EE 460 – Introduction to Communication Systems MATLAB Tutorial #6
Single Sideband Modulation
This tutorial describes the use of MATLAB to simulate a SSB modulator. Most of the methods used here have been introduced in previous MATLAB Tutorials for this course.
Double Sideband Modulators A simple single sideband modulator can be modeled using Simulink. We want to generate a lower sideband signal from a 1 KHz baseband signal and a 20 KHz carrier. The first step is to multiply the baseband signal with the carrier to create a double sideband signal. This can be accomplished with the Simulink model shown in figure 1.
Figure 1. DSB Model in Simulink
Set the sample time so that you have at least 10 samples per cycle of the carrier wave. Before performing the simulation we need to set the Configuration Parameters. Set the simulation time to run for 5 cycles of the baseband signal. Run the simulation and observe the waveform and spectrum of the DSB signal to confirm that you are getting what is expected.
Creating a Sideband Filter The next step is to create and apply a low pass filter to produce a lower sideband signal from the double sideband signal. In this example we use the Lowpass filter block found in the DSP System Toolbox ! Filtering ! Filter Designs sublibrary. Modify your model to match the one shown in Figure 2 below.
Figure 2. Simulink Model of a SSB Modulator
Open the Lowpass Filter block to set the parameters as follows: • Impulse Response: FIR • Order mode: Specify • Order: 20 • Filter type: Single-rate • Frequency constraints: Passband edge and stop band edge • Frequency units: Hz • Input Fs: use the recipricol of the sample time used in the Sine wave blocks • Fpass: 19500 • Fstop: 20500
Run the simulation and observe the resulting waveform and spectrum. By zooming in on the spectrum you should observe that the upper sideband has only been attenuated by about 3 dB below the lower sideband. This is not sufficient sideband suppression. Open the filter block and change the Impulse Response setting to IIR. Re-run the simulation. How far is the upper sideband attenuated now? Next, change the filter order to 10. Re-run the simulation one more time. You should still observe order 60dB of sideband suppression. This tells you something about the difference in performance of IIR and FIR filters.