User:Roberto Sebastian Rosales/Notebook/Physics 307L/Oscilloscope Lab Data - September 1, 2010

^{SJK 12:05, 29 September 2010 (EDT)}

12:05, 29 September 2010 (EDT)
This is a very good primary lab notebook. I like the use of pictures and embedded spreadsheet. Also, the discussion of what you did, along with comments about possible mistakes is good.

I see that you and Matt are sharing this primary notebook. That is fine and a good idea. Just make sure for future labs that you both do the data analysis yourselves (as opposed to sharing that as well). It's OK to work with each other on analysis, just want to make sure you both understand the whole process.

Set Up

^{SJK 12:03, 29 September 2010 (EDT)}

12:03, 29 September 2010 (EDT)
The only thing I see missing is the make and model numbers of the equipment.

• We connected one end of the BNC cable into the Lo Voltage Out on the power supply, and the other into Channel 1 on the oscilloscope.
• We made sure that the power supply was on the lowest voltage setting, and then continued to turn both devices on.
• Initially no waveform was present on the Oscilloscope screen.

Safety

• No extreme safety concerns other than normal electrical hazards

Procedure

Familiarization

• Before attempting to take any measurements with the oscilloscope, my partner and I fiddled around with the different settings on both the generator and the oscilloscope.
• On the generator, we noticed that we could adjust four major settings. These settings were the frequency, the voltage, the type of signal (square, triangular, and sinusoidal), and the DC offset.
• On the oscilloscope, there were many buttons and knobs that we needed to learn how to use properly. The main controls on the oscilloscope that we used were the positioning knobs, the volts and seconds divisions knobs, the cursor and measuring menus to get accurate readings for our signal, the triggering menu to learn what triggering was, and the coupling menu to get measurements for the fall time.

Basic Waveform Measurement

• We switched the power supply to a sine wave and set the frequency to 200Hz as stated in the lab manual
• We had to turn the amplitude knob on the power supply to about 2V in order to get a sine wave to appear on the display (with the screen resolution at 1.00V per grid box). This is the initial signal that we measured (as shown below and labeled 'Initial Signal').
• We also continued to record data for two more signals. One was a larger amplitude sine wave of about 20V peak to peak which was obtained by simply turning the voltage knob on the generator to the right, and the other had a large DC offset which was obtained by turning the DC offset knob to a setting greater than 0. Unfortunately we forgot to get an image for our DC offset waveform, but this mistake will not be made in future labs.
• All data for these three signals can be found below in the spreadsheet.

Initial Sine Waveform

Larger Amplitude Sine Waveform

Raw Data

{{#widget:Google Spreadsheet |key=0At9fxsqtusShdDh4Y0luZkpUeGlURVhMNnowUndWdnc |width=600 |height=300 }}

Triggering

• As mentioned in the summary, my partner and I had a little trouble understanding what oscilloscope was doing when we were adjusting the triggering settings. After a little explanation from Katie and Prof. Koch, we learned that triggering simply describes the process of telling the oscilloscope where to begin tracing the incoming signal. By adjusting the trigger, we could position the starting point of the incoming signal on the display to any point we wanted. As far as I understand, triggering on the rising edges means that the oscilloscope will look for the specified voltage on the signal wave at a point where the slope of the wave is positive. One thing to note is that when we adjusted the triggering knob past some threshold point, it no longer displayed the incoming signal. We figured this out when the TA, Katie, asked us to adjust the settings on the generator with the knob past the threshold point. Nothing happened to the signal which indicated that it was no longer displaying the output from the generator. Triggering seems to be a very useful feature because it allows a user to study an incoming signal in further detail by allowing them to adjust it on the display.

AC and DC Coupling

• This final part of the lab called for us to use the AC coupling feature of the oscilloscope. This feature allows for only the AC part of a AC + DC signal to be analyzed and displayed on the screen. On the other hand, DC coupling allows for both the AC and DC components of a signal to be analyzed and displayed. When using DC coupling, our signal was offset or shifted by a factor of the DC offset value on the generator. For the measuring of the fall time and calculation of [math]\displaystyle{ \tau = RC }[/math], we used AC coupling only.
• For the measuring of the fall time we set our generator to output a 14V signal with a frequency that read "<10Hz." This was probably not a good idea now that I look back on it, because if we were trying to reproduce these results at some later date we would not know exactly what settings the generator was at. I will have to keep this idea in mind for future labs.
• To measure the fall time, we simply zoomed in on the display of the oscilloscope, with AC coupling on, until we found what looked to me to be some kind of discharging of an electrical component (an image of what was displayed on the screen can be seen below). In order to measure the fall time with the cursor function, we simply needed to measure the time for the signal to drop from its maximum or peak value of 14V, to 10% of that value, or 1.4V.

Using AC Coupling to measure Fall Time

• [math]\displaystyle{ Vmax = 14.0V }[/math]
• [math]\displaystyle{ V_1 = 1.4V }[/math]with f<10Hz
• Cursor = 58ms
• Measure Function = 63.31ms ; let [math]\displaystyle{ t_f = fall time = 63.31ms }[/math]
• Using the following formula, we calculated the RC constant which is simply [math]\displaystyle{ \tau=RC }[/math].
• [math]\displaystyle{ V_1 = Vmax*e^{-t_f/\tau} }[/math], or in our case [math]\displaystyle{ 0.1 = e^{-t_f/\tau} }[/math].
• Solving for [math]\displaystyle{ \tau }[/math] yields the following:
  • [math]\displaystyle{ \tau = 29.66 ms }[/math]
  • We could not find the expected value of the time constant for the oscilloscope to make a comparison.

References

I used Randy Lafler's and Brian Josey's summary to help with formatting. We also received help from Katie, the TA for the class, and Prof. Koch when learning about triggering and AC coupling.