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fft [2015/03/19 14:12] schultz |
fft [2015/03/19 14:34] (current) schultz |
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| - If you want a better frequency resolution you need a wider time window. | - If you want a better frequency resolution you need a wider time window. | ||
| - Play around with the time window to see the effect on frequency resolution. | - Play around with the time window to see the effect on frequency resolution. | ||
| - | - Set the time/division to 20 μs. This time resolution produces about 2 periods of the sine wave. This time resolution is what the auto scale will typically change the time resolution to. | + | - Set the time/division to 0.02 ms. This time resolution produces about 2 periods of the sine wave. This time resolution is what the auto scale will typically change the time resolution to. |
| - We have an f=10 kHz signal so set the frequency span to 100 kHz and the Center to 50 kHz. You do this by pushing the Math ‘button’ and using the ‘soft keys’ where is says Span and Center. | - We have an f=10 kHz signal so set the frequency span to 100 kHz and the Center to 50 kHz. You do this by pushing the Math ‘button’ and using the ‘soft keys’ where is says Span and Center. | ||
| - Use the cursor to pin point the f=10 kHz signal. You select the cursor using a ‘button’, change it to the FFT trace using ‘soft key’, and use a ‘knob’ to move the cursor. | - Use the cursor to pin point the f=10 kHz signal. You select the cursor using a ‘button’, change it to the FFT trace using ‘soft key’, and use a ‘knob’ to move the cursor. | ||
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| - Notice how the spike at f=10 kHz gets more narrow as the time/division increases. | - Notice how the spike at f=10 kHz gets more narrow as the time/division increases. | ||
| - In summary, it takes a lot of periods of a signal to produce a good frequency domain representation. Therefore, the signal needs to be zoomed out to get a good frequency domain signal. | - In summary, it takes a lot of periods of a signal to produce a good frequency domain representation. Therefore, the signal needs to be zoomed out to get a good frequency domain signal. | ||
| - | - You can adjust the amplitude and position of the purple signal (this is the FFT signal) by using the knobs in the gray math box. The red box in Figure 3 shows how you change the amplitude and Figure 4 shows how to shift the signal up and down. | + | - You can adjust the amplitude and position of the purple signal (this is the FFT signal) by using the knobs in the gray math box. The red box in the following picture shows how you change the amplitude of the FFT signal {{ :scope3.jpg?100 |}} and the following picture shows how to shift the signal up and down. {{ :scope4.jpg?100 |}} You can push the shifting button to put the FFT signal in the middle of the screen. |
fft.1426795958.txt.gz · Last modified: 2015/03/19 14:12 by schultz
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