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Analog communications lab
 

Analog communications lab

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    Analog communications lab Analog communications lab Document Transcript

    • Analog Communications Lab INDEX S.No 1. 2. 3. Name of the Experiment Page No Date of Performance Date Of Submission Assessment of marks (Max 10M) Sign. Of Faculty Amplitude Modulation & Demodulation Diode Detector Characteristics Frequency Modulation & Demodulation 4. Balanced Modulator 5. Pre-Emphasis & De-emphasis 6. Mixer Characteristics 7. Digital Phase Detector 8. Phase Locked Loop 9. Synchronous Detector 10 AGC Characteristics 11 Frequency Synthesizer 12. Squelch Circuit Average marks Signature of Lab-In-charge HOD INSTRUCTIONS TO STUDENTS Turbomachinery Institute Science & Technology 1 Dept. of ECE
    • Analog Communications Lab Students shall read the points given below for understanding the theoretical concepts and Practical applications. 1. Listen carefully to the lecture given by teacher about importance of subject, curriculum philosophy, Learning structure, skills to be developed, information about equipment, instruments, procedure, method of continuous assessment, tentative plan of work in Laboratory and total amount of work to be done in a semester. 2. Students shall undergo study visit of the laboratory for types of equipment, instruments and material to be used, before performing experiments. 3. Read the write up of each experiment to be performed, a day in advance. 4. Organize the work in the group and make a record of all observations. 5. Understand the purpose of experiment and its practical implications. 6. Student should not hesitate to ask any difficulty faced during conduct of practical / exercise. 7. Student shall develop maintenance skills as expected by the industries. 8. Student should develop the habit of pocket discussion / group discussion related to the experiments/ exercises so that exchanges of knowledge / skills could take place. 9. Student should develop habit to submit the practical, exercise continuously and progressively on the scheduled dates and should get the assessment done. 10. Student shall attempt to develop related hands - on - skills and gain confidence. 11. Student shall focus on development of skills rather than theoretical or codified knowledge. 12. Student shall visit the nearby workshops, workstation, industries, laboratories, technical exhibitions trade fair etc. even not included in the Lab Manual. In short, students should have exposure to the area of work right in the student hood. 13. Student shall develop the habit of evolving more ideas, innovations, skills etc. those included in the scope of the manual. 14. Student shall refer to technical magazines, proceedings of the Seminars, refer websites related to the scope of the subjects and update their knowledge and skills. Turbomachinery Institute Science & Technology 2 Dept. of ECE
    • Analog Communications Lab 15. The student shall study all the questions given in the laboratory manual and practice to write the answers to these questions. Exp No: 1 Date: AMPLITUDE MODULATION & DEMODULATION AIM: To study the function of Amplitude Modulation & Demodulation (under modulation, perfect modulation & over modulation) and also to calculate the modulation index. APPARATUS REQUIRED: S. No. 1 Component Transistor 2 Resistors 3 Capacitors 4 5 6 7 8 9 Diode Functions Generator Regulated Power Supply Bread Board Cathode Ray Oscilloscope Connecting Wires Specification BC107 100KΩ 4.7KΩ 270 Ω 33 Ω 4.7µF 0.001µF 0A79 1Mhz (0-30)V 0-20MHz Single Strand Quantity 2 2 2 1 1 2 1 1 2 1 1 1 As Required THEORY: Modulator section illustrates the circuit of modulating amplifier employing a transistor as an active device in CE mode. R1 & R2 establish a quiescent forward bias for the transistor. The modulating signal fed at the emitter section causes the bias to increase or decrease in accordance with the modulating signal. C3 is bypass capacitor for carrier. Thus the carrier signal applied at the base gets amplified more when the amplitude of the modulating signal is at its maximum and less when the amplitude of the modulating signal is small. C 2 couples the modulated signal to output of the modulator. Demodulation involves two operations. i) Rectification of the modulated signal and ii) Elimination of RF components of the rectified signal. Turbomachinery Institute Science & Technology 3 Dept. of ECE
    • Analog Communications Lab The diode in the circuit diagram of demodulator does half wave rectification. The rectified signal is applied to a low pass filter to extract the modulating signal. CIRCUIT DIAGRAM: VCC = 12V R1 R3 C2 Output C1 B RF Input C L1 100µH BC 107 R2 E R4 C3 AF Input Demodulator: D1 AM Input AF Output OA 79 1KΩ 1nF EXPECTED WAVEFORMS:- Turbomachinery Institute Science & Technology 4 Dept. of ECE
    • Analog Communications Lab PROCEDURE: 1. Made the connections according to Circuit Diagram. 2. Measure and note down the frequency & amplitude (p-p) of the fixed carrier signal. 3. Measure and note down the frequency & amplitude (p-p) of the fixed message signal. 4. Apply fixed frequency carrier signal to RF input terminals. 5. Apply modulating signal to AF input terminals. 6. Note down and trace the modulated signal envelope on the CRO screen. 7. Find the modulation index by measuring Vmax and Vmin from the modulated (detected/ traced) envelope. m=(Vmax –Vmin)/(Vmax+Vmin) 8. Repeat the steps 4,5 & 6 by changing the amplitude of the modulating signal so as to observe over modulation, under modulating and perfect modulation. 9. For demodulation, apply the amplitude modulated signal as an input to the demodulator and verify the demodulated output with respect to the applied modulating signal PRECAUTIONS: RESULT: Turbomachinery Institute Science & Technology 5 Dept. of ECE
    • Analog Communications Lab Turbomachinery Institute Science & Technology 6 Dept. of ECE
    • Analog Communications Lab QUESTIONS Tm 1. AM is Defined as ____________ 2. Draw its spectrum___________ 3. Draw the phase representation of an amplitude modulated wave___ 4. Modulation index is defined as_____ 5. The different degrees of modulation _______ 6. What are the limitations of square law modulator 7. Compare linear and nonlinear modulators 8. Compare base modulation and emitter modulation 9. AM Demodulator is ___________ 10. Detection process _________ 11. The different types of distortions that occur in an envelop detector are__________ 12. Elimination of distortions in Envelope Detector __________________ technique we use. Turbomachinery Institute Science & Technology 7 Dept. of ECE
    • Analog Communications Lab Turbomachinery Institute Science & Technology 8 Dept. of ECE
    • Analog Communications Lab Turbomachinery Institute Science & Technology 9 Dept. of ECE
    • Analog Communications Lab Turbomachinery Institute Science & Technology ECE 10 Dept. of
    • Analog Communications Lab Turbomachinery Institute Science & Technology ECE 11 Dept. of
    • Analog Communications Lab Exp No: 2 Date: DIODE DETECTOR CHARACTERISTICS AIM: To perform demodulation of an amplitude modulated signal using (i) Simple diode detector and (ii) Practical diode detector APPARATUS REQUIRED: S. No. 1 Component Transistor 2 Resistors 3 Capacitors 4 5 6 7 8 9 10 Diode Functions Generator Regulated Power Supply Bread Board Cathode Ray Oscilloscope Potentiometer Connecting Wires THEORY: Specification BC107 100KΩ 4.7KΩ 270 Ω 33 Ω 4.7µF 0.001µF Variable 0A79 1Mhz (0-30)V 0-20MHz 100K Ω Single Strand Quantity 2 2 2 1 1 2 1 2 1 2 1 1 1 1 As Required Demodulation involves two operations: (i) Rectification of the modulated wave and (ii) Elimination of RF components of the rectified modulated wave Simple Diode Detector The diode is the most common device used in AM demodulator. Signal (AM modulated signal) is applied to anode and output is taken from cathode. Diode operates as half wave rectifier and passes only positive half cycle of the modulated wav e. Further signal is applied to a parallel combination of resistor (Rd) and capacitor (Cd) which acts as a low pass filter. This LPF allows only low frequency signal to output and it by passes RF component to the ground. This simple diode detector has the disadvantage that the output voltage, in addition to being proportional to the modulating signal, also has a dc component, which represents the average envelope amplitude (i.e. carrier signal) and a small RF ripple. However these unwanted components are removed in a practical detector leaving only AF signal. CIRCUIT DIAGRAM: Turbomachinery Institute Science & Technology ECE 12 Dept. of
    • Analog Communications Lab EXPECTED WAVEFORMS:- AM Modulated signal Practical Diode Detector: Turbomachinery Institute Science & Technology ECE 13 Dept. of
    • Analog Communications Lab In practical diode detector the cathode terminal of the diode is connected to one end of the secondary of IF transformer. The other end is grounded. Secondary is tuned with the capacitor C1. The capacitors C2 and C3 are used for RF filtering. The modulated signal is applied at the input of IF transformer. The voltage applied is negative and hence the cathode of the diode passes is connected to the IF transformer. So the diode passes both the positive and negative half cycles. The RF filtering is done by C2 and C3. The output is taken at the volume control. PROCEDURE:1. Connect the circuit as per the circuit diagram and switch on the power supply. ( Measure the power supply voltage, +12V and -12V) 2. Observe outputs of RF and AF signal generator using CRO, note that RF voltage is approximately 300mv p-p of 1MHz frequency and AF voltage is 10V p-p 2KHz frequency. 3. Now connect the modulator output to the simple diode detector input. 4. Observe the AF signal at the output to the simple diode detector at approximately 50% modulation using CRO. 5. Compare it with the original AF and observe that the detected signal is same as the AF signal applied. Thus no information is lost in the process of modulation. (Note: Only wave shape and frequency will be same, amplitude will be attenuated and phase may change) 6. To observe AM wave at different frequencies, connect AF signal from external signal generator to the input of modulator and observe demodulated wave at different frequencies. 7. Repeat the experiment using practical diode detector circuit. PRECAUTIONS: RESULT: Turbomachinery Institute Science & Technology ECE 14 Dept. of
    • Analog Communications Lab AF output QUESTIONS Tm Turbomachinery Institute Science & Technology ECE 15 Dept. of
    • Analog Communications Lab 1. Selectivity of receiver is defined as _____________ 2. Sensitivity of a receiver is defined as ____________ 2. The purpose of diode in diode detector circuit is __________ 3. The disadvantages of simple diode detector circuit are _______ 4. The factors influencing the choice of intermediate frequency in receivers _____ 5. The advantages of practical diode detector are________ Turbomachinery Institute Science & Technology ECE 16 Dept. of
    • Analog Communications Lab Turbomachinery Institute Science & Technology ECE 17 Dept. of
    • Analog Communications Lab Turbomachinery Institute Science & Technology ECE 18 Dept. of
    • Analog Communications Lab Turbomachinery Institute Science & Technology ECE 19 Dept. of
    • Analog Communications Lab Exp No: 3 Date: FREQUENCY MODULATION AND DEMODULATION AIM: To study the functioning of frequency modulation & demodulation and to calculate the modulation index. APPARATUS REQUIRED S. No. 1 Component ICs 2 Resistors 3 Capacitors 4 5 6 7 8 Specification XR 2206, LM 565 100KΩ, 4.7KΩ 10KΩ 220Ω 47KΩ 0.01µF,0.001µF 0. 1µF, 470pF, 0.01pF 1µF,10µF 1Mhz (0-12)V Functions Generator Regulated Power Supply Bread Board Cathode Ray Oscilloscope Connecting Wires 0-20MHz Single Strand Quantity 1 Each 2 Each 1 1 1 2 1 1 2 1 1 1 As Required THEORY: 1. AF Generator: This is an op-amp placed wein bridge oscillator. A FET input quad Op-Amp (ICTL084) is used here to generate low frequency signals of 500 Hz and 5KHz to use as modulating signal. In this experiment, a switch is provided to change the frequency. Required amplification is provided to avoid loading effect. 2. Regulated power supply: This consists of bridge rectifier, capacitor filters and three terminal regulators to provide required dc voltages in the circuit i.e. +15 V, -15 V, +5V . 3. Modulator: This has been developed using XR-2206 integrated circuit. The IC XR-2206 is a monolithic Function generator; the output waveforms can be both amplitude and frequency modulated by an external voltage. Frequency of operation can be selected externally over a range of 0.01 MHz. The circuit is ideally suited for communications, instrumentations and function generator applications requiring CIRCUIT DIAGRAM: Frequency Modulation Circuit Diagram: +12v Turbomachinery Institute Science & Technology ECE 4.7KΩ 20 Dept. of
    • Analog Communications Lab 4 100KΩ 7 47KΩ 3 AF I/F 10µF/63v FM O/P 2 10KΩ 4.7 KΩ 0.1µF XR 2206 14 220Ω 5 0.01µF 6 13 1 10 1µF/63v Frequency Demodulation Circuit Diagram: + 5v 10KΩ 10 10KPF 2 0.1µF AF Input 8 680Ω 1KPF LM 565 7 AF Output 4 3 5 1 9 680Ω 470PF – 5V sinusoidal tone, AM, FM or FSK generation. In this experiment, IC XC-2206 is connected to generate sine wave, which is used as a carrier signal. The amplitude of carrier signal is 5vPP of 100 KHz frequencies. 4. Demodulator: Turbomachinery Institute Science & Technology ECE 21 Dept. of
    • Analog Communications Lab This had been developed using LM4565 integrated circuit. The IC LM565 is a generalpurpose phase locked loop containing a stable, highly linear voltage controlled oscillator for low distortion FM demodulation. The VCO free running frequency f0 is adjusted to the center frequency of input frequency modulated signal i.e. carrier frequency which is of 100 KHz. When FM signal is connected to the demodulator input, the deviation in the input signal (FM signal) frequency which creates a DC error voltage at output of the phase comparator which is proportional to the change of frequency δf. This error voltage pulls the VCO to the new point. This error voltage will be the demodulated version of the frequency modulated input signal. PROCEDURE: 1. Connect the circuit as per the given circuit diagram. 2. Switch on the power supply. 3. Measure the frequency of the carrier signal at the FM output terminal with input terminals open and plot the same on graph. 4. Apply the modulating signal of 500HZ with 1Vp-p. 5. Trace the modulated wave on the C.R.O & plot the same on graph. 6. Find the modulation index by measuring minimum and maximum frequency deviations from the carrier frequency using the CRO. Mr = S maximum Frequency deviation = f modulating signal frequency 7. Repeat the steps 5& 6 by changing the amplitude and /or frequency of the modulating Signal. 8. For demodulation apply the modulated signal as an input to demodulator circuit and compare the demodulated signal with the input modulating signal & also draw the same on the graph. NOTE: Note down all the input and output wave forms of the signals applied and obtained respectively. PRECAUTIONS: RESULT: Turbomachinery Institute Science & Technology ECE 22 Dept. of
    • Analog Communications Lab EXPECTED WAVEFORMS: Turbomachinery Institute Science & Technology ECE 23 Dept. of
    • Analog Communications Lab Turbomachinery Institute Science & Technology ECE 24 Dept. of
    • Analog Communications Lab QUESTIONS 1. Frequency modulation is defined as ____ 2. The advantages of indirect method of FM generation are __ 3. Modulation index and frequency deviation of FM is ____ 4. The advantages of FM are ______ 5. Narrow band FM is _____ 6. Compare narrow band FM and wide band FM? 7. Differentiate FM and AM _____ 8. FM wave can be converted into PM wave _____ 9, State the principle of reactance tube modulator _____ 10. The bandwidth of FM system is ___ 11. The function of FM discriminator is_________ 12. Ratio detector differ from foster-seely discriminator _____ 13. Linear detector is ________ 14. The drawbacks of slope detector are ___________ Turbomachinery Institute Science & Technology ECE 25 Dept. of
    • Analog Communications Lab Turbomachinery Institute Science & Technology ECE 26 Dept. of
    • Analog Communications Lab Turbomachinery Institute Science & Technology ECE 27 Dept. of
    • Analog Communications Lab E Turbomachinery Institute Science & Technology ECE 28 Dept. of
    • Analog Communications Lab xp No: 4 Date: BALANCED MODULATOR AIM: To study the following of the Balanced Modulator as a 1. Frequency Doubler 2. DSB-SC Generator. APPARATUS REQUUIRED: S. No. 1 Component Modulator IC 2 Resistors 3 Capacitors 5 6 7 8 9 Functions Generator Regulated Power Supply Bread Board Cathode Ray Oscilloscope Connecting Wires Specification MC1496 10KΩ 4.7KΩ 270 Ω 33 Ω 0.1µF 0.001µF 1Mhz (0-30)V 0-20MHz Single Strand Quantity 2 3 2 1 1 4 1 2 1 1 1 As Required 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. 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, -8v @ 150 MA each. CIRCUIT DIAGRAM: Turbomachinery Institute Science & Technology ECE 29 Dept. of
    • Analog Communications Lab 4. Modulator: Turbomachinery Institute Science & Technology ECE 30 Dept. of
    • Analog Communications Lab 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 AMDSB/ 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 100KHz and a 5 KHz sinusoid multiplied together. Figure 1.2 shows the circuit that you will use for this experiment using MC 1496 balanced modulator/demodulator. Note: In frequency doubling If the input time period is “T” after frequency doubling the time period should be halfed.i.e,”T/2”. 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 500KHz. 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. Repeat the above process by varying the amplitude and frequency of AF but RF maintained constant. NOTE:- Note down all the waveforms for the applied inputs and their respective outputs. PRECAUTIONS: EXPECTED WAVEFORMS: Turbomachinery Institute Science & Technology ECE 31 Dept. of
    • Analog Communications Lab Turbomachinery Institute Science & Technology ECE 32 Dept. of
    • Analog Communications Lab Turbomachinery Institute Science & Technology ECE 33 Dept. of
    • Analog Communications Lab RESULT: QUESTIONS 1. The two ways of generating DSB_SC are ________ 2. The applications of balanced modulator are ________ 3. The advantages of suppressing the carrier ________ 4. The advantages of balanced modulator __________ 5. The advantages of Ring modulator __________ 6. The expression for the output voltage of a balanced modulator is _________ Turbomachinery Institute Science & Technology ECE 34 Dept. of
    • Analog Communications Lab Turbomachinery Institute Science & Technology ECE 35 Dept. of
    • Analog Communications Lab Turbomachinery Institute Science & Technology ECE 36 Dept. of
    • Analog Communications Lab Turbomachinery Institute Science & Technology ECE 37 Dept. of
    • Analog Communications Lab Exp No: 5 Date: PRE-EMPHASIS & DE-EMPHASIS AIM: To study the functioning of Pre-Emphasis and De-Emphasis circuits. APPARATUS REQUIRED: S. No. 1 Component Resistors 2 Capacitors 3 4 5 6 7 Functions Generator Regulated Power Supply Bread Board Cathode Ray Oscilloscope Connecting Wires Specification 1KΩ 0.1µF 0.001µF 1Mhz (0-30)V 0-20MHz Single Strand Quantity 2 2 1 2 1 1 1 As Required 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 booting of the higher frequencies, in accordance with a pre-arranged curve, is termed pre-emphasis, and the compensation at the receiver is called deemphasis. 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 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 preemphasis 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 50 micro second. CIRCUIT DIAGRAM: Turbomachinery Institute Science & Technology ECE 38 Dept. of
    • Analog Communications Lab Turbomachinery Institute Science & Technology ECE 39 Dept. of
    • Analog Communications Lab 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 microseconds. This frequency is given by f=1/2ÐRC and it is therefore 2120 Hz; with 50-microseconds deemphasis it would have been 3180 Hz. Figure I shows pre emphasis and de-emphasis 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 de-emphasis in existing AM services since extensive modifications would be needed, particularly in view of the huge numbers is receivers in use. 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. Repeat steps 2, 3 & 4 of Pre-Emphasis to de-emphasis also. PRECAUTIONS: RESULT: Turbomachinery Institute Science & Technology ECE 40 Dept. of
    • Analog Communications Lab Turbomachinery Institute Science & Technology ECE 41 Dept. of
    • Analog Communications Lab QUESTIONS 1. The need for pre-emphasis __________ 2. The operation of pre-emphasis circuit __________ 3. Pre-emphasis operation is similar to high pass filter explain how_______ 4. De-emphasis operation is similar to low pass filter justify _______ 5. De-emphasis is _______ 6. Draw the frequency response of a pre-emphasis circuit_______ 7. Draw the frequency response of a de-emphasis circuit ______ 8. Give the formula for the cutoff frequency of the pre-emphasis circuit ______ 9. The significance of the 3db down frequency is ______ Turbomachinery Institute Science & Technology ECE 42 Dept. of
    • Analog Communications Lab Turbomachinery Institute Science & Technology ECE 43 Dept. of
    • Analog Communications Lab Turbomachinery Institute Science & Technology ECE 44 Dept. of
    • Analog Communications Lab Turbomachinery Institute Science & Technology ECE 45 Dept. of
    • Analog Communications Lab Turbomachinery Institute Science & Technology ECE 46 Dept. of
    • Analog Communications Lab Exp No: 6 Date: CHARACTERISTICS OF MIXER AIM: To study the functioning of a frequency mixer. APPARATUS REQUIRED: S. No. Component 1 Resistors 2 Capacitors 3 4 5 6 7 8 Transistor Functions Generator Regulated Power Supply Bread Board Cathode Ray Oscilloscope Connecting Wires Specification 4.7KΩ, 22KΩ,3.3KΩ 27KΩ, 6.2KΩ 10KΩ 0.1µF, 50pF 0.1µF 100pF BC 547 1Mhz (0-30)V 0-20MHz Single Strand Quantity 1 3 3 2 2 4 1 2 1 1 1 As Required THEORY: The fundamental principle of modulation involves the mixing or multiplying of a low frequency signal with a higher frequency signal such as an AM or FM carrier. This enables the information contained in the low frequency signal to be transmitted through space as high frequency electromagnetic waves. Commercial radios use as intermediate frequency (455 khz for commercial AM). These if carrier contain all the information available to the receiver but in order to obtain this information mixing must take place to obtain the lower frequency signals ‘riding’ onto IF or RF frequency. This principle is also used in mixing low frequency signals up to IF or RF frequencies. The purpose of this experiment is to observe the effect of mixing two frequencies using a nonlinear single transistor mixer and to demonstrate the use of a band stop and a 2-pole low pass filter in a practical application. BLOCK DIAGRAM DESCRIPTION: 1. RF Generator 1: Colpitts oscillator using FET is used here to generate RF signal of approximately 500 KHz to use as local oscillator in this experiment. Adjustments for amplitude and Frequency are provided on panel for ease of operation. Turbomachinery Institute Science & Technology ECE 47 Dept. of
    • Analog Communications Lab CIRCUIT DIAGRAM: 12V 50pF 50pF 3.3KΩ 22KΩ 4.7KΩ 27KΩ 6.2KΩ 6.2KΩ 27KΩ Output C 0.1µF B BC 547 100pF 100pF 100pF VX E 0.1µF VY 10KΩ 10KΩ 10KΩ EXPECTED WAVEFORMS: 2. RF Generator 2: Turbomachinery Institute Science & Technology ECE 48 Dept. of
    • Analog Communications Lab Colpitts oscillator using FET is used here to generate RF signal of approximately 520 KHz to use as IF Signal in this experiment. Adjustment for Frequency is provided on panel to get exact frequency. 3. Regulated Power Supply: This consists of bridge rectifier, capacitor filters and three terminal regulators to provide required do voltage in the circuit i.e., +12v @150 mA. 4. Mixer: Mixing is the nonlinear combination of two signals to produce sum and difference frequencies and harmonics of the signals. It is primarily used for modulation of demodulation of a signal. Modulation is the translation of the signal information to a higher frequency signal, and demodulation is the translation of signal / information carried by high frequency signal down to a lower frequency. The principle involved in this experiment is the use of the nonlinear portion of a transistor characteristic to mix (combine) two signals. Below figure shows the circuit you will use in this experiment. The Vy input the local oscillator and is made large so that the signal is cut off at the Collector. This forces the transistor to operate in a very nonlinear region. Now that the transistor is operating in a nonlinear regional during part of its cycle the input signal representing an input from the IF of a radio, is introduced at a low level (millvolts). The result is an output at the collector at the transistor that contains all the frequencies (sum, difference and high frequence harmonics) An example if Vyis 500 Khz and Vx is 520 Khsz the output frequencies at the collector will be do, dc, 20 Khz, 20 Khz, I Mhz, 1.092 MHz and other harmonics of the sum frequencies, the difference frequencies, and the original frequencies. In a radio you are interested only in the audio output: therefore you need to eliminate all higher frequencies. In this circuit you are interested only in the 20 Khz signal (which is above the usual audio range but provides a good signal on many spectrum analyzers), so you must design a filter to eliminate all the higher frequency signals. The 500 Khz component is the largest in the output because of the very large signal generated by the local oscillator (Vy). Thus a special notch filter will be used to attenuate this large singnal and a low pass filter will be used to attenuate other harmonics. Turbomachinery Institute Science & Technology ECE 49 Dept. of
    • Analog Communications Lab TABULAR COLOUMN: S.NO Inputs Vx sin (2∏fxt) Turbomachinery Institute Science & Technology ECE Filter Output Vy sin (2∏fyt) 50 Vo sin (2∏fot) Dept. of
    • Analog Communications Lab PROCEDURE: 1. Connect the circuit as per the circuit diagram. 2. Observe the output signal of RF generator 1 and 2 using CRO and note that output voltage of RF generator 1 is approximately 3VPP (frequency between 400 to 550 KHz) and RF generator 2 is approximately 0.6VPP (frequency between 550KHz). 3. Set RF generator 1 output at 500KHz with the help of IFT and connect it to the V Y input (which represents local oscillator) of the mixer circuit. 4. Observe the signal at output using CRO. Vary the amplitude of the RF generator 1 using potentiometer until the signal at output is noticeably nonlinear (note that the signal is clipped). The clipping is necessary to introduce nonlinearities, which is how the sum and difference frequencies and other harmonics are produced. 5. Set RF generator 2 output at 520KHz with the help of IFT and connect it to the V X input (which represents IF signal) of the mixer circuit and observe the signal at mixer output using CRO. 6. Connect output of mixer to filter input and observe output signal. Note that the signal is approximately 0.7VPP of 20KHz frequency. 7. Change VX and VY signal frequencies and observe output signal. PRECAUTIONS: RESULT: QUESTIONS: 1. The need for a frequency mixer is _________ 2. Heterodyning is 3. ___________filter is used at the o/p of transistor circuit in a frequency mixer? 4. The frequency components that appear at the collector of the transistor in the mixer circuit are __ 5. The transistor operated in the nonlinear region in a frequency mixer because ________________ Turbomachinery Institute Science & Technology ECE 51 Dept. of
    • Analog Communications Lab Turbomachinery Institute Science & Technology ECE 52 Dept. of
    • Analog Communications Lab Turbomachinery Institute Science & Technology ECE 53 Dept. of
    • Analog Communications Lab Turbomachinery Institute Science & Technology ECE 54 Dept. of