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Zener Noise (was: 1N4007 varactors)

Discussion in 'Electronic Design' started by RST Engineering, Jan 10, 2006.

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  1. Roy ...

    I've been playing around (ahem, excuse me, heuristically engineering) with
    zener noise sources for a while using the same spectrum analyzer trick and
    as yet I haven't been able to make the noise as "flat" across the passband
    as I'd like. I've tried varying the bias, the voltage, and a few other
    tricks, but as yet, no joy.

    Can you shed some light on what you've found to make the noise power/voltage
    fairly level across the band?

    Jim
     
  2. Roy Lewallen

    Roy Lewallen Guest

    It's not particularly flat over the whole band, and I haven't attempted
    to make it be. My interest has been mostly in relatively narrow band
    filters, or the shape of a main filter rolloff. The noise is adequately
    flat over the bandwidths I've been interested in. It's been a while
    since I've fooled with it, but as I recall, the shape of the noise
    spectral distribution changed all over the map as I changed the diode
    bias. I imagine that it changes with temperature and with individual
    diodes, too. So any circuit used to flatten it would only work for a
    particular diode, current, and probably temperature. The bottom line is
    that it's probably a lousy way to try to generate a flat broadband noise
    spectrum.

    Roy Lewallen, W7EL
     
  3. RST Engineering wrote...
    Good zener noise sources are carefully bred and tested, they
    do _not_ come naturally from garden-variety zener diodes. I
    suggest you go read the hundred or so messages in the famous
    zener oscillation thread a few years back here on s.e.d. In
    this you'll learn of my substantial investigations into the
    topic, some physics - and, very important, learn what a zener
    microplasma is. You'll see my ASCII waveform plots of actual
    bench measurements showing exactly what's going on. After all
    this you may decide to avoid using zener diodes for calibrated
    noise sources. Sorry about that!
     
  4. Sjouke Burry

    Sjouke Burry Guest

    What I did for some project which needed equal amplitude
    uncorrelated noise,is amplify the zener noise with a wide
    band video opamp,with high pass and lowpass filtering.
    Used the low pass as input for a zero cross detector,
    delayed the zerocrossing 10 microseconds,and used that
    for clock to a circulating bit in a shift register.
    each parallel output of that register controlled a
    sample/hold opamp,sampling the highpass signal.
    Voila!! 8 audio frequency, non-correlated noise sources.
    The zerocrossing clock was made this way,to avoid detectable
    clock tones int the output.(2 to 20 microsec between
    crossings)
    the 10 microsecond delay was used to get a voltage at
    the sample and hold opamp which was not correlated to the
    zerocrossing.
    If you need only one signal ,leave out the shift register,
    and just use a 10 and a 1 microsec. oneshot for the s/h
    opamp clock.
    The application? A wind and engine noise generator for a
    car simulator.
     
  5. John Larkin

    John Larkin Guest

    These folks will sell you serious noise diodes...

    http://www.noisecom.com/NC/default.htm

    John
     
  6. JeffM

    JeffM Guest

  7. JeffM wrote... That's one thread, perhaps the first in a series. That thread
    doesn't have the waveforms I was referring to (although there
    are some waveforms in posts 51 and 66). Tony, Bill, Roy and I,
    and some others here wasted masses of time on this subject over
    a period of a few months, eight and a half years ago. We took
    bench measurements, did calculations, found the scientific
    literature (it was a subject that occupied physicists in the
    late 50s, see posts 72-76), and we did plenty of speculation.
     
  8. Winfield Hill wrote...
    All of which led Roy McCammon to remark (post 90), "I'd have to
    say that it is the best thread this year." He said that Aug 5th,
    after 3 weeks of posts, and yet the followup threads in Aug and
    Sept on the same topic were just as long, and perhaps even more
    interesting. Ah, those were that days!
     
  9. OK, then. A zener makes a poor noise source according to what I'm reading.
    Noise.com used to sell off-spec diodes by the onesies for we poor peons to
    play with, but for whatever reason that doesn't seem to be the case any
    more.

    Given that a zener (at whatever current) is a poor noise source, what is a
    good source of electronic broadband noise from low HF through high UHF --
    say, 5 to 500 MHz.? (No smart remarks about spark gaps.)

    Jim





    We took
     
  10. John Larkin

    John Larkin Guest


    A hot resistor.

    How about a thermistor or a lamp filament that was 50 ohms at some
    high temperature. You could heat it with DC, sense its
    resistance/temp, and let it make noise, all in a single part.

    Old vintage noise figure meters used gas tubes. And I think there was
    a pencil tube that mounted in a waveguide and made shot noise.

    And, of course, the old photomultiplier trick.

    John
     
  11. Mark

    Mark Guest

    I have seen a small "gain of wheat" type light bulb used as a noise
    source...

    Feed it with DC through a choke and AC couple the noise out ....

    and you can vary it too!!

    Mark
     
  12. Immediately you say "choke" you have modified and peaked the bandwidth.
    I'll buy that you can feed it with a resistor for broadband, but in my
    humble opinion the construction of a grain of wheat bulb won't get up into
    the UHF region with noise. Do you have any idea of the output level of this
    circuit?

    Jim
     
  13. Phil Hobbs

    Phil Hobbs Guest

    I still like the flashlight/photodiode trick. You can get a really good
    calibration just from the dc, and can calibrate the frequency response
    with a spark plug.

    Cheers,

    Phil Hobbs
     
  14. Bill Turner

    Bill Turner Guest

    ORIGINAL MESSAGE:


    "Phil Hobbs" < wrote

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    I generally use a pipe wrench, but I'll try anything once.

    Bill, W6WRT
     
  15. Tim Williams

    Tim Williams Guest

    I've got a gaussian noise generator, some SS in the power supply, tubes
    everywhere else, found it on the curb and apparently works. Uses a pair of
    6D4 thyratrons in magnetic fields for the noise.

    Tim
     
  16. John Larkin

    John Larkin Guest

    GR?

    John
     
  17. John Larkin

    John Larkin Guest


    What's the light-flash waveform look like from a spark plug? What do
    you drive it with?

    Don't you have gobs of femtosecond lasers around your place?

    John
     
  18. Clark

    Clark Guest

  19. Tim Williams

    Tim Williams Guest

    Come again?

    Tim
     
  20. K7ITM

    K7ITM Guest

    Jim wrote: "...what is a good source of electronic broadband noise
    from low HF through high UHF -- say, 5 to 500 MHz.?"

    A linear feedback shift register. Small, repeatable. 500MHz should be
    no particular problem these days. (There's an idea for some IC
    manufacturer...32 bits clocked at 1G/sec repeats every 4 seconds, which
    would be OK, but I'd prefer 40 or more bits. Should fit nicely into a
    5 pin SOT-23: power, gnd, reset, out, ...)
     
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