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Darn those oscilloscopes!

  1. Dec 6, 2006 #1
    I had my lab final today where we were given a mystery box containing either an unknown R and an unknown C, or an unknown R and an unknown L. The task was to figure out what elements were in the boxes and give their values as well, using a function generator and an oscilloscope. I thought mine was broken because i was getting ZERO phase shift between the source voltage and the mystery-box voltage. It turns out that there WAS a phase shift of 4.6 degrees....4.6!!! How on earth can I distinguish 4.6 degrees on a dinky little screen containing 360 of them?

    I even tried to change the horizontal scale to "zoom in" on the phase shift, but it wouldn't zoom in near the axis....

    Does anyone else think this was cruel? How stressed I was
     
  2. jcsd
  3. Dec 6, 2006 #2
    btw this was not an oscilloscope with a zoom-in function...and i've never been taught to use a Lissajous pattern
     
  4. Dec 6, 2006 #3

    berkeman

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    That is a pretty small phase shift, but measurable. I don't know what you mean about "wouldn't zoom in near the axis...." Do you mean the trigger point? You can use the delay trigger function to move the trigger point, or change the trigger amplitude to slide the two waveforms left and right. You may also have to turn down the display brightness to cut down on the trace blooming if you have to look for really fine structures.

    But yeah, using 4.6 degrees in a teaching exercise like this is probably not a good use of the students' time.
     
  5. Dec 6, 2006 #4

    berkeman

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    Oh, and of course you can zoom in to measure the time delay between the two waveforms at a zero crossing. Measure that time delay zoomed way in, then zoom back out to measure the overall period. Also, what frequencies were you using?
     
  6. Dec 6, 2006 #5

    chroot

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    Every oscilloscope on the planet has "zooms" in both axes.

    - Warren
     
  7. Dec 6, 2006 #6
    to Berkeman - 1kHz, and i'm not sure what you mean by "time delay function" but I'm sure we haven't been introduced to it.

    To the other guy: yes this meter lets you adjust the seconds per division (horizontal) and volts per division (vertical), but if i zoomed in with the horizontal...it would give me a zoomed in picture of the waveforms near the peaks but not quite there. I always have measured phase shift from either the peaks or the x intercepts, and I couldn't for the life of me get the scope positioned on either of these, no matter how much twittling I did for the hour given.

    haha, I thought I must of had a pure resistor..but when I measured the resistance it didn't match up. Oh well at least it's not a class where you fail the final lab and you failed the class
     
  8. Dec 6, 2006 #7

    Gokul43201

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    You could turn the time axis knob to increase the sensitivity. Smarter yet, might be to actually turn down the sensitivity real low and count the number of waves it took to make a phase shift of 2pi. It's actually pretty easy once you've used the cursors to find the time period (or frequency).
     
  9. Dec 6, 2006 #8

    berkeman

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    :rofl: Now chroot's "the other guy" :rofl: #2pencil -- that other guy's got some skills.

    Anyway, the delay trigger function that I'm talking about is in the trigger control area on your oscilloscope. turning it adjusts where the trigger time is with respect to the left edge of the oscilloscope. The default is zero delay, and you can generally adjust the trigger point from the right edge of the display all the way to many milliseconds to the left of the left edge of the display. If you have trouble finding this control, ask your TA in the next lab to show it to you, or just download the User's Guide for your lab oscilloscope from the web.

    And most signal generators will go at least up to 1MHz or so -- use higher frequencies to help you figure out reactive component values.
     
  10. Dec 6, 2006 #9

    chroot

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    I guess I just don't understand the problem. You seem to have the right idea -- zoom in close enough so that you can measure the delay between the two sine waves, and then convert that time into phase (you know the period of the sine wave already).

    The reasons you're giving for why this method didn't work ("but it wouldn't zoom in near the axis...." and "but not quite there.") are unfortunately pretty vague, so I honestly don't know what was causing you trouble.

    Are you just complaining that you could not "zoom in" on the part of the waveform you wanted to see? You do realize there are knobs to move the waveform left and right across the display (as well as up and down), right?

    - Warren
     
  11. Dec 6, 2006 #10

    Gokul43201

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    Hmmm...yes, that sounds like it.

    I suppose you couldn't just ask for a lock-in ("please...thank you very much"), could you? :biggrin:
     
  12. Dec 6, 2006 #11

    es1

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    Where you allowed to change the frequency the function generator outputs?
     
  13. Dec 6, 2006 #12
    that's exactly what i'm complaining about! Yes, I tried moving it left and right with knobs (and I think I did try the triggering knob as berkeman stated). Maybe (correct me if I'm wrong), my problem was that I tried moving the waveforms around AFTER I zoomed in, which actually did not allow for much movement.

    I think (again correct me if I'm wrong), maybe I should of moved it before the zoom, which makes more sense overall anyway. I now officially have an oscilloscope phobia
     
  14. Dec 6, 2006 #13
    No, he wanted us to specifically use 1kHz
     
  15. Dec 7, 2006 #14

    berkeman

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    No, no. Don't do that. You should make it a goal to get familiar and comfortable with all the instruments that you'll be using. Even the ones that tend to have dorky user interfaces (like some of our LeCroy oscilloscopes with the badly overloaded context-dependent buttons).

    One of the ways that I learned to be comfortable with instruments like oscilloscopes and curve tracers was to set up the instrument completely before connecting it up to the signal source (or device, in the case of the curve tracer). That meant no fiddling with dials to try to get the display to look the way I wanted. If I knew basically what the signal (or device curves) would look like, I'd set all the dials and buttons up once, and then connect up to make the measurement. That way, you have to have a good idea of what all the controls do, without trying to turn them back and forth randomly searching for the correct setup.

    I still use this technique in a lot of my work in the lab. Give it a try!
     
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