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I've got parasites

Discussion in 'Electronic Design' started by Tim Williams, Nov 21, 2005.

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  1. Tim Williams

    Tim Williams Guest

    I know no one can tell me exactly why, but someone can at least give me some
    thoughts here. Ok, my problem. First of all, when I set my Tek 475 to
    2mV/div, 200MHz BW with an open circuit probe, I get about 20mV (it's a 10x
    probe) of noise, primarily at approximately 90MHz. I've tried turning off
    basically everything in the house and that hasn't stopped it. (Amazingly, I
    didn't see any change at all when turning off my computer and monitor just
    five feet away!)

    So I've reassembled my medium power induction test circuit.
    (All neet and shiny with tight 12AWG hookups and bypass caps.
    Same old sextet of STW11NB80's.)

    When I turn on the drive circuit,
    I immediately get something like 0.5V of amplitude modulated noise across
    the ground-return resistor (0.1 ohm, eh probably inductive). It's a perfect
    keyed CW envelope, modulated by the 5-200kHz drive signal as it is.

    I tried sniffing around with a balanced twisted pair with 1" dia. loop on
    the end, but that didn't tell me much besides the circuit is somewhere
    radiating shit!

  2. John Miles

    John Miles Guest

    FM radio station, most likely.
    Sometimes the easiest way to track down low-level stability problems is
    to "build" your circuit in LTSpice. LTSpice has the advantage of
    perfect immunity to environmental effects.

    -- jm
  3. Tim Williams

    Tim Williams Guest

    - I don't know of one any of siginificant power nearby. I'm not in a big
    city. The college has one but I don't think it's that strong.

    There is a local AM geezer (WGEZ, really!) radio station that gets into just
    about everything, but being a non-relatavistic rate of voltage change, I can
    squash that easily enough in most cases, including here.

    For Pete's sake I can't even short the VHF signal at the scope connector,
    how does that work!?

    At any rate, as much as it has to be an external signal, how would you
    explain the keying effect when the drive circuit is turned on? The local
    circuit would have to be acting as a passive resonator or something!
    Well, that won't help much, because I already know the circuit works, on
    paper and in reality. Besides which, simulators tend to go with perfect
    connections. I don't know about LTSpice in particular, but I'm guessing it
    wouldn't know the difference if I had one of the MOSFETs on the other side
    of the room on some ten foot pigtail leads.

  4. Mark

    Mark Guest

    90 MHz would be about right for a college station..

    how far away is it?

    And what about if you disconnect the probe complely from the
    scope...see it then?

  5. John - kd5yi

    John - kd5yi Guest

    You mean, you remove probes and all connections to the scope and short the
    BNC connector at the scope with, say, a screwdriver and the problem is
    still there? If yes, the problem is internal to the scope.

    There is usually a switch near the input that selects DC/AC/GND. Put it to
    GND. Still there? Yes = scope internal problem.

    2 cents, please.

  6. Ian Stirling

    Ian Stirling Guest

    Set the trigger right, and see if you can resolve the frequency accurately.
    Now, tune a FM radio to that freq, and see if it is or isn't.
  7. Tim Williams

    Tim Williams Guest

    Sorry for the confusion, I was using a tee connector. Still, that's
    balls-all inductance, I don't see how the signal can get past a short like
    that, at least for another few hundred MHz.
    With no antenna, or the input GND'd, it shows no signal. Even a 2" stub of
    wire will register a signal.

    87.5MHz is the exact value I get, something like 7 cycles in 8 div at
    10ns/div. Ten percent error smears it over half the damned frequency band,
    so it could be only what, 40 channels on the dial, or one of 40 more that
    don't exist! (under 88MHz) I don't have a frequency counter high enough, or
    any other way to lock it more precisely.

    BTW: there is a college radio station a few blocks down the street(!), but I
    don't know how strong it is (tell me, how many FM radios do you own that
    came with AVC and an indicator? ;).

    Does anyone have an explanation how the induction drive circuit appears to
    be amplifying the signal in modulation to the drive output?

  8. Rich Grise

    Rich Grise Guest

    Turn your TV to channel 6, and listen as you watch the trace. :)

    Or, turn your TV _off_. (or maybe your computer. and unplug the microwave
    and VCR.) ;-)

    Good Luck!
  9. Tim Williams

    Tim Williams Guest

    I tried just about every appliance in the house, including the network (the
    CAT5 does happen to run quite near the experiment... which caused network
    troubles when I was playing with small CW Tesla coils ;), but there was no

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