Exp 1 DSBSC HL24nn(1).pdf

University of Sharjah College of Engineering Electrical Engineering Department TELECOMMUNICATION SYSTEMS I LAB. 0402347 Experiment # 1: Double Sideband Modulation and Demodulation (DSBSC) Telecommunication Systems 1 Lab Table of Contents 1. Objectives..........

University of Sharjah College of Engineering Electrical Engineering Department TELECOMMUNICATION SYSTEMS I LAB. 0402347 Experiment # 1: Double Sideband Modulation and Demodulation (DSBSC) Telecommunication Systems 1 Lab Table of Contents 1. Objectives................................................................................................................... 3 2. Introduction................................................................................................................ 3 3. Double-sideband suppressed-carrier (DSBSC)............................................................... 4 3.1 DSBSC Modulation.............................................................................................................5 3.2 DSBSC Demodulation..........................................................................................................7 4. Lab Work (using TIMS)................................................................................................ 8 Part I Generation of DSBSC.............................................................................................................9 Part II DSBSC Demodulation.......................................................................................................... 10 Part III Effect of the Phase shift..................................................................................................... 11 5. Lab Work (using SIMULINK) Optional......................................................................... 12 DSBSC Modulation........................................................................................................................ 12 DSBSC Demodulation.................................................................................................................... 16 Effect of Phase Shift...................................................................................................................... 17 2 Telecommunication Systems 1 Lab Double Sideband Modulation and Demodulation (DSBSC) 1. Objectives In this experiment, the student will be able to ▪ Use the Emona Telecoms-Trainer and Simulink to implement the DSBSC modulator and demodulator. ▪ Observe the impact of the phase error between the transmitter’s and the receiver’s carrier on the recovered signal. 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 Telecommunication Systems 1 Lab 3. Double-sideband suppressed-carrier (DSBSC) In amplitude modulation (AM), the sidebands refer to the two bands of frequencies that are generated on both sides (above and below) of the carrier frequency. The sidebands contain the information that is being transmitted, and they are typically symmetrically located with respect to the carrier frequency. There are two sidebands in AM modulation: Upper Sideband (USB): This sideband is located above the carrier frequency and contains the information in the form of variations in amplitude. Lower Sideband (LSB): This sideband is located below the carrier frequency and also contains the same information but in a slightly different phase. The sidebands are a result of the modulation process, where the carrier signal's amplitude is varied based on the information signal (audio or data) being transmitted. This variation in amplitude results in the creation of sidebands that are centered around the carrier frequency. A transmission of a signal that includes a carrier and two sidebands is termed a Double Sideband Large Carrier, or DSBLC for short. When the carrier is suppressed, and the saved power is distributed to the two sidebands, this is referred to as a Double Sideband Suppressed Carrier system, or DSBSC. The Figure below shows a simple message signal and an unmodulated carrier. It also shows the result of modulating the carrier with the message using DSBSC. 4 Telecommunication Systems 1 Lab 3.1 DSBSC Modulation The signal is obtained by the multiplication of baseband signal m(t) with carrier signal 𝑨𝒄 𝐜𝐨𝐬⁡(𝟐𝝅𝒇𝒄 𝒕) Figure 1. Block diagram of DSBSC modulation In Figure 2, you can observe a baseband signal, a carrier, and their corresponding DSBSC modulated waveforms. Figure 2 DSBSC modulation for a band limited message signal and its spectrum. Consider a modulating wave (message signal) m(t) that consists of a single tone or single frequency component. That is 𝑚(𝑡) = 𝐴𝑚 cos⁡(2𝜋𝑓𝑚 𝑡), then the DSBSC modulated wave is described by 𝒔(𝒕) = 𝑨𝒎 𝐜𝐨𝐬(𝟐𝝅𝒇𝒎 𝒕). 𝑨𝒄 𝐜𝐨𝐬⁡(𝟐𝝅𝒇𝒄 𝒕) 𝟏 𝟏 𝒔(𝒕) = 𝑨𝒄 𝑨𝒎 𝐜𝐨𝐬(𝟐𝝅(𝒇𝒄 − 𝒇𝒎 )𝒕) + 𝑨𝒄 𝑨𝒎 𝐜𝐨𝐬(𝟐𝝅(𝒇𝒄 + 𝒇𝒎 )𝒕) 𝟐 𝟐 5 Telecommunication Systems 1 Lab Figure 3 shows the graphical representation of single tone DSBSC signal Figure 3 Single Tone DSBSC signal and its Spectrum The spectrum of the DSBSC modulated signal is depicted in the figure. Within the modulated signal, the portion containing spectral components at a frequency of (𝑓𝑐 − 𝑓𝑚 ) below the carrier frequency 𝑓𝑐 , is referred to as the lower sideband signal (LSB) frequency and the portion above the carrier frequency⁡𝑓𝑐 is called the upper sideband signal (USB) frequency (𝑓𝑐 + 𝑓𝑚 ). The transmission bandwidth by DSBSC is equal to twice the highest frequency present in the message signal. In the case of message signal m(t) band limited to ‘W’ Hz, the transmission bandwidth is 2W Hz. 6 Telecommunication Systems 1 Lab 3.2 DSBSC Demodulation Recovering the message signal from the modulated signal is performed coherently. That is, the modulated signal is multiplied by a high-frequency sinusoid in perfect synchronization (in phase and frequency) with the incoming carrier. Multiplying the modulated signal with a local carrier will produce a baseband signal (message signal) as well as a signal modulated at double the carrier frequency. Therefore, an LPF is needed at the end of the demodulator to recover the baseband signal. Figure 4: Product demodulator In this process, the message signal can be extracted from DSBSC wave by multiplying it with a carrier, having the same frequency and the phase of the carrier used in DSBSC modulation. The resulting signal is then passed through a Low Pass Filter. Output of this filter is the desired message signal. Let the DSBSC wave be 𝑠(𝑡) = 𝐴𝑐 cos(2𝜋𝑓𝑐 𝑡) 𝑚(𝑡) The output of the local oscillator is 𝑐(𝑡) = 𝐴𝑐 cos(2𝜋𝑓𝑐 𝑡 + 𝜑) Where, 𝜙 is the phase difference between the local oscillator signal and the carrier signal, which is used for DSBSC modulation. From the figure, we can write the output of product modulator as 𝑣(𝑡) = 𝑠(𝑡)𝑐(𝑡) Substitute, 𝑠(𝑡) and 𝑐(𝑡) values in the above equation. 7 Telecommunication Systems 1 Lab 𝑣(𝑡) = 𝐴𝑐 cos(2𝜋𝑓𝑐 𝑡) 𝑚(𝑡)⁡𝐴𝑐 cos(2𝜋𝑓𝑐 𝑡 + 𝜑) = 𝐴2𝑐 cos(2𝜋𝑓𝑐 𝑡) cos(2𝜋𝑓𝑐 𝑡 + 𝜑) 𝑚(𝑡) = 𝐴2𝑐 [cos(4𝜋𝑓𝑐 𝑡 + 𝜑) cos(𝜑)] 𝑚(𝑡) 𝐴2𝑐 𝐴2𝑐 = cos(𝜑) 𝑚(𝑡) + cos(4𝜋𝑓𝑐 𝑡 + 𝜑)𝑚(𝑡) 2 2 In the above equation, the first term is the scaled version of the message signal. It can be extracted by passing the above signal through a low-pass filter. Therefore, the output of low pass filter is 𝐴2𝑐 cos(𝜑) 𝑚(𝑡) 2 The demodulated signal amplitude will be maximum when 𝜙=0°. That’s why the local oscillator signal and the carrier signal should be in phase, i.e., there should not be any phase difference between these two signals. The demodulated signal amplitude will be zero, when 𝜙=±90°. This effect is called the quadrature null effect. 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. 2X 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. 1X Phase Shifter. 8 Telecommunication Systems 1 Lab Part I Generation of DSBSC The objective of this part is to implement the complete equation DSBSC (Double-Sideband Suppressed Carrier) = message × carrier using the block diagram shown in Figure 5. The goal is to generate a DSBSC signal with specific characteristics, observe its waveform and frequency spectrum, and analyze the frequency components present. Figure 5 DSBSC Modulator The block diagram in Figure 5 implements the equation: DSBSC = the message × the carrier. 1. Use available TIMS modules and establish the connection, as shown in Figure 5, to implement the DSBSC equation. 2. Generate a DSBSC signal with a sinusoidal message of 2 kHz (from the MASTER SIGNALS module) and a carrier frequency of 10 kHz (from the AUDIO OSCILLATOR module). Use one multiplier from the QUADRATURE UTILITIES module. 3. Configure the oscilloscope, ensuring the time-based control displays three cycles of the Master Signals module's 2 kHz SINE output. Enabling channels A and B, and set both channels to DC coupling mode. 4. Display the waveforms of the original message and the DSBSC signal. Capture a screenshot of the results. 5. Activate the frequency spectrum mode with a suitable frequency range. 6. Observe and capture a screenshot of the spectrum of the DSBSC signal, choosing an appropriate frequency range for the spectrum analyzer. 7. Comment on the frequency components observed in the spectrum, analyzing their significance in the DSBSC signal. 9 Telecommunication Systems 1 Lab Part II DSBSC Demodulation The main goal of DSBSC demodulation is to retrieve the original message signal from a modulated waveform. By reversing the modulation process used during transmission and extracting the encoded information in both sidebands while suppressing the carrier. Figure 6 DSBSC demodulator The block diagram in Figure 6 represents the entire set-up of the DSBSC demodulator. Notably, the modulator's carrier is 'borrowed' to serve as the local carrier for the product detector. This synchronization between the two carriers is essential for the proper functioning of Double-Sideband Suppressed Carrier (DSBSC) communications. 1. Identify and locate the second multiplier within the QUADRATURE UTILITIES module. 2. Find the Tunable Low-pass Filter module and adjust its Gain control to approximately the middle of its travel. 3. Set the Cut-off Frequency of the Tunable Low-pass Filter module to 4 kHz. (Note: Measure the 3-dB cutoff frequency by connecting the TTL (CLK) output of the filter to the TTL input of the FREQUENCY COUNTER. Divide the reading by 100.) 4. Adjust the setup according to the configuration shown in Figure 6. 5. Observe the signal in both time and frequency domains before and after the Low-pass Filter, capturing screenshots of the results. 6. Explain the observed spectrum alterations before and after applying the Low-pass Filter, highlighting the impact of this filtering process on the signal characteristics. 7. Is the message signal recovered? 10 Telecommunication Systems 1 Lab Part III Effect of the Phase Shift The objective of this part is to introduce a controlled phase error between the carrier signals at the transmitter and receiver using the PHASE SHIFTER module. By adjusting the phase shift, we aim to explore the impact of intentional phase differences on the demodulation process. The procedure involves modifying the carrier path, utilizing the PHASE SHIFTER module, and observing the effects on the recovered signal. 1. Set the Tunable Low-pass Filter module's Gain to the middle of its travel and adjust its Cut- off Frequency to 4 kHz. 2. Instead of using the transmitter's carrier directly, route it to the PHASE SHIFTER module, and connect the module's output to the demodulator circuit's multiplier. 3. Locate the Phase Shifter module and set its Phase Change control to the 180° position 4. Vary the Phase Shifter module’s Phase Adjust control left and right while watching the effect on the recovered message. 5. Adjust the Phase Shifter module’s Phase Adjust control until the recovered message is the biggest. 6. What is the likely phase error between the two carriers? Tip: If you’re not sure about the answer to this question (and the next one), reread the notes at the top of pages 7-8. 7. Adjust the Phase Shifter module’s Phase Adjust control until the recovered message is the smallest. 8. What is the likely phase error between the two carriers? 11 Telecommunication Systems 1 Lab The phase difference can be calculated using the below equation 360° ∆𝜃 = ∆𝑡⁡𝑥⁡ 𝑇𝑃𝑒𝑟𝑖𝑜𝑑 5. Lab Work (using SIMULINK) Optional DSBSC Modulation This part of the experiment lets you build a Simulink model to simulate a DSBSC modulator- demodulator. Figure 7 Two sin wave blocks are used to model the message and the carrier signals. The output from the multiplier is the DBSB-modulated signal. 12 Telecommunication Systems 1 Lab 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 with a sinusoidal carrier signal in order to transmit the AM-modulated signal. ▪ Click on the Simulink Library icon or type Simulink at the MATLAB COMMAND prompt. * The Simulink Library Browser window is opened. ▪ Create a new blank model. ▪ Click to expand the Simulink folder at the Library Browser window. ▪ Build the SIMULINK model setup shown in Figure 7. ▪ Message signal and TX Carrier are sine waveforms. The product, Scope, Spectrum Analyzer, and zero-Order-Hold blocks can be found by Searching “product”, “Scope” and “spectrum Analyzer”, “zero-Order-Hold” respectively within the library browser: ▪ Set the parameters of the different blocks in your model as follows: Block Model Parameters to be set Message Signal Waveform type: Sine Amplitude: 1 Frequency: 2000 Hz Transmitters Carrie (TX carrier) Waveform type: Sine Amplitude: 1 Frequency: 10000 Hz 13 Telecommunication Systems 1 Lab ▪ Set the simulation parameters (Simulation >> Model Configuration Parameters) , See the following configurations: 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 continuous or infinite sample time. ▪ Double-click on the Zero-Order-Hold block and set the sampling time at 1e-5. ▪ 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 14 Telecommunication Systems 1 Lab Suggested Spectrum Analyzer parameters setting MATAB 2023 Analyzer Tab Estimation Tab Spectrum Tab 15 Telecommunication Systems 1 Lab ▪ Form the SIMULINK window and run the simulation. 1. View the message signal, the TX carrier, and the Modulated signal on one scope with three axes. (Double-click on the scope and then adjust the configuration properties ) 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 on your result. DSBSC Demodulation 1. Modify the model as shown in Figure 8 below. 16 Telecommunication Systems 1 Lab Figure 8 The Multiplier and Low-pass Filter (LPF) modules are used to implement a product detector that demodulates the original message from the DSBSC signal. ▪ Search “Analog Filter Design” within the library browser. 1. Adjust the cutoff frequency of the LPF for 20Khz, Observe the signal in time and frequency domains before and after the LPF. Comment on your result. 2. Adjust the cutoff frequency of the LPF for 2KHz, Observe the signal in time and frequency domains before and after the LPF. Comment on your result. Comment on your result. Effect of Phase Shift In this part, we will initiate a phase error between the carrier at the transmitter and the carrier at the receiver. 1. Set the cutoff frequency of the LPF in the demodulation circuit to any value in the good range for recovery. 2. Observe the original message signal and the recovered signal in the time domain. Vary the phase parameter of the receiver’s carrier (RX carrier) and describe the effect on the recovered signal at 0, 45, and 90 degrees. 17