Physics307L F09:People/Ritter/Notebook/070827

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Basic waveform measurement

  1. Hook up the output of the function generator to the oscilloscope
         * Set the function generator to output a sine wave. Say 200 Hz maybe, or pick your frequency.
         * Use BNC cables (why are they called BNC cables?)
         * Should you use a T connector and a terminator? 
A T connector will be necessary to view signals on Ch. 1 and Ch. 2. However, I don't think it will be necessary to use a terminator since my function generator has a 50ohm output, and generally this is what a terminator supplies.see comment
Steven J. Koch 02:49, 29 August 2007 (EDT):Good analysis.  I agree that the terminator is not important.  I think, though, it could be important for high frequency signals and high power.  For high power, you need something to dissipate the energy (besides the scope), and for high frequency, I think you could possibly get troubling reflections, because even though the cable is 50 ohm, the scope is high impedence.  A 50 ohm terminator on a "T" could thus minimize reflections from impedance mismatching.
Steven J. Koch 02:49, 29 August 2007 (EDT):Good analysis. I agree that the terminator is not important. I think, though, it could be important for high frequency signals and high power. For high power, you need something to dissipate the energy (besides the scope), and for high frequency, I think you could possibly get troubling reflections, because even though the cable is 50 ohm, the scope is high impedence. A 50 ohm terminator on a "T" could thus minimize reflections from impedance mismatching.
  2. Fiddle with the oscilloscope settings so you can see the sine wave on the screen.
  3. Measure characteristics of a sine wave
         * Measure peak to peak voltage (thus amplitude) and measure the period (thus frequency)
              1. First, use the grid on the oscilloscope screen ("divisions" are the dotted lines)
              2. Next, use the cursors
              3. Finally, use the "measure" functions. 
         * Repeat this for a few different waves: Very large amplitude; very low amplitdue; large DC offset
               o Are there waveforms that the oscilloscope cannot measure properly? 
The oscilloscope seems to be incapable of measuring any large signals. I seem to be unable to measure any voltage over 1V with any accuracy.see comment
Steven J. Koch 02:50, 29 August 2007 (EDT):That is strange.  Definitely you should be able to...not sure why you couldn't
Steven J. Koch 02:50, 29 August 2007 (EDT):That is strange. Definitely you should be able to...not sure why you couldn't
Triggering
  1. Re-read Wikipedia section about triggering
  2. Common way to trigger is on a rising edge (what does this mean?). What happens to the signal when you use different triggers? Be able to explain this orally. 

According to Wikipedia: an edge-detector that generates a pulse when the input signal crosses a specified threshold voltage in a specified direction. From what I can see, rising edge triggering takes the signal and records it once it rises above a voltage that I preset.

AC Coupling

This is a tricky concept at first!

  1. This Wikipedia article on capacitive coupling isn't too helpful
  2. Apply a large DC voltage to the oscilloscope input (we'll have to figure out how to do this). Compare DC coupling with AC coupling. You may need to adjust the triggering. Which mode is better for viewing any "ripple" on the DC voltage?

The AC coupling is the only setting that shows any change.

     Measure the fall time of the AC coupling
         * Function generator: Square wave; zero DC offset; amplitude about 8.6 V (I used 720mV)
         * Use cursors to measure fall time (peak to 10% value)

Using the cursors I got a fall time of 52.4ms. Initially I was getting very small numbers for this. It was pointed out that I was not allowing the signal to drop off to the necessary 10%. I had to lower the input frequency until I could get the entire signal.

         * Use "measure" function to measure fall time

I got a measured fall time of 62.5ms

  4. What RC constant does this imply? (See Wikipedia article on rise time

One stage low pass RC network. This is the answer I got from Wikipedia. This RC constant seems to be a formula for relating the designed bandwith of a given oscilloscope to an expect rise time.

  5. How does this compare with the expected value for the oscilloscope? (Can you find the answer on Google?) 
FFT
  1. Find the frequency of a sine wave using FFT "Math" function

I got a frequency of 200Hz. This is exactly what I have my function generator is set to.

  2. Look at the harmonics in triangle and square wave
  3. Compare with what you see on this applet: Fourier series applet
  4. Be able to explain what is going on with an FFT and when it may be useful
The FFT is the fast fourier transform. A perfect sign wave would produce a perfect delta function. This delta function should appear on the oscilloscope at the position of our input frequency. This would be useful if you have a signal that you wanted to find but it was buried in a bunch of other signals.see comment
Steven J. Koch 02:52, 29 August 2007 (EDT):Great description!  The best I have seen & I think you have a good understanding of the FFT.
Steven J. Koch 02:52, 29 August 2007 (EDT):Great description! The best I have seen & I think you have a good understanding of the FFT.
Other
  1. Play with XY mode to make some fun patterns
  2. Build your own low or high pass filter using resistors, capacitors and breadboard.
  3. Measure something else you find in the lab


Lab Summary: Overall I found this to be a very worthwhile experience. Everyday at work I use oscilloscopes, or rather read the the data displayed on them, but I never have really played with them. The most important thing that came out of this lab was the chance to run through a "junior" lab for the first time. The much more open ended format to this type of learning experience is a little difficult, but very worthwhile. I also find myself to be possibly a little behind the curve on my knowledge of electronics. AC coupling vs. DC coupling really doesn't mean much to me and I find myself to be a little confused with some other aspects of electronics. I do, however, believe this type of hands on approach will be the best way for me to learn.

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