UC Berkeley Basic Semiconductor Circuits Labs - LabVIEW Programming

For many decades, it was sufficient to read the signal on a meter, or display the signal on an oscilloscope. Sometimes hybrid methods were used; for my Ph.D. thesis, I took about ten thousand photographs of oscilloscope screens, and analyzed the information on the photos with calipers. Nowadays, most data is collected by computer. Computers have become astonishingly powerful, and data acquisition hardware has become cheap, fast and accurate.

Excerpt from the Physics 111 Laboratory Manual by Dr. James L Siegrist & Donald Orlando

Background

This tutorial provides a brief outline of the LabVIEW Programming Lab written at UC Berkeley. The complete lab content is available for download at:

http://socrates.berkeley.edu/~phylabs/bsc/PDFFiles/bscLV-10.pdf

Data Sources

Originally, physicists made measurements by hand; lengths were measured with rulers, events counted by penciling tick marks, and events timed with stopwatches. But hand and eye techniques failed as experiments became more sophisticated; they were too slow, too inaccurate, and too imprecise. Experiments began making measurements electronically.

Computerized Data Acquisition

For many decades, it was sufficient to read the signal on a meter, or display the signal on an oscilloscope. Sometimes hybrid methods were used; for my Ph.D. thesis, I took about ten thousand photographs of oscilloscope screens, and analyzed the information on the photos with calipers. Nowadays, most data is collected by computer. Computers have become astonishingly powerful, and data acquisition hardware has become cheap, fast and accurate.

Noise, Signal Processing, and Data Acquisition

Unfortunately, it’s a rare experiment that produces noise-free data. Noise comes from many sources. Some are intrinsic, like the Johnson Noise discussed later in these Background notes, while others are extrinsic, like the 60Hz harmonics picked up from the power lines. It is always best to minimize noise before collecting data, but often we need to “see into the noise”…to recover a valid signal from a noisy signal.

Data Acquisition Devices

Modern instruments like oscilloscopes, signal sources, and multimeters can often send their measurements to computers. The most common hardware interface protocol is called the GPIB bus, sometimes known as the HP-IB or IEEE bus. Powerful in its time, the GPIB interface is slow, expensive, difficult to use, and archaic. Recently, some instruments have been designed to communicate over Ethernet or USB. Whatever the bus, each instrument has its own set of programming commands, and recovering data from the instrument is generally painful.

Data Acquisition Environments

Standalone instruments can be used independently via their front panel interfaces, but data acquisition cards must be used in a data acquisition environment. Most cards come with a debugging interface that may be used in as a simple data logger, but is insufficient for more sophisticated applications.

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Johnson Noise

In 1928, J.B. Johnson discovered that the RMS voltage across an isolated resistor is not zero, but fluctuates in proportion to the square root of the temperature and resistance. Later that year, H. Nyquist showed that the voltage is due to the thermal fluctuations in the resistors.

In the Lab

This procedure outline presents a brief overview of the complete lab. Download the full procedure at:

http://socrates.berkeley.edu/~phylabs/bsc/PDFFiles/bscLV-10.pdf

Discerning Signals in the Presence of Noise

Load the program Noisy Signal Generator.vi. Set the Noise Level to 0, and run the program. (Enter values into the Noise Level control by left clicking inside the box and typing a number, by left clicking on the arrow indicator on the left side of the box, or by left clicking on the box and using the up and down arrows. Run the program by left clicking the run button and stop it by left clicking on the stop sign or by left clicking on the Stop button.)

This program generates several hundred cycles of a 100Hz, 1V RMS sine wave. The first four cycles of the wave are displayed in the top graph, and its spectrum in the bottom graph.

LabVIEW Programming

Learn to program in LabVIEW by doing all the exercises in National Instrument’s Six Hour Tutorial. The exercises are given in Appendix I at the end of this write-up. Commentary associated with the tutorial can be downloaded from:

http://socrates.berkeley.edu/~phylabs/bsc/LabView

While you may work with your partner, both of you will be expected to learn to program in LabVIEW.

Measuring Boltzman’s Constant

Build the circuit shown in the schematic below. Debug the circuit with a 1kHz, 0.1V signal connected to the voltage divider. To measure Boltzmann’s constant accurately, this circuit work well. Lay it out cleanly, and use decoupling capacitors next to both Op Amps. Make sure all the resistor values are correct. If the circuit is unusually noisy, you may find that it becomes quieter if you move it to another part of the breadboard.

Student Evaluation of Lab Write-up

Now that you have completed this lab, we would appreciate your comments. Please take a few moments to answer the questions below, and feel free to add any other comments. Since you have just finished the lab it is your critique that will be the most helpful. Your thoughts and suggestions will help to change the lab and improve the experiments.

Return to UC Berkeley Semiconductor Circuits Labs


Acknowledgment and Disclaimer

This material is based upon work supported by the National Science Foundation under Grant No. 0411367. Any opinions, findings and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation (NSF).

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