Press shift then freq to set the message signal

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Press “Shift”, then “Freq” to set the message signal frequency to 880 Hz. Press “Shift”, then “Level” to set the modulation depth to 80%. This adjusts the DC component added to the message signal before modulation. 3. Set your oscilloscope to display the frequency spectrum of the input: Press the “Auto-Scale” button and acquire in “High Resolution” mode. Set the time/div to 1 ms, the Volt/div to 500 mV, and adjust the Offset, so that the time domain signal stays in the top part of the display. Remember that the time domain signal has to be completely inside the scope display before performing the FFT. To synchronize the scope with the AM signal you can rise the threshold level of the trigger to about 450 mV . Press “FFT” button. Inside “More FFT” press the Auto-Setup button. Set the center frequency to 14 kHz and the frequency span to 20 kHz. Set the Scale to 15 dB/div (You need to turn on the “Fine” scaling by pressing once the “Scale” knob) and adjust the Offset so that the frequency spectrum stays in the bottom of the display, without interfering with the time domain.. 4. Sketch both, the time domain AM signal and its frequency spectrum and explain what you see in terms of the modulation property of the Fourier transform. (Hint: How is the frequency spectrum of the message signal (co-sinusoid of 880 Hz) plus a DC component in base band, i.e. before modulation?) Explain the shape of the frequency spectrum. ( / 4)
5. Change the shape of the message signal in the modulation menu from SINE to SQUARE (In the function generator press “Shift” then “ < ” then ” to select the shape of the message signal. You can select the waveform by pressing “ > ”.). You are not changing the carrier signal, only the message. In the scope the center frequency should remain at 14 kHz , the frequency span at 20 kHz and the dB/div at 15 dB . Sketch the AM signal in time domain and its frequency spectrum and explain what you see. (Hint: Remember the Fourier analysis of the square wave performed in Lab #3.) Explain the shape of the frequency spectrum. ( / 4)
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