DSBSC Signal Modulation and Demodulation using Costa's Loop in ECE440 - Prof. David J. Lov, Lab Reports of Electrical and Electronics Engineering

Download DSBSC Signal Modulation and Demodulation using Costa's Loop in ECE440 - Prof. David J. Lov and more Lab Reports Electrical and Electronics Engineering in PDF only on Docsity! ECE440, Transmission of Information 4-1 udupa05 ECE 440L Experiment 4: DSBSC (1 week) I. OBJECTIVES Upon completion of this experiment, you should be able to: 1. Modulate a carrier signal with a message to generate a DSBSC (double side-band suppressed carrier) signal. 2. Demodulate a DSBSC signal using a Costa’s Loop. II. INTRODUCTION DSBSC (Double Side-band Suppressed Carrier) is a linear analog modulation technique. Given a message ( )m t , the corresponding DSBSC signal is given by
( ) ( ) cos(2 ) ( ) cos(2 ). max(| ( ) |)c c c c m t s t A f t A m t f t m t π π = =   


















This process is illustrated in the frequency domain as shown in Figure 1. f f Fig. 1: DSBSC Modulation. 042308 ECE440, Transmission of Information 4-2 udupa05 II. Introduction. (continued) If the message is a tone, the resultant DSBSC signal will be as shown in Figure 2. cf c mf f+c mf f− 2 2 cA 2 2 cA Fig. 2: DSBSC signal with sinusoidal message. The advantage of DSBSC is that it is power-efficient since no power is wasted transmitting the carrier. Demodulation is however much more complicated than for AM since the carrier needs to be regenerated at the receiver. Carrier regeneration can be accomplished by using techniques like Costa’s loop or the squaring loop. In some cases, the carrier is not perfectly suppressed to simplify the process of carrier regeneration. Such signals can be considered to lie between perfect DSBSC and DSBLC (AM). For example, stereo FM broadcast radio signals have a DSBSC signal (with a 38 kHz carrier frequency) embedded in them. A sub-harmonic of the carrier (at 19 kHz) is also embedded in the FM signal to simplify the carrier recovery; such a signal is called a pilot tone. ECE440, Transmission of Information 4-5 udupa05 2. Observing Amplitude Modulated Signals. (Figure 5) 2 a. Which of the signals is AM (with 100% modulation index) and which is DSBSC? How can you tell the difference? Hint: same time scale, check phase reversal. 2 b. What is the message signal in each case? Hint: they are different! 2 c. Re-examine the signal generated in 1, above. Do you see the phase reversal? SIGNAL 1 SIGNAL 2 2 Fig. 5. Signals for 2a and 2b. ECE440, Transmission of Information 4-6 udupa05 3. DSBSC Demodulation: Using a Stolen Carrier To synchronously detect DSBSC, the modulated signal must be multiplied by a local oscillator at the same frequency and preferably the same phase as the carrier. Remember that the receiver does not have a copy of the carrier available to it; it needs to generate a copy of the carrier. 3 a. Stealing the Carrier. Create a local oscillator(LO) using the 3314A; Set the 3314A output to be a 20Vpp, 100 kHz sine wave; Connect the <SYNC> output of the generator that produces the carrier to the <TRIG> input of the 3314A; Set the 3314A to the external trigger mode (use <INT/EXT> button); Press the <ф-lock> button; Press the <PHASE> button and adjust the knob until the phase of the output of the 3314A matches the phase of the carrier. (You have now officially stolen the carrier!) 3 b. Build a synchronous demodulator using the stolen carrier! Use another multiplier and the stolen carrier to demodulate the DSBSC waveform. Don’t forget the low pass filter. 3 c. Determine the affect of phase difference between the actual carrier and the stolen carrier on the demodulated signal: Use the <PHASE> button on the 3314A and determine the effect of changing the phase of the 3314A on the demodulated signal. Compare with the theoretical result. 3 d. Determine the effect of frequency difference between the carrier and local oscillator experimentally and compare to theory: Disable the phase-lock using the <INT/EXT> button on the 3314A then determine the effect of changing the frequency of the 3314A on the demodulated signal. Compare with the theoretical result. ECE440, Transmission of Information 4-7 udupa05 4. DSBSC Demodulation: Carrier Recovery using Costa’s Loop The Costa’s loop is frequently used to recover the carrier in suppressed-carrier systems. A detailed diagram of the Costa’s loop is shown below in Figure. 6. Fig. 6: Implementation of Costa’s Loop. The major steps are: (i) Generate the DSBSC signal with a 100 kHz (20 Vpp) carrier. Use a 1 kHz (2 Vpp) triangular wave message initially. After the set-up is working correctly, replace the triangular wave with music from a CD player. (ii) Set the TIMS VCO to the ‘HI’ mode and adjust 0f to 100 kHz. The TIMS VCO operates at around 10 kHz in ‘LO’ and at around 100 kHz ‘HI’. (iii) Adjust the TIMS phase-shifters to produce sinusoids that are in phase quadrature (90° apart). The best way to test this is by generating a Lissajous figure using the XY mode of the scope. ( X = I, Y = Q ) R2 C2 R1 Low Pass Filter TIMS Tunable LPF Low Pass Filter TIMS Tunable LPF θ Phase Shift TIMS Shifter θ + π/2 Phase Shift TIMS Shifter × TIMS VCO at Carrier Frequency Low Pass Loop Filter (See Below) × TIMS × TIMS + DSBSC Signal Output Signal Low Pass Loop Filter