Jawaharlal Nehru Engineering College

Transcription

1 Jawaharlal Nehru Engineering College Laboratory Manual COMMUNICATION ENGINEERING For Author JNEC, Aurangabad. Second Year Students Lab manual made by PROF. S.A. ANNANDATE PROF. P. B. MURMUDE PROF. P.B.YADAV

2 MGM S Jawaharlal Nehru Engineering College N-6, CIDCO, Aurangabad Department of Electronics &Telecommunication Vision of the Department: To develop GREAT technocrats and to establish centre of excellence in the field of Electronics and Telecommunications. Global technocrats with human values Research and lifelong learning attitude, Excellent ability to tackle challenges Awareness of the needs of society Technical expertise Mission of the Department: 1. To provide good technical education and enhance technical competency by providing good infrastructure, resources, effective teaching learning process and competent, caring and committed faculty. 2. To provide various platforms to students for cultivating professional attitude and ethical values. 3. Creating a strong foundation among students which will enable them to pursue their career choice.

3 Jawaharlal Nehru Engineering College Technical Document This technical document is a series of Laboratory manuals of Electronics & Telecommunication and is a certified document of Jawaharlal Nehru Engineering College. The care has been taken to make the document error free but still if any error is found kindly bring it to the notice of subject teacher and HOD. Recommended by, HOD Approved by, Principal

4 FOREWORD It is my great pleasure to present this laboratory manual for second year engineering students for the subject of communication engineering keeping in view the vast coverage required for visualization of concepts of communication engineering. As a student, many of you may be wondering with some of the questions in your mind regarding the subject and exactly what has been tried is to answer through this manual. Faculty members are also advised that covering these aspects in initial stage itself will greatly relieve them in future as much of the load will be taken care by the enthusiastic energies of the students once they are conceptually clear. HOD

5 LABORATORY MANUAL CONTENTS This manual is intended for the second year students of ECT/IE branches in the subject of Communication Engineering. This manual typically contains practical/lab Sessions related to communication engineering, covering various aspects, related to the subject to enhance understanding. Students are advised to thoroughly go through this manual rather than only topics mentioned in the syllabus, as practical aspects are the key to understanding and conceptual visualization of theoretical aspects covered in the books. Good Luck for your Enjoyable Laboratory Sessions.

6 SUBJECT INDEX 1. DO s and DON Ts in Laboratory. 2. Lab exercise 1. Study of RF Signal Generator. 2. Modulation index calculation by AM wave and Trapezoidal pattern. 3. Pre-emphasis and De-emphasis. 4. Pulse width modulation. 5. Pulse position modulation. 6. Sampling and reconstruction. 7. AM Modulation & Demodulation 8. Pulse code modulation. 9. Study of Time Division Multiplexing. 3. Quiz on the subject. 4. Conduction Viva-Voce Examination. 5. Evaluation and Marking Systems.

7 DO s and DON Ts in Laboratory: 1. Do not handle any equipment before reading the instructions/instruction manuals. 2. Read carefully the power ratings of the equipment before it is switched on whether ratings 230 V/50Hz or 115V/60 Hz. For Indian equipments, the power ratings are normally 230V/50Hz. If you have equipment with 115/60 Hz ratings, do not insert power plug, as our normal supply is 230V/50 Hz, which will damage the equipment. 3. Observe type of sockets of equipment power to avoid mechanical damage. 4. Do not forcefully place connectors to avoid the damage. 5. Strictly observe the instructions given by the teacher/lab Instructor. Instruction for Laboratory Teachers: 1. Submission related to whatever lab work has been completed should be done during the next lab session. 2. The promptness of submission should be encouraged by way of marking and evaluation patterns that will benefit the sincere students.

8 Experiment No.1 Aim: To study RF signal generator. Apparatus: Oscilloscope, RF signal generator, function generator, probes, etc. Theory: RADIO-FREQUENCY (RF) SIGNAL GENERATORS In addition to the necessary power supply, a typical RF signal generator contains three other main sections: an OSCILLATOR CIRCUIT, a MODULATOR, and an OUTPUT CONTROL CIRCUIT. The modulator modulates the RF signal of the oscillator. In addition, most RF generators are provided with connections through which an external source of modulation of any desired waveform can be applied to the generated signal. Metal shielding surrounds the unit to prevent signals from the oscillator from affecting the circuit under test. A block diagram of a representative RF signal generator is shown in figure. The function of the oscillator stage is to produce a signal that can be accurately set in frequency at any point within the range of the generator. The type of oscillator circuit used depends on the range of frequencies for which the generator is designed. In lower frequency RF signal generators, the oscillating circuit consists of one of a group of coils combined with a variable capacitor. One of the coils is selected by the position of a range selector switch that connects the coil to a capacitor to provide an inductance-capacitance circuit. The inductive-capacitance circuit then has the correct range of resonant frequencies. In amplitude modulation, the amplitude of carrier is changed in accordance with Instantaneous value of modulating signal.the function of the modulator is to produce an audio (or video) modulating signal that can be superimposed on the RF signal produced by the oscillator. The modulating signal may be provided by an audio oscillator within the generator. This is termed INTERNAL MODULATION. It may also be derived from an external source. This is termed EXTERNAL MODULATION. In some signal generators, either of these two methods of modulation can be employed. In addition, a means of disabling the modulator section is available so that the pure, un-modulated signal from the oscillator can be used when desired.

9 Waveform: Fig: RF signal generator block diagram

10 Procedure: 1. Connect the probe at output terminal of RF generator and another to the oscilloscope. 2. Observe the waveforms for different RF frequency range (KHz-MHz). 3. Check the different waveforms on oscilloscope. Conclusion: We have observed AM waveform for different modulation indices. RF signal generator internally combines message signal & carrier signal & produces modulated waveform.

11 Experiment No.2 Aim: To calculate modulation index of AM wave by using AM wave and Trapezoidal pattern. Apparatus: AM modulator kit, oscilloscope, AF function generator, probes, wires etc. Theory: Modulation means varying amplitude, frequency, phase of a higher frequency continuous wave carrier signal with information waveform. In amplitude modulation, the amplitude means signal strength of the carrier wave is varied in proportion to the waveform being transmitted. %m = L max L min L max +L min 100 Where, L max, L min are lengths of trapezoidal Pattern. Procedure: 1. Give supply to the kit of AM/ FM generator. 2. Using function generator give information Signal to AM/ FM generator External Nob. 3. Measure the information wave & draw the waveform. 4. Connect the information signal to oscilloscope Channel Connect AM/ FM generator output to Channel 1of oscilloscope 6. Calculate E max (p-p) &E min (p-p) for 3 set of different readings. 5. Press the X-Y Nob.

12 Observation: %m = L max L min L max + L min 100 Sr. No. LMax (p-p) LMin (p-p) % M Conclusion: It has been observed that as modulating signal voltage increases, modulation index also increases i.e. Em α m.

13 Experiment No.3 Aim: I) To observe the effects of pre-emphasis on given input signal. II) To observe the effects of De-emphasis on given input signal. Apparatus: Resistors (10 KΩ, 7.5 KΩ, 6.8 KΩ), Capacitors (10 nf, 0.1 µf), CRO, Function Generator Theory: The noise has an effect on the higher modulating frequencies than on the lower ones. Thus, if the higher frequencies were artificially boosted at the transmitter and correspondingly cut at the receiver, an improvement in noise immunity could be expected, thereby increasing the SNR ratio. This boosting of the higher modulating frequencies at the transmitter is known as pre-emphasis and the compensation at the receiver is called de-emphasis. Circuit Diagrams: For Pre-emphasis: Fig.1. Pre-emphasis circuit

14 For De-emphasis: Fig.2. De-emphasis circuit Procedure: 1. Connect the circuit as per circuit diagram as shown in Fig Apply the sinusoidal signal of amplitude 20mV as input signal to pre emphasis circuit. 3. Then by increasing the input signal frequency from 500Hz to 20 KHz, observe the output voltage (vo) and calculate gain (20 log ( vo /vi). 4. Plot the graph between gain Vs frequency. 5. Repeat above steps 2 to 4 for de-emphasis circuit (shown in Fig.2). by applying the sinusoidal signal of 5V as input signal Observation: Table1: Pre-emphasis Vi = 20mV Frequency(KHz) Vo(mV) Gain in db(20 log Vo/Vi)

15 Table2: De-emphasis Vi = 5v Frequency(KHz) Vo(Volts) Gain in db(20 log Vo/Vi) Graphs: Conclusion: - Hence we have studied pre-emphasis and de-emphasis.

16 Aim: To study pulse width modulation Experiment No.4 Apparatus: PWM kit, oscilloscope, function generator, probes. Theory: Pulse width Modulation (PWM) is also known as Pulse duration Modulation (PDM). Three variations of PWM are possible. In One variation, the leading edge of the pulse is held constant and change in the pulse width with signal is measured with respect to the leading edge. In other Variable, the tail edge is held in constant and with respect to it, the pulse width is measured in the third variation, the centre of the pulse is held constant and pulse width changes on either side of the centre of the pulse. The PWM has the disadvantage when compared to PDM that its pulses are of varying width and therefore of varying power content, this means the transmitter must be powerful enough to handle the max width pulses. The PWM is actually a square wave modulated. This modulation infects on the frequency (clock cycle) and the duty cycle of the signal. Both of those parameters will be explained in details later but keep in mind that a PWM signal is characterized from the duty clock and the duty cycle. The amplitude of the signal remains stable during time (except of course from the rising and falling ramps). The clock cycle is measured in Hz and the duty cycle is measured in hundred percent. Fig. Circuit diagram of Pulse Width Modulation

17 Source signal Procedure: PWM signal 1. Connect the probes to CRO and PWM kit. 2. Check its output at pin no Take the reading for different voltages. Observation: Sr. No Modulating Voltage (tp in units) tp Conclusion: Hence we have studied that for change in voltage there will be change width. For maximum voltage output is having large width & at minimum voltage pulse having very small width.

18 Experiment No.5 Aim: To study pulse position modulation Apparatus: PPM kit, function generator, probes. Theory: In Pulse Position Modulation, both the pulse amplitude and pulse duration are held constant but the position of the pulse is varied in proportional to the sampled values of the message signal. Pulse time modulation is a class of signaling techniques that encodes the sample values of an analog signal on to the time axis of a digital signal and it is analogous to angle modulation techniques. The two main types of PTM are PWM and PPM. In PPM the analog sample value determines the position of a narrow pulse relative to the clocking time. In PPM rise time of pulse decides the channel bandwidth. It has low noise interference. Pulse-position modulation (PPM) is a signal modulation used for both analog and digital signal transmissions. This method is widely used for optical communication systems such as optic fiber and IR remote controls, where efficiency is required and little or no external interference occurs. In PPM, data are transmitted with short pulses. All pulses have both the same width and amplitude. The parameter that changes is the delay between each pulse.

19 Fig. Circuit diagram of Pulse Position Modulation

20 Procedure: 1. Connect the probes to CRO and PPM kit. 2. Check its output at pin no Take the reading for different voltages. Observation: Sr. No Modulating Voltage (tp in units) tp Conclusion: Hence we have studied that for change in voltage there will be change position. In positive cycles pulses are dense it becomes rare in negative cycle.

21 Experiment No.6 Aim: To study sampling of a signal and its re-construction. Apparatus: Analog Signal Sampling & Reconstruction Kit, 20 MHz Dual Trace Oscilloscope, Patch Chords Theory: SAMPLING FREQUENCY: The 6.4 MHz Crystal oscillator generates the 6.4 MHz clock. The decade counter divides the frequency by 10 and the ripple counter generate the basic sampling frequencies KHz to 20 KHz and the other control frequencies. The basic sampling frequencies is given to a multiplexer For each "Press" on the frequency select switch, the output of the state counter increases by one and it counts from 000 to 100. As the state counter counts from 000 to 100, the corresponding input of the multiplexer is switched to the output. As soon as the count reaches 101, the output of the 3 to 8 decoder resets the state counter and the whole cycle repeats. Also LED connected to the output of the decoder is switch ON, which indicates the sampling frequency selected. Refer the truth table for better understanding. Procedure: 1. Connect power supply in proper polarity to the kit & switch on. 2. Connect the l KHz, 5V pp Sine wave signal, generated onboard, to the ANALOG INPUT, by means of the patch-cords provided. 3. Connect the sampling frequency signal in the internal mode, by means of the shorting pin provided. 4. Using switch SW1 select 50% duty cycle as shown in the table. 5. Connect the SAMPLE OUTPUT to the input of the 2nd Order Low Pass Filter. 6. Using frequency selector switch, select desired sampling frequency. The selected sampling frequency is indicated by the glowing LED. 7.Take observation as mentioned below for various sampling frequency: 32 KHz,

22 16KHz, 8KHz, 4KHz, 2KHz. Conclusion: From the above observations we conclude that as the Sampling Frequency is increased, the reconstructed output is less distorted and almost original signal is reconstructed. For a sampling frequency of 2KHz, only 2 samples of the 1KHz signal are taken; whereas that for a sampling frequency of 8KHz, 8 samples of 1KHz signal are taken. Hence, as the number of samples taken of the signal increases, the distortion of the

23 Re-constructed signal decreases. As per the Nyquist Criterion at least two samples are required for the reconstruction if the signal. If the Nyquist Criterion is not satisfied, or if the signal is not band limited, then spectral overlap, called "aliasing" occurs, causing higher frequencies to show up at lower frequencies in the recovered message, and specially in voice transmission intelligibility is seriously degraded Thus, universally for the voice band (300Hz to 3300Hz), the sampling frequency used is 8KHz, which satisfies the Nyquist Criterion.

24 Experiment No. 7 Aim: 1. To generate amplitude modulated wave and determine the % modulation. 2. To Demodulate the modulated wave using envelope detector. Apparatus: Transistor (BC 107), Diode (0A79), Resistors (1KΩ, 2KΩ, 6.8KΩ, 10KΩ), Capacitor (0.01µF), Inductor (130mH), Function Generator, Regulated Power Supply Theory: Amplitude Modulation is defined as a process in which the amplitude of the carrier wave c(t) is varied linearly with the instantaneous amplitude of the message signal m(t).the standard form of an amplitude modulated (AM) wave is defined by s t Ac 1 K am t cos 2 f c t Where, Ka is a constant called the amplitude sensitivity of the modulator. The demodulation circuit is used to recover the message signal from the incoming AM wave at the receiver. An envelope detector is a simple and yet highly effective device that is well suited for the demodulation of AM wave, for which the percentage modulation is less than 100%.Ideally, an envelope detector produces an output signal that follows the envelop of the input signal wave form exactly; hence, the name. Some version of this circuit is used in almost all commercial AM radio receivers. The Modulation Index is defined as, m = Emax Emin Emax + Emin Where, Emax and Emin are the maximum and minimum amplitudes of the modulated wave.

25 Circuit Diagrams: For modulation: Fig. AM modulator For demodulation: Fig. AM demodulator Procedure: 1. The circuit is connected as per the circuit diagram shown in Fig Switch on + 12 volts VCC supply. 3. Apply sinusoidal signal of 1 KHz frequency and amplitude 2 Vp-p as modulating

26 signal, and carrier signal of frequency 11 KHz and amplitude 15 Vp-p. 4. Now slowly increase the amplitude of the modulating signal up to 7V and note down values of Emax and Emin. 5. Calculate modulation index using equation 6. Repeat step 5 by varying frequency of the modulating signal. 7. Plot the graphs: Modulation index vs Amplitude & Frequency 8. Find the value of R, fm= 1 2πRC. Where, C=0.001uF. 9. Connect the circuit diagram as shown in Fig Feed the AM wave to the demodulator circuit and observe the output. 11. Note down frequency and amplitude of the demodulated output waveform. 12. Draw the demodulated wave form m=1. Observation: Table 1: fm= 1KHz, fc=11khz, Ac=15 V p-p. Sr. No. Vm(Volts) Emax(volts) Emin (Volts) m %m (m x100) Table 2: Am= 4 Vp-p fc =11KHz, Ac=15 V p-p. Sr. No. fm(khz) Emax(volts) Emin(Volts) m %m (m x100)

27 Waveforms and graphs: Precautions: 1. Check the connections before giving the power supply 2. Observations should be done carefully. Conclusion: According to the graphical representation of AM modulator & demodulator AM is generated at the transmitter and modulating signal is recovered back at the demodulator.

28 Experiment No.8 Aim: To study pulse code modulation Apparatus: PCM kit, oscilloscope, function generator, probes. Theory: Pulse Code Modulation is also known as a digital pulse modulation technique. The pulse modulation (PCM) is quite complex compared to the analog pulse modulation techniques(i.e. PAM, PWM, PPM) in these sense that the message signal is subjected to a great number of operations. Fig. shows the basic elements of a PCM system. It consists of three main parts i.e. transmitter, transmission path and receiver. The essential operations in the transmitter of a PCM system are sampling, quantizing and encoding as shown in fig.1.sampling is operation in which an analog (i.e. continuous -time) signal is sampled according to the sampling theorem resulting in a discrete-time signal. The quantizing and encoding operations are usually performed in the same circuit which is known as an analog-to-digital converter (ADC). Also the essential operations in the receiver are regeneration of impaired signals, decoding and demodulation of the train of quantized samples. These operations are usually performed in the same circuit which is known as digital-to-analog converter (DAC). To transmit digitally an analogue signal such as speech or music, the analogue signal must be regularly sampled. The stream of samples is then converted into digital form, i.e. digitised, by an analogue-to-digital converter (ADC). This process is known as pulse code modulation. On reception of the digital signal, digital-to-analogue conversion takes place. It was explained in topic that an ADC is an integrated circuit (IC) that has one input pin, which accepts an analogue voltage of any value, and n output pins, which produce a corresponding n-bit binary number. The diagram below shows how this IC can be used to transmit an analogue signal in digital form.

29 The sampling gate circuit repeatedly samples and stores for a brief period, the value of the analogue voltage at some moment in time (remember that the analogue signal varies continuously with time). The rate at which such samples are taken is governed by the frequency of the sampling clock. The ADC converts each sample of the analogue information into a code of n bits. These n-bit codes are transmitted in parallel along n wires, one sample after another, to a matching n-bit digital-to-analogue converter (DAC). This produces a stepped analogue voltage from the incoming codes. The output is often referred to as a staircase approximation to the original analogue signal. The reconstitution filter is essentially a filter circuit that smoothes out the quantisation steps from the DAC. It operates in such a way that it appears to change the staircase waveform into a smooth curve.

30 Conclusion: Hence we have studied Pulse Code Modulation.

31 Aim: To study Time division Multiplexing Theory: Experiment No 9: Why multiplexing? It is best on several observations. First one is it has been found that most individual data communication devices typically require modest data rates. For example, when you are sending the requirement is only 4 Kbps and obviously 4 KHz that is the bandwidth of course whenever you convert it in digital form then it comes to 64 Kbps then of course that one is also not very weak. Similarly the average data rate required is also not very high. Fig: Block diagram of TDM Time-division multiplexing is used primarily for digital signals, but may be applied in analog multiplexing in which two or more signals or bit streams are transferred appearing simultaneously as sub-channels in one communication channel, but are physically taking turns on the channel. The time domain is divided into several recurrent time slots of fixed length, one for each sub-channel. A sample byte or data block of sub-channel 1 is transmitted during time slot 1, sub-channel 2 during time slot 2, etc. One TDM frame consists of one time slot per sub-channel plus a synchronization channel and sometimes error correction channel before the synchronization. After the last sub-channel, error correction, and synchronization, the cycle starts all over again with a new frame,

32 starting with the second sample, byte or data block from sub-channel 1, etc. Fig: Waveforms of TDM Conclusion: Time Division Multiplexing allows equal time period for each channel.

33 4. Quiz on the subject: 1) What is standard IF value for AM receiver & FM receiver? 2) In a receiver which stage rectifies the IF signal? 3) From which component the three point tracking is achieved? 4) Define double spotting, image frequency, sensitivity, selectivity & fidelity. 5) Define carrier signal, modulating signal & modulated signal. 6) How many side bands exist in AM&FM? 7) Define frequency spectrum in AM. 8) What is the use of Bessel s function? 9) What is the disadvantage of FM over AM? 10) What are the different types of detectors in AM & FM? 11)What are the different forms of AM? 12) What is AGC? What are its types? State use of it? 13) What is band spreading? 15) What is ganged tuning? 16) Which stage of AM & FM receiver determines the gain? 17) Define bandwidth. State the B.W. of AM & FM signal. 18) Define balanced modulator. 19) What are the o/p components of BM.? 20) What are the different types of BM? 21) ) What is SSB? What are its types? 22) State the different AM & FM generation methods?

34 5. Conduction of Viva-Voce Examinations: Teacher should conduct oral exams of the students with full preparation. Normally, the objective questions with guess are to be avoided. To make it meaningful, the questions should be such that depth of the students in the subject is tested. Oral examinations are to be conducted in co-cordial environment amongst the teachers taking the examination. Teachers taking such examinations should not have ill thoughts about each other and courtesies should be offered to each other in case of difference of opinion, which should be critically suppressed in front of the students. 6. Evaluation and marking system: Basic honesty in the evaluation and marking system is absolutely essential and in the process, impartial nature of the evaluator is required in the examination system. It is a wrong approach or concept to award the students by way of easy marking to get cheap popularity among the students, which they do not deserve. It is a primary responsibility of the teacher that right students who are really putting up lot of hard work with right kind of intelligence are correctly awarded. The marking patterns should be justifiable to the students without any ambiguity and teacher should see that students are faced with just circumstances.