1 Security Exercise 17: Amplitude Modulation Introduction to Amplitude Modulated Signals 1: Set Up Part Step One : Function generator setup. Turn on power to the lab bench (the switch on the right that says "120V OUTLETS.") □ Turn on the function generator. □ □ Select the sinusoidal function by pressing the button with the Sine wave on it. The function generator display should indicate a small sine wave. □ Turn any other lit buttons off. Utility function and Output Setup soft key. Load should be High Z. □ Select the Push Done. □ □ Press the output button. Step Two : Oscilloscope setup. □ Turn on the oscilloscope. □ Locate the CH 1 input on your oscilloscope. It will have a two -input (RED / BLACK) banana plug adaptor installed. Connect banana plug cables (which can be found under the bench on □ the plastic Quad board) from the function generator’s output to the CH 1 input on the oscilloscope (red to red and black to black). 2: Amplitude Modulated Signal in the Frequency Domain Part An Amplitude Modulated (AM) signal looks somewhat like the figure that follows. This particular depiction includes the AM (message amplitude), V (carrier amplitude), V (max envelope voltage) and V signal’s envelope, and the definitions of max c m (min envelope voltage). The information signal (message) modulates the amplitude of the carrier. V min □ On the Function Generator select the Store/Recall button and then push Recall State and then push State 3 and then Recall State again. Make sure the output button is lit.
2 □ Push AUTOSET on the Oscilloscope and you will see an AM signal. □ CH 1 MENU on the oscilloscope and make the following settings. Press Coupling: AC BW Limit: OFF Volts/Div: COARSE Probe: 1X Invert: Off □ CH2 Menu button twice If a trace appears for CH2, then press the to turn off the CH 2 trace, since nothing is connected to CH 2 for this lab. With the CH 1 menu selected, adjust the position of the vertical axis zero □ level by rotating the position knob under the vertical section in the CH 1 column, so that the “1→” on the left side of the LCD is adjacent to the major horizontal axis (centered vertically on the display). Adjust the vertical scale (Volts/Div) to enla rge the □ displayed signal so that it occupies more than half of the oscilloscope display. Adjust the horizontal range (Sec/Div) so that you can □ e one shown see the AM waveform similar to th above. You may have to adjust the knob for trigger level (it is to the far right below the AUTOSET button) to clean up the signal. Turn the knob so that you see the trigger level (arrow to far right of screen) rise from the center line. Hint: if you have trouble removing the “jitter” from the graph, use the “Run/Stop” but ton in the upper right hand corner of the oscilloscope to pause the capture. Once the display is properly adjusted, use the boxes on the oscilloscope to measure the following parameters. Record your Question 1 results in on your answer sheet. . and V Measure 1. V max min (higher frequency signal). You will have to adjust the horizontal Measure the period of the carrier, 2. T c (sec/div) scale to accurately measure a the period of a cycle of the high frequency carrier signal. 3. Measure T sage (lower frequency signal). You will need to adjust back the horizontal , the period of the mes m measures the period of the envelope of the AM signal. (sec/div). Note: T m m f the modulation index ), the message signal's frequency ( f , the carrier frequency ( ), the amplitude of the carrier Calculate c m ). Use the equations shown in Table 1 below along with your measured V ( Vc ) and the amplitude of the message signal ( m Question 1 on your answer sheet. values. Record your results in −+ − VVV VV VV min max min max min max m = = V Vm = = mc 22 + VV V c min max 11 ff = = cm TT cm : the equation of the AM waveform displayed on the oscilloscope. Write your answer on your answer Question 2 Determine V , V , sheet using the form below, but with numerical values replacing f : and f c m c m v t ) cos(2 ) = + cos(2 ( ) V V f t ft ππ ] [ m c AM c m
3 Part 3: Amplitude Modulated Signal in the Frequency Domain Let’s look at the freq uency spectrum of this signal. In class we manipulated the above equation to show us what frequencies will show up in the frequency domain: VV mm π ππ f ft f ft v t V ft =+++− cos 2 cos 2 ( ) cos(2 ) ( ) ( ) ) ( ) ( c m m c AM c c 22 There are 3 frequencies in the AM signal: the carrier and its 2 frequencies . So let us try to see this signal in terms of sideband Again, the oscilloscope performs a fast Fourier transform (FFT) and displays the magnitudes of the its frequency content. quencies present in the signal vs. frequency, so the horizontal scale shows frequency instead of time. The y -axis will show fre values in decibels, which can be ignored for this lab. □ Press AUTOSET and you will see the options to display the signal in time or in frequency (FFT). (NOTE: If the FFT option does appear when you press AUTOSET, then press the MATH MENU button.) not Push the button next to the FFT (or stay in FFT operation) and you will see spikes at specific frequencies □ . Set the horizon tal scale to read 12.5 kHz per division using the Sec/Div knob. □ Frequency cursor (recall that when using Question 3: Determine the value of the center and sideband frequencies. Use the be MATH, not CH 1 o r CH 2 to use the frequency plot cursors) must the FFT to show frequency content, the source : Question 4 as displayed on your o- scope. Label the frequency axis using the frequencies of the Sketch your frequency plot tones present in the AM signal. Indicate the heights of the AM signal spikes with the expected amplitude of the cosines in the -scope, you just need to indicate them on your signal (note: the amplitudes are not actually displayed on the FFT on your o plot.) Have your instructor check it. : What is the Question 5 bandwidth of the amplitude modulated waveform? Question 6 : Rewrite your AM signal as the sum of three cosines as in the example below: cos 2 f t V =++ πππ ( ) cos(2 ) ft V cos 2 ft v t V ( ( ) ) 12 1 2 3 3 AM . , f , , f f , V and V V filling in numerical values for 2 1 3 1 3 2 Question 7 : Is this AM signal within the range of frequencies that commercial AM radio stations transmit? Question 8 : If the information signal from above contained many frequencies (including frequencies higher than the f you m measured), how would this affect the bandwidth of th eform? e amplitude modulated wav 4: Another AM signal Part An evil user offers to You have a message signal you would like to transmit, but do not have access to an AM transmitter. your signal (for a small fee). You pay the fee. Your customers start complaining tha t your signal sounds terrible. transmit You examine the amplitude modulated signal that the evil user has generated for you: □ On the Function Generator select the Store/Recall button and then push Recall State and then push State 4 and then Recall State again. Make sure the output button is lit. □ AUTOSET on the Oscilloscope and you will see an AM signal. Push □ Eliminate the CH 2 signal (if necessary), vertically center the waveform and adjust the horizontal range and trigger level to stabilize the AM signal. Quest ion 9 : Explain the problem with the evil user AM signal (hint: look at the shape of the envelope).
4 Secu rity E se 17 Answer Sheet xe rci Na me: _____________________________________________________________________________________________________ Question 1: (kHz) V f (kHz) V m V V f m c max c min m ( s) T s) μ ( μ T c m _____________________________________________________________________________________________________ Question 2: _____________________________________________________________________________________________________ = _____________ f = ___________ f = _____________ f Question 3: lsb c usb ____________ _________________________________________________________________________________________ 4: Question __________________________________ Instructor / Lab Tech _____________________________________________________________________________________________________ Question 5: _____________________________________________________________________________________________________ Question 6: _____________________________________________________________________________________________________ Question 7: _____________________________________________________________________________________________________ Question 8: _________________________________________________________________________________________ ____________ Question 9: ____________________________________________________________________________________________________