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Tektronix 6064 Differential Probe question

Discussion in 'Electronic Equipment' started by moonlightbambi, Apr 6, 2004.

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  1. Hi,

    I recently acquired a 6046 differential probe from tektronix.

    I'm trying to use it measure current , across a .1 ohm resistor
    current sensor.

    I notice that there is over and under shoot. Coesn't really look real
    to me.

    I don't have a manual, unfortunately.
    Looking for advice about how to adjust this thing, and what to expect
    steps and results:
    1. Turning a laser diode on and off at 3 kHz.
    2. When I put it across the current sense resistor, I see an
    overshoot on the rising edge, but shen I turn the probe around, I
    don't see it. THe signal is about 50 mV It seems that the channels act
    differently, not differentially.......
    3. I adjusted the compensation on the .4 V square wave on the scope.
    I left one side floating (or grounded) and adjusted the other to make
    it square. It has a little overshoot too, and I can't get rid of it.
    4. Strangly, when I turn the laser on, It ramps up, but the
    "overshoot" is there even during ramp up.Perhaps it's pickup on some
    other signal. But I don't see why a differential probe would do this.

    Any ideas on the cause of this overshoot? I'm trying to measure in the
    2 mv accuracy range.

    Anyway, anyone have a manual?

    thanks,

    Adam
     
  2. Bob Koller

    Bob Koller Guest

    Do you have a ground from the metal shell of the probe tip(s) to the circuit
    ground?

    This is necessary to prevent ringing.

    bob
     
  3. There is a ground from the probe ( a wire and an alligator clip) which
    is grounded to circuit ground.

    I, unfortunately, don't have the tip accessories, ( I'm working on
    it). The tip of the probe does have a cylindrical ground around the
    pointy tip. since I'm going directly from the tips to the circuit,
    that point in the grounds isn't grounded.

    Are you implying that they need to be grounded independently?

    thanks!!!!!
    adam
     
  4. TekMan

    TekMan Guest


    the p6064 needs to be grounded seperately.
    And do not drive out the common mode voltage range (appr. 4 Volt
    without tips, higher with 10:1 or 100:1 divider probe tips).


    hth,
    Andreas
     
  5. Jon Goguen

    Jon Goguen Guest

    The probe will ring at a frequency determined by the parallel RC circuit
    consisting of the probe capacitance and the inductance of the ground
    reference path. To keep the ringing frequency high--so it will only
    distort signals with very fast transitions--you want short ground leads.
    That said, how short is short enough? A good rule of thumb is that a
    ground lead provides about 25 nH of inductance per inch. Using the 5"
    lead provided with the 6046 gives 125 nH. The input capacitance is 3
    pf, which gives a resonant frequency of ~ 300 MHz. This is well above
    the 100 MHz bandwidth of the probe. I think the lack if local grounding
    of the probe tips is not likely to be the problem. Tektronix doesn't
    seem to be much concerned with provisions for local grounding of the
    tips either. I have the full accessory kit, and, with the exception of
    a pair of coaxial probe tip jacks intended for circuit board mounting,
    no local ground other than the ground lead, which comes in 5 and 12"
    inversions, is provided.

    I think the high frequency response of the two inputs is probably
    unbalanced. The Tek procedure for checking high frequency CMRR
    performance involves applying an identical 50 MHz signal to both probe
    tips and ensuring the output is less than 0.1% of the input. They make
    a special probe tip fixture for this. If you have a signal generator, I
    think you could cobble something together with a 50 ohm feed-through
    terminator and three goal post style BNC Ts (three Ys might be even
    better). Connect the terminator to the generator followed by the base
    of one of the Ts. Connect the other two Ts to the arms of the first one.
    Turn these outer Ts so the open ports of each are aligned vertically.
    Connect the two bottom ports together with a BNC jumper. Now, twist the
    Ts so the jacks in the terminators are at the correct distance to accept
    the tips of the 6046. Connect the ground lead from the probe to the
    generator ground. Set the vertical deflection factor on the probe amp
    to 1 mV and the generator for 2 V p-p. You should see a waveform of
    less than two divisions on the scope.

    If you try this and it fails, don't despair. The probe may be fixable
    by calibration, which you could probably manage if you invest in a
    manual.


    Good luck!

    Jon

    Cheers,
     
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