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Help me with my oscilloscope

Discussion in 'General Electronics Discussion' started by kap, Jul 17, 2013.

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  1. kap

    kap

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    Jul 15, 2013
    Oscilloscope Problem: probes are connected to oscilloscope but I am not measuring any thing by probes.when I drill down to the smallest rang of voltage in my oscilloscope I see periodic voltage spikes. I carefully checked ground connection and it is well connected. So what could be the reason?
     
    Last edited by a moderator: Jul 17, 2013
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Does your scope have a calibration output?

    Most scopes have an output on the front panel to allow you to check the calibration of your probes.

    If you connect th probe tip to this, you should see a square wave.
     
  3. kap

    kap

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    Jul 15, 2013
    yes it has a calibration output clip on the front panel.

    yes i see the square wave when i connect the probe to the calibration output.

    ok, one more thing related to that, when i remove the probe wire from oscilloscope then there is no spikes,only when i connect the probe wire to Oscilloscope it shows the spikes,
     
    Last edited: Jul 17, 2013
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    OK, since you get a square wave with the probe connected to the calibration point, we know your scope is working (always a good thing to know).

    What are you connecting your probe to?

    Do you see a spike when you connect and disconnect it? (If so, the problem might be that you have selected AC coupling rather than DC coupling).
     
  5. kap

    kap

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    Jul 15, 2013
    I am not connecting any thing to measure with prob,
    coupling is DC.
     
  6. gorgon

    gorgon

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    Jun 6, 2011
    Have you shorted your probes to GND when you 'drill down'? If not they may pick up ambient noise from around you. From the Power grid or light fixtures.
     
    Last edited: Jul 17, 2013
  7. kap

    kap

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    Jul 15, 2013
    I think i have not shorted the probes to GND.
    so How to short probes to ground ?
     
  8. gorgon

    gorgon

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    Jun 6, 2011
    Connect the tip of the probe to the ground clip.
     
  9. kap

    kap

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    Jul 15, 2013
    Ok, but if i will connect the tip of the prob to ground clip it shows square wave.
     
  10. BobK

    BobK

    7,682
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    Jan 5, 2010
    Really? Are you sure you are not connecting it to the calibration lug?

    Bob
     
  11. gorgon

    gorgon

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    Jun 6, 2011
    What is the frequency of the square wave?
     
  12. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Your probe will typically have an alligator clip and a pointy probe.

    Clip the alligator clip to the pointy part.

    Now what do you see?

    Oh, and what make/model is your scope?
     
  13. kap

    kap

    44
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    Jul 15, 2013

    yes, really
     
  14. kap

    kap

    44
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    Jul 15, 2013
    sorry i mean i connect the tip of the prob to calibration clip and it shows square waves, and if i connect the tip of the probe to GND clip which is alligator type clip the wave form shows NO SPIKES at all, so from where it is picking the spikes when it is not connected to GND?
     
    Last edited: Jul 18, 2013
  15. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    From the air...

    Have you heard of radio?

    Electromagnetic noise is inducing small voltages in the scopes probes which are being displayed by the scope. Some of them may not be quite so tiny.

    They come from things like electric motors (especially things like compressors turning on and off), and many other electronic devices.

    You live in a sea of electromagnetic noise. Now you have a way to see it.
     
  16. kap

    kap

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    0
    Jul 15, 2013
    O YES, I got it,thank you for opening my eyes. wooooo!

    But one question comes up in my mind: if the prob is able to catch Electromagnetic noise waves from the atmosphere ,then the oscilloscope measurements will never be correct because it will show the the signal of the point you are measuring+Electromagnetic noise in the atmosphere,I think it maters a lot when you working on micro Volts.
     
    Last edited: Jul 18, 2013
  17. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    It can be a problem, but remember that you see this noise when your probe is open circuit. The only load is the 10M or so input impedance of the scope.

    In practice, when you are measuring something, the impedance will be (generally) low enough to effectively short out the noise, OR your readings will be a higher voltage and will swamp the noise.

    If you have a high impedance source AND low level signals, then the noise can become significant -- as also can the effect o the probe on the circuit (in fact, all this requires is a high impedance source).

    It is similar to the problems you can have measuring voltage using your voltmeter. The input impedance of your meter can load down the source and give you an incorrect reading.
     
  18. kap

    kap

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    0
    Jul 15, 2013



    I understand what you are saying but not 100%,
     
  19. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    OK, here is something to try.

    Get a 9V battery and use your multimeter to measure the voltage across the battery.

    Make a table with 2 columns, Ohms and Voltage. In the first row place 0 in the ohms column (because you didn't have a resistor) and whatever your measured voltage was.

    Now get a resistor between 100 ohm and 1k and place it in series with the battery and the multimeter (let''s say you use 220 ohms) Note in your table the resistor value and the measured voltage. The voltage is the same, right?

    Now repeat this again with a resistor between 1k and 10k, and again with one between 10k and 100k. You might start to see a small and practically incosequential change.

    Now do it again with resistances between 100k and 1M, and then with a resistance between 1M and 10M.

    What do you see now?

    If your first reading was 9.601 volts and your meter was perfect, the readings would always be the same 9.601 volts.

    However your meter is not perfect and forms (in conjunction with the series resistor) what is effectively a potential divider. This means that the meter only sees part of the voltage.

    This series resistance works exactly the same way as the impedance of a signal source. There may be a significant voltage present, but you cannot measure it because the impedance of your measuring equipment is too low.

    This is often the case with noise that is picked up by some stray wire. It will only affect high impedance devices because the signal itself has a high impedance. A low impedance device will only see a very small fraction.

    Go back to the table you drew above and add a third column. In the third column place the difference between the voltage measure in that row and the voltage measured with no resistor.

    This swaps things around. If you had a source of 9V with the impedance of your meter, the third column indicates how much of that you would see across a load represented by the resistor.

    As you can see, for low to moderate resistance values the voltage is very, very tiny. But it increases dramatically as the resistance rises.

    If that 9V were now a noise source generating peaks of 9V, that third column would indicate the height that these noise spikes could reach on your oscilloscope.

    If you're following along, you might ask "How does this relate to the oscilloscope probe problem, because there was no resistance there -- it was open circuit?"

    The answer is that the signal is AC and there are stray capacitances and inductances which effectively close the circuit and induce the current. Impedance is more than just resistance -- the example above used DC where impedance = resistance.

    If you're interested, you can attach various resistors across between the probe and its ground connection and see how the noise varies. You could also do the same with various capacitances and inductances, but don't try to make too much sense out of what happens with them as this is actually very complex.

    edit: you'll never understand it 100%, but you'll get closer and closer to that.
     
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