Your labs will require you do several different things: simulate your circuit, construct your circuit, characterize (measure voltages and currents)
your circuit, and compare your experimental results with the results from the simulation.
Your lab report has to show that you did all these things and it should demonstrate that you understood the concepts behind the circuits and the data.
- The most important part: You should already have a good idea what results you should get.
- Setting up experimental circuits requires to pay attention to details. A single wrong connection can still give you results.
But they are virtually always different from the ones you should have observed. If you don't know what to expect, you don't know when it is wrong.
- Electronic components fail.
- A burned out fuse of a voltage supply line is only easy to find when you search for it. But when you don't know that your results are wrong,
you will not look for it.
- A broken transistor or burned out diode is not going to tell you that it is broken. If you don't know what to expect, you will not know that.
- If you just follow the instructions, take data, and then try to make sense of all this later when you write your lab report,
you will be surprised how much can go wrong.
- How do you know what to expect?
- Understand the circuits before you come to class.
- Estimate what to expect often.
- Use the data sheets.
- During the experiments:
- Measure voltages not only at the outputs but also at other locations in the circuit. Do these values make sense. Put them in your lab book.
- It is often useful to modify the circuit a bit to check if the result changes as expected. For example: A transistor is often used
to amplify a current. But this can only work if the collector is powered. Disconnect the power from the collector. If nothing changes,
your transistor is probably broken.
The Parts of a Lab Report
You should organize your information into several categories:
- The Title identifies the topic of your report:
- Lab 1: Simple sensors / DC etc.
- You are the Author and your lab partner is listed as "partner."
- Author: John Doe, Partner: Jane Doe
- In case you did parts of the lab alone or with a different partner because your original partner missed parts of the class,
mention here which parts you did alone or with a different partner.
- Subheading, indicating what is being done. Generally, each lab consists of a series of small experiments.
For example, in Lab. 2 you study a phototransistor, a temperature sensor, the oscilloscope, the function generator, a high-pass filter,
a low-pass filter, and finally a microphone with a coupling circuit. Cover each of these experiments completely as described in the
following sub-bullets before describing the next experiment.
- Paragraph about the motivation for this experiment: What is the goal? What am I expected to learn? It is important
in physics to not loose sight of the goal among all the details.
- Circuit diagram: This can be a copy of the circuit built in the simulator.
- It should include names for all components and nodes in your circuit.
- Add a list of components used. Maybe in form of a table. Use the real values wherever possible (as measured resistances or capacitances).
- Measured Voltages from DC power supplies should be listed as well.
- Experimental Procedure describes the process in a short clear paragraph, explaining all steps in the order they actually happened,
not as they were supposed to happen. Refer to the circuit diagram to support your explanations. The procedure should not include the measurement of the
resistors, capacitors, and supply voltages listed above (we just assume you did this correctly). Do not copy text from the lab manual to the report,
but do describe what you did. Be sure you describe those occasions when you did not follow the manual exactly (e.g., "At step 4 we performed four repetitions
instead of three, and ignored the data from the second repetition"). If you've done it right, another researcher should be able to duplicate your experiment.
- Results include results from the simulation and from your experiment. Results can be given in the form of data in a table or as a graph.
- Graphics need to be clear, easily read, and well labeled (e.g., "Figure 1: Output voltage as a function of signal frequency").
An important strategy for making your results effective is to draw the reader's attention to them with a sentence or two within the normal text, so the reader has a focus when
reading the graph. One very common mistake are the 'screenshots' from the scope. They are often not labelled correctly and neither you nor I want to guess
later which scale is the correct one.
- Compare your results with your expectations. Graphs should include the data as data points and a curve based on the expected result
should be fitted to the data if possible.
- Follow with a brief discussion how the circuit works. Show that you understand the experiment. Do you think that the differences
between experiment and theory are acceptable? Are they consistent with uncertainties in the measurements, within range of data sheets? Any other excuses why the results might be off?
- Summary/General discussion:
- Summarize what you observed and what was learned when you look at a larger set of experiments. What is the point of this set of experiments?
- Put the experiments/results in a larger context. This could be some fundamental physics background: For example one of Kirchhoff's laws is
a consequence of charge conservation. Or the larger context could be potential applications. But keep it short. There are no bonus points for long novels.
- Lab Report Guidelines
- Label all figures and tables with a descriptive sentence.
- e.g., "Figure 1: Output from a high-pass filter, using as input a 60 Hz, 2 V, square wave. R = 10k, C = 100 pF."
- When you compare your observed value with your calculated value, do the comparison as a percent difference.
- Percent deviation (difference): ( ( |measured - expected| / expected) * 100 )
- e.g., "We calculated the driving current of the current mirror to be 1.0 mA. The measured output is 1.6 mA. This is a 60% deviation
from the theoretical expectation."
- Show the complete analysis of your work. "Yes." is never accepted as an answer.
- Example: the lab asks you if the output from a circuit is what you expect. "yes" is not appropriate. What is more appropriate is:
"We expect that the half-wave circuit will only pass the positive swing of the input AC current. We displayed the input AC wave on the scope, along with
the output of the circuit. We observed the positive swing at the output of our circuit, and a flat line at 0 during the time when the input is swinging
negative. Our output matches our expectation."
- Show all data to support your statements.
- Example: you are asked to vary frequency of the input AC wave and check that the time constant of your circuit does not change.
Stating that "we varied the frequency and found tau didn't change" is not good. What's your proof? Why should I trust you?
More importantly, how do I know you didn't copy the work from someone else? You have tables of results in your lab notebook.
Turn those tables into plots for your report. You can also take scope images. All of these things should be used to defend your conclusions.
- Answer all questions for each part of the lab. Please ask if you aren't sure if something should be answered. Put the questions and
the answers in the part of the report where they are relevant.
- Lab partners results must be consistent. You should have the same images and the same data points (2.90 and 2.88 are fine. 2.02 and 2.19 are not).
If there are any problems sharing information please let me know.
- If you miss a day, you may not copy data taken by your lab partner while you were absent. You will need to repeat these measurements
yourself at a time when your partner does not need the equipment.
- You must have correct spelling, proper grammar, and use correct scientific terms to describe effects.