SCHOOL OF ENGINEERING SCIENCE SIMON FRASER UNIVERSITY

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1 SCHOOL OF NGINRING SCINC SIMON FRASR UNIVRSITY NSC 37 Communiation Systems Lab # Amplitude Modulation Report Due Date: Wed., Ot 19, 011, 4:00pm Please submit to the drop box outside the NSC Offie 1. OBJCTIVS To study the modulation and demodulation of AM. To measure modulation fator in time domain and frequeny domain. To understand the funtions of various omponents in the superheterodyne reeiver.. LIST OF QUIPMNT LabVolt Power Suppoly and Dual Audio Amplifier Model 9401 LabVolt Dual Funtion Generator Model 940 LabVolt Frequeny Counter Model 9403 LabVolt AM/DSB/SSB Generator Model 94 LabVolt AM/DSM Reeiver Model 9411 Osillosope Spetrum Analyzer User manuals are available in the ourse website. 3. TH THORY Amplitude modulation results when a DC bias is added to the message signal m(t) prior to the modulation proess. The transmitted signal an be written as [ 1+ k m( t) ] os(πf ) = a. s( t) A t For a baseband signal of the form m(t) = Am os(πfmt), we have [ 1+ µ os(πf t) ] os(πf ), s( t) = A t m where µ = k A a is alled the modulation fator or modulation index. m DSB-SC signal an be obtained if there is no DC bias in the AM formula. 1

2 The modulation fator an be measure from the observed waveform on the sope when the modulating signal is a pure sine or osine wave, as shown in Fig. 1. Fig. 1. Sope output for AM modulation. In this ase, the max amplitude max = (1+µ) A, and the min amplitude min = (1- µ) A. Therefore max min 1+ µ = 1 µ, -min µ = -max max max + In pratial measurement, we an use the marker funtion of the sope to measure the differene between max and max, as well as the differene between min and min. In this way there is no need to enter the waveform around a referene line, and the auray of the measurement an be improved. The spetrum analyzer an also be used to measure the modulation fator in the frequeny domain. min min P1 P Fig.. Spetrum analyzer output for AM modulation. Fig. shows an output of the spetrum analyzer. The three peaks orrespond to the arrier and the two sidebands for a single tone input. When the modulating signal is a normalized sine or osine wave, the reading of the spetrum analyzer (in log sale) is in dbm sale: log 1 x mw. Therefore from the readings P1 and P in Fig, we have

3 P P 1 = log A / 1mW. log µ A /8 = log 1mW 4 µ The value of µ is thus given by µ = P1 P 0 For further details please refer to the following links Agilent Test & Measurement Appliation Note 150-1: Spetrum Analysis for AM & FM Anritsu s Guide to Spetrum Analyzer 4. AM MODULATION WITH LAB-VOLT 4. 1 Proedure: 1. Make sure all gain and level ontrols of different LabVolt modules are turned to the MIN position before ranking up the power.. On the AM/DSB/SSB generator, push in the arrier level knob to enable linear amplifiation and linear overmodulation. Turn up the arrier-level and RF gain ontrols to the MAX position. Adjust the RF tuning knob so that the arrier frequeny (measured by the frequeny ounter or sope) oming out of the RF output port (#6) is 10 khz. 3. Set up the funtion generator to produe a 0.5V peak peak khz sine wave (whih will be used as the message signal). 4. Use a T-onnetor (an be found in the file abinet near the door of the lab) to onnet the output of the funtion generator to the input of the AM/DSB/SSB generator as well as Channel of the sope. Connet the AM/DSB output to Channel 1 of the sope. Set up the sope so that it is triggered by Channel (the message signal). Compute and reord the modulation fator. 5. Repeat steps 3 4 for a square-wave message signal. 6. Again keep the arrier-level at the MAX position. Using a khz sine wave as the message signal. By varying the message level, generate AM 3

4 waveforms with five different perentage modulations between 0 0% and ompare their spetra. Use the Spetrum Analyzer to reord the power level of the arrier and eah of the two sidebands. Compute the modulation fator using the frequeny domain formula. Compute the effiieny using the power measure. For a sinusoidal message signal with modulation fator µ = 1, verify the effiieny is 33%. 7. Vary the frequeny of a sinusoidal message signal and observe how the signal spetrum hanges. 4. Analysis: 1. Sketh the envelope of the modulated signal for eah of the modulation fators tested in part 6.. xplain the hanges in the spetrum observed when the modulation fator is varied as in part 6 of the experiment. 3. Plot the power of the sideband signal as a funtion of the modulation fator as measured in part 6 and show that the result agrees with theory. What does this say about the effiieny of the modulation? 4. How does the modulation fator vary with the frequeny of the message signal? 5. AM DMODULATION WITH LAB-VOLT The Lab-Volt AM / DSB Reeiver, Model 9411, is a superheterodyne reeiver using three different detetion iruits. In this part of the lab, we will look at the envelope detetor for the demodulation of standard AM signals. The blok diagram of the reeiver is shown in the figure below. 4

5 The reeption of an AM signal using a superheterodyne reeiver is initiated by tuning the RF filter so that it passes the desired modulated message signal but eliminates the image signal. The frequeny of the loal osillator (LO) is adjusted at the same time so that the desired message signal is frequeny shifted from its position around the RF arrier to the enter of the intermediate frequeny (IF) band (455 khz in AM). The system uses high-side tuning, i.e., the loal frequeny is given by flo = f + fif, where f is the arrier frequeny, and fif is 455 khz. The highly seletive IF filter then removes unwanted frequenies and passes the signal on to an amplifier and a demodulator designed to operate at IF. The passband of the IF filter is khz (the bandwidth of eah station) in AM radio, and the passband of the RF filter is wider than the IF filter, sine the image signal is quite far from the desired signal. In the ase of standard AM with µ 1, an envelope detetor an be used as the demodulator (an output filter is also used to smooth unwanted high frequeny ripple). Automati Gain Control (AGC) AM is a linear modulation, so the sound volume is proportional to the radio signal strength. However, the strength of the reeived signal usually varies widely, depending on the power and distane of the transmitter, and signal path attenuation. Another fator that affets the quality of the demodulated signal is the linearity of the amplifier in the reeiver. The amplifier only has linear operating urve in a ertain range. The output will be distorted by the iruit if the signal strength is not in the desired range. The automati gain ontrol (AGC) iruit in AM reeiver alleviates the two problems by deteting the overall strength of the signal and automatially adjusting the gain of the reeiver to maintain an approximately onstant average output level in a larger range, i.e., it will raise a signal if it is too weak and redues it if it is too strong. Further information an be found at Proedure: 1 Inlude the AM/DSB reeiver Model 9411 in your system onfiguration if it is not there already. Use the frequeny ounter, the sope, or the spetrum analyzer to set the LO frequeny at point 4 of the reeiver to MHz, so the desired input is around 1 MHz. Adjust the spetrum analyzer so that it has a entre frequeny of 1 MHz and a span of 50 khz/div. 5

6 3 Measure the frequeny response of the RF filter: Disonnet the message signal and onnet a BNC able from the AM/DSB/SSB generator output to the input of the reeiver, Turn off the AGC by pulling out the pushbutton at Point 9. Set the generator arrier level to MAX (linear modulation mode) and adjust the RF gain (amplifier A) ontrol to approx. 11 o lok. 4 Connet the RF output signal (point 3) of the reeiver to the sope and observe the display as you slowly vary the arrier frequeny of the AM generator from 950 khz to 50 khz. Next, onnet the RF output at point 3 to the spetrum analyzer, observe the signal on the spetrum analyzer, and reord the frequeny at whih the peak signal is obtained. Also, in order to find out the bandwidth of the RF filter, reord the frequenies at whih the signal is 3 db below the peak. 5 Study of the IF output: Next, onnet the output of the AM generator to the input of the AM reeiver. Turn on the AGC and onnet the LabVolt frequeny ounter (Model 9403) to the IF output (Point 7) of the reeiver. Adjust the RF tuning knob for an IF output frequeny of exatly 455 khz. Apply a 0 mvp-p,.5 khz sinusoid to the input of AM generator (You may need to hange the amplitude and the frequeny of the message signal for a better view. You an observe the generator output on the sope, and use a modulation fator lose to 0% to have a better view of the AM signal. You may need to redue the input amplitude if the signal is overmodulated). Set the analyzer to have a entre frequeny of 455 khz and a span of khz/div. Reord the signal at IF output (point 7) in both the time and frequeny domains, be areful to trigger the sope as well as possible. 6 Demodulated signal: Connet the sope to the detetor output (point 8) of the reeiver and selet envelope detetor method in Point 11. Use a T- onnetor from the funtion generator to trigger the sope via hannel #. Keep the input message amplitude, fix the modulation fator to be around 50%, vary the input frequeny from 1 khz in khz steps, and reord the output waveforms and maximum levels in eah ase. 7 Set the message signal frequeny to.5 khz at about 0% modulation and adjust the RF gain ontrol of the AM generator from MIN to MAX, observe the hange of the amplitude and distortion of the demodulated output waveform. Reord the results at 1/4, 1/, 3/4 and max RF gains. 8 Turn off the AGC and repeat the last step. 5. Analysis: xplain the effets observed in Step 4 to Step 8. 6