University of Sharjah Electrical Engineering Telecom Lab PDF

University of Sharjah College of Engineering Electrical Engineering Department TELECOMMUNICATION SYSTEMS I LAB. 0402347 Experiment # 2: AM TRANSMISSION/RECEPTION (DSB-LC) Table of Contents 1 Objectives.....................................................

University of Sharjah College of Engineering Electrical Engineering Department TELECOMMUNICATION SYSTEMS I LAB. 0402347 Experiment # 2: AM TRANSMISSION/RECEPTION (DSB-LC) Table of Contents 1 Objectives......................................................................................................................... 3 2 Introduction...................................................................................................................... 3 3 Amplitude Modulation Demodulation (AM) or DSB-LC...................................................... 3 3.1 AM Modulation.................................................................................................................... 3 Modulation Index........................................................................................................................... 5 3.2 AM Demodulation................................................................................................................ 7 4. Lab Work (Using TIMS).................................................................................................. 8 AM Modulation................................................................................................................................ 8 AM Demodulation.......................................................................................................................... 10 5. Lab Work (Using SIMULINK) Mandatory..................................................................... 11 AM Modulation.............................................................................................................................. 11 AM Demodulation.......................................................................................................................... 14 2  AM TRANSMISSION/RECEPTION (DSB-LC) 1 Objectives In this experiment, the student will be able to: ▪ Use the Emona Telecoms-Trainer and Simulink to implement the DSB-LC modulator and demodulator. 2 Introduction In communication systems information is transmitted from one place to another using electrical signals (telephone, TV and radio broadcast etc.). Usually the information bearing signals (message signals) are not suitable for transmission due to its propagation qualities (a large wavelength). Also, since these signals generally exist in the same frequency range it is necessary to transmit them using different frequency allocations to avoid interference. One of the methods used to solve these problems is linear modulation, which is merely the frequency translation of the spectrum of the information (or message) signal to a usually much higher frequency. The translated spectrum can be modified before transmission in different forms resulting in different linear modulation schemes. Specifically, there are four linear modulation methods: Double-sideband with suppressed carrier DSB-SC. Amplitude modulation (AM) or DSB-LC (large carrier), Single-sideband (SSB). Vestigial-sideband (VSB). 3 Amplitude Modulation Demodulation (AM) or DSB-LC 3.1 AM Modulation A continuous-wave goes on continuously without any intervals and it is the baseband message signal, which contains the information. This wave has to be modulated. According to the standard definition, “The amplitude of the carrier signal varies in accordance with the instantaneous amplitude of the modulating signal.” Which means, the amplitude of the carrier signal containing no information varies as per the amplitude of the signal containing information, at each instant. This can be well explained by the following figures. 3 The first figure shows the modulating wave, which is the message signal. The next one is the carrier wave, which is a high-frequency signal and contains no information. While the last one is the resultant modulated wave. It can be observed that the positive and negative peaks of the carrier wave, are interconnected with an imaginary line. This line helps to recreate the exact shape of the modulating signal. This imaginary line on the carrier wave is called an Envelope. It is the same as that of the message signal. Consider a modulating wave (message signal) m(t) that consists of a single tone or single frequency component. 𝑚(𝑡) = 𝐴𝑐 cos⁡(2𝜋𝑓𝑚 𝑡) and the carrier signal be, 𝑐(𝑡) = 𝐴𝑐 cos⁡(2𝜋𝑓𝑐 𝑡) Where, Am and Ac are the amplitude of the modulating signal and the carrier signal respectively. fm and fc are the frequency of the modulating signal and the carrier signal respectively. 4 Then, the equation of Amplitude Modulated wave will be 𝑠(𝑡) = (𝐴𝑐 +𝐴𝑚 cos(2𝜋𝑓𝑚 𝑡))cos⁡(2𝜋𝑓𝑐 𝑡) (Equation 1) Thus, the carrier waveform is transmitted along with the sidebands to make it available at the receiver for coherent demodulation without needing complex carrier recovery circuits. When the message is a simple sinewave the equation’s solution tells us that the AM signal consists of three sinewaves: 1 𝑠(𝑡) = 𝐴𝑐 cos⁡(2𝜋𝑓𝑐 𝑡) + [𝐴𝑚 𝐴𝑐 cos(2𝜋(𝑓𝑐 − 𝑓𝑚 )𝑡) + 𝐴𝑚 𝐴𝑐 cos(2𝜋(𝑓𝑐 + 𝑓𝑚 )𝑡)] 2 ▪ One at the carrier frequency ▪ One with a frequency equal to the sum of the carrier and message frequencies ▪ One with a frequency equal to the difference between the carrier and message frequencies Modulation Index The modulation index 𝜇⁡of an amplitude modulated signal is defined as the measure or extent of amplitude variation about an un-modulated carrier. 𝐴𝑚 μ= 𝐴𝑐 Rearrange the Equation 1 as below. 𝐴𝑚 𝑠(𝑡) = 𝐴𝑐 [1 + cos(2𝜋𝑓𝑚 𝑡)] cos⁡(2𝜋𝑓𝑐 𝑡) 𝐴𝑐 𝑠(𝑡) = 𝐴𝑐 [1 + µ cos(2𝜋𝑓𝑚 𝑡)]cos⁡(2𝜋𝑓𝑐 𝑡) The modulation index or modulation depth is often denoted in percentage called as Percentage of Modulation. We will get the percentage of modulation, just by multiplying the modulation index value with 100. 5 For a perfect modulation, the value of the modulation index should be 1, which implies the percentage of modulation should be 100%. For instance, if this value is less than 1, i.e., the modulation index is 0.5, then the modulated output would look like the following figure. It is called as Under-modulation. Such a wave is called as an under-modulated wave. If the value of the modulation index is greater than 1, i.e., 1.5 or so, then the wave will be over- modulated. It would look like the following figure. As the value of the modulation index increases, the carrier experiences a 180o phase reversal, which causes additional sidebands, and hence, the wave gets distorted. Such an over-modulated wave causes interference, which cannot be eliminated. The total power in an AM signal is the sum of the carrier power and the lower and the upper sideband power. Transmission efficiency is the fraction of total power that is contained in the sidebands. 𝑃𝑆𝐵 ɳ= 𝑃𝑇 6  Figure 3: DSB-LC AM Generation A symbolic diagram for DSB-LC AM is shown in Figure 3. Note that the adder has adjustable gain controls on both of its inputs. For this diagram to correspond to the equations shown above, the G gain control should be set to the value m (modulation index), and the g gain control should be set so that the DC offset has unity magnitude. Modulation index can be measured by using the below formula 3.2 AM Demodulation The demodulation of AM signal can be achieved by an envelope detector circuit which is an electronic circuit that takes a high-frequency signal as input and provides an output which is the "envelope" of the original signal. The capacitor in the circuit stores up charge on the rising edge and releases it slowly through the resistor when the signal falls. The diode in series rectifies the incoming signal, allowing current flow only when the positive input terminal is at a higher potential than the negative input terminal. Figure 4 Envelop detector circuit 7 4. Lab Work (Using TIMS) The following plug-in modules will be needed to run this experiment: 1X Audio oscillator – Generates an adjustable-frequency clock in the audio range. 1X Adder – Adds two signals together. 1X Multiplier – Multiplies two signals together. 1X Utilities Module – Contains several useful parts including a rectifier. 1X Tunable Low-pass Filter – Filters the input to allow only low frequencies to pass. The cutoff frequency is adjustable. Part I AM Modulation The objective of this procedure is to explore and understand Amplitude Modulation (AM) through the implementation of the block diagram shown in Figure 6. The modulator stays to the equation AM = (DC + message) × the carrier. The goal is to set up and configure the modulation system, adjusting parameters such as DC voltage and modulation index to observe and analyze the resulting AM signal. Figure 6 1. Configure the oscilloscope with the Trigger mode set to Auto, Source control on CH1(A), and Time-based control adjusted to display three cycles of the Master Signals module's 5kHz SINE output. Enable both channels A and B in DUAL mode with both channels set to DC coupling. 8  2. Locate the Adder module and turn its G and g controls fully anti-clockwise. 3. Find the Variable DCV module and set its DC Voltage control almost fully anti-clockwise. 4. Connect the setup as shown in Figure 6. Set the DC voltage to 1. I. Disconnect the cable from the A input of the ADDER. II. Set the DC voltage to 1, adjusting the DC Variable and/or “g” gain until the signal at the ADDER's output is approximately 1 DC volt. Set the modulation index m=0.5. I. Disconnect the cable from the B input of the ADDER. Reconnect the cables that were previously removed from the A input of the ADDER. II. Set the modulation index m=0.5, adjusting the “G” gain control of the ADDER until the output signal is about 0.5-volt peak (i.e. 1 peak -to peak.) Note ;Make sure the MULTIPLIER coupling is set to DC) Modulation index = (A-B)/(A+B) Where A and B as shown in the figure below. 5. Display the two waveforms (Original message and AM signal) and capture a screenshot of the results. 6. Activate the frequency spectrum mode with a suitable frequency range. 7. Observe and capture a screenshot of the spectrum of the AM signal, analyzing the frequency components present. 8. Comment on the observed frequency components in the spectrum of the AM signal. 9 Part II AM Demodulation In this procedure, we are adding an envelope detector to the setup shown in Figure 6, as illustrated in Figure 7. Our goal is to understand how this addition influences the demodulation process and evaluate how well the message signal is recovered. Figure 7 1. Display the waveform and the spectrum of the demodulated signal. Is the message signal perfectly recovered? Capture a screenshot of the results. 2. Supply the demodulated signal to the Tunable Low-pass Filter module and observe the signal at the output of the module. 3. Adjust the Tunable Low-pass Filter module's Gain control to the middle of its travel and set the Cut-off Frequency to 6 kHz. Assess whether the recovery of the message signal improves and provide an explanation. Capture a screenshot of the results. 4. Repeat the procedure for a modulation index of m=1 and explain the effect of modulation index variations on the recovery process. Capture a screenshot of the results. 10 5. Lab Work (Using SIMULINK) Mandatory AM Modulation This part of the experiment lets you build a simulink model to simulate a DSB AM modulator demodulator. Figure 8 It is clearly seen that the AM model is exactly based upon the mathematical foundation provided in the theoretical section. The message signal is multiplied by the modulation index, then it is added a DC carrier, and finally is multiplied with a sinusoidal carrier signal in order to transmit the AM modulated signal. ▪ Build the SIMULINK model setup shown in Figure 8. ▪ Message signal and Carrier are sine waveforms. The product, adder, Modulation Index, DC, Scope, Spectrum Analyzer, and zero-Order-Hold blocks can be found by Searching “product”, “Adder”,” Gain”,” constant”, “Scope” and “spectrum Analyzer”, “zero-Order-Hold” respectively within the library browser. ▪ Set the parameters of the different blocks in your model as follows: 11  Block Model Parameters to be set Message Signal Waveform type: Sine Amplitude: 1 Frequency: 5000 Hz Transmitters Carrie (TX carrier) Waveform type: Sine Amplitude: 1 Frequency: 100000 Hz Modulation Index 1 ▪ Set the simulation parameters (Simulation >> Model Configuration Parameters) as follows: Scope settings ▪ In the scope menu, select View > Configuration Properties > logging and uncheck the limit data points ▪ In the scope menu, select View > Configuration Properties > main and set the sampling time tab to 1e-6 Spectrum Analyzer settings ▪ Zero-Order-Hold block must be used since the spectrum cannot be displayed for continues or infinite sample time. ▪ Double click on the Zero-Order-Hold block and se the sampling time at 1e-6. ▪ The Spectrum Settings pane appears at the right side of the Spectrum Analyzer window. These settings control how the spectrum is calculated. To show the Spectrum Settings, in the Spectrum Analyzer menu, select View > Spectrum Settings or use the button in the toolbar. ▪ Set the parameters of the spectrum as shown in the figure below 12 Suggested Spectrum Analyzer parameters setting MATAB 2023 Analyzer Tab Estimation Tab Spectrum Tab 13  1. View the Message signal, the TX carrier and the Modulated signal on one scope with three axes. ▪ In the scope menu, select View > Configuration Properties > main and set the Number of input ports on the Main tab to 3) ▪ In the scope toolbar, select View > Layout, or select the Layout button ( ). 2. Title the signals, Right click on each waveform then >> axes properties or configuration properties (depends on the MATLAB version that you have) 3. Comment your result. In order to observe the spectrum analyzer, please increase the simulation time to 1 or 2 seconds. 4. Observe the 3 spectrum analyzers, On the toolbar, click the Peak Finder button, observe the peaks and record their frequencies, then explain the waveforms from the frequency point of view. Comment your result. AM Demodulation 1. Modify the model as shown in Figure 9 below. 14  Figure 9 Square-Law and Envelope Detector is used to demodulate low level AM wave. This demodulator contains a square law device and low pass filter. The AM wave is applied as an input to this demodulator. The output of the square law device is 1 𝑠(𝑡)2 = [(1 + 𝜇𝑚(𝑡)) cos(2𝜋𝑓𝑐 𝑡)]2 , where cos(2𝜋𝑓𝑐 𝑡)2 = [1 + cos(4𝜋𝑓𝑐 𝑡)] 2 1 𝑠(𝑡)2 = [(1 + 𝜇𝑚(𝑡))2 + (1 + 𝜇𝑚(𝑡))2 cos(4𝜋𝑓𝑐 𝑡)] 2 1 𝑠(𝑡)2 = [(1 + 2⁡𝜇𝑚(𝑡) + 𝑚(𝑡)2 ) + (1 + 𝜇𝑚(𝑡))2 cos(4𝜋𝑓𝑐 𝑡)] 2 The high frequency is removed after filtering and a scaled version of the message signal can be extracted. The output of the low pass filter is 1 𝑀(𝑡) = [(1 + 2⁡𝜇𝑚(𝑡)] 2 ▪ Drag and drop Math function from Math Operation toolbox, double click and select magnitude^2 to perform the |U2| function. 2. Set the analog filter’s order to 2 and the passband edge frequency at a frequency close to the message signal’s frequency 3. Observe the signal in the time domain before the LPF. Comment your result. 4. View the Message signal and the recovered signal on one scope with two axes. 5. Title the signals, Right click on each waveform then >> axes properties or configuration properties (depends on the MATLAB version that you have) 6. Comment your result. 7. Observe and explain the spectrum of the recovered signal. 8. Repeat this part for modulation index 0.5 and 4 respectively. ▪ What happens to the modulated signal’s waveform for each case? ▪ In which values, the demodulation can be performed correctly? Why? 15