I began this experiment much like the last by forking and cloning onto the desktop the given repository ,2012-Physics-308L-Lab-5. After locating and acquiring the necessary materials, I began by developing the necessary VI in LabView.
The goal of the this experiment was measure the peak to peak voltage of the RC circuit when the function generator was set to different frequencies. The VI was supposed to record this data in such a way that a *.PNG file capturing the front face with a time and date stamp would be generated every time the program with run. Additionally, the program would display the current reading alongside the most recent reading. The instructions detailed that we should begin by modifying the buffered data acquisition VI from Lab 4, however, I failed to read the instructions carefully and ended up creating the VI from scratch, while referencing the example provided by Dr. Koch. The front panel and the block diagram for the new VI can be viewed below. ( Once again I do not have the *.PNG files for these images as I must have mistakenly deleted them while deleting the test screen captures from testing out the VI before I began taking measurements.)
The RC circuit was constructed with the materials listed above. Two different views of the the board can be viewed below, as well as the quick sketch of the RC circuit I drew to quickly explain what we were constructing to one of my fellow students.
In the above photos the green wires are connected to the voltage probe that is plugged into the function generator. The red wires are connected to the voltage probe that is plugged into the oscilloscope. The yellow wires are connected to the channel 1 and channel two ports of the MCC DAQ card. I now realized that I should have taken a wider angle photo or maybe even a video to better display the nature of the circuit.
Testing the Circuit
I began by testing the circuit by checking to see if the proper sine wave would be displayed by the oscilloscope when it was hooked up to the circuit as described above. It initially displayed a lot of noise and it was deduced by Dr. Koch that this was due to the order in the circuits that the grounds of the voltage probes were connected. They needed to be reoriented so that they were directly after one another in the sequence in order for the reading to be shown correctly. As of now the true reason why this must be so is unknown. In the future I will take additional photographs to improve upon the explanation.
While testing different frequencies the sine wave became distorted and then dissappeared completely. Dr. Koch explained that I needed to simply adjust the time scale on the oscilloscope to correct this.
Testing the VI
I hooked up the MCC DAQ card to the RC circuit as described above and ran the VI. Initially, it did not show any data in the either graph and I quickly realized that it was displaying an error code “1” nothing that I had the “incorrect board number”. I redid the board test under measurement computing from last week and everything seemed to be in order. From here I attempted to use the example VI provided by Dr. Koch and it reported the same error. I tried to remedy this error by adding an input for the board number and connected it to the “Aln ScFg” VI that was imbedded in the loop of the larger VI. This did not have any effect. Finally, after consulting with Dr. Koch, he had me close down LabView completely and the reopen it. This solved the problem and it also turns out that the addition of the “board number” input was unnecessary because for the VI to work it needed to be set to “0”. A screen shot of acquired data can be seen below.
After all the debugging the VI was complete I began to take readings of the Peak-Peak voltage. I started with a frequency of .4 Hz and moved this incremently higher until I reached ~100 Hz. I then jumped much higher to the the ~300 Hz range, to the ~600 Hz range, and finally to the about ~900 Hz range. There was very little deviation in the readings, so I returned to the frequency range near the cutoff frequency of 159 Hz, that was determined form the equation (f = 1/(2pi RC). I also recorded the acquired data, along with the corresponding frequencies in an excel spreadsheet. This data was then uploaded under my account to Figshare. From this data I created a graph plotting the peak-peak voltage vs the frequency that can be seen below.
From this it is easy to extrapolate that the prediction of the cutoff value of 159 Hz is very similar to the data that was acquired during the experiment. It is also pertinent to mention that the peak-peak voltage data displayed on the y-axis is not in terms of volts. Dr. Koch explained to us that the data acquired by the MCC DAQ card was still in raw bit form and had not yet been transformed to volts. This does not, however, preclude one from determining the cutoff frequency, as the trend stays the same whether in bits or volts.
Details of Materials
USB-1208LS USB-based Analog and Digital I/O Module (Board Serial #163)
BK Precision 4017A 10 MHz Sweep/Function Generator
Tektronix P2220 Voltage Probe
Tektronix TDS 1002 Two Channel Digital Storage Oscilloscope
Wikipedia: Articles on Capacitance and Resistance