# fft34 - ELEC 352 FFT Lab Experiments Experiment 3&4 Report...

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ELEC 352 FFT Lab Experiments Experiment 3&4 Report Department: FST Major: EEE Name: TENG CHENG (DA928240) LEONG MAN PIO(DA927978)

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Experiment#3 Objective This experiment demonstrates the use of the FFT for analyzing the spectral content of a square wave and a triangle wave. Data analysis (1) Square wave part Use function generator to produce a 500 kHz square wave (2 V pp ). Activate the Math Function Menu. Select On under the function 2 portion of the display menu. At this point, the effective sampling rate is 200 MSa/s, and thus many of the square wave harmonics are displayed. Figure a square wave with 500 kHz and 2Vpp Figure the harmonics of square wave display in time domain And then use the Time/Div setting to lower the sampling rate to 100MSa/s. Adjust FFT Setting and use the Cursors function to measure relative amplitude difference between the peaks, and then we need to calculate the theoretical values of the magnitude of square wave harmonics, since the Fourier transform of any period signal is: And the signal is a 2V pp square wave, the magnitude of coefficient which shown on the above equation is: Use the equation to calculate the theoretical values, and the error percentage is: All the data are shown in table 1:
Square wave harmonics Experiment values(dB) Theoretical values(dB) Error percentage (%) 1 0 0 0 3 -10.62 -9.54 11.32 5 -15.86 -13.98 13.45 7 -18.9 -16.9 11.83 9 -20.00 -19.09 4.8 Table 1 experiment and theoretical values of square wave harmonics From the table we can see, the experiment values of square wave harmonics we got are larger than the theoretical values, and the error percentages are a little larger except for the 9 th harmonic. Use the Time/Div control to select an effective sampling rate of 5MSa/s, the phenomenon is shown in fig.3 Figure The square wave harmonics in 5MSa/s sampling rate

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