User:Paul V Klimov/Notebook/JuniorLab307L/2008/09/08

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Oscilloscope Lab

SJK 23:31, 16 September 2008 (EDT)
23:31, 16 September 2008 (EDT)
OVERALL FEEDBACK. Very nice raw data notebook. You have careful notes and I think you would have an easy time remembering what you were doing. The most serious thing missing from your raw data is a clear description of the equipment you were using. Notably: what model oscilloscope? What kind of function generators? Any details about cables necessary? Of course a picture would help, especially if you could read the model numbers. The next most serious thing missing was the description of your settings you were using on your function generator, at least in the beginning. This is a must do for the future. Record as much of this information as possible, imagining the kinds of things you would need to know if you were going to try to exactly repeat these experiments next fall.

In this lab, I will learn how to use an oscilloscope and become familiar with its features. After doing so, I will attempt to find the time constant of the oscilloscope. This will be done by taking advantage of the AC coupling feature and a low frequency signal provided by the generator.

Click here to skip to the lab summary

SJK 23:17, 16 September 2008 (EDT)
23:17, 16 September 2008 (EDT)
You do have a link to your summary, and in your summary you link to the oscilloscope lab instructions ... but it would be appropriate to link to your methods here in your raw data to make it easier to follow what you're doing.

Becoming familiar with the oscilloscope

  • The oscilloscope controls are divided into 3 sections that let you change the appearance of the wave: vertical, horizontal and trigger.
  • There are also useful controls at the top of the oscilloscope which let you take measurements. These controls will not only make calculations about the wave for you, such as frequencies, periods, peak to peak voltages, etc., but they will also let you use a cursor to obtain these values yourself. This will probably be useful when I check my own calculations.
  • The Position controls will let you move the waveform and rescale it.
  • There are several menu buttons: "Ch1 Menu", "Ch2 Menu", "Horiz Menu", and "Trig Menu". After pressing one of these buttons, you can adjust various settings using the 'ATM buttons' on the side of the oscilloscopes screen.
  • The trigger controls let you 'stop' the waveform at the position targeted by the vertical and horizontal cursors. However, mine only stops when it is on 'Normal' mode. I'm not sure if I understand exactly what is going on here (I will read about this later)

Characteristics of the sine wave

SJK 23:13, 16 September 2008 (EDT)
23:13, 16 September 2008 (EDT)
It's good how you record the oscilloscope data, including units, measurement method, and estimate of uncertainty. That is great. However, you did not record anything about your function generator setting. At least as far as I can see. That would have been essential if you wanted to be able to repeat this in the future and to know what you had actually done.

Period:

  • Measure:5+-.01ms
  • Cursor:5ms
  • Grid:5+-.1ms

Peak to Peak Voltage:

  • Measure:12.8+-.2V
  • Cursor:12.8V
  • Grid:13+-.2V

Characteristics of the triangle wave

Period

  • Measure:5.01ms+-.1ms
  • Cursor:5.00ms
  • Grid:5.00ms

Peak to Peak Voltage

  • Measure:12.9+-.1V
  • Cursor:12.9V
  • Grid:13.1+-.2V

Characteristics of the square wave (unfinished)

Period:

  • Measure:34.1+-.1ms
  • Cursor:34.40ms+-.2ms
  • Grid:

Peak to Peak Voltage

  • Measure:
  • Cursor:
  • Grid:
  • In investigating the different wave forms, I was unable to find one that was measured incorrectly by the oscilloscope. But, i was short on time so I would not be surprised if there are such discrepancies and I was just unable to discover them. I will have to check on this later.
  • Edit: Having some time to think about the lab, I would assume that the oscilloscope could have some troubles making correct measurements in AC coupling mode. In this mode, a capacitor filters out the DC signal, which causes the wave to lose its form at lower frequencies. I will discuss AC coupling later on in the report.

Triggering

  • Triggering will stop the waveform at some threshold value on the display. For it to halt the waveform on its 'downward' oscillations, you must select falling. If you want to stop the wave on its 'upward' oscillations, the required setting is 'rising'. Therefore, if the cursor is at the very center of the waveform, by switching between rising and falling you will progress the wave by a pi radian interval -- This was noted by Dr.Koch and it is obvious why this is so. The next closest 'rising' part, after a 'falling' part of the wave (or visa versa) must be exactly pi radians away, if you are at the center of the wave (this would not be the case if you were not at the vertical center of the wave!). Triggering is very useful because it allows you to observe the wave in detail without it moving on the screen.

AC Coupling and DC Coupling

  • In AC coupling mode, the oscilloscope filters out the DC signal. AC coupling is useful in analyzing small ripples because it allows one to obtain a high resolution 'picture' of the ripple. With DC coupling enabled, this is not always possible because the signal is shifted to a higher voltage on the screen due to the presence of the DC signal. This DC voltage shift makes it hard to zoom in on small ripples, because the wave form very easily jumps off of the screen.
  • Perhaps a disadvantage to AC coupling is that it will cause the waveform to become distorted. These aberrations are most easily seen when your generator is producing a square wave -- after extrema in the wave are reached, the waveform declines exponentially to some lower absolute voltage value. These distortions are caused by the charging and discharging capacitor that filters out the DC signal.
  • In the next portion of the lab, I will study this voltage drop and from it I will attempt to find out the time constant of the capacitor in the oscilloscope. It is important to note that these waveform aberrations are most easily seen at lower frequencies because each half period of the wave will exceed several capacitor-relaxation times.

(I learned about some of the above information from: [| brief summary of AC and DC coupling] and from: [| Another short summary of AC coupling])

Measuring the time constant of the oscilloscope

Wrong Results:

SJK 23:20, 16 September 2008 (EDT)
23:20, 16 September 2008 (EDT)
It is great that you keep these notes in here and then explain why you think they are wrong! That is very good scientific practice that I am impressed by.

First, I generated a square wave at a fairly low frequency (not low enough as I found out later!) with 0 DC offset. I adjusted the wave to get an amplitude of about 8.6V, as suggested in the problem.The Voltage drop due to the capacitor was about 4.8V. Using the cursor controls, I found where the voltage had gone down by 90%. From here, I was able to find the time required for this voltage drop using both the 'cursor' and 'measure' techniques. The results:

Fall Time:

  • Measure: 12.56±3ms
  • Cursor:14.7ms

To get the time constant, τ, I used the equation: Vf=Vo*exp(-t/τ)). Here Vf/Vo is .1, since we have waited for the voltage to drop by 90%. The fall time that I decided to use was 14.7ms because the 'measure' value of 12.56ms seemed a bit unreasonable looking at the wave in higher resolution. After discussing my results with Dr. Koch, I realized that I hadn't measured my fall time as well as it should have been measured. The time was measured again after changing my experimental procedure:

Better Results:

In the above measurements, my input wave's frequency was too high and so in the new setup, the frequency of the input wave was lowered to its lowest possible value. SJK 23:30, 16 September 2008 (EDT)
23:30, 16 September 2008 (EDT)
Without knowing the model of function generator, you won't be able to know what "lowest possible value" is if you wanted to repeat in the future.
repeating the same procedure as above, I obtained the following results:

Fall Time:

  • Cursor: 51.20ms
  • Measure: 48.24ms

The discrepancy between the cursor and measure times is surprising because I was observing the wave at a high resolution. However, given that I have a very small amount of data (1 measurement), my uncertainty in the accuracy of this measurement is fairly high. Because of this, I believe that the 'measure' fall time is more accurate, and so i will use it when I calculate the time constant. To get the time constant, τ, I will need to solve: Vf=Vo*exp(-t/τ).

Time Constant, τ: 20.95ms

I searched around the web to find the real time-constant of the oscilloscope but was not successful. Therefore, I have no idea whether or not my result is reasonable.

SJK 23:25, 16 September 2008 (EDT)
23:25, 16 September 2008 (EDT)
The Tektronix manual says that frequencies below xx Hz (did they say 10 Hz? (I think so)) are cutoff, but they don't exactly specify whether that is the 3 dB point or what. You can convert your tau to omega, and then omega to frequency, and you'll see it's pretty close to that number.
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