User:Elizabeth Allen/Notebook/Physics307L

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Oscilloscope Lab


Summary
The oscilloscope is an important instrument for analyzing experimental data, as it receives electrical signals and displays them as a wave form that can be analyed with respect to amplitude, frequency, continuity and duration, which reflect aspects of the input. The oscilloscope functions by allowing the input current to pass through a cathode tube and two pairs of charged electric plates. One pair causes the electron beam to be deflected vertically depending on its voltage. The other pair drives the vertical movement of the display across the screen over time. In this lab, the input comes from a wave generator, and various aspects of the input and display can be measured or adjusted through different functions of either the generator or the oscilloscope. The purpose of the lab is to gain knowledge and skill with these capabilities. 

Equipment

 * Day One: Textronix TDS 1002 Two Channel Digital Storage Oscilloscope, 60 MHz 1GS/s Wavetek Sweep/Function Generator Model 180
 * Day Two: Textronix TDS 1002 Two Channel Digital Storage Oscilloscope BK Precision 4017A, 10MHz Sweep/Function Generator

Explorations

 * Day One: Initial explorations included working with the Channel I Menu selections and the Volts/Div and Sec/Div controls. This Menu brings up the option to switch among reading the DC, AC, and Ground inputs. DC gives the full signal whereas the AC coupling displays only the AC variations that ride on the DC signal. Ground gives only a flat line display. The acquire button enables one to find the signal when the initial screen setting does not allow it to be displayed. Then the Volts/Div and Sec/Div controls adjust the calibrations of the screen to allow changes in the signal display, altering the displayed altitude and wavelength respectively.


 * Day Two: Worked with David Weiss this day. At first the DC sinusoidal waveform was clipped and the AC display was distorted on the lower part. I thought this was due to a shift in the DC offset on the function generator, but couldn't recreate the form when in the lab another time. We found that the triggering control adjusts where the wave display begins, and it shifts from being on a rising or falling slope through the Trigger Menu, Slope. When it is set to trigger within the waveform, it stabilizes the display, as long as the trigger mode is set to normal. We also looked at a square wave from the function generator displayed on the oscilloscope. With DC coupling, the wave form was clearly squared off but with AC coupling the wave form was distorted on the horizontal sections due to overshooting. Adjusting the grid to expand this portion allowed us to measure the fall time of the wave.

Basic waveform measurement

 * We set the function generator to output a sine wave at 1KHz and tuned the frequency to 200Hz, then centered the wave on the grid with vertical grid divisions of 5.00V and horizontal grid of 5ms due to the chosen frequency. Estimating by sighting on the grid we found an amplitude of 6V. Using the Measure function gave us a Pk-Pk measurement of 11.8V. The cursor function opens a menu that allows to measure by time or voltage. On day three, using different equipment, I measured a sine function set up in a similar manner but with horizontal grid of 1ms, that displayed with a grid estimate of 1.4V, a cursor measurement of 1.4V and a Pk-Pk measurement of 1.4V. This was a good way to learn just where to set the cursors on the waveform to get the proper measurement.


 * Adjusting the function generator to a very low amplitude and adjusting the vertical grid to 200mV, I estimated the amplitude at300mV, measured with the cursor an amplitude of 296mV, with Pk-Pk measurement display varying from 600-632mV.

Triggering

 * We adjusted the trigering to the rising slope through the Trigger Menu funtion. Triggering on the rising edge means that the input for display begins on the rising portion of the wave form. In this mode, the rising edge of the wave form is at the left edge of the screen and at the trigger point markings. Switching to falling moves the display until a falling edge of the wave form is at these points.

AC Coupling

 * The DC voltage is increased by the output level of the function generator. The DC coupling displays the basic wave form, whereas the AC coupling displays the ripple on the DC voltage.


 * The Square wave display allowed us to use the AC coupling to measure the fall time for the overshoot of the wave form that occurs from the Fourier series. We were unclear about this process at the time but were able to adjust the grid and lower the frequency to better display the falling slope and measured the fall time to be 2.3 ms at 200 Hz and 293.0 ms at 1 Hz. Reading from a sight on Wikipedia, we found rise time, tr, to be approximately 2.197 tao, where tao is the RC constant. The rise time is also defined as the time from 10% to 90% of the amplitude. I considered the fall time to be a similar relationship, from 90% to 10% of the amplitude. This fall time is extended by the delay in the signal display that is inherent in the processing by the oscilloscope, so the fall time measures a bit larger than it actually is. This may be important for other labs where the rise time of the signal is close to the delay time of the oscilloscope.


 * We did not have time to do repeated measurements to determine the expected error. In such a calculation, we would need to also include the expected influence of the oscilloscope as mentioned above.

Resources
http://en.wikipedia.org/wiki/Oscilloscope
 * An extensive article about oscilloscopes can be found in Wikipedia, and a shorter introduction can be found in the lab manual by Dr. Gold. The Wikipedia article on rise time was also very helpful. It was good to look at the Fourier series applet to see a visual representation of how the displayed wave form becomes more accurate with increase in the number of terms. Links to these sites can be found in the Oscilloscope lab instructions. I was unsuccessful in my attempts to enter them here.

Reflections
In this lab I gained insight into the workings of both the wave generator and the oscilloscope, as well as developing some skills interfacing with the Openwetware and organizing observations into a presentable format. Also, in preparing for this lab, I learned about the structure of the oscilloscope and the process that I am working with to analyze data.


 * -http://en.wikipedia.org/wiki/Rise_time#One_stage_low_pass_RC_network
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http://www.falstad.com/fourier/