

    ?-/
     
  16. John S

    John S Guest

    According to:
    http://www.arrl.org/files/file/Ham Radio License Manual/Comm w Other Hams-Tech Band chart back.pdf
    (watch out for the wrap)...

    QRM Your transmission is being interfered with _________
    (1. Nil; 2. Slightly; 3. Moderately; 4. Severely; 5. Extremely.)
    Is my transmission being interfered with?

    QRN I am troubled by static _________. (1 to 5 as under QRM.)
    Are you troubled by static?
     
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