Physics307L F08:People/Smith/Notebook/1

From OpenWetWare

Jump to: navigation, search

Contents

Lab 1: Oscilloscope

Setup

Insert one end of a BNC connector into CH1 of the Digital Storage Oscilloscope (hereafter referred to as the "DSO". Yes, I am the sort of person that reads the EULA when I install software. Don't ask me why.) Insert the other end of the BNC connector into the function generator, I just put it in the SYNC connector. Turn on the DSO and function generator, select German as language in the DSO and realize your mistake. Correct it. On the function generator, depress the "100" pushbutton in the RANGE Hz section, turn the Frequency dial to around 2, and depress the pushbutton that looks like a sinewave in the Function section. Then hit the auto set button in the upper right hand corner of the button panel of the DSO. The amplitude, offset, trigger phase and sym dials were untouched, just to see what the function generator would output.

I chose to directly insert the BNC connector into the DSO and function generator, forgoing a T-connector and terminator; if I ran into problems, I could always unhook my connector and insert the T-connector and terminator.

After fiddling around with the 'scope a bit, pressing some buttons on it's interface, I realized that I had made a mistake - I should have inserted the BNC connector into the V(pp) port instead of the SYNC port, as the SYNC port wasn't giving the "sinewave" function that I was looking for, but gave a "sawtooth" function. I don't know why.

Data, measurements, etc.

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)
    • volts per vertical division = 5.00 V
    • Around 2.2 divisions counted, above and below the "axis" (which you might consider "ground") + or - 0.1 divisions or so
    • Vmax = | Vmin | = 5.00Voltsperdivision * 2.2divisions = 11.0Volts
  2. Next, use the cursors
    • By turning the "vertical position" knob for channel 1, and the "horizontal position" knob, you can line up a maximum with the origin. Then, the DSO will give a vertical offset reading both in divisions and in volts. My DSO output 2.16 divisions, or -10.8 volts, meaning that the axis (ground) of the sinewave form was 2.16 divisions in below the origin, or -10.8 volts. The resolution of the DSO's cursors seems to be around 0.04 divisions, which works out to be 5.00Voltsperdivision * 0.04divisions = 0.2Volts, which implies the uncertainty in measurement is 0.2 Volts.
    • Repeating the procedure for the minimum yields 2.04 divisions, or 10.2 V volts. This is surprisingly different from the value of the maximum.
  3. Finally, use the "measure" functions.
    • Pressing the "auto set" button on the DSO in the upper right hand corner of the panel again centers the sinewave form once again. Pressing the "measure" button on the DSO brings up nice measurements on the right hand side of the screen, including the frequency, period, Pk-Pk (peak to peak, a measurement that we happen to be looking for), Max and Min (which I measured in the previous two steps.) The Max displayed is 10.8 V, the Min displayed is -10.4 V, and the Pk-Pk measurement displayed is 21.2 V.
    • Measuring the period: counting some horizontal divisions is easy after pressing "auto set", as the wave has a period of almost exactly 2 full divisions. The DSO also notes that the vertical divisions are 2.50 ms each, although you have to guess that "M 2.50ms" means that.
      • (2.50 * ms / division)(2divisions) = 5.0ms
    • The DSO also measures the period (AND frequency) for you, if you ask it to. In my case, my wave had a frequency of around 202.8 Hz (the reading kept fluctuating, from between 202.4Hz - 203.2Hz), and a period of around 4.935ms (again, it fluctuated between 4.925ms and 4.950ms).

Repeating for different amplitude waves

  1. Just for fun, I'm going to change the frequency as well.
    • Fiddled around with the function generator knobs, pressed "auto set" on DSO again.
    • Waveform looks purty.
    • Press measure, frequency between 310.6Hz and 312.5Hz
    • Period between 3.202ms and 3.220 ms
    • Pk-pk measurement=2.10V, Max=1.06V, Min=-1.04V
  2. It looks like the DSO has problems measuring small DC voltages, as the wave form becomes rather fuzzy when I turn down the voltage. Also, very low frequencies yield an absolutely useless picture on the scope, and the frequency reading just displays "<10Hz". As the frequency is increased to very high numbers, like 150 kHz, even if the voltage is high the waveform looks kind of fuzzy.

Triggering

  • Messing around with the trigger level changes when the DSO will start to measure. It's a little strange to describe, but Koch reminded me of how 'scopes used to work (I read this one Wikipedia this morning, also) - they were CRT, and would begin scanning once a threshold voltage was reached. Our DSO is a little nicer, because it can differentiate between when the waveform is rising and when it is falling, and different coupling types can be used, etc...
  • Messing around with the DC Offset on the function generator is kind of interesting. While leaving the DSO trigger level untouched, increasing the DC offset will raise the entire waveform on the screen, but it will also appear to move to the right - this is necessary because the DSO likes the waveform to cross the t=0 axis where the trigger level is met, as long as the M Pos: 0.00 ms (or, in non-DSO, real English as long as the time offset is zero.)

AC Coupling

  • Note: the Wikipedia page on this subject isn't useful here.
  • Low pass filter, as explained by Koch in class.
  • Remember from Physics 161, how capacitors charge with respect to time when a voltage is applied - the charge on the plates build up until the V in Q=CV is the same as the Voltage applied. This happens in an exponential decay way; asymptotically, the voltage of the plates will approach the applied voltage. For this to happen, the charge Q on the plates does the same thing. The graph of I vs. t will look like the derivative of Q vs. t with respect to t, since I = dQ/dt. (Koch also drew some nice graphs on the blackboard to demonstrate these concepts.)
  • The oscilloscope really has a low pass filter built in - it's a capacitor and a resistor in series. The capacitor charges, or discharges, since the function generator is applying some voltage across the 'scope's lead. Using the sawtooth function makes this a bit easier; sudden changes in voltage will make "AC coupling" show stuff.
  • T.A. showed me how to T off the output (I don't know why it got mad, but for some reason it works. Ok. Bad pun.) Put T on output plug of function generator, put in the lead from channel 1 of the DSO to one side of the T, hook up another BNC connector from the other side of the T to channel 2. Set channel 2 to AC coupling, make sure channel 1 is on DC coupling, make sure function on function generator is sawtooth, mess around with the frequency, amplitude on function generator and volts/div of each channel on the DSO. Then you get a picture something like is on the F08:Labs/Oscilloscope Lab manual
  1. Measure the fall time
    1. After getting the picture as described above, and as directed in the lab manual, set the DSO to measure the fall time of channel 2. It was displayed to be 239.6ms to 241.0ms.
    2. Using the cursor to measure the distance between the peak and 10% value (as directed in the lab manual) yields around 8.0 volts peak value at t=0, 800 mv 10% value at 58.00 ms.
      • This is drastically different than what the DSO's "fall time" measurement is. Why?
      • Because I was measuring it very badly, somehow.
      1. A new measurement, using the cursors and using the Type "Time" was much better. This time, I used the cursor with type "voltage" and set the cursor to 800 mv (which is, coincidentally, 10% of 8.00 v), and used the horizontal position knob to move the waveform until the channel 2 waveform intersected the t=0 axis at the cursor. Then, setting the Type to "Time", I set the cursor to the t=0 point, and read the Delta given.The good measurement yields a delta of 200.0ms, meaning a fall time of 200.0ms.
        • On second look, this doesn't appear to be valid. I liked the first measurement I got with my cursors, and so did instructor Koch. Neither he nor I could quite figure out why the fall time being presented by the DSO was around 4 times as large as it appears to be.
      • I will assume the fall time is around 58.0ms. That was at frequency ~ 2.343Hz - 2.430Hz. I'll try to find an error here. As instructor Koch suggested, I'll change the frequency.
      • Frequency ~ 4.798Hz - 5.061Hz: 57.00ms.
      • Frequency ~ 7.257Hz - 7.262Hz: 55.00ms

Oscilloscope Lab Summary

  1. Brief summary of what you did (linking to the lab manual page is OK)
  2. Add links to all wiki pages that contain your notebook entries. This is likely only one page.
  3. Report your value for the fall time using AC coupling.
    • Include error bars!
    • Explain how you measured this (briefly)
  4. What did you learn?
    • It's OK if you didn't learn anything. But include things you are still confusing.
  5. What did you explore outside of the standard lab procedure? Anything interesting?
  6. What could make the lab better next year?

Links to other entries

Wednesday Labs
Personal tools