Fatih University Electrical and Electronics Engineering Department EEE Communications I EXPERIMENT 4 AM DEMODULATORS

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1 Fatih University Eletrial and Eletronis Engineering Departent EEE Couniations I EXPERIMENT 4 AM DEMODULATORS 4.1 OBJECTIVES 1. Understanding the priniple of aplitude odulation and deodulation.. Ipleenting an aplitude deodulator with a diode. 3. Ipleenting an aplitude deodulator with a produt detetor. 4. PRELIMINARY WORK Read hapter 4 in the textbook. 4.3 EQUIPMENT AND MATERIALS The equipent list that will be used in this experient is shown in Table 4.1. Before starting the experient, reord the odel, serial nuber and offie stok nuber of the equipent that will be used throughout the experient. Also, reord any daages. Table 4.1 Equipent list for experient 4. Ite No Equipent Model Serial No Offie Stok No 1 Digital Osillosope (DO) Funtion Generator (1) 3 Funtion Generator () 4 DC Power Supply 5 Conneting Probes and Cables 6 Diode Detetor Unit 7 Produt Detetor Unit Reord of daages or other oents: 4.4 THEORY Deodulation is the opposite proess of odulation. Aplitude Modulation (AM) is a proess that a high-frequeny arrier signal is odulated by a low-frequeny odulating signal (usually an audio). In order to reover the audio signal at the reeiver, it is neessary to extrat the audio signal fro the AM signal. The proess of extrating a odulating signal fro a odulated signal is alled deodulation or detetion (see Figure 4.1). In general, EEE Couniations I Experient 4 Page 1 of 9

2 detetors an be ategorized into two types: synhronous (oheren and asynhronous (nonoheren detetors. Figure 4.1 Illustration of an aplitude deodulation Diode Detetor Sine AM odulated signal is the signal that the arrier aplitude varies with the odulating aplitude, a deodulator is used to extrat the original odulating signal fro the AM signal. Figure 4. Blok diagra of a retified deodulator. The blok diagra of a diode detetor whih is shown in Figure 4., is a typial asynhronous detetor. The AM odulated signal inluding both positive-half and negative-half envelope waves is applied to the input of the retifier. The retified output signal is the positive half envelope plus a DC level and is fed into a low-pass filter whose output is the original odulating signal with a DC level. The odulating signal is finally reovered bak by reoving the DC level fro this signal. Figure 4.3 shows a pratial diode detetor iruit. The oponents R 1, R, R 3, R 4, U 1 and U onstitute two inverting aplifiers onneted in asade to offer a proper gain for the AM signal. The aplified AM signal is retified by D 1 diode and then fed into the input of the low-pass filter onstruted by C, C 3 and R 5. The output signal of low-pass filter is the positive-half envelope with a DC level. The apaitor C 4 is used to pass the AC oponents while bloking the DC oponent. EEE Couniations I Experient 4 Page of 9

3 Figure 4.3 Diode detetor iruit Produt Detetor Deodulation of an AM signal an be also aoplished with a balaned odulator. This type of deodulator is alled synhronous detetor or produt detetor. Figure 4.4 shows t, the AM signal the internal iruit of MC1496 balaned odulator. The equations for x AM ( ) and x (, the arrier signal, an be written as x x AM ( V [ 1+ µ os( πf ][ V os( πf ] = (4.1) DC ( V os( πf = (4.) If these two signals are onneted to the inputs of a balaned deodulator, then its output will be as follows: x out ( = kx ( x AM ( = kv V [ 1+ µ os( πf ] os ( πf DC kvdcv = kvdcv + kvdcv + µ os ( πf [ 1+ µ os( πf ] os[ ( πf ] (4.3) where k is the gain of the balaned odulator. The first ter on the right side of Eq. (4.3) represents DC level, the seond ter is the odulating signal, and the third ter is the seond-order haroni signal. To reover the odulating signal, the intelligene ust be x out t. extrated fro the AM signal ( ) EEE Couniations I Experient 4 Page 3 of 9

4 Figure 4.4 MC1496 internal iruit. Figure 4.5 shows a pratial produt detetor iruit. The VR 1 ontrols the input level of the arrier signal. The output signal fro the MC1496 pin 1 is expressed by Eq. (4.3). The low-pass filter onstruted by C 7, C 9 and R 9 is used to reove the third ter, whih is the seond-order haroni signal in the AM odulated signal. The first ter of Eq. (4.3) is the DC level that an be bloked by the apaitor C 10. The aplitude of the deodulated output whih represents the audio signal an be written as x out kv V = (4.4) DC ( µ os( πf Fro the above disussion, we an onlude that the diode detetor is an asynhronous detetor whose iruit is siple but the quality of detetion is low. On the other hand, the produt detetor is a synhronous detetor whose iruit is ore opliated and the arrier signal ust be exatly synhronized with the AM signal. However, the quality of detetion is uh superior. EEE Couniations I Experient 4 Page 4 of 9

5 Figure 4.5 Produt detetor iruit. 4.5 EXPERIMENTAL PROCEDURE AND RESULTS Note: When using digital osillosope (DO), reord the ritial data related with any observation on DO, for exaple DC level, peak values, period and frequeny for different regions; then draw the wavefor based on this data on sale. The ritial data should appear just below the assoiated figure. You an take the DO display by holding and taking it in storage Diode Detetor Ciruit 1. Connet the proper supply voltages into DSB-SC odulator iruit. Set SW 1 to R 11 and SW to R 1. Set the CARRIER INPUT to 50 Vpp, 00 khz sine wave and the AUDIO INPUT to 150 Vpp, 3 khz sine wave. Adjust the VR 1 of DSB-SC odulator to get %100 perent odulation.. Connet proper supply voltages into diode detetor iruit. Connet the DSB-SC OUTPUT signal of odulator into the AM INPUT of diode detetor iruit. 3. Observe the output wavefor of the DEMODULATION OUTPUT. Reord the AUDIO INPUT and DEMODULATION OUTPUT signals in Table 4.. Measure the DEMODULATION OUTPUT frequeny and peak to peak voltage by using your osillosope and reord the results in Table Change the AUDIO INPUT frequeny of odulator iruit to khz and 1 khz, respetively and reord the results in Table 4.. EEE Couniations I Experient 4 Page 5 of 9

6 4.5. Produt Detetor 1. Connet the proper supply voltages into DSB-SC odulator iruit. Set SW 1 to R 11 and SW to R 1. Set the CARRIER INPUT to 50 Vpp, 500 khz sine wave and the AUDIO INPUT to 150 Vpp, 3 khz sine wave. Adjust VR 1 of DSB-SC odulator to get 50% perent odulation.. Connet the proper supply voltages into produt detetor iruit. Connet the DSB-SC OUTPUT signal of odulator into the AM INPUT of the produt detetor iruit and onnet the sae arrier of the odulator iruit into the CARRIER INPUT. 3. Observe the output wavefor of the DEMODULATION OUTPUT. Adjust VR 1 and VR of the produt detetor iruit until you get the axiu peak to peak voltage. Reord the AUDIO INPUT and DEMODULATION OUTPUT signals in Table 4.3. Measure the DEMODULATION OUTPUT frequeny and peak to peak voltage by using your osillosope and reord the results in Table Change the AUDIO INPUT frequeny of odulator iruit to khz and 1 khz, respetively and reord the results in Table Change the CARRIER INPUT to a 50 Vpp, 1MHz sine wave, and the AUDIO INPUT to a 150Vpp, khz sine wave. Adjust the VR 1 of DSB-SC odulator to get 50% perent odulation. 6. Observe the output wavefor of the DEMODULATION OUTPUT. Reord the AUDIO INPUT and DEMODULATION OUTPUT signals in Table Change the CARRIER INPUT frequenies to 1.5 MHz and MHz, respetively and reord the results in Table 4.4. Table 4. (V = 50 V pp, V = 150 V pp, f = 00 khz, = 100%) Audio Input Frequeny Audio Input Wavefor Deodulated Output Wavefor Y1: 50 V A: 00 µs. Y1: 1 V A: 00 µs. 3kHz Frequeny: EEE Couniations I Experient 4 Page 6 of 9

7 Y1: 50 V A: 00 µs. Y1: 1 V A: 00 µs. khz Frequeny: Y1: 50 V A: 500 µs. Y1: 1 V A: 500 µs. 1kHz Frequeny: Table 4.3 (V = 50 V pp, V = 150 V pp, f = 500 khz, = 50%) Audio Input Frequeny Audio Input Wavefor Deodulated Output Wavefor Y1: 50 V A: 00 µs. Y1: 500 V A: 00 µs. 3kHz Frequeny: EEE Couniations I Experient 4 Page 7 of 9

8 Y1: 50 V A: 00 µs. Y1: 500 V A: 00 µs. khz Frequeny: Y1: 50 V A: 00 µs. Y1: 500 V A: 00 µs. 1kHz Frequeny: Table 4.4 (V = 50 V pp, V = 150 V pp, f = khz, = 50%) Carrier Input Frequeny Audio Input Wavefor Deodulated Output Wavefor Y1: 50 V A: 00 µs. Y1: 500 V A: 00 µs. 1 MHz Frequeny: EEE Couniations I Experient 4 Page 8 of 9

9 Y1: 50 V A: 00 µs. Y1: 500 V A: 00 µs. 1.5 MHz Frequeny: Y1: 50 V A: 00 µs. Y1: 500 V A: 00 µs. MHz Frequeny: EEE Couniations I Experient 4 Page 9 of 9