Introduction
This experiment will explore amplitude modulation (AM) and frequency modulation (FM) of waveforms.

Apparatus
Stanford Research DS345 Function Generator
Stanford Research SR760 FFT Spectrum Analyzer
Tektronix TDS210 Digital Oscilloscope
Radio

Procedure

1.  AM Radio
The function generator can provide both AM and FM waveforms.  Gather around the radio at the end of the lab.  The lab instructor will use the long gray wire attached to the BNC cable as a transmitting antenna. To transmit AM radio waves, she will press the AM (Int) key, setting the depth to 50%, with a rate of 600 Hz.  She will set the frequency of the sine wave to 750 kHz and the amplitude of the sine wave to 9.5 Vpp.  To recieve the 750 kHz frequency she will turn on the radio and tune it to 750AM.

Question 1. What happens as she changes the rate and depth?  What happens as she changes the amplitude of the sine wave?  What happens as she tunes the radio to a higher or lower frequency?

To start, set the function generator frequency to provide sine waves at 1 kHz, with an amplitude of 0.1 V. The oscilloscope can be set to 50 mV/division on the vertical scale, and 1 mS/division on the horizontal scale. The spectrum analyzer display should be "linear," with a frequency span of either 1.56 kHz or 3.125 kHz. Check that the sine wave output of the function generator is consistent with what you see on both the oscilloscope and spectrum analyzer, and describe in your lab notebook that this is so.

3. Amplitude Modulation
Press the AM (Int) key in the Sweep/Modulate section of the function generator. Check the Depth and set it to 50%, with a Rate of 0.1 Hz.

Question 2. What happens to the oscilloscope display? Does it make sense that this wave is "amplitude modulated" with a depth of 50%? Is the output of the spectrum analyzer consistent with that of the oscilloscope?

Now observe what happens as you adjust the Rate to each of the following values: 1 Hz, 10 Hz, 100 Hz. Describe what you see in the same manner as you answer question 1 above, for each frequency. Print a hard copy of the spectrum for the 100 Hz rate. Use the "marker" key and knob to determine the frequencies of major peaks in the spectrum, and write those frequencies above the relevant peaks on your hard copy.

Question 3. What are the "sideband" frequencies in this spectrum?

4. Frequency Modulation
Set the Rate back to 0.1 Hz, and press the "FM" key. Check to see that the function generator "span" is 90 Hz.

Question 4. What is the waveform on the oscilloscope doing? Is this reasonably called a "frequency modulation?"

Note what's happening to the spectrum analyzer display. Does this make sense? Use the "pause" key to check the frequency of the peak at the extreme ends of its range.

Question 5. What are the lowest and highest frequencies of the spectral peak during its "motion?" Are these consistent with a "span" of 90 Hz in the frequency modulation?

Now make similar observations of both displays for the same rate frequencies as before, 1 Hz, 10 Hz, and 100 Hz, and print a copy of the 100 Hz spectrum, noting the frequencies of the peaks as before. As time permits, make any additional adjustments to the waveform and comment.

Question 6. What frequencies are present in the 100 Hz spectrum? Do these make sense?