Communications Lab Manual Amplitude modulation and DSB-SC

BIRZEIT UNIVERSITY ELECTRICAL ENGINEERING DEPARTMENT ANALOG AND DIGITAL COMMUNICATION LAB (ENEE 411)

Last Update: January -2013

Table of Contents Experiment 1 AM Modulation and Detection .............................................................. 4 Experiment 2 DSB-SC and SSB ................................................................................. 11 Experiment 3 FM Modulation and Demodulation .................................................... 19 Experiment 4 FDM ..................................................................................................... 26 Experiment 5 ADC ...................................................................................................... 34 Experiment 6 DAC ...................................................................................................... 42 Experiment 7 PCM ...................................................................................................... 52 Experiment 8 TDM ..................................................................................................... 57 Experiment 9 ASK (Amplitude Shift Keying) ............................................................ 63 Experiment 10 FSK (Frequency Shift Keying) .......................................................... 70 Experiment 11 BPSK(Binary Phase Shift Keying) .................................................... 75 Experiment 12 QPSK(Quadri- Phase Shift Keying) .................................................. 80 Experiment 13 Delta Modulation and Demodulation ............................................... 83

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EXPERIMENT.1

AM Modulation and Demodulation Objectives:  To understand the theory of amplitude modulation and demodulation.  To design and implement the two types of AM modulator: transistor and balanced modulator.  To design and implement the two types of AM demodulator: the diode detection and the product detection.  To understand the measurements and adjustments of AM modulator and demodulator.

PrelaB Work: Use MATLAB command and M files to draw the demodulated signal after the envelope detector given that: S AM (t )  Ac [1   cos(m t )] cos(c t )

1. Write the mathematical expression for the demodulated signal. 2. Use MATLAB command and M files to draw the demodulated signal for the following three cases: a. Ac=16v, modulation index=0.22, modulating signal frequency=800Hz b. Ac=16v, modulation index=1, modulating signal frequency=800Hz c. Ac=16v, modulation index=1.85, modulating signal frequency=800Hz 3. Discuss your result in each part .you must write the commands which are used in the Pre-lab.

Equipment Required:    

2 AC Function Generators DC Power Supply ETEK ACS-3000-02 Module Connection wires

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Theory: Modulation: the process by which some characteristic (like: amplitude, frequency or phase) of a carrier signal is varied in accordance with the modulating signal (message signal).The signal modulation is used in order to transmit messages over long distances and also to transmit signals from various sources simultaneously over a common channel. Amplitude Modulation (AM): The process in which the amplitude of the carrier varies linearly with message signal. The general formula for the modulated AM signal:

 A  S AM (t )  Ac 1  m cos(2f m t ) cos(2fct )............(1  1)  Ac  A  : M odulation index whic h is equal to m . Ac Am : Amplitude of the message signal with volt unit. f m : Frequency of the message signal. fc : Frequency of the carrier signal. Ac : Amplitude of the carrier signal with volt unit. From the above formula we find that in order to generate an AM signal we just need to add a DC signal with the message signal then multiply the added signal with the carrier signal. The analog multiplier is the basic modulator that is used to generate AM signal as shown in fig1.1:

Fig(1.1): Analog Multiplier.

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The Modulation Index µ: There is an important parameter in the AM modulation which is called the modulation index (µ) which is equal to Am/Ac .

The Frequency Spectrum for the AM modulated signal: Equation (1-1) can be written as : ( )

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The first term of equation (1-2) represents the double side band signals .While the second term represents the carrier signal. Since the audio signal is hidden in the double side bands and the carrier signal does have no data, the AM modulation is lower efficiency than double side band suppressed carrier (DSB-SC) modulation but its demodulation circuit is much simpler. If the double side bands get stronger then the transmission efficiency is getting better. From equation (1-2) we find that the double side bands are proportional to µ so larger µ is getting better efficiency. The transmission power efficiency η: (

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The modulation index is smaller or equal to .So if µ >Tm .Where Tc=1/fc, Tm=1/fm, RC is the time constant of the RC low pass filter .

Fig(1.2): Envelope detector.

If there is an over modulation we can use the product detector in order to recover the signal.

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Detection Using Product detector:

Fig(1.3): Product detector.

The output of the product as shown in fig (1-3): (

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The first term of eq(1-4) is a DC signal while the second is the message signal , the third term is the second harmonic of the AM signal which is rejected by the low pass filer. The two types of the detectors have its own advantages and disadvantages. For the envelope detector which is asynchronous detector, its circuit is simple but its performance is not better as the product detector .However, the product detector’s circuit is more complicated and requires synchronous for both carrier signal and AM signal (same phase and same frequency), otherwise the quality of the output will be affected.

Procedure: Transistor AM modulator: 1- Refer to ACS3-1 on ETEK kit ACS-3000-02 module. 2- At the audio input port (Audio I/P) ,use the function generator to input a sine wave 600mV amplitude and 1KHz frequency. At the carrier signal port (Carrier I/P) ,input a sine wave 1.7 amplitude and 500 KHz frequency. 3- By using the oscilloscope, observe the AM modulated signal at the modulator output port (AM O/P).Adjust VR1 so that the AM signal is maximum without distortion (VR1 is used to change the operation point of the transistor and it also controls the magnitude of the carrier) . 4- Observe the signals at TP1,TP2 and TP3. 7

5- Try to change the frequency and the amplitude of the message signal. record your results.

Balanced AM modulator: 1- Refer to ACS3-2 on ETEK kit ACS-3000-02 module. 2- Let J1 short,J2 open so that R10=6.8KΩ (R10 determines the magnitude of the bias current for the modulator). 3- At the audio input port (Audio I/P) ,use the function generator to input a sine wave 500mV amplitude and 1KHz frequency. At the carrier signal port (Carrier I/P) ,input a sine wave 2V amplitude and 500 KHz frequency. 4- By using the oscilloscope, observe the AM modulated signal at the modulator output port (AM O/P).Adjust VR2 so that the AM signal is maximum without distortion (VR2 controls the gain of the modulator) . Adjust VR1 so that the value of µ is less than 1 (VR1 controls the value of µ). Record your results. 5- Change the value of VR1 until µ=1 (100% modulation). Let µ