Lab 4: Detect an unknown signal
The goal: How can you figure out if a signal is just noise? Or perhaps it is a signal? One very powerful method is to use fourier analysis.
About FFT analysis
Make sure to work through this tutorial on basic FFT analysis here, before stating: FFT tutorial
Overview:
Our signal will be a generated brightness level on a computer screen. You can build a simple detector using the photoresistor from your kit. Set it up as an analog voltage divider (using the 4.7 kΩ resistor as the reference). Then, hold the photoresistor as close the screen as possible and record the values. Here you can see the measurement in progress.
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You will need to record your values with very accurate millisecond time deltas. To do this, use the code shown in the notes here: Signals Notes.
You will need to record about 20 seconds of data to get enough to have a decent resolution for the frequency spectrum.
Your signals will be made up of 4 different sin functions, all between 1 and 10 Hz, added together. This means you will detect 4 main frequencies in your FFT, which will appear as 4 peaks in your FFT graph.
Here is a known signal to practice on. Test Signal Generator →. It is created by a sinusoidal function with a frequency of 2.5 Hz. You can test your code and measurements methods with this one. You should be able to reproduce the following graph of the FFT spectrum from the test signal.
A 2.5 Hz signal in the test experiment.
Each student will have their own 'mystery signal'. Make sure to use the link that corresponds to your student ID number (the last 4 digits are shown).
| Student ID Number (last 4) | Your Link |
|---|---|
| 0167 | Signal ID 1 |
| 1304 | Signal ID 2 |
| 8784 | Signal ID 3 |
| 0163 | Signal ID 4 |
| 4927 | Signal ID 5 |
| 4436 | Signal ID 6 |
| 9624 | Signal ID 7 |
| 5998 | Signal ID 8 |
| 9628 | Signal ID 9 |
| 1584 | Signal ID 10 |
| 8992 | Signal ID 11 |
| 6302 | Signal ID 12 |
| 9872 | Signal ID 13 |
| 6338 | Signal ID 14 |
| 7935 | Signal ID 15 |
| 5941 | Signal ID 16 |
| 0398 | Signal ID 17 |
| 2708 | Signal ID 18 |
| 9095 | Signal ID 19 |
| 9863 | Signal ID 20 |
| 3806 | Signal ID 21 |
To be included in your report
- Brief Introduction. Think of a real world example of where an FFT analysis might be useful. (And don't just copy something from here. Be original.)
- Circuit Diagram
- Your results of the FFT analysis (plots and frequency values)
- Links to Data
- Links to your analysis code
Some further reading on FFT can be found on these pages: