User:Garrett E. McMath/Notebook/Junior Lab/2008/09/08

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Oscilloscope Lab 1

SJK 13:35, 17 September 2008 (EDT)
13:35, 17 September 2008 (EDT)
Overall comments: in terms of the goals you state, I think you solidly achieved the first goal. Your in-class work was good and you gained a lot of understanding. However, there is a lot of significant information missing from your raw data notes, as I describe in my comments below. Make sure in future labs to record much more information, always keeping in mind that the goal would be for someone in the future to be able to replicate your experiments by reading your notes!
  • To learn how to use an oscilloscope and write a lab report for physics 307L.
SJK 13:23, 17 September 2008 (EDT)
13:23, 17 September 2008 (EDT)
An important section missing is "equipment." It is very important to write down the details (manufacturer, model number) of the equipment you used. Notably, the type of function generator and the type of oscilloscope. The goal is for a reader of your notebook to be able to understand what you did and to be able to replicate your experiments in the future. For example, the RC constant is likely to be very specific to this model oscilloscope.

Lab Notes

Caution: A terminator should be used when doing this lab given the resistance of 50 Ω a danger of reflected RF signal is possible according to Wiki webpage 13:21, 17 September 2008 (EDT)
13:21, 17 September 2008 (EDT)
This is a good thing to know about. It is important when you have higher frequencies and the coaxial cable is acting like a transmission line for an EM wave. The coax cable impedance is 50 ohm, whereas the scope is much higher impedance. Thus, there will be a reflection at the cable / scope interface (much like there is a reflection of light at air / glass interface). This reflection can screw up your measurements or even damage instruments if you're using high power. Probably the time you would have noticed this effect is with cable TV and seeing "ghost" images.

In our lab, however, we were doing stuff at low frequencies, and thus the wave propagation effects were not significant.
  • I measured a peak to peak voltage of my sine wave to be 3.88V, and a frequency of 203.3mHz using the cursors.
  • Using the measure feature on the ocsilloscope i got values of 3.88V, and 202.8 +/- .4 (due to fluxuations during measurement)
  • Using different waves from the generator the oscilloscope measured very similar values for all.
  • Obviously I believe that measuring by grid lines is the least accurate way to take these measurements, the next being the cursor menu, and finally the most accurate being the measure feature on the oscilloscope.
DC vs. AC 
  • The first noticible difference between AC and DC coupling was the amount and speed of fluxuation in the ongoing measurement of both peak to peak Voltage and frequency.
  • Probably the biggest problem with AC vs DC was with the square wave. DC coupling allowed for near perfect box representation of the wave while AC was asymtotic on both top and bottom of the wave function. I believe this has to do with the AC couplings high pass filter but I don't know specifically why.
  • When I measured the fall time of of the AC coupling I noticed a huge difference depending on the graph choice.
Fall time
  • I measured the fall time using AC coupling with zero DC offset and an amplitude of 8.6V. The value I got using the cursors was 51 milliseconds, and with the measure feature I got a value of 52.2 milliseconds, though it did fluxuate by approximately +/- 2 milliseconds.SJK 13:26, 17 September 2008 (EDT)
    13:26, 17 September 2008 (EDT)
    It is really good here and above how you record the ± values when using the measure function! You could also have done this w/ the cursor method by providing the step size of the cursor.
RC Value 
  • Using the formulas on wikipedia for RC value, I found a calculated fall time of 48 milliseconds which is very close to my expirimental value of 52.2 which was calculated by the oscilloscope.
SJK 13:28, 17 September 2008 (EDT)
13:28, 17 September 2008 (EDT)
I do not understand what you did in this section. How did you calculate it without knowing R or C? Definitely a link to the wikipedia article and inclusion of specific values and formulas you were using to calculate it would be necessary.

Lab Summary

  • The oscilloscope lab
  • SJK 13:33, 17 September 2008 (EDT)
    13:33, 17 September 2008 (EDT)
    I am really glad you learned a lot in this lab--that of course was a major purpose as you stated above.
    This lab was very informative as I had never used an oscilloscope before. The menus and tools on the oscilloscope were surprisingly intuitive and easy to use. As the closest instrument I have used to the oscilloscope is a simple multimeter; the oscilloscope was incredibly more informative with how the readings one gets from a multimeter are attained. Also with all the features on the oscilloscope one can measure the readings three different ways in most cases.
  • The main measurement in this lab was the AC coupling fall time. I measured the fall time using AC coupling with zero DC offset and an amplitude of 8.6V. The value I got using the cursors was 51 milliseconds, and with the measure feature I got a value of 52.2 milliseconds, though it did fluxuate by approximately +/- 2 milliseconds.SJK 13:32, 17 September 2008 (EDT)
    13:32, 17 September 2008 (EDT)
    Good inclusion of uncertainty estimates
    Later using the formulas on the wiki page for RC values and rise time (See Wikipedia article on rise time) I got a calculated a fall time of 48 milliseconds which was reasonably close to my experimental value of 52.2 milliseconds.SJK 13:32, 17 September 2008 (EDT)
    13:32, 17 September 2008 (EDT)
    OK, I do see the link to wikipedia now, which is good. However, I still do not know how you calculated 48 milliseconds. Any reader of your raw data and summary would have no idea, and in fact you probably wouldn't remember either at some near point in the future! This is a really good example of why more details are needed in the raw data notebook. Make sure to keep trying to imagine, "how would someone else replicate what I am doing by reading this notebook?"