Dc Lab Manual

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Digital Communication Lab

DIGITAL COMMUNICATION LAB (07EC61)CYCLE-1 1. Modulation and demodulation of a BPSK signal. 2. Determination of modes, transit time, electronic tuning range and Sensitivity of a Reflex Klystron. 3. Determination of V-I characteristics of a Gunn diode, measurement of guide wavelength (g), frequency and VSWR. 4. a. DPSK encoder and decoder using module. b. QPSK encoder and decoder using module. CYCLE-2 5. a. Design and implementation of ASK modulator and demodulator. b. FSK Generation and Detection using module. 6. Measurement of Directivity and Gain of a Horn Antenna (X-Band). 7. Characterization of optical Fibers: Calculation of Launching angle, Critical angle, Numerical aperture and different types of losses. 8. a. Measurement of resonance characteristics of Microstrip ring resonator (C-Band). b. Measurement of Power division and isolation Characteristics of Microstrip 3dB Power divider (C-Band). CYCLE-3 9. Determination of coupling coefficient, Insertion loss and Isolation of Magic Tee and Directional coupler(Waveguide) 10. Measurement of Directivity and Gain of Printed Dipole antenna and Rectangular Microstrip patch antenna (X-Band). 11.Time Division Multiplexing& Demultiplexing, Frequency Division multiplexing & demultiplexing using OFC link.

Prof and Head

Dept of E&C, RVCE

Digital Communication Lab

EXPERIMENT 1 MODULATION AND DEMODULATION OF A BPSK SIGNALAim: To generate and detect a BPSK signal using kit. Apparatus: BPSK modulator kit, Op-Amp (A741), DC power supply, OA79 Diode, Cathode ray oscilloscope, Resistors and capacitors as indicated in the circuit diagram. BPSK Generation:

Fig1.1 BPSK generation circuit. BPSK detection:

Fig1.2: BPSK detector circuit.

Dept of E&C, RVCE

Digital Communication Lab Procedure 1. Apply sinusoidal signal (Carrier) and Square wave (Message) signal to the Modulator kit as shown in fig1.1. 2. Connect -6V and +12V DC supply to the modulator kit. 3. Observe the BPSK modulated signal at the output of the kit. Amplitude and frequency of the modulated signal are noted down. 4. Apply the BPSK modulated signal and a carrier signal to the input of the demodulated circuit as shown in fig1.2. 5. Verify the outputs of the adder and the envelope detector before verifying the output of the comparator. 6. Plot the input and output waveforms displayed on the CRO on a graph. Input waveform and expected output waveforms are shown in fig 1.3 and 1.4 respectively.

Fig 1.3.Base band information sequence 0010110010

Fig1.4. Binary PSK modulated signal (180 phase shifts at bit edges)


Dept of E&C, RVCE

Digital Communication Lab

EXPERIMENT 2 TO STUDY THE CHARACTERISTICS OF REFLEX KLYSTRONAim: To conduct a suitable experiment on reflex klystron, plot its mode curves and determine its Transit time, Electronic tuning range, Sensitivity, Peak output power for different modes, and Frequency variation for any one mode. Apparatus: Klystron power supply, Isolator, Frequency meter, Variable attenuator, X-band detector, Waveguide-to-BNC adaptor and Oscilloscope. Block diagram

Fig2.1: Experimental setup of a reflex klystron oscillator. Procedure: 1. Equipments are connected as shown in the Fig. 2.1. 2. Keep the repeller voltage at maximum, beam voltage at minimum before switching on powersupply and also switch on the fan.

3. Switch on klystron power supply and increase the beam voltage to 250V. Note down beam4. current. Adjust the repeller voltage and detector knob to get maximum output on CRO/SWR meter keeping frequency meter detuned. Repeller voltage is increased and slowly reduced in steps of 10V and at each step note down the output voltage on CRO or output power on SWR meter along with frequency in frequency meter. To measure operating frequency, the frequency meter is tuned to get the dip on the CRO and frequency is read directly from the frequency meter. To find the guided wave length, move the carriage on the slotted line to get the maximum output and note down the reading on the scale on slotted line and vernier scale, say d1 in cm. Move the carriage to the right or to the left to get the next maximum output position, say d2 in cm. The guide wave length = 2(d1~d2) cm

5. 6. 7.

8. To find VSWR, move the carriage to the maximum output position and set the VSWR to 1 onthe VSWR meter by adjusting the gain. Move the carriage to minimum output position. The reading of the VSWR meter gives the VSWR. 9. Note down the repeller voltage, SWR power and Frequency for different modes in the tabular column. 10. The VSWR can also be found alternatively. Find the maximum output voltage and minimum output voltage. VSWR = Vmax / Vmin

Dept of E&C, RVCE

Digital Communication Lab11. Calculate the mode number, transit time of each mode, electronic tuning range and electronic tuning sensitivity. Sample calculation is shown. 12. Plot the output power versus repeller voltage to get mode curves. Also plot the frequency versus repeller voltage. Expected graphs are shown in fig 2.2. Tabular column: S.No. Repeller voltage (V) SWR power Frequency meter reading (GHz).

Fig2.2: Mode curves of a klystron

Dept of E&C, RVCE

Digital Communication Lab

NOTE:Even though there should be little danger from microwave radiation hazards in the lab, the following work habits are recommended whenever working with RF or microwave equipment: Never look into the open end of a waveguide or transmission line that is connected to other equipment. Do not place any part of your body against the open end of a waveguide or transmission line. Turn off the microwave power source when assembling or disassembling components. ------------------00000-------------------


Digital Communication Lab

Determination of V-I characteristics of a Gunn diode, measurement of guide wavelength, frequency and VSWRAim: Conduct an experiment to plot the V-I characteristics of a GUNN diode and to determine the threshold voltage, measure operating frequency, guided wavelength and VSWR. Apparatus: GUNN power supply, GUNN oscillator, PIN modulator, Attenuator, Frequency meter, VSWR meter/ power supply. Block Diagram: GUNN power supply GUNN oscillator Ferrite isolatorPIN ModulatorSWR / Power meter / CRO

Crystal detector


Frequency meter

Slotted section with carriage

Matched Load Fig3.1: GUNN diode experimental setup. Procedure: 1. Setup the equipment as shown in fig 3.1. 2. Bias the GUNN diode and P-I-N diode. 3. Adjust the attenuator and P-I-N modulator to get maximum output on the CRO. Change the GUNN biasing in steps of 0.5V and record the corresponding current in table. 4. Draw the V-I characteristics and find the threshold voltage VTH and compare it with the ideal graph (Fig 3.2). 5. To measure operating frequency, tune the frequency meter to get the dip on CRO and frequency is read directly from the frequency meter making GUNN diode to operate in negative resistance region. 6. Find the guided wavelength and cutoff wavelength then calculate the theoretical operating frequency and compare it with the measured frequency. 7. Measure the VSWR by using VSWR meter.

I (mA)

Dept of E&C, RVCE

Digital Communication Lab

V Vth VV

Fig 3.2: V-I Characteristics of GUNN diode

Tabular column Bias Voltage (Volts) Current (mA)



Digital Communication Lab

a. GENERATION AND DETECTION OF QPSKAim: To generate QPSK wave and detect it using QPSK module. Apparatus: QPSK Modem Kit, Power supply with regulated supply of +5V, + 12V, CRO, andJumper wires.

QPSK Kit Description QPSK TransmitterXR 2206 generates a master clock of frequency 10 KHz. The clock divider circuit consists of two numbers of 74HC161. Patch cord is used for selecting data rate between 600 bps or 300 bps. A 1C

74HC161 form a divide by 8 circuits which is used for getting a word pulse (WP).CD4014 generates the data. It converts the 8 bit parallel data in serial form. Data pattern can be selected through the DIP switch SW1. QPSK system (fig4.1) requires four signals sin, sin, cos and cos which are generated using four number of UA741 through signal processing operation like inversion and differentiation. 74HC161 and CD 4094 form a two bit shift register used for serial to parallel conversion (Bit splitter). The two bit parallel output is used for selecting one of the four signal generated by ICs UA741. CD14053 and UA741 form a multiplexer and adder which is used for selecting one of the four signals of the QPSK modulation depends on the value of two bits of B0 and B1.

Fig4.1: Block diagram of a QPSK transmitter module. QPSK Receiver QPSK receiver (fig4.2) consists of band pass filter and two parallel branches of multiplier, a low pass filter and a comparator. In coming QPSK signal is fed two parallel branches of multiplier, LPF and level converter. Multiplier is used to remove the carrier frequency and recover the and band signal. Then it is passed through a LPF to restrict the noise and made to fall within the base band signal bandwidth. The level converter is enables the decision circuit to Dept of E&C, RVCE

Digital Communication Lab decide 1 or 0 of transmitted bits. The local carrier is used in the multiplier are sine and cos signal, which ultimately generates two bit streams B0 and B1 at the output of level converters. These outputs are then fed to 2 bit parallel to serial bit combiner to recover the original data.

Fig4.2: Block diagram of a QPSK receiver module Procedure Transmitter 1. Connec