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Tek 475 trace/probe problem

Discussion in 'Electronic Design' started by amdx, Sep 11, 2013.

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  1. amdx

    amdx Guest

    Hi Guys,
    I recently acquired a Tektronix 475 scope.
    While testing it with the internal squarewave calibrator
    I noted the trace was wide, I tried the second probe, same thing.
    Then I happened set the probe cable across the top of the scope,
    when I did that the trace got thin or more normal.
    I tried a third probe it is better but the trace width varies somewhat
    depending where the probe is positioned, any ideas?

  2. mike

    mike Guest

    As in horizontal width on screen?
    As in thickness of the green line?
    As on RF pickup?
    It's a sensitive scope with wide bandwidth.
  3. amdx

    amdx Guest

    Don't know the cause, trace width Ok with no probe attached.
    Yes, pulling bandwidth limiter to 20Mhz solves the problem.

    Here's a photo of calibrator output.
    Here's a photo of calibrator output with probe cable laid across top of
    scope case.
    Note difference in trace width.

    Thanks, Mikek
  4. John S

    John S Guest

    What is your vertical sensitivity setting?

    Try connecting your ground lead to the probe tip and move the probe
    around. You may have some pickup from a florescent lamp or something.
  5. amdx

    amdx Guest

    Calibrator output is 300mv P to P.
    Scope is set to 10mv scale with a X10 probe.

    Connecting ground lead to tip, the trace looks OK.
    Moving it around doesn't change trace.

    My probes are B&K PR-45. I thought these were 35Mhz bandwidth, but a
    quick search yielded no reference, so now I'm not sure.
    I'll search more later, gotta leave for work.

    Thanks, Mikek
  6. John S

    John S Guest

    Do you see this on both input channels?

    The two pics you posted show that the voltage amplitude is not the same
    with probe position (assuming you did not change vertical sensitivity).
    You might have an open input on channel 1. Otherwise, you may have some
    open probes.
  7. amdx

    amdx Guest

    I'll check the other channel this evening.
    I suspect I did change amplitude between pictures.
    BTW, the scope has a 20Mhz bandwidth limiter and that does get rid of
    the wide trace.
  8. Artemus

    Artemus Guest

    Perhaps an internal broken ground to the input connector?
    Laying the probe on top would capacitivly couple the probe
    shield to that ground.
  9. amdx

    amdx Guest

    It has the same problem on both channels.
    It's late so I'll do some more checking in the morning.
  10. Guest

    could be RF pickup.

    are you located near any AM FM TV or other kind for RF high power transmitter?

  11. John S

    John S Guest

    Okay. It sure looks like a signal getting in somewhere. Probably via the
    probe based on your reports.

    What is confusing is that the probe calibration source must be a very
    high impedance (if your probe is good) to have this kind of
    interference. You still had the probe attached to the calibrator when
    you laid the probe across the top of the instrument, yes?

    The 20MHz filter is telling you that you have an HF signal getting in
    somewhere, I think. Try moving the scope to some other place and see if
    the problem follows. Turn off your lights and anything else while
    experimenting with tracing the source.

    With the probe not shorted and not connected to your calibrator, move it
    all around the area and watch the amplitude. Maybe it will lead you to a

    I have a florescent desk lamp that must be turned off for sensitive
  12. Guest

    I think it's probably just a cheap probe picking something up. For anything to be wrong with the scope I think it would have to be some wierd ass grounding problem.
  13. mike

    mike Guest

    Assume you've done obvious stuff, like verify continuity between the
    connector and the ground wire at the probe end?

    Some probes and some scope inputs have resistance in the ground
    lead. If you put current into the ground, it blows the resistor
    and the ground is no more.

    Have you tried to trigger the scope on the noise?

    I noticed ambient noise went way up when I switched from
    incandescent to CFL lighting.
  14. amdx

    amdx Guest

    Yes, the probe was attached to the calibrator.
    The calibrator is low impedance, 300mv at 30ma.

    There is not much turned on around my bench, but I understand your
    point. I have two CFLs that both create noise at about 45khz, but they
    are of.
    I used delayed sweep to zero in on the noise, it is about
    approximately 1Mhz.
    I used a 4ft piece of coax with a BNC connector on one end, the other
    end I frayed about two inches of the shield out as a ground plane with
    the center condutor as the vertical. It picked up the CFLs fine but I
    don't see the 1Mhz.
    I'll take the scope on a road trip and see what happens.

    BTW, I received a P6136 350Mhz probe yesterday, It has the same noise
    problem. Also, My Tektronix 2465 scope at the same position does NOT
    have this noise problem.

    Thanks, Mikek
  15. amdx

    amdx Guest

    Well it is a B&K probe, but I have two of those and a third other
    brand, and yesterday I received a P6136 Tektronix probe that has the
    shows the same noise when I attached it.

    Thanks, Mikek
  16. amdx

    amdx Guest

    I'm not, But I'm taking the scope on a road trip shortly to see if
    I'm getting away from a noise source.

    Thanks, Mikek
  17. amdx

    amdx Guest

    I've tried four different probes, plus they all work fine on another
    I haven't, I can try but I think the amplitude is to low.
    I did use delayed sweep and it is roughly 1 Mhz.
    Yes, I have two CFLs that sing at about 45khz.

    Thanks, Mikek
  18. amdx

    amdx Guest

    I have a squarewave from the calibrator displayed.
    It has noise. I measured the frequency of the noise,
    it is about 110Mhz.
    I took the scope to another location, it had the same problem.
    Here again, is a picture of the noise.
    This happens on both channels, both probes.

    I also notice something else, and this again happens on both channels,
    both probes. I have the probe tip connected to the ground of Channel 2
    the BNC connector ground. On the screen note the 110Mhz signal.

    Increasing the timebase for a longer time period, you can see the signal
    has some modulation. The modulation is unchanging.

    Any Ideas?

  19. Tim Williams

    Tim Williams Guest

    110MHz is either top of the FM band (modulation would be multipath, but
    wouldn't be modulated that fast) or bottom of the aviation band
    (specifically, radionav at 108-118; comms are 118-137). Got any stations

    Try ferrite beads on the probe cable, as near the scope as possible.

    If you're concerned that it's from the scope, try taking off the cover and
    probing around in air with the probe tip, see if you can find something
    squealing. If you're concerned that it's something else, attach a loop
    antenna to a long enough BNC cable and probe around your area.

  20. Artemus

    Artemus Guest

    Since you have 2 (or more) scopes, use the second one to look at the cal out
    on the noisy scope. This will rule out, or point to, a noisy cal generator. Be
    sure to get your grounds connected. I still think bad connector grounds are
    more likely on the noisy scope.
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