DIGITAL COMMUNICATIONS (INTRODUCTION TO MULTISIM SOFTWARE)

Transcription

1 PROJECT 1B DIGITAL COMMUNICATIONS (INTRODUCTION TO MULTISIM SOFTWARE) (i) FSK SYSTEM (MODULATOR / DEMODULATOR) Abstract: In this project, students are required to design a complete circuit of FSK SYSTEM. Nowadays, FSK circuit is used in a new commercial data communication, it is similar to the FM modulation technique.it means that FSK can be replaced with the FM in new digital based radio receivers. Students are required to perform simulation using Electronics Workbench Multisim. 1.0 PROJECT OBJECTIVES: (i) Student will be able to design and implement the electronic circuit for FSK System. (ii) Students will be familiar with the use of design and simulation tools in the design process. (iii) Students will be able to determine the bandwidth of an FSK signal. (iv) Students will be able to demonstrate how digital pulses are transmitted over an analog communication system using FSK. 2.0 PROJECT METHODOLOGY & INSTRUCTIONS; 2.1 Components lists Function Generator (1) Dual-trace oscilloscope (1) Voltage-controlled sine oscillator (1) Diode (1) Inductor 1mH (1) Capacitors : 200 nf (1), 15 nf (1) Resistors : 500 Ω (1), 20 kω (1) 1

2 2.2 Circuit Diagram & Project specification such as By using simulation tools Electronic Workbench Multisim, simulate your design and observe the result of the FSK Systems. A B G T Mod Input Mod Output Demod Input 500ohm R1 1 D 2 Demod Output -10V 10V 1mH L1 200nF C1 20kohm R 15nF C VCO FSK Modulator FM Slope Detector FSK Demodulator Figure PROCEDURE: STEP 1 (A) Make sure that the following settings are selected at oscilloscope: (i) Time Base ( Scale = 500µs/Div, Xpos = 0, Y/T (ii) Ch A ( Scale = 5V/Div, Ypos = -2.0, DC) (iii) Ch B ( Scale = 10V/Div, Ypos = 0.0, DC) (iv) Trigger ( Pos edge, Level = 0, Auto) (B) Double click the function generator to bring down the enlargement. Make sure that following settings are selected: (i) Square wave, Frequency = 250 Hz (ii) Duty cycle = 50 % (iii) Amplitude = 2.5 V (iv) Offset = 2.5 V 2

3 (C) Run the simulation for one full screen display, then pause the simulation. Next, the FSK modulator input (red) and output (blue) on the oscilloscope screen display. (D) Answer all the questions STEP 1. STEP 2 Expand the oscilloscope horizontal time base by changing the time base scale 200µs/Div. Measure the modulator output ( blue curve plot) time period for one cycle when the input (red) is low (T s ) and when the input is high (T m ). Record your answers at STEP 2. STEP 3 Change the function generator frequency to 500 Hz. Run the simulation for one full screen display, and then stop the simulation. Answer all the questions STEP 3. STEP 4 Move the oscilloscope Channel B input to the demodulator output. Change the function generator frequency back to 250Hz. Change the oscilloscope horizontal time base scale to 2ms/Div and the Channel B scale to 5 V/Div. Run the simulation for one full screen display, then pause the simulation. (The modulator input (bottom curve plot) and the demodulator output (top curve plot) on the oscilloscope screen) STEP 5 Move the oscilloscope Channel A output to the demodulator input. Change the function generator frequency back to 250Hz. Change the oscilloscope horizontal time base scale to 1ms/Div and the Channel B scale to 5 V/Div. Run the simulation for one full screen display, then pause the simulation. (The demodulator input (bottom curve plot) and the demodulator output (top curve plot) on the oscilloscope screen) 3

4 NAME : I/C Number : Date : 40 RESULT : Sketch and observe the input and output wave shape of the FSK System with complete label. 3 Mark QUESTION: STEP 1 1) Is the input (red) a digital or analog wave shape? Explain why. 2) Is the input (blue) a digital or analog wave shape? Explain why. 3) Is this a form of frequency modulation? What is this form of modulation called? 4

5 4) Measure the bit time (t b ) of one of the input (red) bits and calculate the input bit rate (f b ) and baud rate. Assume the input consist of alternating ones and zeros ( 1010 ); 5) Is this bit rate considered to be slow or fast? STEP 2 10 Mark 6) T s = ; T m = 7) Based on the time periods measured in 6, calculate the modulator output frequencies when the input is low (f s ) and when the input is high (f m ). 8) What is the relationship between the modulator input voltage and output frequency? 9) Based on the input bit rate (f b ), the mark frequency (f m ), and the space frequency (f s ), determine the bandwidth (B) required for the FSK signal. 8 Mark 5

6 STEP 3 10) Measure the bit time (t b ) of one of the input (red) bits and calculate the input bit rate (f b ) and baud rate. How did the bit rate compare with the bit rate in Step 1? 11) Measure the modulator output ( blue curve plot) time period for one cycle when the input (red) is low (T s ) and when the input is high (T m ). Record your answers. T s = ; T m = 12) Based on the time periods measured in 11, calculate the modulator output frequencies when the input is low (f s ) and when the input is high (f m ). Did the mark and space frequencies change when the bit rate was changed? Explain why. STEP 4 8 Mark 13) Is the modulator input waveshape digital or analog? 14) Is the demodulator output waveshape digital or analog? Was this expected? 6

7 15) Sketch the demodulator output and the modulator input waveshape? 16) Change the function generator frequency to 100 Hz. Observe the result. How does the demodulator output waveshape compare with the modulator input waveshape? 7 Mark STEP 5 17) What is the relationship between the demodulator input frequency and the demodulator output voltage level? Sketch the demodulator input frequency and the demodulator output voltage level waveshape. 4 Mark 7