How is a function generator connected to an oscilloscope?

Discussion in 'General Electronics Chat' started by zero_coke, Jun 3, 2010.

  1. zero_coke

    Thread Starter Active Member

    Apr 22, 2009
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    I'm having trouble making this circuit work:

    [​IMG]
    How come I'm not getting a reading in any of the multimeters??? Does a function generator output an AC or DC signal? I clicked AC as its output for the oscilloscope, but these meters still don't give me a reading. Oh, and when I switch from a sine wave to a square wave, I get readings on the meters, why does this happen?
     
    Last edited: Jun 3, 2010
  2. cyberfish

    New Member

    Jun 3, 2010
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    Have you tried grounding the - (in addition to the common) of the signal generator?
     
  3. zero_coke

    Thread Starter Active Member

    Apr 22, 2009
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    How can I do that? And where do I put it??
     
  4. Jojo_B

    New Member

    Nov 17, 2009
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    Seems to me you can either put another ground symbol at any node along the -(negative) path...or have the negative wire and the ground wire coming out of the functional generator tie into a node together. I haven't used Multisim in a while though.

    It looks like you checked the right box on the meters...I am assuming they spit out the RMS value.
     
    Last edited: Jun 3, 2010
  5. zero_coke

    Thread Starter Active Member

    Apr 22, 2009
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    But how come I'm getting a value of 0 amps and 0 volts?
     
  6. Jojo_B

    New Member

    Nov 17, 2009
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    Function Generators typically don't have a negative output...as the general output fluxuates from positive to negative voltage (if there's no DC offset). I'm not sure why your Multisim model has one, unless it can double as a DC source or produce multiple waveforms.

    You are getting 0 because there is no path to common.
     
  7. zero_coke

    Thread Starter Active Member

    Apr 22, 2009
    294
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    Yeah I was wondering the same too, until I saw those BNC cables having one side that goes into the 50 ohm port in the function generator, and the other end had 2 prongs of a red and black wire. Can you suggest anything at the moment? I need to make this LC circuit resonate at 117 KHz, but I'm not getting any readings on any of the multimeters. What do you mean by "no path to common"?
     
  8. Jojo_B

    New Member

    Nov 17, 2009
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    Create a new wire that connects like the attached picture. Or don't even connect to the negative side of the Function Generator at all...just combine with the common port.

    untitled.JPG
     
  9. zero_coke

    Thread Starter Active Member

    Apr 22, 2009
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    This is really strange. When I did what you said Jojo, and I clicked SQUARE WAVE, the voltmeter seemed to output the correct value, but now the currents are like 85 amps and 0 amps, and I'm confused as to why. I'm not even sure if I placed these multimeters in the correct spots to read the current and voltage. I mean, the capacitor stores energy, and the inductor stores as well, so I don't know ... I'm just confused. When I switch to SINE WAVE, I get completely different and absurd numbers for the current and voltage...maybe the current is in mA and mV for volts when its a sine wave?
     
  10. Jojo_B

    New Member

    Nov 17, 2009
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    Well, the RMS voltage/current is going to be different for the different waveforms, for instance:

    VRms = Vpeak/sqrt(2) - Sine Wave
    and
    VRms = Vpeak - Square Wave

    So it makes sense that the values are different for different waveforms.
    Your ammeters and voltmeters look to be in correct spots, if those
    are the values you are intending to measure. Since those components are in parallel, your voltmeter should be the same waveform as seen in your oscilliscope.
     
    Last edited: Jun 3, 2010
  11. Jojo_B

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    Nov 17, 2009
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    Last edited: Jun 3, 2010
  12. zero_coke

    Thread Starter Active Member

    Apr 22, 2009
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    I've read the link you gave me Jojo, but what does it mean that "the current drops to 0 at resonance"? Does that mean the LC tank circuit just oscillates back and forth between the inductor and the capacitor and loses no energy? Is that an ideal LC circuit or in real life it has to take "some" current from the power source to maintain the oscillation because in a real life inductor/capacitor some stuff is lost during charging/discharing right?
     
  13. cyberfish

    New Member

    Jun 3, 2010
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    In real life, everything has resistance, so some energy will be burnt out as heat.

    And yes, ideally they oscillate back and forth indefinitely.

    Note that only resistances can consume energy. Ideally, both inductor and capacitor have purely imaginary impedance (purely reactive), so they will never consume energy.
     
  14. zero_coke

    Thread Starter Active Member

    Apr 22, 2009
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    So then in real life, this LC circuit will consume "very little" current at resonance right?
     
  15. cyberfish

    New Member

    Jun 3, 2010
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    Yeap.

    It's like plugging a huge capacitor into the wall.

    On the rising edge it will charge up and on the falling edge it will discharge. It won't be consuming any energy even though there is a huge current going back and forth.

    Of course, the electricity company won't be too happy about that, because while you won't be paying anything (since you didn't use any net power), the currents flowing back and forth will cause energy loss on their lines, and force them to use thicker wires.
     
  16. zero_coke

    Thread Starter Active Member

    Apr 22, 2009
    294
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    Wait so ok I'm a bit confused on the topic of capacitors in AC circuits...doesn't the current change direction, say, 60 times per second in conventional outlet? If you put in a capacitor into a wall outlet, the capacitor would get charge and discharge 60 times a second?

    And in the link you sent me Jojo, what is the AC voltage they used in that LC tank circuit?
     
  17. cyberfish

    New Member

    Jun 3, 2010
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    Exactly. The current will be changing direction 60 times per second (60Hz), too.
     
  18. Jojo_B

    New Member

    Nov 17, 2009
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    According to the Spice Model:
    v1 1 0 ac 1 sin
    .ac lin 20 100 200

    So, a standard sine wave with an amplitude of 1 volt, with a frequency sweep from 100 to 200 Hz.
     
    Last edited: Jun 4, 2010
  19. zero_coke

    Thread Starter Active Member

    Apr 22, 2009
    294
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    Hey Jojo, in that same link, what does this row mean?

    freq i(v1) 3.162E-04 1.000E-03 3.162E-03 1.0E-02
    - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
    1.000E+02 9.632E-03 . . . . *
    1.053E+02 8.506E-03 . . . . * .
    1.105E+02 7.455E-03 . . . . * .
    1.158E+02 6.470E-03 . . . . * .
    1.211E+02 5.542E-03 . . . . * .
    1.263E+02 4.663E-03 . . . . * .
    1.316E+02 3.828E-03 . . . .* .
    1.368E+02 3.033E-03 . . . *. .
    1.421E+02 2.271E-03 . . . * . .
    1.474E+02 1.540E-03 . . . * . .
    1.526E+02 8.373E-04 . . * . . .
    1.579E+02 1.590E-04 . * . . . .
    1.632E+02 4.969E-04 . . * . . .
    1.684E+02 1.132E-03 . . . * . .
    1.737E+02 1.749E-03 . . . * . .
    1.789E+02 2.350E-03 . . . * . .
    1.842E+02 2.934E-03 . . . *. .
    1.895E+02 3.505E-03 . . . .* .
    1.947E+02 4.063E-03 . . . . * .
    2.000E+02 4.609E-03 . . . . * .
    - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -


    I get the freq column, and I get the i(vl) column, but what are those bolded values to the right? What do they represent?

    I'm trying to simulate the same experiment, and at 100 Hz I'm getting a current reading of 9.632 mA. It seems correct my simulation, as I am getting the same reading for all the currents at all of the frequencies from 100 to 200. However, I don't know what those bolded values mean.

    And one more question:

    If I need a really strong magnetic field in an LC circuit, do I need to increase the capacitance or decrease it? I know a higher current will produce a stronger field, what about voltage? And frequency? How do these affect the magnetic field produced?
     
    Last edited: Jun 5, 2010
  20. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,536
    It is a graph shown sideways. Resonance is a funny phenomena, it does some strange stuff. If you have the math down try looking at a series resonant LC circuit, and figure the voltages across the capacitor or coil at resonance.
     
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