EE352 Lab 4 Solved

In this week, we continue amplitude modulation with double sideband suppressed carrier amplitude mod- ulation (DSB-SC AM) method. In laboratory works, we will construct the message and carrier signals as we did in the previous labworks. In order to construct a signal in Matlab, it is essential to generate a time vector by using various parameters such as the sampling period (Ts) which is given by Ts = 1/fs where fs is the sampling frequency, message frequency (fm) and the signal duration (t). Then, the message and carrier signals can be generated by using this time vector.

In DSB-SC AM, the modulated signal is created by using a product modulator that multiplies the message signal with a carrier wave. As a result, the phase of modulated signal is reversed each time message signal crosses zero. Therefore, the DSB-SC modulated signal has a different envelope than the message signal and we should observe this in the labwork.

In demodulation part of the lab, a coherent detection is employed to recover the message signal from DSB-SC modulated signal. Modulated signal is multiplied with a sinusoidal wave coherent/synchronized with the carrier wave and filtered to obtain the message signal. The overall block diagram of DSB-SC modulation and demodulation is given in Figure 1.

Figure 1: DSB-SC Modulation and Demodulation System Design

EE 352 – Lab 4: Double Sideband Supressed Carrier Modulation and Demodulation

2 Labwork

Read the preliminaries given above carefully before doing the experiment given below.

2.1 DSB-SC Modulation

1. Construct a message signal m(t) = cos(2πfmt) where fm = 100Hz and a carrier signal c(t) = cos(2πfct) where fc = 1kHz. The sampling frequency is fs = 100kHz and the durations of the both signals (m(t) and c(t)) is 0.08 s.
2. Employ the DSB-SC modulation for message signal m(t).
3. Plot m(t), c(t) and the modulatedsignals in time domain in the same figure using the subplot( )

   function.

4. Plot the message and modulated signal in the frequency domain in the same figure using the subplot(

   ) function. Comment on the frequency content and magnitude of the obtained signal.

2.2 DSB-SC Demodulation

1. The overall DSB-SC system is presented in Figure 1. Follow the block diagram to obtain the demod- ulated signal v(t) (before filtering). (Hint: Pay attention to the magnitude of your signals and adjust your operations accordingly.)
2. Plot the frequency response of v(t). Comment on the frequency content and magnitude of the obtained signal.
3. Construct a non-ideal low pass filter (LPF) by using butter() function to obtain the message signal. Choose the cut-off frequency and filter order accordingly. Comment on your filter design.
4. Plot the recovered signal after filtering, vo(t), in time and frequency domain using subplot() function. Compare and comment on the frequency content and magnitude of the obtained signal.