Government Polytechnic, Muzaffarpur

Transcription

1 Government Polytechnic, Muzaffarpur ADVANCE COMMUNICATION SYSTEM LAB. Subject Code: AIM: To observe an AM wave on CRO produced by standard signal generator using internal and external modulation. The depth of modulation is to be measured with the above experiment. APPARATUS: 1. Amplitude Modulation & Demodulation trainer kit. 2. C.R.O (20MHz 3. Function generator (1MHz). 4. Connecting chords & probes. THEORY: Amplitude modulation is defined as the process in which the amplitude of the carrier wave c(t) is varied about a mean value, linearly with the baseband signal. An AM wave may thus be dscribed, in the most general form, as a function of time as follows. S(t)=Ac{1+Kam(t)}cos(2πfct) Where Ka- Amplitude sensitivity of the modulator S(t) Modulated signal Ac- carrier signal m(t) modulating signal The amplitude of Ka m(t) is always less than unity, that is Ka m(t) 1 for any carrier wave becomes over modulated,resulting in carrier phase reversal whenever the factor 1+Kam(t) crosses zero. The modulate wave then exhibits envelope distortion. The absolute maximum value of Ka m(t) multiplied by 100 is referred to as the percentage modulation. Or percentage modulation = Vmax Vmin Vmax Vmin x100 1

2 The carrier frequency fc is much greater than the highest frequency component ω of the message signal m(t),that is fc >>W Where W is themessage bandwidth. If the condition is not satisfied, and envelope cannot be visualized satisfactorily. The trainer kit has a carrier generator, which can generate the carrier wave of 100 KHz when the trainer is switched on. The circuit s carrier generator, modulator and demodulator are provided with the built in supplies, no supply connections are to be given externally Circuit diagram: PROCEDURE: 1. Switch on the trainer kit and check the O/P of the carrier generator on oscilloscope. 2. Connect around 1KHz with 2Volts.A.F signal at A.F I/P to the modulator circuit. 2

3 3. Connect the carrier signal at carrier I/P of the modulator circuit. 4. Observe the modulator output signal at AM O/p Spring by making necessary changes in A.F signal 5. Vary the modulating frequency and amplitude and observe the effects on the modulated waveform. 6. The depth of modulation can be varied using the variable knob provided at A.F input. 7. The percentage modulation can be calculated using the formula. Percentage modulation = Vmax Vmin Vmax Vmin x100 Modulation factor= Vmax Vmin Vmax Vmin 8. Connect the output of the modulator to the input of the demodulator circuit and observe the output. VI. EXPECTED WAVEFORMS: 3

4 VII. RESULT: 4

5 VII. RESULT: VIII. APPLICATIONS: 1. Tele communications. 2. TV Transmitters. 5

6 Government Polytechnic, Muzaffarpur ADVANCE COMMUNICATION SYSTEM LAB. Subject Code: Double side band suppressed carrier (DSB-SC) modulated Signal. AIM: To generate and study double side band suppressed carrier (DSB-SC) modulated Signal. I. APPARATUS: 1. Balanced modulator trainer kit 2. C.R.O (20MHz) 3. Connecting chords and probes 4. Function generator (1MHz) II. THEORY: 1. RF Generator: Colpitts oscillator using FET is used here to generate RF signal of approximately 100 KHz Frequency to use as carrier signal in this experiment. Adjustments for Amplitude and Frequency are provided in panel for ease of operation. 2. AF Generator: Low Frequency signal of approximately 5KHz is generated using OP-AMP based wein bridge oscillator. IC TL 084 is used as an active component; TL 084 is FET input general purpose quad OP-AMP integrated circuit. One of the OP-AMP has been used as amplifier to improve signal level. Facility is provided to change output voltage. 6

7 3. Regulated Power Supply: This consists of bridge rectifier, capacitor filters and three terminal regulators to provide required dc voltage in the circuit i.e. +12v, 150 ma each. 4. Modulator: The IC MC 1496 is used as Modulator in this experiment. MC 1496 is a monolithic integrated circuit balanced modulator/demodulator, is versatile and can be used up to 200 MHz. Multiplier: A balanced modulator is essentially a multiplier. The output of the MC 1496 balanced modulator is proportional to the product of the two input signals. If you apply the same sinusoidal signal to both inputs of a ballooned modulator, the output will be the square of the input signal AM-DSB/SC: If you use two sinusoidal signals with deferent frequencies at the two inputs of a balanced modulator (multiplier) you can produce AM-DSB/SC modulation. This is generally accomplished using a high- frequency carrier sinusoid and a lower frequency modulation waveform (such as an audio signal from microphone). The figure 1.1 is a plot of a DSB-SC waveform, this figure is the graph of a 100 KHz and a 5 KHz sinusoid multiplied together. Figure 1.2 shows the circuit that we will use for this experiment using MC 1496-balanced modulator/demodulator. III. CIRCUIT DIAGRAM: 7

8 IV. PROCEDURE: (i)-frequency Doubler 1. Connect the circuit as per the given circuit diagram. 2. Switch on the power to the trainer kit. 3. Apply a 5 KHz signal to both RF and AF inputs of 0.1VP-P. 4. Measure the output signal frequency and amplitude by connecting the output to CRO. 5. Repeat the steps 3 and 4 by changing the applied input signal frequency to 100KHZ and 500 KHz. And note down the output signals. NOTE:- Amplitude decreases with increase in the applied input frequency. (ii)-generation of DSB-SC 1. For the same circuit apply the modulating signal(af) frequency in between 1Khz to 5Khz having 0.4 VP-P and a carrier signal(rf) of 100KHz having a 0.1 VP-P. 2. Adjust the RF carrier null potentiometer to observe a DSB-SC waveform at the output terminal on CRO and plot the same. 3. Repeat the above process by varying the amplitude and frequency of AF but RF maintained constant. V. EXPECTED WAVEFORMS: 8

9 VI. RESULT: VII. APPLICATIONS: 1. Tele communications. 2. TV Transmitters. 9

10 Government Polytechnic, Muzaffarpur ADVANCE COMMUNICATION SYSTEM LAB. Subject Code: PRE-EMPHASIS & DE-EMPHASIS AIM:To perform the characteristics of Pre-Emphasis and De-Emphasis circuits. APPARATUS: 1. Pre-emphasis & De-emphasis trainer kits. 2. C.R.O (20MHz) 3. Function generator (1MHz). 4. Patch chords and Probes THEORY Frequency modulation is much immune to noise than amplitude modulation and significantly more immune than phase modulation. A single noise frequency will affect the output of the receiver only if it falls with in its pass band. The noise has a greater effect on the higher modulating frequencies than on lower ones. Thus, if the higher frequencies were artificially boosted at the transmitter and correspondingly cut at the receiver, improvement in noise immunity could be expected. This boosting of the higher frequencies, in accordance with a pre-arranged curve, is termed pre-emphasis, and the compensation at the receiver is called de-emphasis. If the two modulating signals have the same initial amplitude, and one of them is pre-emphasized to (say) twice this amplitude, whereas the other is unaffected (being at a much lower frequency) then the receiver will naturally have to de-emphasize the first signal by a factor of 2, to ensure that both signals have the same amplitude in the output of the receiver. Before demodulation, I.e. while susceptible to noise interference the emphasized signal had twice the deviation it would 10

11 have had without pre-emphasis, and was thus more immune to noise. Alternatively, it is seen that when this signal is de-emphasized any noise sideband voltages are de-emphasized with it, and therefore have a correspondingly lower amplitude than they would have had without emphasis again their effect on the output is reduced. The amount of pre-emphasis in U.S FM broadcasting, and in the sound transmissions accompanying television, has been standardized at 75 microseconds, whereas a number of other services, notably CCIR and Australian TV sound transmission, use 50micro second. The usage of microseconds for defining emphasis is standard. 75 microseconds de-emphasis corresponds to a frequency response curve that is 3 db down at the frequency whose time constant is RC is 75 RC and it is therefore 2120 Hz; microseconds. This frequency is given by f=1/2 with 50- microseconds de-emphasis it would have been 3180 Hz. Figure I shows pre emphasis and deemphasis curves for a 7 microseconds emphasis, as used in the united states. If emphasis is applied to amplitude modulation, some improvement will also result, but it is not as great as in FM because the highest modulating frequencies in AM are no more affected by noise than any others. Apart from that, it would be difficult to introduce pre-emphasis and deemphasis in existing AM services since extensive modifications would be needed, particularly in view of the huge numbers is receivers in use. CIRCUIT DIAGRAM: 11

12 PROCEDURE: I-PRE-EMPHASIS 1. Connect the circuit as per the circuit diagram 2. Apply a sine wave to the input terminals of 2 VP-P (Vi) 3. By varying the input frequency with fixed amplitude, note down the output amplitude (Vo) with respect to the input frequency. 4. Calculate the gain using the formula Gain = 20 log (VO/ VI) db Where VO = output voltage in volts. VI = Input voltage in volts. And plot the frequency response. II-DE-EMPHASIS 1. Connect the circuit as per circuit diagram. 2. Repeat steps 2,3 & 4 of Pre-Emphasis to de-emphasis also EXPECTED WAVEFORMS: RESULT: APPLICATIONS: 1.FM transmitters. 2.FM Receivers. 3.FM Stereo systems. 12

13 Government Polytechnic, Muzaffarpur ADVANCE COMMUNICATION SYSTEM LAB. Subject Code: TIME DIVISION MULTIPLEXING AIM: To Study time-division multiplexing technique and observe cross-talk. EQUIPMENTS AND COMPONENTS (i) Apparatus: 1. TDM Trainer 2. Function generator 3. CRO 4. Bread Board 5. Power supply (ii) Description of Apparatus: CRO: The 20 MHz dual channel oscilloscope 201 is a compact, low line and light weight instrument. It is a general purpose Dual Trace Oscilloscope having both vertical amplifiers offering a bandwidth of DC- 20 MHz and maximum sensitivity of 2mv/cm. The 201 offers five separate add-on modules. - frequency counter - Curve tracer - Power supply - Function generator - Digital voltmeter The add-on modules enhance measuring capabilities of instrument at low cost. This instrument is meant for giving three types of periodic waveforms SINUSOIDAL, SQUARE and TRIANGULAR waveforms where frequency can be selected from 0.1 Hz to 1 MHz and whose amplitude also can be selected from 0 to 20 volts peak to peak independently. The power on switch in pressed position will connect supply to the instrument. The amplitude switch varies the amplitude of output waveforms from 20 mv to 20 v(p-p). The function is a interlocked 3 station push button which switches to select the desired waveform for output. 13

14 Wire Connections are usually carried out using a system called Bread Board. It is a rectangular board divided into a number of nodes. This component has a provision on which any circuit can be constructed by interconnecting components such as resistors, capacitors, diodes, transistors etc., for testing the circuit. (iii). Components K - resistor 2 No K - resistor 2 No K - resistor 1 No. 4. 1K - resistor 2 No K - resistor 1 No F capacitor 1 No F capacitor 1 No. 8. BC 107 transistor 1 No. (iv) Description of Components: a. 100K - resistor Most circuits need contrast resistances. There are different types of resistors available fordifferent applications. Typical specifications of resistor are Rating : 10 to 10M Wattage : ¼ W to 2 W Tolerance : Normally 5% and above b. 4.7K - resistor Same as above c. 5.6K - resistor Same as above d. 1K - resistor Same as above e. 10K - resistor Same as above f F capacitor and 0.1 F capacitor Capacitors are made by sandwiching an insulating material between two conductors which form the electrodes. There are rated by their maximum working voltage. The breakdown voltage depends upon temperature and hence upon the losses in the dielectric. The factors to be considered in the choice of capacitors are 14

15 1. Required capacity 2. Working voltage 3. Tolerances The specifications of 0.01 F capacitor are 1. capacity 0.01 F voltage range 16v to 3kv tolerance 10% g. BC 107 transistor A bipolar junction transistor has two junctions. The conduction through the device involving two types of charge carriers holes and elements. BJT s are available in two varieties: PNP and NPN. Either type can be treated as equivalent to two diodes connected back to back with three terminals leads, emitter, base and collector. Width of the base region is smaller than that of emitter or collector layers. THEORY:- Time division multiplexing enables the joint utilization of a common transmission channel by a plurality of independent message sources without mutual inference. The circuit has 555 timer which generates a square wave which is then fed to the transistors to provide the bias current. Two message signals are square wave and some wave generated from frequency generator and they are time division multiplexed when square wave has ON and OFF Cycles. The multiplexed output is viewed on the CRO. 15

16 CIRCUIT DIAGRAM:- 16

17 17

18 PROCEDURE:. i.connections can be made as per circuit diagram ii. Switch on the trainer kit and observe the multiplexed signal at transmitter data work output. iii. Transmitter data output is connected to receiver data input of receiver TDM section. iv. Observe the demultiplexed signal on individual channels Ch0, Ch1, Ch2,Ch3 v. Draw the graph for input signals, multiplexed signal, and demultiplexed signals. OBSERVATIONS: GRAPHS: Input signals = Frequency = Amplitude = Multiplexed Output signal = RESULT: Thus the time division multiplexing of a square wave and sine wave is generated and observed. INFERENCES: From the above observation, we can infer that it is possible to covey different signals in different time slots using a single channel PRECAUTIOS: 1. Power handling capacity of resistor should be kept in mind while selecting RL. 2. Contact wires must be checked before use. 3. Maximum forward current should not exceed value given in data sheet. 4. Reverse voltage across diode should not exceed peak inverse voltage (PIV). APPLICATIONS: Telephone Channel 18

19 Government Polytechnic, Muzaffarpur ADVANCE COMMUNICATION SYSTEM LAB. Subject Code: VERIFICATION OF SAMPLING THEOREM AIM: To verify the sampling theorem and to observe aliasing effect. EQUIPMENTS AND COMPONENTS: I. Apparatus: 1. Sampling theorem trainer kit 2. Function generator 3. CRO 4. Patch Cards II. Description of Apparatus: 1. CRO: The 20 MHz dual channel oscilloscope 201 is a compact, low line and light weight instrument. It is a general purpose Dual Trace Oscilloscope having both vertical amplifiers offering a bandwidth of DC- 20 MHz and maximum sensitivity of 2mv/cm. The 201 offers five separate add-on modules. - frequency counter - Curve tracer - Power supply - Function generator - Digital voltmeter The add-on modules enhance measuring capabilities of instrument at low cost. 2. This instrument is meant for giving three types of periodic waveforms SINUSOIDAL, SQUARE and TRIANGULAR waveforms where frequency can be 19

20 selected from 0.1 Hz to 1 MHz and whose amplitude also can be selected from 0 to 20 volts peak to peak independently. The power on switch in pressed position will connect supply to the instrument. The amplitude switch varies the amplitude of output waveforms from 20 mv to 20 v(p-p). The function is a interlocked 3 station push button which switches to select the desired waveform for output. 3. Wire Connections are usually carried out using a system called Bread Board. It is a rectangular board divided into a number of nodes. This component has a provision on which any circuit can be constructed by interconnecting components such as resistors, capacitors, diodes, transistors etc., for testing the circuit. THEORY: When an analog signal message is conveyed over an analog communication system, the full message is typically used at all times. To send the same analog signal over a digicom system requires that only its samples are transmitted at periodic intervals. Because the receiver can therefore receive only samples of the message, it must attempt to reconstruct the original message at all times from only its samples. Methods exist whereby this desired end can be accomplished which include the sampling theorem. Sampling theorem can be stated as follows: A signal f(t), band limited such that it has no frequency components above fm, can be uniquely determined and reconstructed by its values at regularly spaced intervals of Ts if and only if Ts < ½ Tm. CIRCUIT DIAGRAM 20

21 21

22 PROCEDURE: 1. Connect the 2 KHz 5 V p-p signal generator on board to the analog signal input,by means of the patch chords provided. 2. Connect the sampling frequency signal in the internal mode, by means of shorting pin provided. 3. Connect the S/H output to the input of the 2/4th order LPF. 4. If the external sampling frequency signal is used,then connect the signal generator output to the sampling contrl input. OBSERVATIONS: Message signal voltage = Message signal frequency = Sampling signal voltage = Sampling signal frequency = GRAPHS: 22

23 23

24 RESULT: A signal is naturally sampled and the reconstructed signal is observed and plotted. INFERENCES: We infer that the use of sample and hold logic compensates for the poor amplitude at the input of the low pass filter resulting in faithful reconstruction. Pulse amplitude modulated signal and demodulated signals are observed. PRECAUTIONS: 1.Power handling capacity of resistor should be kept in mind while selecting RL. 2.Contact wires must be checked before use. 3.Maximum forward current should not exceed value given in data sheet. 4.Reverse voltage across diode should not exceed peak inverse voltage (PIV). Applications: Broad Casting, Digital communication. EXTENSION: We find the signal is reconstructed more faith fully in the case of sample-and hold waveform rather in case of natural sampled waveform where the reconstructed signal suffers an amplitude distortion. Trouble Shooting: S.No. Fault Diagnosis 1 Output signal is same as input signal 2 Output appears and suddenly disappear Absence of carrier signal Check the contact wires whether they are placed properly 24

25 Government Polytechnic, Muzaffarpur ADVANCE COMMUNICATION SYSTEM LAB. Subject Code: Pulse Code Modulation and Demodulation Objective : To Set-up circuits for pulse code modulation and demodulation and to study the modulator and demodulator with the study of quantization noise. Apparatus:: PCM kit, Dual traces CRO, Connecting leads. BLOCK DIAGRAM: Fig.1 : PCM Modulator 25

26 Theory: Fig.2 : PCM De-Modulator Pulse code modulation pulse code modulation system comprises the following steps: 1. Sampling 2. Quantization 3. Encoding 4. Formatting Let these four steps should be detailed before circuit description. 1. Sampling: The input is first sampled according to Nyquist criteria. The Nyquist criteria state that a signal must be sampled at a rate of >2fm, means the sampling frequency must become than the twice of input information highest frequency. For a voice signal it must be 2 * 3.4 KHz=>6.8 KHz or say 8 KHz. The sampled signal is now in the form of pulse amplitude modulated signal. Thus in a pulse code modulation first step is to obtain a PAM signal at Nyquist rate. 2. Quantization: The quantization means to compare the stepped PAM signal height with a known reference value. 3. Encoding: The sampled input is compared and encoded into equivalent binary word.here is to note that a binary 0 represent absence of pulse and 1 presence of the pulse. 4. Formatting: The ready data is now transmitted through cables or being modulated by carrier component. Let take the cable system.to send this data four lines of data stream and one common line is required.if the encoded data eight word length than there must be nine and lines and it is not economical same case it is not possible to modulate this data lines (4 & 8) with a carrier component.thus prior to send the parallel data is converted into serial form with governing bits. The process is called data formatting. In the block diagram it is drawn as shown below. Waveforms: 26

27 Procedure: 1. Keep DC ADJ control to minimum (fully controlled clock wise).switch on the power. 2. Connect the CRO ground with the ground point provided between Tx CK and TX DO sockets. 27

28 3. Connect CRO live lead with the TP1, adjust CRO for 2V/div and 1 Microsecond/ div. Observe the clock signal there. It is the clock signal which is used in conversion of analog to digital function. 4. Connect the CRO at TP2; adjusting time 50 microsecond/div. It is the main clock signal which is used performs all functions. Remain CRO one channel connected here. 5. Connect other channel of CRO with the TP3.Observe the signal there. Adjust CRO time base to appear o ne complete frame upon the signal there.adjust CRO time base to appear one complete frame upon the screen. Trace the clock signal with the TP3 signal. 6. Disconnected TP2, probe and connect it with the TP4, TP5 and TP6 signals. Trace these all signals as shown in the figure. 7. Connect the CRO with the TP7, while other input connected with TP3.Observe and trace this signal. 8. Connect CRO with the TxDO output socket and observe the signal there 9. Trigger CRO with this signal and measure the time T between two successive leading signals. It is the transmission frame time measure the start bit time. 10. Now increase the analog input (DC ADJ control) signal gradually till LSB, LED(D0) glows. Observe the modulated signal. Result: Pulse Code Modulation & Demodulation has been studied. Precautions: 1. Switch off the experimental kit during making connections. 2. Adjust the frequency of pulse trains carefully to get reasonable PCM waveforms. 3. Use the CRO carefully. 28

29 Government Polytechnic, Muzaffarpur ADVANCE COMMUNICATION SYSTEM LAB. Subject Code: Phase Locked Loop as FM Demodulator Objective : To Study phase locked characteristics and its application as FM demodulator Theory: There are many methods available for FM demodulation like slope detector, Foster-Seeley discriminator, Phase Lock Loop detector, Quadrature method, Ratio detector. In our project we are using the PLL demodulation method using IC LM565 which is FM demodulation IC. The LM565 and LM565C are general purpose phase locked loops containing a stable, highly linear voltage controlled oscillator for low distortion FM demodulation, and a double balanced phase detector with good carrier suppression. The VCO frequency is set with an external resistor and capacitor, and a tuning range of 10:1 can be obtained with the same capacitor. The characteristics of the closed loop system bandwidth, response speed, capture and pull in range may be adjusted over a wide range with an external resistor and capacitor. The loop may be broken between the VCO and the phase detector for insertion of a digital frequency divider to obtain frequency multiplication. 1. FM DEMODULATION The process of removing the information signal from the carrier is termed demodulation. The challenge is to design a circuit (or algorithm) that will achieve this task optimally in the presence of noise, interference and varying signal strength, frequency and phase, whilst being compact, power efficient and cheap. To detect an FM signal, it is necessary to have a circuit whose output voltage varies linearly with the frequency of the input signal. The slope detector is a very basic form of such a circuit, 29

30 although its linearity of response is not a good. The tuning circuit tune to receive the signal on the slope of the response curve. The carrier amplitude is caused to vary with the frequency. There are four methods for the frequency demodulation technique. These are as follows: 1. Foster-Seeley Discriminator 2. Ratio Detector 3. Phase Lock Loop Detector 4. Quadrature Detector Here IC LM565 is used as a Phase Lock Loop Detector. Mainly two sections are present as follows: 1. FM Demodulation by PLL. 2. Output audio amplifier section. 2. FM DEMODULATION BY PLL An IC565 PLL is used for FM demodulation. It contain voltage controlled oscillator which produces the frequency, which is proportional to the voltage applied to it. The frequency of oscillation is determined by resistance and capacitance at pin 8 and 9 by 4k7 preset. IC TBA810 is used as output audio amplifier. The block diagram for Phase locked loop demodulator is shown in fig. The phase detector which is basically balance modulator, produce an average output voltage that is a linear function of the phase difference between the two input signals.the frequency component is selected by the low pass filter which also remove much of the noise. The filtered signal is given is amplified through amplifier A and pass as a control voltage to the VCO where it result in frequency modulation of the VCO frequency.when the loop is in lock the VCO frequency follows or track the incoming 30

31 frequency.for example when the instantaneous frequency increases. The control voltage will cause the VCO frequency to increase. 3. OUTPUT AUDIO AMPLIFIER SECTION: The Output audio amplifier consists of Pre-amplifier and output amplifier stage. This section is used to amplify low-level audio signal coming from Mike/Loudspeaker and give it to the F.M. modulator section for live F.M. modulation. The pre-amplifier consists of one transistor Q1. Transistor Q1 is connected in C-E configuration. The input signal from mike is connected to the base of Q1 through coupling capacitor 10/16 EC. The amplified audio signal obtained at the collector of Q1 is given to the output driver amplifier consisting of IC 810 at pin 10 through volume control pot P1. The IC 810 performs the functions of the audio amplifier, driver and the o/p stage. The amplified signal obtained from the transistor Q1 is given at the audio input (pin 10) of the IC. This amplified by the IC and the o/p is available at the pin 16 of the IC. The feedback is given from the output to the emitter of pre-amplifier transistor in the IC (through resistor 4k provided internally).the feedback voltages develops over R6 (22Ω), C12 (220/10) being used for dc blocking at pin 8.the gain of amplifier depends on this feedback. The amplified output is available at pin 16 and in turn at output terminals. 4. CIRCUIT DIAGRAM 31

32 5. COMPONENTS Resistor Capacitors 1) 100kΩ : 1 1) 47Nf dc : 1 2) 22kΩ: 2 2) 1nf dc : 1 3) 10kΩ: 1 3) 1nf ppc : 1 4) 4k7Ω : 1 4) 330pf dc : 1 5) 1k5Ω : 1 5) 470pf : 1 6) 47kΩ : 4 6) 10μf/16V : 2 7) 1Ω : 1 7) 220μf/16V : 3 8) 220kΩ: 1 8) 470μf/16V : 2 9) 1kΩ : 1 10) 22Ω : 1 11) 100Ω: 1 12) 1Ω : 1 13) 1Ω, 0.5w: 1 14)330k Ω : 1 PRESETS : 1) 10k pot ICs: 2) 565( fm demodulator) 3) 741 ( amplifier) 6. SPECIFICATIONS 1) Power supply requirement : +5,-5,+15,-15 DC 2) On board RF carrier signal generator : 200 KHz to 1 MHz (VCO) frequency range 3) Demodulator type : Phase lock loop 4) Vp-p 0 to 5v 5) On board input audio amplifier with volume control for modulating external signal from mike. 7. EXPERIMENT RESULT 32

33 9. CONCLUSION FM demodulation in which PLL demodulator is working up to 200 KHz to 6MHz frequency. The output of the PLL demodulator is somewhat distorted so to get the proper output Low Pass Filter is required. In the output FM demodulated wave in the form of sine wave is obtain. 33

34 Government Polytechnic, Muzaffarpur ADVANCE COMMUNICATION SYSTEM LAB. Subject Code: Frequency Modulation Using the Voltage Controlled Oscillator and a Function Generator Circuit Aim:- To generator frequency modulated signal using VCO (Voltage controlled oscillator) Theory: The voltage controlled oscillator (VCO) is a device whose frequency changes linearly with an input voltage. It is used to perform direct frequency modulation on signals. VCO has a center frequency fc and the input (control) voltage m(t) modulates the instantaneous frequency around this center frequency. Issues The main issue is modulating the frequency around a center frequency that is within the specifications made by the Federal Communications Commission (FCC) regulation to avoid any electromagnetic compatibility issues. The frequency of this VCO ranges from 2315MHz to 2536MHz. A band of frequencies around 2400MHz has been designated as the Industrial, Scientific and Medical (ISM) radio bands and the devices using this band does not need to be 34

35 licensed for instance, cordless phones, WiFi and routers. The design must not exceed the allowed band frequency. Steps A signal needs to be fed to the input of the VCO by the function generator circuit. The function generator circuit can output either a sine, square or triangle wave depending on the designer. Figure 2 shows the circuit for a sine, triangle or square wave generation with minimum harmonic distortion. R1 at pin 7 provides the desired frequency tuning. R3 determines the output swing which is the positive and negative peak of the waveform. The selection of R3 value can be done by referring to Figure 3. RA adjusts the sine-shaping resistor and RB provides the fine adjustment for the waveform symmetry. 35

36 Figure 4. Performance Data and Curves of ZX C-S+ Voltage Controlled Oscillator Figure 4 shows the performance and data curves of the VCO. All of the parameters on the table are at 25 C (room temperature) unless mentioned otherwise. Voltage tune is the output voltage of the waveform generator. The voltage tune corresponds to the desired frequency. Therefore, R1 and R3 on the waveform generator circuit should be tuned to the list of voltage tune listed on the table in order to get the corresponding frequency. Example This is an example of having the minimum voltage at 1V, average voltage at 2V and maximum voltage at 3V. According to figure 4, 1V, 2V and 3V correspond to 2.334GHz, 2.408GHz and 2.481GHz respectively. Therefore the center frequency of the VCO is approximately 2.4GHz. Figure 5. ZX C-S+ Frequency and Tuning Sensitivity 36

37 Hardware Developed A hardware that was developed using this method is the radar kit that was done by a design team from ECE480 Fall 2011 and modified by Design Team 5 from ECE480 Spring 2012 (Figure). This kit produces a triangle waveform from the function generator circuit using Figure. The potentiometer R3 is tuned to get 3V as the max voltage and 1V as the minimum voltage and potentiometer R1 is tuned to get 2V as the average voltage. Figure 6. ZX C-S+ Voltage Controlled Oscillator and XR-2206cp Function Generator 37

38 Figure 7. Radar Kit Developed Design parameters of the kit are as follows: Frequency = 2.4GHz Bandwidth = 80MHz Waveform = Continuous wave triangle Antenna isolation = 50dB DC power is less than 1W RF power is less than 1W Conclusions The function of VCO is to do direct frequency modulation at a specific center frequency that is determined by the waveform produced by the function generator. The frequency of the VCO depends on the voltage tune input of the VCO. The voltage output swing of the function generator depends on the value of potentiometer at pin 3. When designing a device, it is important to ensure that the frequency band of the device is permitted by the FCC regulation. 38

39 Government Polytechnic, Muzaffarpur ADVANCE COMMUNICATION SYSTEM LAB. Subject Code: Monochrome Television Receiver Fault Finding Aim:- To find out various faults and trace circuits in monochrome TV receiver Apparatus:- Monochrome TV receiver, CRO, CRO probe, digital Multimeter Theory :- The first step towards finding the cause of failure of a receiver is visual inspection of the components, tubes, transistors and ICs mounted on various printed circuit boards, modules and chassis of the receiver. Such an inspection sometimes leads quickly to the defective device or part in the receiver. For example, burnt or charred resistors can often be spotted by visual observation. Similarly overheated transformers, oil or wax filled capacitors and coils can be located by a peculiar small caused by overheating and shorting. In addition a disoldered wire of a coil or component, broken load of a transistor or other similar faults can be found out by a preliminary visual inspection. Fault Localization 1. The picture and sound signals have a common path from antenna to the video detector after which the two separate out to their respective channels. Another section of the receiver provides necessary signals for producing the raster and maintaining synchronism between the televised scene and reproduced picture. Thus the known path of the video, audio, and sync signals together with the symptoms observed on the screen and noted from the sound output can form a basis for localizing any trouble in one or more sections of the receiver. 2. If a receiver exhibits a distorted picture accompanied by a distorted audio output, it becomes obvious that the defect is in a circuit which is common to both the signals. Thus the fault is localized to the RF and IF sections of the receiver. On the other hand, if only the sound output is missing or appears to be distorted, the fault lies in the sound section following the point of separation of the two signals. 3. Similarly if nothing but a white horizontal line appears on the screen, it becomes evident, that the trouble lies somewhere in the vertical circuit. However, if the picture holds vertically but tears diagonally, the trouble probably lies either in the horizontal sync or horizontal sweep circuit. Thus the method of observing symptoms from picture and sound is very efficient for fault location and saves much servicing time. 39

40 Signal Source for Observing Faults 1. Though any transmission from a local TV station can be used for observing possible faults but continuous change of picture details on the screen makes it difficult to observe minor irregularities in the reproduced picture. 2. A test pattern obtained either from a test chart generator or an analyst generator shows much more than a picture and so is very useful for detecting all types of faults in the picture. 3. However, if these instruments are not available, a pattern generator set to produce a cross-hatched pattern on the screen can be used as input signal source. In any case, an initial observation by tuning in any one of the local channels is helpful for localizing trouble to one of the major functional areas of the receiver. 4. Once this has been done, the normal trouble shooting tools can be used to isolate defective component or components of that section. SAFETY PRECAUTIONS IN TELEVISION SERVICING The following general safety precautions should be observed during operation of test equipment and servicing of television receivers: (i) A contact with ac line can be fatal. Line connected receivers must have insulation so that no chassis point is available to the user. After servicing all insulators, bushes, knobs etc. must be replaced in their original position. The technician must use an isolation transformer whenever a line connected receiver is serviced. (ii) Voltage in the receiver, such as B+ and EHT can also be dangerous. The service man should stand or sit on an insulated surface and use only one hand when probing a receiver. The interlock and the back cover of the receiver must always be replaced properly to ensure that the receiver s high voltage points are not accessible to the user. (iii) Fire hazard is another major problem and must get the attention it deserves. Technicians should be very careful not to introduce a fire hazard in the process of repairing TV receivers. The parts replaced must have correct or higher power rating to avoid overheating. This is particularly important in high power circuits. (iv) The picture tube is another source of danger by implosion. If the envelope is damaged, the glass may shatter violently and its pieces fly great distance with force. It can cause serious injury on hitting any part of the body. Though in modern picture tubes internal protection is provided but it is necessary to handle it carefully and not to strike it with any hard object. (v) Many service instruments are housed in metal cases. For proper operation, the ground terminal of the instrument is always connected to the ground of the receiver being serviced. It should be made certain that both the instrument box and the receiver chassis are not connected to the hot side of the ac line or any point above ground potential. (vi) All connections with test leads or otherwise to the high-voltage points must be made after disconnecting the receiver from ac mains

41 (vii) High voltage capacitors may store charge large enough to be hazardous. Such capacitors must be discharged before connecting test leads. (viii) Only shielded wires and probes should be used. Fingers should never be allowed to slip down to the meter probe tip when the probe is in contact with a high voltage circuit. (ix) The receiver should not be connected to a power source which does not have a suitable fuse to interrupt supply in case of a short circuit in the line cord or at any other point in the receiver. (x) Another hazard is that the receiver may produce X-radiation from the picture tube screen and high voltage rectifier. It is of utmost importance that the voltages in these circuits are maintained at the designed values and are not exceeded. Following flow chart (1) illustrates General procedure for trouble shooting a monochrome or colour television receiver. In this experiment we have localized five faults in monochrome TV system illustrated in flow chart related with various circuits as listed below in Table 1. Sr. No Name of Faults in monochrome TV receiver Related Circuits in monochrome TV system 1 No sound but picture normal Video pre amplifier and video amplifier 2 No sound but picture normal Sound IF circuit to the audio amplifier circuit 3 Vertical rolling of picture on TV screen Sync separator circuit to Vertical frequency oscillator circuit 4 Set dead 5 No sound but picture normal. Power supply section / horizontal output stage Sound IF section to the audio amplifier circuit Flow Chart (1) General procedure for trouble shooting a Monochrome or Colour Television receiver. 41

42 Fault 1: White Raster with retrace lines Stages: From video pre amplifier to video amplifier stage Flow Chart (2) Fault White Raster with retrace lines 42

43 Fault 2 : No sound but picture normal Stages: From sound if section to the audio amplifier section Flow Chart (3) No sound but picture normal 43

44 Fault 3 : Vertical rolling of picture on TV screen Stages: From Sync separator circuit to Vertical frequency oscillator Flow Chart (4) Vertical rolling of picture on TV screen 44

45 Fault 4 : Set dead Stages: Power supply section or horizontal output stage Flow chart (5) Set Dead 45

46 Fault 5: No sound but picture normal. Stages: From sound if section to the audio amplifier section. Flow Chart (6) No sound but picture normal 46

47 Conclusion: Thus in this experiment we have gone through procedure of troubleshooting of monochrome television receiver localized five faults in monochrome television system. 47

48 Government Polytechnic, Muzaffarpur ADVANCE COMMUNICATION SYSTEM LAB. Subject Code: Colour Television Receiver Fault Finding Aim:- To study the trouble shooting of colour TV receiver with normal defects with their remedy. Apparatus:- PAL colour TV receiver, CRO, CRO probe, digital Multimeter. Theory:- The first step towards finding the cause of failure of a receiver is visual inspection of the components, tubes, transistors and ICs mounted on various printed circuit boards, modules and chassis of the receiver. Such an inspection sometimes leads quickly to the defective device or part in the receiver. For example, burnt or charred resistors can often be spotted by visual observation. Similarly overheated transformers, oil or wax filled capacitors and coils can be located by a peculiar small caused by overheating and shorting. In addition a disoldered wire of a coil or component, broken load of a transistor or other similar faults can be found out by a preliminary visual inspection. In this experiment we have localized five faults in Colour TV system illustrated in flow chart related with various circuits as listed below in Table 1. 48

49 Table 1 Faults in Colour TV system Sr. No Name of Faults in Colour TV receiver Related Circuits in Colour TV system 1 Colour Fading effect Luminance chrominance processor with horizontal & vertical oscillator, Chroma section, DS 5 switch 2 Video Amplifier supply open (No color signal only Luminance signal) Chroma Section to video amplifier, DS 8 Switch 3 No Raster & No picture Luminance Chrominance Processor with horizontal and Vertical oscillator stage, Horizontal output stage, Link L 17 4 Only Raster No picture & Sound Electronic Tuner, VIF & SIF section, Luminance Chrominance Processor with horizontal and Vertical oscillator stage, Link L6 5 Luminance information disappears on screen Luminance Chrominance Processor with horizontal and Vertical oscillator stage, Horizontal output stage, Link L10 Fault 1: Colour Fading Effect Stages: Luminance chrominance processor with horizontal & vertical oscillator, Chroma section Flow Chart (1) Colour Fading Effect 49

50 Fault 2 : Video Amplifier supply open (No color signal only Luminance signal) Stages: Chroma section and video amplifier Flow Chart (2) No Color signal only Luminance signal 50

51 Fault 3 : No Raster & No picture Stages: Luminance Chrominance Processor with horizontal and Vertical oscillator stage, Horizontal output stage Flow Chart (3) No Raster & No picture 51

52 Fault 4 : Only Raster No picture & sound Stages: Electronic Tuner, VIF & SIF section, Luminance Chrominance Processor with horizontal and Vertical oscillator stage Flow chart (4) Only Raster No picture & sound 52

53 Fault 5: Luminance information disappears on screen Stages: Luminance Chrominance Processor with horizontal and Vertical oscillator stage Flow Chart (5) Luminance information disappears 53

54 Conclusion: Thus in this experiment we have studied trouble shooting procedure for colour television receiver and localized five faults in PAL colour television system. 54

55 Government Polytechnic, Muzaffarpur ADVANCE COMMUNICATION SYSTEM LAB. Subject Code: Study of CRO and its application Objective : Study of CRO, and its application for measurement of phase, frequency, and amplitude such that it can be used for the communication System Apparatus: CRO, Function generator, Digital Voltmeter, Connecting Wires Circuit Diagram: An oscilloscope is a measuring device used commonly for measurement of voltage, current, frequency, phase difference and time intervals. The heart of the oscilloscope is the cathode ray tube, which generates the electron beam, accelerates the beam to high velocity, deflects the beam to create the image, and contains the phosphor screen where the electron beam eventually 55

56 becomes visible. To accomplish these tasks, various electrical signals and voltages are required. The power supply block provides the voltages required by the cathode ray tube to generate and accelerate the electron beam, as well as to supply the required operating voltages for the other circuits of the oscilloscope. Relatively high voltages are required by the cathode tubes, on the order of a few thousand volts, for acceleration, as well as a low voltage for the heater of the electron gun, which emits the electrons. Supply voltages for the other circuits are various values usually not more than few hundred volts. The oscilloscope has a time base, which generates the correct voltage to supply the cathode ray tube to deflect this part at a constant time dependent rate. The signal to be view is fed to you vertical amplifier, which increases the potential of the input signal to a level that will provide a usable deflection of the electron beam. To synchronize the that the horizontal deflection starts at the same point of the input vertical signal each time it sweeps, a synchronizing or triggering circuit is used. This circuit is the link between the vertical input and the horizontal time base. Procedure: Phase Measurement using Lissajous Patterns (X-Y Mode): To Measure the phase difference of two sine waves their frequencies must be equal. 1. Connect a 1Volt peak-peak, 1KHz sine wave signal from the function generator to the horizontal input of the CRO. 2. Connect the output of phase shift network to the vertical input as shown in figure. 3. Adjust the vertical and horizontal gains properly for good display. 4. Observe Lissajous Patterns for different combinations of R and C values. 56

57 Calculate the phase angle as Sine θ = A/B A: Distance between the points where the ellipse crosses the y-axis and the origin. B: Distance between the origin and the y co-ordinate of the maxima of the ellipse. Calculate theoretical phase difference as! = tan-1 (f1/f2) Where f2 = 1/2"RC f1 = input signal frequency. LISSAJOUS FIGURES 57

58 Conclusion: 58

59 Government Polytechnic, Muzaffarpur ADVANCE COMMUNICATION SYSTEM LAB Subject Code: Intersymbol Interference (ISI) Aim: Observation of dependence of intersymbol Interference (ISI) on band- width of the channel and the eye pattern due to noise in the channel Apparatus Required: 1.pseudo random binary sequence (PRBS) generator 2.time domain viewing: snap shot and eye patterns 3.two generator synchronization and 4. alignment with the sliding window correlator. 5.WIDEBAND TRUE RMS METER 6. NOISE GENERATOR 7. BASEBAND CHANNEL FILTERS module 59

60 Theory: Digital Messsages In analog work the standard test message is the sine wave, followed by the twotone signal for more rigorous tests. The property being optimized is generally signal-to-noise ratio (SNR). Speech is interesting, but does not lend itself easily to mathematical analysis, or measurement. In digital work a binary sequence, with a known pattern of '1' and '0', is common. It is more common to measure bit error rates (BER) than SNR, and this is simplified by the fact that known binary sequences are easy to generate and reproduce. A common sequence is the pseudo random binary sequence. Random Binary Sequences The output from a pseudo random binary sequence generator is a bit stream of binary pulses; ie., a sequence of 1`s (HI) or 0`s (LO), of a known and reproducible pattern. The bit rate, or number of bits per second, is determined by the frequency of an external clock, which is used to drive the generator. For each clock period a single bit is emitted from the generator; either at the '1' or '0' level, and of a width equal to the clock period. For this reason the external clock is referred to as a bit clock. For a long sequence the 1`s and 0`s are distributed in a (pseudo) random manner. The sequence pattern repeats after a defined number of clock periods. In a typical generator the length of the sequence may be set to 2n clock periods, where n is an integer. In the TIMS SEQUENCE GENERATOR the value of n may be switched to one of three values, namely 5, 8, or 11. There are two switch positions for the case n = 8, giving different independent patterns. The SYNCH output provides a reference pulse generated once per sequence repetition period. This is the start-of-sequence pulse. It is invaluable as a trigger source for an oscilloscope. Observation: There are two important methods of viewing a sequence in the time domain. The Snapshot A short section, about 16 clock periods of a TTL sequence, is illustrated in Figure 1 below. 60

61 Suppose the output of the generator which produced the TTL sequence, of which this is a part, was viewed with an oscilloscope, with the horizontal sweep triggered by the display itself. As stated above, it gives a start-of-sequence pulse at the beginning of the sequence. This can be used to start (trigger) the oscilloscope sweep. At the end of the sweep the oscilloscope will wait until the next start-of-sequence is received before being triggered to give the next sweep. Thus the beginning 'n' bits of the sequence are displayed, where 'n' is determined by the sweep speed. For a sequence length of many-times-n bits, there would be a long delay between sweeps. The persistence of the screen of a general purpose oscilloscope would be too short to show a steady display, so it may blink. Alternately, the oscilloscope may decide that it has waited too long and automatically triggers resulting in an unstable display. The Eye Pattern A long sequence is useful for examining 'eye patterns'. These are defined and examined in the part of this experiment entitled Eye patterns. Applications One important application of the PRBS is for supplying a known binary sequence. This is used as a test signal (message) when making bit error rate (BER) measurements. For this purpose a perfect copy of the transmitted sequence is required at the receiver, for direct comparison with the received sequence. This perfect copy is obtained from a second, identical, PRBS generator. The second generator requires: 1. B bit clock information, so that it runs at the same rate as the first 61

62 2. A method of aligning its output sequence with the received sequence. Due to transmission through a band limited channel, it will be delayed in time with respect to the sequence at the transmitter. Bit clock acquisition In a laboratory environment it is a simple matter to use a 'stolen carrier' for bit clock synchronization purposes, and this will be done in most TIMS experiments. In commercial practice this bit clock must be regenerated from the received signal. EXPERIMENT Since TIMS is about modelling communication systems it is not surprising that it can model a communications channel. Two types of channels are frequently required, namely lowpass and bandpass. Lowpass (or baseband) channels A lowpass channel by definition should have a bandwidth extending from DC to some upper frequency limit. Thus it would have the characteristics of a lowpass filter. A speech channel is often referred to as a lowpass channel, although it does not necessarily extend down to DC. More commonly it is called a baseband channel. Bandpass channels A bandpass channel by definition should have a bandwidth covering a range of frequencies not including DC. Thus it would have the characteristics of a bandpass filter. Typically its bandwidth is often much less than an octave, but this restriction is not mandatory. Such a channel has been called narrow band. Strictly an analog voice channel is a bandpass channel, rather than lowpass, as suggested above, since it does not extend down to DC. So the distinction between baseband and bandpass channels can be blurred on occasion. Designers of active circuits often prefer bandpass channels, since there is no need to be concerned with the minimization of DC offsets. The above description is an oversimplification of a practical system. It has concentrated all the bandlimiting in the channel, and introduced no intentional pulse shaping. In practice the bandlimiting, and pulse shaping, is distributed between filters in the transmitter and the receiver, and the channel itself. The transmitter and receiver filters are designed, knowing the characteristics of the channel. The signal reaches the detector having the desired characteristics. 62

63 Noise A representative noisy, band limited channel model is shown in block diagram form in Figure 1 of the following page. Band limitation is implemented by any appropriate filter. The noise is added before the filter so that it becomes band limited by the same filter that band limits the signal. If this is not acceptable then the adder can be moved to the output of the filter, or perhaps the noise can have its own band limiting filter. Figure 1: Channel model block diagram Controllable amounts of random noise, from the noise source, can be inserted into the channel model, using the calibrated attenuator. This is non signal-dependent noise. For lowpass channels lowpass filters are used. For bandpass channels bandpass filters are used. Signal dependent noise is typically introduced by channel non-linearities, and includes intermodulation noise between different signals sharing the channel (cross talk). Unless expressly stated otherwise, in TIMS experiments signal dependent noise is considered negligible. That is, the systems must be operated under linear conditions. Diagrammatic representation In patching diagrams, if it is necessary to save space, the noisy channel will be represented by the block illustrated in Figure 2 below. 63

64 This macro module is modelled with five real TIMS modules, namely: 1. An INPUT ADDER module. 2. A NOISE GENERATOR module. 3. A bandlimiting module. For example, it could be: a. Any single filter module; such as a TUNEABLE LPF (for a baseband channel). b. A BASEBAND CHANNEL FILTERS module, in which case it contains three filters, as well as a direct through connection. Any of these four paths may be selected by a front panel switch. Each path has a gain of unity. This module can be used in a baseband channel. 64

65 The filters all have the same slot bandwidth (40 db at 4 khz), but differing passband widths and phase characteristics. c. A 100 khz CHANNEL FILTERS module, in which case it contains two filters, as well as a direct through connection. Any of these three paths may be selected by a front panel switch. Each path has a gain of unity. This module can be used in a bandpass channel. 4. An OUTPUT ADDER module, not shown in Figure 1, to compensate for any accumulated DC offsets, or to match the DECISION MAKER module threshold. 5. A source of DC, from the VARIABLE DC module. This is a fixed module, so does not require a slot in the system frame. Thus the CHANNEL MODEL is built according to the patching diagram illustrated in Figure 3 below, and (noting item 5 above) requires four slots in a system unit. Noise level Figure 3: details of the macro CHANNEL MODEL module The noise level is adjusted by both the lower gain control 'g' of the INPUT ADDER, and the front panel calibrated attenuator of the NOISE GENERATOR module. Typically the gain would be set to zero [g fully anti-clockwise] until noise is required. Then the general noise level is set by g, and changes of precise magnitude introduced by the calibrated attenuator. Theory often suggests to us the means of making small improvements to SNR in a particular system. Although small, they can be of value, especially when combined with other small improvements implemented elsewhere. An improvement of 6 db in received SNR can mean a doubling of the range for reception from a satellite, for example. 65

66 Signal to noise ratio This next part of the experiment will introduce you to some of the problems and techniques of signal-to-noise ratio measurements. The maximum output amplitude available from the NOISE GENERATOR is about the TIMS ANALOG REFERENCE LEVEL when measured over a wide bandwidth - that is, wide in the TIMS environment, or say about 1 MHz. This means that, as soon as the noise is bandlimited, as it will be in this experiment, the rms value will drop significantly. You will measure both, (ie, SNR) and, and compare calculations of one from a measurement of the other. The uncalibrated gain control of the ADDER is used for the adjustment of noise level to give a specific SNR. The TIMS NOISE GENERATOR module has a calibrated attenuator which allows the noise level to be changed in small calibrated steps. Within the test set up you will use the macro CHANNEL MODEL module already defined. It is shown embedded in the test setup in Figure 5 below. As in the filter response measurement, the oscilloscope is not essential, but certainly good practice, in an analog environment. It is used to monitor waveforms, as a check that overload is not occurring. The oscilloscope display will also give you an appreciation of what signals look like with random noise added. 66

67 Set up the arrangement of Figure 5 above. Use the channel model of Figure 3. In this experiment use a BASEBAND CHANNEL FILTERS module. Before commencing the experiment proper have a look at the noise alone; first wideband, then filtered. Switch the BASEBAND CHANNEL FILTERS module to the straight-through connection - switch position #1. Look at the noise on the oscilloscope. Switch the BASEBAND CHANNEL FILTERS module to any or all of the lowpass characteristics. Look at the noise on the oscilloscope. Probably you saw what you expected when the channel was not bandlimiting the noise - an approximation to wideband white noise. But when the noise was severely bandlimited there is quite a large change. For example: 1. The amplitude dropped significantly. Knowing the filter bandwidth you could make an estimate of the noise bandwidth before bandlimiting? 2.The appearance of the noise in the time domain changed quite significantly.. Observations We are now going to set up independent levels of signal and noise, as recorded by the WIDEBAND TRUE RMS METER., and then predict the meter reading when they are present together. After bandlimiting there will be only a small rms noise voltage available, so this will be set up first. Reduce to zero the amplitude of the sinusoidal signal into the channel, using the 'G' gain control of the INPUT ADDER. Set the front panel attenuator of the NOISE GENERATOR to maximum output. Keep the filter in its pass through state and adjust the gain control 'g' of the INPUT ADDER to maximum. Adjust the 'G' control of the OUTPUT ADDER for about 1 volt rms. Record the reading. The level of signal into the BASEBAND CHANNEL FILTERS module may exceed the TIMS ANALOG REFERENCE LEVEL, and be close to overloading it - but we need as much noise out as possible. If you suspect overloading, then reduce the noise 2 db with the attenuator, and check that the expected change is reflected by the rms meter reading. If not, use the INPUT ADDER to reduce the level a little, and check again. Switch to one of the filter positions and record the rms voltage level of the noise through the filter. Reduce to zero the amplitude of the noise into the channel by removing its patch cord from the INPUT ADDER, thus not disturbing the ADDER adjustment. 67

68 Set the AUDIO OSCILLATOR to any convenient frequency within the passband of the channel. Adjust the gain 'G' of the INPUT ADDER until the WIDEBAND TRUE RMS METER reads the same value as it did for the noise level in step T30. Replace the noise patch cord into the INPUT ADDER. Record what the meter reads. Calculate and record the signal-to-noise ratio in db. Measure the signal-plus-noise, then the noise alone, and calculate the SNR in db. Compare with the result of the previous Task. Increase the signal level, thus changing the SNR. Measure both, and, and predict each from the measurement of the other. Repeat for two additional SNR by changing the signal gain. EYE PATTERNS: Pulse Transmission It is well known that, when a signal passes via a bandlimited channel it will suffer waveform distortion. As an example, refer to Figure 1. As the data rate increases the waveform distortion increases, until transmission becomes impossible. Figure 1: Waveforms before and after moderate bandlimiting The effect of ISI becomes apparent at the receiver when the incoming signal has to be 'read' and decoded; ie., a detector decides whether the value at a certain time instant is, say, 'HI' or 'LO' (in a binary decision situation). A decision error may occur as a result of noise. Even though ISI may not itself cause an error in the absence of noise, it is nevertheless undesirable because it decreases the margin relative to the decision threshold, ie., a given level of noise, that may be harmless in the absence if ISI, may lead to a high error rate when ISI is present. 68

69 EXPERIMENT Set up the model of Figure 2. The AUDIO OSCILLATOR serves as the bit clock for the SEQUENCE GENERATOR. A convenient rate to start with is 2 khz. Select CHANNEL #1. Select a short sequence (both toggles of the on-board switch SW2 UP) Synchronize the oscilloscope to the 'start-of-sequence' synchronizing signal from the SEQUENCE GENERATOR. Set the sweep speed to display between 10 and 20 sequence pulses (say 1 ms/cm). This is the 'snap shot' mode. Both traces should be displaying the same picture, since CHANNEL #1 is a 'straight through' connection. The remaining three channels (#2, #3, and #4) in the BASEBAND CHANNEL FILTERS module represent channels having the same slot bandwidth 3 (40 db stopband attenuation at 4 khz), but otherwise different transmission characteristics, and, in particular, different 3 db frequencies. Change the oscilloscope synchronizing signal from the start-of-sequence SYNC output of the SEQUENCE GENERATOR to the sequence bit clock. Increase the sequence length (both toggles of the on-board switch SW2 DOWN). Make sure the oscilloscope is set to pass DC. Select CHANNEL #2. Use a data rate of about 2 khz. We should have a display on CH2-A similar to that of Figure 3 below. 69

70 Figure 3: A 'good' eye pattern Increase the data rate until the eye starts to close. Figure 4 shows an eye not nearly as clearly defined as that of Figure 3. Take some time to examine the display, and consider what it is what are looking at! There is one 'eye' per bit period. Those shown in Figure 3 are considered to be 'wide open'. But as the data rate increases the eye begins to close. The actual shape of an eye is determined (in a linear system) primarily by the filter (channel) amplitude and phase characteristics (for a given input waveform). Conclusion: 70

71 Government Polytechnic,Muzaffarpur ADVANCE COMMUNICATION SYSTEM LAB. Subject Code: Aim of experiment: To generate and Study wide band and narrow band noise Theory: A random process X(t) is bandpass or narrowband random process if its power spectral density SX(f) is nonzero only in a small neighborhood of some high frequency fc Deterministic signals: defined by its Fourier transform Random processes: defined by its power spectral density. 1. Since X(t) is band pass, it has zero mean: E[(X(t)] = fc needs not be the center of the signal bandwidth, or in the signal bandwidth at all. Narrowband Noise Representation: In most communication systems, we are often dealing with band-pass filtering of signals. Wideband noise will be shaped into bandlimited noise. If the bandwidth of the bandlimited noise is relatively small compared to the carrier frequency, we refer to this as narrowband noise. We can derive the power spectral density Gn(f) and the auto-correlation function Rnn(τ) of the narrowband noise and use them to analyse the performance of linear systems. In practice, we often deal with mixing (multiplication), which is a non-linear operation, and the system analysis becomes difficult. In such a case, it is useful to express the narrowband noise as n(t) = x(t) cos 2πfct - y(t) sin 2πfct. where fc is the carrier frequency within the band occupied by the noise. x(t) and y(t) are known as the quadrature components of the noise n(t). The Hibert transform of n(t) is n^ (t) = H[n(t)] = x(t) sin 2πfct + y(t) cos 2πfct. Generation of quadrature components of n(t). x(t) and y(t) have the following properties: 71

72 1. E[x(t) y(t)] = 0. x(t) and y(t) are uncorrelated with each other. 2. x(t) and y(t) have the same means and variances as n(t). 3. If n(t) is Gaussian, then x(t) and y(t) are also Gaussian. 4. x(t) and y(t) have identical power spectral densities, related to the power spectral density of n(t) by Gx(f) = Gy(f) = Gn(f- fc) + Gn(f+ fc) (28.5) for fc - 0.5B < f < fc + 0.5B and B is the bandwidth of n(t). Inphase and Quadrature Components: In-Phase & Quadrature Sinusoidal Components From this we may conclude that every sinusoid can be expressed as the sum of a sine function phase zero) and a cosine function (phase 2). If the sine part is called the ``in-phase'' component, the cosine part can be called the ``phase-quadrature'' component. In general, ``phase quadrature'' means ``90 degrees out of phase,'' i.e., a relative phase shift of ± 2. It is also the case that every 72

73 sum of an in-phase and quadrature component can be expressed as a single sinusoid at some amplitude and phase. Ideal BPF white noise: 73

74 Wide-Band Analog White-Noise Generator: Commercially available white-noise generators are rather expensive. The circuit presented here, however, is an inexpensive version that produces frequencies up to about 300 MHz. Its operation is based on the noise generated by the Zener breakdown phenomenon in the BJT inversely polarized base-collector junction. In other words, such shot noise involves the statistical fluctuations of the current flow present in the bipolar transistor. The generator shown makes use of a common 2N2907 biased by the constant current source supplied by a 2N2222 (Fig. 1). To increase the amount of shot noise attainable, the collector of the 2N2907 is left open and the base-emitter is reverse-biased. That is, the BJT is connected as a Zener diode to exploit the reverse breakdown phenomenon. 74

75 With this configuration, the reverse breakdown voltage exhibited by the emitter-base junction can be easily observed using an ordinary spectrum analyzer. The attainable bandwidth is about 300 MHz, and the power output is about -70 dbm. To increase the noise power, one or more amplifiers, such as the monolithic MAV-11 from Mini- Circuits, can be added. The 50-MHz low-pass filter (the PLP-50 from Mini-Circuits) inserted between the generator and the first MAV-11 is necessary to maintain the amplifier output power compression at an acceptable value. But, of course, with this configuration, the bandwidth is restricted to the 0-50 MHz range, i.e., the power spectrum vanishes outside the cutoff frequency of the filter. In Figure 1, R4 is needed to limit the current delivered to the amplifier. L1 provides high impedance to isolate the dc source from the RF signal. C3 removes any dc content from the 75

76 output of the generator. The 20k trimmer connected between the base and ground in Figure 1 permits a wide range of the attainable output noise up to -60 dbm (Fig. 2). At the Istituto di Radio astronomia del C.N.R., this circuit is currently being used to simulate cosmic white noise, in which the radio astronomical signal (a coherent white noise) to be extracted is buried. Applications Bit error rate and SINAD testing Modem and receiver characterization Channel impairment tests Conclusion: 76

77 Government Polytechnic, Muzaffarpur ADVANCE COMMUNICATION SYSTEM LAB. Subject Code: Amplitude modulated Signal using a square-law modulator Objective: To generate an amplitude modulated Signal using a square-law modulator and study the spectra of AM wave Apparatus: a single diode, an inductor, a capacitor and two auto transformers,cro. Theory: The design was based on the square law approach which makes use of a single diode, an inductor, a capacitor and two auto transformers to couple the modulating and carrier signals to the circuit, as depicted in the circuit diagram in Figure 1 below. When a small alternating voltage vd is applied across a forward biased diode, the ac diode current id will be a function of this voltage [5]. Expanding this function by the stirling s or maclaurin s theorem and retaining up to the second order terms in the series expansion, we have: where A1 and A2 are the series expansion coefficients and id and vd are the instantaneous values of the diode current and the voltage across it. Let the carrier voltage be given by (1) and the modulating voltage by where (2) (3) vc and vm correspond to the instantaneous values Vc and Vm are the corresponding peak values, and 77

78 ωc and ωm are the respective angular frequencies. The vd will be given by since the two voltage sources are connected in series. Substituting equation (4) in (1) gives: (4) or (5) but (6) therefore substituting equation (6) in (5) gives: (7) The various frequency components in this equation are: a. = carrier frequency component of the carrier wave b. = modulating frequency component of the modulating wave c. There is a direct current (dc) component and a carrier frequency second harmonic component. d. 78

79 There is a dc component and a modulating frequency second harmonic. e. The first part is the upper sideband term of frequency given by (ωc + ωm) and the second is the lower sideband term of frequency given by (ωc ωm). Thus equation (5) gives rise to six terms of different frequencies in addition to the dc component. The load in this case is a tuned circuit which is adjusted to the carrier frequency ωc. Hence it will respond to a narrow frequency band centered about ωc. If ωm<< ωc, the tuned circuit load will mainly respond to the frequency components (ωc ωm), ωc and (ωc + ωm). The remaining terms in the circuit expression will not produce enough output voltage across the load. Hence the desired diode current (mainly effective to produce the output voltage across the load) is (8) Where the modulation index. The modulated output voltage is then given by where Rt is the impedance of the tuned circuit at resonance. The modulation index of the signal can be calculated using equation (10) below: (9) (10) Circuit Design Parameters: 79

80 Calculating the Capacitance and Inductance To calculate the values of the component used, a carrier frequency of 200 KHz and modulating frequency of 1KHz were considered such that the tuned circuit load will mainly respond to the frequency component Considering the lower sideband, the resonant frequency of 199 KHz was considered. Assuming L = 10 mh, C can be calculated thus Using ω 2 = 1 / LC Figure 1. Circuit diagram of a Square Law modulator using audio-transformer coupling Signal Diode An IN4148 signal diode was used. Signal diodes are so called because they are commonly found in circuits like those in radios or televisions, to pass very low-power but high-frequency currents. They pass current up to 100 milliamps, and are often used to process information found in electrical signals. By allowing the current to flow only in one direction, they block half of the 80

81 oscillations of the radio wave, converting the alternating-current wave to direct-current and allowing its varying strength to be read as an audio signal and converted to sound. This diode is a silicon signal type which has a lower resistance and vulnerability to heat. Audio Transformer The audio transformers used are those specifically designed for use in audio circuits. They can be used to block radio frequency interference or the DC component of an audio signal, to split or combine audio signals, or to provide impedance matching between high and low impedance circuits, such as between a high impedance tube (valve) amplifier output and a low impedance loudspeaker, or between a high impedance instrument output and the low impedance input of a mixing console. Such transformers were originally designed to connect different telephone systems to one another while keeping their respective power supplies isolated, and are still commonly used to interconnect professional audio systems or system components. Being magnetic devices, audio transformers are susceptible to external magnetic fields such as those generated by AC current-carrying conductors. "Hum" is a term commonly used to describe unwanted signals originating from the "mains" power supply (typically 50 or 60 Hz). Audio transformers used for low-level signals, such as those from microphones, often include magnetic shielding to protect against extraneous magnetically-coupled signals. Results & Discussion: It is pertinent to establish and ascertain some highly efficient testing techniques in order to minimize cost. Testing involved trouble shooting the system to detect, isolate and correct internal or external fault such as malfunction in the internal circuitry, input or output shorted to ground or any open circuit. The following results and waveforms were obtained from the oscilloscope. Figure 2. Modulating Signal displayed at CRO 81

82 The Time/Div was set to 0.5 ms and the period span through 2 division while the volt/div was set to 1 V. The period is calculated thus: The frequency is calculated as Figure 3. Carrier Frequency displayed on CRO The Time/Div was set to 20 µs and the signal span through 0.25 division while the volt/div was set to 0.5 V. The period is calculated as The frequency is calculated as 82

83 Figure 4. Modulated Signal at 50% modulation The frequency and amplitude of the carrier were held constant while the frequency of the modulating signal was varied. The result obtained is depicted in Table 1 below. Table 1. Varying the modulating signal frequency while carrier frequency remains constant The frequency and amplitude of the modulating signal were held constant while the carrier frequency was varied. The results obtained were shown in Table 2 below Table 2. Varying the carrier frequency while the modulating frequency remains constant 83