Physics307L:People/Cochran/Oscilloscope
SJK 11:22, 29 September 2010 (EDT)
This is a good summary. You report your result and discuss the error, even though not statistically like you will in future labs. For future labs, you'll report the result along with a statistical estimate of uncertainty. Also, you'll compare to accepted result and discuss whether your measurements are in agreement. That's not easy with this lab, but will be in future labs.
Oscilloscope Lab Summary
The purpose of this lab was to become familiar with the operation of an oscilloscope and learn about fall time and AC coupling by messing around. I used a Tektronix TDS 1002 oscilloscope and a BK Precision 4017A Function Generator. My procedure for this lab is described in my lab notebook.
Results
I set the function generator to an amplitude of 8.56 V, displayed a square wave, and switched off the DC offset. Using the cursors, I measured the fall time to be approximately 56ms. Using the measure function, which Cristhian showed me how to use to calculate fall time, I measured the fall time to be about 131.8ms. I assume that this means that the oscilloscope is calculating fall time with the 5% value instead of the 10%. I remeasured the fall time with cursors for the 5% value and got 134 ms.
According to Wikipedia, rise time is proportional to the RC constant: [math]\displaystyle{ t_r\cong 2.197\tau }[/math]
So the RC constant should be [math]\displaystyle{ 131.8ms/2.197\cong 60ms }[/math]
Also according to Wikipedia, [math]\displaystyle{ t_r\cong 0.35/BW }[/math], where BW is the bandwidth. For the TDS1002, bandwidth is 60 MHz according to the manual, so its rise time is [math]\displaystyle{ 0.35 / 60MHz = 5.8ns }[/math], and its RC constant is [math]\displaystyle{ 5.8ns / 2.197 = 2.66ns }[/math]
I don't understand why my fall time with cursors was so much smaller than my fall time with the measure function, or how this value should compare with the RC constant for the oscilloscope. Overall, I do not feel entirely solid in my understanding of the concept of fall time, but most of my measurement error probably came from placing the cursors inaccurately. I would have liked to have tried measuring rise time and seen if the numbers for cursors and the measure function were similarly discrepant.
Conclusion
This lab was a good introduction to the course format and to the oscilloscope. I also learned a lot about RC constants and had my memory of circuits refreshed.
