student content of experiments

8/13/2019 Student Content of Experiments

1/59

Microwave & Optical Communication Lab Electronics & Communication Engineering

SASI Institute of Technology & Engineering Page 1

1. REFLEX KLYSTRON CHARACTERISTICS

AIM: To Study the Repeller mode characteristics of the Reflex Klystron tube and plot its

mode characteristics.

APPARATUS:

S.No. Name of the item Specifications Quantity

1 Klystron Power SupplyBeam voltage 240-420 Vdc

Repeller Voltage 10-270 Vdc1

2 Reflex Klystron tube with mount --- 1

3 Isolator

Min Isolation: 20dB;

Min Insertion loss:0.4dB 1

4 Frequency Meter 8.2 to 12.4 GHz 1

5 Variable AttenuatorMax Insertion loss: 0.2dB

Attenuation:0-15dB1

6 Slotted Section Max VSWR:1.01 1

7 Probe Detector --- 1

8 Matched TerminationAvg power-2 watts

Better than 1.021

9 Oscilloscope 30 MHz 1

10 BNC Connector ---- 1

11 Wave Guide Stands---- As per

required

12Cooling fan for Reflex Klystron

Oscillator ----1

THEORY:

The Reflex Klystron makes the use of velocity modulation to transform a continuous

electron beam into microwave power. Electrons emitted from the cathode are accelerated &

passed through the positive resonator towards negative reflector, which retards and finally

reflects the electrons and the electrons turn back through the resonator. Suppose an RF-field

exists between the resonators, the electrons traveling for-ward will be accelerated or retarded,


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as the voltage at the resonator changes in amplitude. The accelerated electrons leave the

resonator at an increased velocity and the retarded electrons leave at the reduced velocity.

The electrons leaving the resonator will need different time to return, due to change in

velocities. As a result, returning electrons group together in bunches, as the electron bunches

pass through resonator, they interact with voltage at resonator grids. If the bunches pass the

grid at such a time that the electrons are slowed down by the voltage then energy will be

delivered to the resonator; and Klystron will oscillate.

The frequency is primarily determined by the dimensions of resonant cavity. Hence,

by changing the volume of resonator, mechanical tuning of Klystron is possible. Also, a small

frequency change can be obtained by adjusting the reflector voltage. This is called Electronic

Tuning.

BLOCK DIAGRAM:

Figure 1.1: Setup for measurement of mode characteristics of Reflex klystron.

MODEL GRAPH:

Figure 1.2: Modes of typical Klystron tube


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PROCEDURE:

1. Connect the components and equipments as shown in figure 1.1.

2. Set the Variable Attenuator at minimum position.

3. Keep the Control Knobs of Klystron power supply as below

a. Beam voltage - OFF

b. Mod- switch - AM

c. Beam Voltage knob - fully anticlockwise

d. Reflector Voltage knob - fully clockwise

e. AM- Amplitude Knob - around fully clockwise

f. AM- Frequency Knob - Mid position.

4.

Switch ON the Klystron power supply and Cooling Fan.5. Switch ON Beam voltage switch and rotate the beam voltage knob clockwise

slowly up to 300V meter reading and observe beam current position. The beam

current should not increase more than 30mA.

6. Change the repeller voltage slowly and observe amplitude of square wave in CRO

and modes of Klystron tube can be seen on CRO.

7. Rotate the frequency meter slowly and stop at the position, where there is lowest

output amplitude (dip) on CRO. Read directly the frequency of signal between

two horizontal lines and vertical marker for every repeller voltage change.

OBSERVATIONS: Beam Voltage=300 V

Mode

No.

Repeller Voltage

(Volts)

Amplitude

(mv) ( Power)Frequency

(GHz)

1.

2.

3.


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PRECAUTIONS:

1. Keep the cooling fan towards the reflex Klystron.

2. Beam voltage in Klystron power supply is first kept in min and repeller voltage at max.

3. Power supply kept in high tension mode with AM mode.

4. Measurement bench is kept horizontal without any loosely coupling.

RESULT:

VIVA QUESTIONS:

1. What is the range of x band frequencies?

2. Why is it necessary to modulate the Klystron when the VSWR meter is used as an indicator?

3. Why does the Klystron oscillate only within certain intervals of the reflector voltage?

4. What is meant by electronic bandwidth and the tuning sensitivity of the Klystron?

5. What are the specifications of the tube you have used in the lab?


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2. V-I CHARACTERISTICS OF GUNN DIODE

AIM:

To study the volt-amp characteristics and find threshold voltage of the Gunn Diode.

APPARATUS:


1 Gunn Power Supply Min. Output Power of:10mW 1

2 Gunn Oscillator 8.6 to 11.6 GHz 1

3 Gunn diode mount --- 1

4 PIN modulator Max. RF Power of 1 W 1

5 Isolator Min Isolation: 20dB;Min Insertion loss:0.4dB

1



Attenuation:0-15dB1


9 Detector Mount8.2-12.4 GHz

BNC type connector1

10 Oscilloscope 30MHz 1

11 BNC Connector --- 2

12 Wave Guide Stands--- As per

required

13 Cooling Fan for Gunn oscillator --- 1

THEORY:

The Gunn Oscillator is based on negative differential conductivity effect in bulk

semiconductors, which has two conduction bands, separated by an energy gap (greater than

thermal energies). A disturbance at the cathode gives rise to high field region which TQ3vels

towards the anode. When this field domain reaches the anode, it disappears and another

domain -is formed at the cathode and starts moving towards anode and so on. The time

required for domain to travel from cathode to anode (transit time) gives oscillation frequency.


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In a Gunn Oscillator, the Gunn diode is placed in a resonant cavity. Cavity

dimensions determine the Oscillation frequency. Although Gunn Oscillator can be amplitude

modulated with the bias voltage. We have used a PIN modulator for square wave modulation

of the signal coming from Gunn diode. A measure of the square wave modulation capability

is the modulation depth the output ratio between 'ON' and 'OFF' state.

BLOCK DIAGRAM:

Figure 2.1: Set for the V-I Characteristics of Gunn diode

MODEL GRAPH:

Figure 2.2: I-V characteristics of Gunn diode


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PROCEDURE:

1. Set the components as shown in the figure 2.1.

2. Keep the control knobs of Gunn Power Supply as below:

Meter Switch -- OFF'

Gunn bias knob -- fully anticlockwise

PIN bias knob -- fully anticlockwise

PIN Mode frequency -- any position

3. Switch 'ON' the Gunn Power Supply.

4. Turn the meter switch of GUNN power supply to voltage position.

5. Measure the Gunn diode current corresponding to the various Gunn bias voltages

from the Panel meter by turning meter switch to voltage and current positions. Do notexceed the bias voltage above 10 volts.

6. Plot the voltage and current readings on the graph as shown in figure.

7. Measure the threshold voltage which corresponds to maximum current.

OBSERVATIONS:

S.No Gunn Bias Voltage (Volts) Gunn Diode Current

(mA)1

2

3

4

5

6

7

89

10

11

12

13

14

Threshold voltage = VMaximum Current = mA


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PRECAUTIONS:

1. Care should be taken to avoid microwave radiations.


3. Do not exceed the bias voltage above 10 volts.

4. Do not keep gun bias knob position at threshold position for more than 10-15 seconds.

RESULT:

VIVA QUESTIONS:

1. What is the material used in the Gunn diode?

2. What is meant by negative resistance region?

3. How can you vary the frequency of Gunn oscillator?

4. Calculate the efficiency of the Gunn oscillator?

5. Describe various modes in which Gunn diode can be operated.


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3. ATTENUATION MEASUREMENT

AIM:

To measure the attenuation factor of Microwave components (fixed attenuators).

APPARATUS:



Repeller Supply 10-270 Vdc1


3 Isolator Min Isolation: 20dB;

Min Insertion loss:0.4dB1



Attenuation:0-15dB1



BNC type connector

1

8 Fixed attenuators 5dB and 10dB 1




required


Oscillator ---

1

THEORY:

The attenuators are two-port bi-directional devices, which attenuates some power

when inserted into transmission line. Attenuation in dB is given by the ratio of power

absorbed or detected by the load without the attenuation in line to the power absorbed or

detected by the load with the attenuator in line.Attenuation A(dB)=10 log P1/P2


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Where P1=power detected by the load without load in the line

P2= power detected by the load with the attenuator in the line

The attenuator consists of a rectangular wave-guide with a resistive vane inside it to

absorb the microwave power according to their position with respect to sidewall of the wave-

guide. As electric field is maximum at centre in TE10mode, the attenuation will be maximum

if the vane is placed at the centre of the waveguide. Moving from the centre towards the side

walls attenuation decreases. In the fixed attenuator the vane position is fixed where as in

variable attenuator, its position can be changed by the help of micrometer of by other

methods.

BLOCK DIAGRAM:

Figure 3.1: Setup-1 for Attenuation measurement

Figure 3.2: Setup-2 for Attenuation measurement

PROCEDURE:

1. Connect the components and equipments as shown in figure 3.1 & 3.2.



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b. Mod- switch - AM





4. Rotate the knob of frequency meter at one side fully.

5. Switch ON the Klystron Power Supply, CRO and cooling fan for the klystron tube.

6. Switch ON Beam voltage switch and rotate the beam voltage knob clockwise slowly

up to 300V meter reading and observe beam current position. The beam current

should not increase more than 30mA.

7. Change the repeller voltage slowly until get maximum amplitude of square wave on

CRO.

8. Without fixed attenuator measure the amplitude (V1) of detector output voltage on

CRO.

9. Connect the fixed attenuator (5dB) and measure the amplitude (V2) of detector output

voltage on CRO.

10.Calculate the attenuation of connected attenuator by 20 log(V1/V2) dB.

11.Repeat the procedure for fixed attenuator 10 dB.

OBSERVATIONS: Beam Voltage = 300 V

Attenuator

Type

Without fixed attenuator

Amplitude(V1)

With fixed attenuator

Amplitude (V2)

Attenuation in dB

5 dB

10 dB

CALCULATIONS: Attenuation (A) = 20 log (V1/V2)

For 5 dB attenuator:

A = 20 log ( ) dB

= dB

For 10 dB attenuator:

A = 20 log ( ) dB

= dB


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PRECAUTIONS:


2. Beam voltage in Klystron power supply is first kept in min and repeller voltage at

max.



5. Output signal voltage is kept as high as possible.

RESULT:

VIVA QUESTIONS:

1. What is mean by attenuators?

2. What are the different types of attenuators?

3. What is mean by fixed attenuator?

4. What is mean by flap attenuator?

5. What is mean by rotary vane attenuator?


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4. DIRECTIONAL COUPLER CHARACTERISTICS

AIM:

To determine the coupling factor, directivity and Isolation of a Two hole directional

coupler.

APPARATUS:





3 IsolatorMin Isolation: 20dB;




Attenuation:0-15dB1


7 Matched TerminationAvg. power-2 watts

VSWR better than 1.021


BNC type connector1

9 Two hole Directional Coupler Directivity: 35 1




required


Oscillator

---1

THEORY:

A directional coupler is a device with which it is possible to measure the incident and

reflected wave separately. It consist of two transmission lines the main arm and auxiliary

arm, electro magnetically coupled to each other. The power entering, in the main-arm gets


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divided between port 2 and 3, and almost no power comes out in port 4. Power entering at

port 2 is divided between port 1 and 4. Figure bellow shows the Directional coupler with port

designations.

Figure 4.1: Two hole Directional Coupler

The coupling factor and Isolation are defined as

Coupling Factor (dB)= 1

3

10log pp

where port 2 and 4 are terminated with matched load.

Isolation (dB) = 1

4

10log p

p Where Port 1and 3 are terminated with matched load.

With built-in termination and power entering at Port1, the directivity of the coupler is

a measure of how well Coupler distinguishes forward port and backward port . Directivity is

measured indirectly as follows. Hence

Directivity D(dB) = 10 log10[P3/P4] =Isolation (dB)coupling factor (dB).

BLOCK DIAGRAM:

Figure 4.2: Setup for measurement of coupling factor of a Directional coupler


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Figure 4.3: Setup for measurement of Isolation of a Directional coupler

PROCEDURE:

1. Connect the components and equipments as shown in figure.




b. Mod- switch - AM

c.

Beam Voltage knob - fully anticlockwised. Repeller Voltage knob - fully clockwise



4. Rotate the knob of frequency meter at one side fully.

5. Switch "ON" the Klystron Power Supply, CRO and cooling fan for the klystron tube.

6. Put on Beam voltage switch and rotate the beam voltage knob clockwise slowly up to

300V meter reading and observe beam current position. The beam current should not

increase more than 30mA.

7. Change the repeller voltage slowly until get maximum amplitude of square wave on

CRO.

8. Measure the input signal voltage (V1) on CRO without connecting Directional

coupler.

9. Connect the input to the port 1 of directional coupler and matched terminations to port

2, 4 and measure the output voltages at port 3 (V3) and using V1 and V3 calculate

Coupling factor.


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10.Similarly Connect the input to the port 1 of directional coupler and matched

terminations to port 2, 3 and measure the output voltages at port 4 (V4) and using V 1

and V4calculate Isolation..

11.Now calculate Directivity using the expression Isolation =Directivity(dB) + Coupling

factor (dB).

OBSERVATIONS:

Voltage V1= V

Voltage V3= V

Voltage V4= V

CALCULATIONS:

Coupling Factor (dB) = 20 log (V1/V3) =

Isolation (dB) = 20 log (V1/V4) =

Directivity (dB)=Isolation (dB)-Coupling Factor (dB)=

PRECAUTIONS:


2. Beam voltage in Klystron power supply is first kept in min and repeller voltage at

max.



5. Output signal voltage is kept as high as possible.

RESULT:

VIVA QUESTIONS:

1. What is coupling factor?

2. What is the directivity of the directional coupler?

3. What is the difference between the forward and backward directional coupler

4. List the different types of directional couplers

5. What are factors that control the coupling factor?


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5 VSWR MEASUREMENTAIM:

To determine the standing wave ratio and reflection coefficient of various microwave

components.

APPARATUS:









Attenuation:0-15dB1


7 Tunable Probe --- 1

8 Matched Termination Avg. power-2 watts



BNC type connector1




required


Oscillator ---1

THEORY:

The electromagnetic field at any point of transmission line, may be considered as the

sum of two traveling waves the Incident Wave, which Propagates from the source tothe load

and the reflected wave which propagates towards the generator. The reflected wave is set up


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by reflection of incident wave from a discontinuity in the line or from the load impedance.

The superposition of the two traveling waves, gives rise to a standing wave along the line.

The maximum field strength is found where the waves are in phase and minimum where the

two waves add in opposite phase. The distance between two successive minimas (or

maximas) is half the guide wavelength on the line. The ratio of electrical field strength of

reflected and incident wave is called reflection coefficient.

The voltage standing wave Ratio (VSWR) is defined as ratio between maximum and

minimum field strength along the line.

Hence VSWR denoted by S is as follows

S = MAX

MIN

E

E

=I R

I R

E E

E E

Where Ei = Incident Voltage and Er = Reflected Voltage

Reflection Coefficient, is

=0

0

Lr

i L

Z ZE

E Z Z

Where ZLis the load impedance and Zo is characteristics impedance.

The above equation, which gives Reflection Coefficient equation:

= 11

S

S

BLOCK DIAGRAM:

Figure 5.1: Setup for VSWR measurement


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PROCEDURE:

1. Set up the equipment as shown in the figure 4.1.

2. Keep variable attenuator in the minimum attenuation position.

3. Keep the control knobs of VSWR Meter as below

a. Range db - 40-db/50 db

b. Input Switch - Low Impedance

c. Meter Switch - Normal

d. Gain (Coarse-Fine) - Mid position approx.



b. Mod- switch - AM





5. Switch ON the Klystron power supply, VSWR Meter and Cooling Fan.

6. Switch ON the Beam Voltage Switch position and set beam voltage at 300 V.

7. Rotate the reflector voltage knob to get deflection in VSWR meter.

8. Tune the output by turning the reflector voltage, amplitude and frequency of AM

Modulation.

9. Tune plunger of Klystron Mount and probe for maximum deflection in VSWR

meter.

10.If required, change the range db-switch, variable attenuator position and gain

control knob to get maximum deflection in the scale of VSWR meter.

11.As you move probe along the slotted line, the deflection will change.

Measurement of Low and Medium VSWR

(a) using VSWR meter1. Move the probe along the slotted line to get maximum deflection in VSWR Meter.

2. Adjust the VSWR Meter gain control knob or variable attenuator until the meter

indicates 1.0 on normal VSWR Scale.

3. Keep the all control knobs as it is, move the probe to next minimum position.

Read the VSWR value directly from the VSWR meter.


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(B) Using CRO1. Set the probe along the slotted line and adjust for minimum reading on CRO. Note

down the corresponding voltage reading Vmin.

2. Set the probe along the slotted line and adjust for maximum reading on CRO.

Note down the corresponding voltage reading Vmax

3. Calculate VSWR

VSWR =

max

min

V

V

OBSREVATIONS:

S.No. Microwave

Component

Maximum

Voltage

(Vmax)

Minimum

Voltage

(Vmin)

VSWR

S = ( max

min

V

V)

VSWR

with

Meter

Reflection

Coefficient

( )

1 Matched Load

2 Detector Mount

CALCULATIONS:

Beam voltage =

Repeller Voltage =

VSWR for matched load =max

min

V

V =

VSWR for detector Mount =max

min

V

V=

Reflection Coefficient for Matched load = = 1

1

S

S

=

Reflection Coefficient for detector Mount = = 11

S

S

=

PRECAUTIONS:


2. Beam voltage in Klystron power supply is first kept in minimum and repeller voltage at

maximum.


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RESULT:

VIVA QUESTIONS:

1. What is VSWR? Why VSWR is different for different loads? What are the minimum and

maximum values of VSWR?

2. What are the different methods of VSWR measurement?

3. What is slotted wave guide? In VSWR measurement how slotted section used?

4. Explain why successive minima of a standing wave pattern are separated by half wavelength?

5. Give the expression for VSWR in terms of reflection coefficient.


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6. MEASUREMENT OF WAVEGUIDE PARAMETERS

AIM:

To determine the frequency and guide wavelength of a rectangular waveguide

working in TE10 mode.

APPARATUS:





3 Isolator Min Isolation: 20dB;Min Insertion loss:0.4dB

1



Attenuation:0-15dB1



8 Matched Termination Avg. power-2 wattsVSWR better than 1.02

1




required


Oscillator ---1

THEORY:

For the dominant TE10 mode of rectangular wave-guide o, g and care related by

the expression.

0

2 2 2

1 1 1

g c

Where 0is free space wavelength

gis

guide wavelength


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'c' is cutoff wavelength and is given by cc

c

f , where

cf is the cutoff frequency

Cutoff frequency is defined as the lowest frequency of a particular mode and is given by

2 2

2c

c m nfa b

For TE10 mode m=1 and n=0, so cutoff frequency2

c

cf

a and c= 2a where 'a' is inner

broad dimension of wave-guide and which is the known value.

We know that the frequency is given by0

cf

BLOCK DIAGRAM:

Figure 6.1: Setup for measurement of waveguide parameters.

PROCEDURE:

1. Connect the components and equipments as shown in figure.




b. Mod- switch - AM


d. Repeller Voltage knob - fully clockwise



4.

Rotate the knob of frequency meter at one side fully.5. Switch "ON" the Klystron Power Supply, CRO and cooling fan for the klystron tube.


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6. Put on Beam voltage switch and rotate the beam voltage knob clockwise slowly up to

300V meter reading and observe beam current position. The beam current should not

increase more than 30mA.

7. Change the repeller voltage slowly to get perfect square wave output on CRO.

8. Rotate the knob of frequency meter slowly and stop at the position, where there is lowest

output amplitude on CRO. Read directly the frequency of signal between two horizontal

lines and vertical marker.

9. Move probe along the slotted line to maximum position and record the corresponding

distance d1 on slotted line scale.

10.Again move probe along the slotted line to next maximum position and record the

corresponding distance d2 on slotted line scale

11.Calculate the guided wavelength g as twice the distance between two successive

maximum positions.

12.Using g, c calculate 0 and frequency of the signal.

OBSERVATIONS:

Distance1d = cm

Distance2d = cm

Frequency 0f= GHz

CALCULATIONS:

Wavelengths relation is given by

0

2 2 2

1 1 1

g c

For X band frequencies and to propagate TE10 mode a= 2.286 cm

0 is free space wavelength 0o

c

f = cm

0f= GHz is frequency measured directly by frequency meter.

c = 2a= 4.472 cm

g(theoretical) =2

1

o

o

c

= cm

g (practical)= 2( 1 2d d ) cm

= cm


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PRECAUTIONS:


2. Beam voltage in Klystron power supply is first kept in min and repeller voltage at max.

3.

Care should be taken to avoid microwave radiations.4. Measurement bench is kept horizontal without any loosely coupling.

RESULT:

VIVA QUESTIONS:

1. What are the different methods of frequency measurement?

2. What is the difference between the direct frequency meter and indirect frequency

meter?

3. What is meant by critical frequency and critical wavelength?

4. What are the different wavelengths associated with waveguides?

5. Define a dominant mode.


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7. SCATTERING PARAMETERS OF MAGIC TEE

AIM: To find the Scattering Parameters of Magic Tee.

APPARATUS:









Attenuation:0-15dB1




BNC type connector

1



10 Magic Tee --- 1




required


Oscillator ---1

THEORY:

The device magic Tee is a combination of the E and H plane tees, Arm 3 is the H- arm

and arm 4 is the E-arm. If the power is fed, into arm 3 (H- arm) the electric field divides

equally between arm 1 and 2 with the same phase, and no electric field exits in arm 4. If


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power is fed in arm 4 (E- arm), it divides equally in to arm 1 and 2 but out of phase with no

power to arm 3, further, if the power is fed in arm 1 and 2 simultaneously it is added in arm 3

(H-arm) and it is subtracted in E-arm, i. e. in arm 4. The scattering matrix of lossless magic

tee is given by

Figure bellow shows the schematic of magic Tee junction.

Figure 7.1: Magic Tee

BLOCK DIAGRAM:

Figure 7.2: Set up to measure input voltage.


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Figure 7.3: Set up to measuring Scattering parameters of magic Tee

PROCEDURE:

1. Connect the components and equipments as shown in figure 7.2 and 7.3.




b. Mod- switch - AM


d. Repeller Voltage knob - fully clockwise



4. Switch ON the Klystron power supply and Cooling Fan.

5. Switch ON Beam voltage switch and rotate the beam voltage knob clockwiseslowly up to 300V meter reading and observe beam current position. The beam

current should not increase more than 30mA.

6. Change the repeller voltage slowly and observe amplitude of square wave in CRO

and modes of Klystron tube can be seen on CRO.

7. In step-1, without connecting magic tee measure input voltage as V1.


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8. In step-2, connect input to port-1 and measure output at port-2 (V2), port-3 (V3)

and port-4 (V4). Now calculate scattering coefficients21

s (=12

s ),31

s (=13

s ) and

41s (=

14s ).

9. In step-3, Connect input to port-2 and measure output at port-3(V3) and Port-

4(V4). Now calculate scattering coefficients32s (= 23s ) and 42s (= 24s ).

10. In step-4, connect input to port-3 and measure output at port-4(V4). Now calculate

scattering coefficient43

s (=34

s ) .

11. In step-5, to calculate11s , 22s , 33s and 44s by connect input to port-1 and

remaining all ports terminated by matched load. Now measure Max. Voltage and

Min. voltage and calculate standing wave ratio and reflection coefficients11s , 22s ,

33s and

44s .

OBSERVATIONS & CALCULATIONS:

Step-1: input voltage V1=

Step-2: With input to port-1 (V1= )

V2= V3= V4=

Scattering parameter 221 121

vs s

v =


vs s

v =


vs s

v =


V3= V4=

Scattering parameter3

32 23

2

v

s s v =


vs s

v =


V4=


vs s

v =

Step-5: With input to port-1


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maxv = , minv =

max

min

vS

v = ,

11 22 33 44

1

1

Ss s s s

S

=

PRECAUTIONS:1. Keep the cooling fan towards the reflex Klystron.

2. Do not look into the transmitting Horn.

3. Do not apply ever repellar voltages as zero volts.

RESULT:

VIVA QUESTIONS:

1. Explain how the power is coupled (field line) in magic Tee.

2. What are the various applications of magic Tee? What is magic in magic Tee.

3. Obtain the scattering matrix of magic Tee.

4. In magic Tee which port is called E port, and which is called H port?

5. What are the applications of MAGIC Tee.


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8. CHARACTERIZATION OF LED

AIM:

a) To plot the volt-ampere characteristics of a LED.

b) To determine the cut-in voltage, dynamic & static forward bias resistance.

APPARATUS:

THEORY:

In optical fiber communication system, electrical signal is first converted into optical

signal with the help of E/O conversion device as LED. After this optical signal is transmitted

through optical fiber, it is retrieved in its original electrical form with the help O/E

conversion device as photo detector.

Different technologies employed in chip fabrication lead to significant variation in

parameters for the various emitter diodes. All the emitters distinguish themselves in offering

high output power coupled into the plastic fiber. Data sheets for LEDs usually specify

electrical and optical characteristics, out of which are important peak wavelength of emission,

conversion efficiency (usually specified in terms of power launched in optical fiber for

specified forward current), optical rise and fall times which put the limitation on operating

frequency, maximum forward current through LED and typical forward voltage across LED.

Photo detectors usually come in variety of forms like photoconductive, photovoltaic,

transistor type output and diode type output. Here also characteristics to be taken into account

are response time of the detector, which puts the imitation on the operating frequency,

wavelength sensitivity and response.


1 Bread Board --- 1

2 DC power supply 0-15 V 1

3 Digital Ammeter 0-200 mA 1

4 Digital Voltmeter --- 1

5 LED (CQ424) 1

6 Resistor 1K, 1/2W 1

7 Single stand wires --- ---


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CIRCUIT DAIGRAM:

Figure 8.1: Set up for LED Characterization

PROCEDURE:

1. Make the connections as per circuit diagram in the figure 9.1.

2. Keep the voltage control knob at minimum and turn the regulated power supply.

3. Adjust the input voltage to zero note the Vfand Ifvalues.

4. Then increase the input voltage in steps of 0.1 V and note down the corresponding

voltmeter and ammeter readings.

5. Repeat step 4 for different values of input voltage and tabulate the readings.

6. Plot the graph forward voltage VS forward current by taking forward voltage on x-

axis and forward current on y-axis.

7. From the graph identify the cut-in voltage of diode.


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OBSERVATIONS:

S.No. Forward

Voltage (Vf) (v)

Forward

Current (If) (mA)

Electrical Power

Pi = V * I (mW)

Optical power of

LED Po = Pi *

1.15% (W)

1

2

3

4

5

6

7

8

9

10

11

12

MODEL GRAPH:

Figure 8.2: V-I Characteristics of LED


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Figure 8.3: Optical power verses Current characteristics of LED

PRECAUTIONS:

Before switching ON RPS, voltage control knob shall be kept at minimum position

and current control knob at maximum position.

CALCULATIONS:

Static Resistance = Vf/ If = K

Dynamic Resistance = Vf/ If =

RESULT:

VIVA QUESTIONS:

1. Give the characteristics of LED?

2. What are the different types of LEDs?

3. What is mean by labertian radiation pattern?

4. What are the advantages of LED compared to laser?

5. What are the applications of LED?


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9. CHARACTERIZATION OF LASER DIODE

AIM:

To study optical power (P0) of a laser diode Vs Laser diode forward current (I f).

APPARATUS:

THEORY:

An important feature of laser diodes is their ability to respond to direct high-speed

modulation.

In pulse drive operation, if the DC bias current Ib is less than threshold current, Ith

(threshold current) a time delay will result between the drive current pulse and the optical

power output pulse. Therefore, the DC bias current is normally set just above the threshold

current to obtain quick response.

In this circuit, IC U5 is use as a current comparator, which detects current flowing

through photo diode thus the APC function uses the average optical power output. Therefore,

even if the frequency of the optical power output pulse is the same, a change in the duty ratio

will cause the maximum optical power output to change.

As the forward current applied to a semiconductor laser is increased, laser oscillation

begins at a certain threshold, causing optical emission. This threshold current is called

starting current. The optical O/P power is proportional to the forward current in the region

above the starting current.

The photo diode should have a sensitivity of approx0.5mA/mW for GaAlAs laser

diode. The photo receiver module uses a standard In GaAs photo diode. Receiver saturates at

about 0.5mW(-3db)

Photo detector post amplifier IC U6 SA5214 is used as TTL amplifier. Also analog

signal can be received through photodiode is then amplified by amplifier section which uses

IC U7 TL084 and LF357 as given in the circuit diagram.


1 Optical fiber cable PMMA 1

2 Laser diode Modulation trainer (Tx) kit. =650nm 1

3 Laser diode Dmodulation trainer (Rx) kit =650nm 1

4 Digital multi meters --- 2


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BLOCK DIAGRAM:

Figure 9.1: Setup for Po VS If measurement

Figure 9.2: Internal block diagram

PROCEDURE:

1. Connect 2-meter PMMA cable Fo(cab 1) to Txunit of TNS 20EL and couple the laser

light to power meter on Rxunit as shown. Select ACC mode of operation.

2. Set DMM 2 to 2000mv range on Rxside and connect to the terminal marked P0to it.

Turn it on. The power meter is now ready for use. (P0= reading /10dbm).

3. Set DMM2 to the 200mv range and connect it between V0 and GND on the

transmitter unit. (If= V0/100).

4. Adjust the SET IF on Txknob to extreme anticlockwise direction to reduce Ifto zero.

The power meter reading will normally be below -40 dbm or out of range.


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5. Slowly turn the SET P0 knob clockwise to increase If & P0 readings. Take closer

readings prior to and above threshold.

6. Plot the graph P0VS If on a semi log graph sheet.

OBSERVATIONS:

S.N0 V0 (V) If =V0/100 (mA) P0(watts) P0(dbm)=P0(mw)/10

1

2

3

4

5

6

7

89

10

11

MODEL GRAPH:

Figure 9.3: Laser diode characteristics

RESULT:


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VIVA QUESTIONS:

1. What is the principle of laser?

2. What are the advantages of laser compared to LED?

3. What is mean by stimulated emission?

4. What are the different types of laser?5. What is mean by Quantum efficiency?


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10. MEASUREMENT OF DATA RATE FOR DIGITAL OPTICAL LINK

AIM:To study fiber optic digital link and measure Rise Time, Fall Time and Propagation

delay.

APPARATUS:

THEORY:

Transmitter:

Fiber optic links can be used for transmission of digital as well as analog signals.

Basically a fiber optic link contains three elements, a transmitter, an optical fiber & a receiver

.the transmitter module takes the input signal in electrical form & then transforms it in to

optical (light) energy congaing the same information. The optical fiber is the medium, which

carries this energy to the receiver. At the receiver, light is converted back into electrical form

with the same pattern as originally fed to the transmitter.

Fiber optic transmitters are typically composed of a buffer, driver & optical source. The

buffer provides both an electrical connection & isolation between the transmitter and the

electrical system supplying the data. The driver provides electrical power to the optical

source in a fashion that duplicates the pattern of data being fed to the transmitter. Finally the

optical source (LED) converts the electrical current to light energy with the same pattern. The

LED SFH756V supplied with the link operates within the visible light spectrum. Its optical

O/P is centered at wavelength of 660nm.

The LED SFH756V used in the link is coupled to the transistor driver in common emitter

mode. The buffer in this case is a 74HCT14-inverting gate configured as voltage follower

precedes the driver. In the absence of input signal no voltage appears at the base of the

transistor. This biases the transistor to the cut off region for the linear applications. Thus LED


1 Fiber optical Transmitter kit =660nm 1

2 Fiber optical Receiver kit =660nm 1

3 Cathode Ray Oscilloscope. 30MHz 1

4 Optical fiber cable PMMA Type 1

5 Connecting patch cords --- ---


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emits no intensity of light at this time. When the signal is applied to the input post it biases

the transistor to the active region for linear applications

Thus LED emits full intensity of light at this time. This variation in the intensity has linear

relation with the input electrical signal. Optical signal is then carried over by the optical fiber.

Receiver:

There are various methods to configure detectors to extracts digital data usually

detectors are of linear nature .We have used a photo detector SFH551V having TTL type

output. Usually it consists of PIN diode, trans impedance amplifier and level shifter.

BLOCK DIAGRAM:

Figure 10.1: Block diagram for FIBER OPTIC DIGITAL LINK


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Figure: 10.2 Fiber Optic Digital Transmitters

Figure: 10.3 Fiber Optic Digital Receivers


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MODEL WAVEFORMS:

PROCEDURE:

1. Connect one end of the optical fiber cable to LED part of FT-2106 Tx and another end

of Fiber Fo part of FT-2106 Rx. please note that minimum force should be applied

and at the same time ensure that the connector is not loosely couple to the LED.

2. Connect the NRZ encoder output to Vin on Tx side. Also connect it to CH-1 of dual

trace CRO. Connect Vo on the RX side to CH 2 of the oscilloscope.

3. Set Rinto 200 using DMM to measure resistance.

4. Now turn on the power for Tx and Rx units. The NRZ waveform should appear on

CH 1. It should be 5Khz square wave.

5. Next adjust Rthuntil waveform on Rx side is almost identical to input and draw the

Tx and Rx waveforms.

6. Now measure the time difference between the rising edges of the transmitted and

received pulses accurately. This is the RTD for the selected settings. Next measure the

transmitted and received falling edges. This is the FTD for the specific settings of the

link.

7. The larger of the two is the propagation delay for the settings of optical power, Rth

and Rin.


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8. Next adjust Rth such that RTD and FTD are equal. In this setting, PD is set optimally

for the given Rin.

9. By measuring signal time period calculate the bit rate.

OBSERVATIONS:

a) At Transmitter:

Signal amplitude: Volts (Peak to Peak)

Signal time period: msec

Signal frequency: KHz

b) At Receiver:

Signal amplitude: Volts (Peak to Peak)

Output signal time period: msec

Output signal Frequency: KHz

Propagation Delay : usec

Rise Time Distortion: usec

Fall Time Distortion: usec

Bit Rate=4*1/T= = Kbits/sec

PRECAUTIONS:

It is very important that the optical sources be properly aligned with the cable and the

distance from the launched point and the cable be properly selected to ensure that the

maximum amount of optical power is transferred to the cable.

RESULT:

VIVA QUESTIONS:

1. What are the different encoding formats?

2. What is mean by NRZ encoding format?

3. What is mean by bite rate?

4. Define rise time distortion.

5. Define fall time distortion.


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11. MEASUREMENT OF NUMERICAL APERTURE (NA)

AIM:

Measure the numerical aperture of the plastic fiber provided with the kit using 660 nm

wavelengths LED.

APPARATUS:

THEORY:

Numerical aperture refers to the maximum angle at which the light incident on the

fiber end is totally internally reflected and is transmitted properly along the fiber. The cone

formed by the rotation of this angle along the axis of the fiber is the acceptance cone of the

fiber. The light ray should strike the fiber end within its cone of acceptance, else it is

refracted out of the fiber core.

Numerical aperture gives light collecting capability or light gathering capability of the

fiber. Numerical aperture is given by

NA=SinA = 2 21 2n n =

1/2

1 2n

Where1

n is refractive index of core .

2n is refractive index of cladding.

is relative refractive index.

PROCEDURE:

1. Connect one end of the fiber to the transmitter through a connector.

2. Insert the other end of the fiber into the numerical aperture measurement jig. Adjust

the fiber such that its cut face is perpendicular to the axis of the fiber.

3. Keep the distance of about 10 mm between the fiber tip and the screen. Gently tighten

the screw and thus fix the fiber in the place.


1 Fiber optic transmitter =660nm 1

2 Fiber optic Cable (1 m) PMMA Type 1

3 Fiber holding fixture --- 14 Ruler --- 1


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4. Now observe the illuminated circular patch of light on the screen.

5. Measure exactly the distance d and also the vertical and horizontal diameters BC and

DE indicated in the block diagram.

6. Mean radius is calculated using the following formula. x = (BC + DE) / 4

7. Find the numerical aperture of the fiber using the formula. NA =

sinmax = x / d2+ x2

Where maxis the maximum angle at which the light incident is properly transmitted

through the fiber

8. Repeat the steps for different values of d compute the average value of numerical

aperture.

BLOCK DIAGRAM:

Figure 11.1: Set up for measurement of numerical aperture (NA)

OBSERVATIONS:

S.No Distance between source

and center of circular

patch AO = d (cm)

Vertical

Diameter

BC (cm)

Horizontal

diameter

DE (cm)

x = (BC + DE) / 4

NA = sinmax

= x / d2+ x2

1.

2.

3.

4 Average Numerical Aperture(NA)


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CONSIDERATIONS IN NA MEASUREMENT:

1. It is very important that the optical source should be properly aligned with the cable

and distance from the launched point and the cable is properly selected to ensure that

the maximum amount of optical power is transferred to the cable.

2. This experiment is best performed in a less illuminated room.

RESULT:

VIVA QUESTION:

1. What is mean by numerical aperture?

2. What is the significance of numerical aperture?

3. What is mean by acceptance angle?

4. Give the expression for numerical aperture.

5. Give the numerical aperture expression for graded index fiber.


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BLOCK DIAGRAM:

Figure 12.1: Block diagram for Attenuation losses

Figure 12.2: Block diagram for bending losses


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PROCEDURE:

(A) MEASUREMENT OF ATTENUATION

1. Setup 756 nm analog link using 1m fiber optic cable.

2. Feed 2Vp-p sinusoidal signal of 10 KHz, zero DC at analog Transmitter and observe

the received signal at analog Receiver on the oscilloscope.

3. Observe the received signal voltage by using CRO and Observe make it as V1

4. Repeat the same for 3m fiber cable also note the output voltage V2.

5. Calculate Cable attenuation by using the formula

2

2 1 1

10log

dB

v

L L v

(B) MEASUREMENT OF BENDING LOSSES

1.Setup 756nm analog link using 1m fiber cable.

2.Feed 2 Vp-p sinusoidal signal of 10KHz zero dc at analog in Tx& observe the

received signal frequency at analog out Rxon oscilloscope.

3.Bend the fiber in a loop as shown in figure and reduce the diameter of the loop

slowly, measure the signal received at analog out Txfor each value of the

diameter of the loop.

4.Plot the signal received (V0) VS diameter of the loop.

5. Repeat the same procedure for 3m optical fiber cable also.

(A) ATTENUATIONCALCULATIONS:

(a)For 0.5 mt cableInput voltage = Volt

Output voltage (V1) = Volt

Frequency = KHz

(b)For 1.35meter cable.Input voltage = Volt

Output voltage (V2) = Volt

Frequency = KHz

Attenuation loss = = dB


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(B) BENDING LOSSES:

OBSERVATIONS:

Input voltage (Vi) = Volt

S.No Diameter

(cm)

Output Voltage

(volts)

Loss in dB = -20 log10

( V0/ Vi)

1

2

3

4

MODEL GRAPH:

Loss in dB

Diameter in cm

PRECAUTIONS:

It is very important that the optical sources be properly aligned with the cable and the

distance from the launched point and the cable be properly selected to ensure that the

maximum amount of optical power is transferred to the cable.

RESULT:

VIVA QUESTIONS:

1. What are different attenuation mechanisms?

2. What is mean by macro bending losses?

3. What is mean by micro bending losses?

4. Give the expression for attenuation loss.

5. What is mean by absorption loss?


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13. ISOLATOR CHARACTERISTICS

AIM: To determine the Characteristics Insertion loss, Isolation and Scattering matrix of

ferrite isolator.

APPARATUS:









Attenuation:0-15dB1



BNC type connector1

8 Test Isolator

Min Isolation: 20dB;







required


Oscillator ---1

THEORY:

Ferrites are non-metallic materials with magnetic properties. Ferrites have one more

peculiar property, which is useful at microwave frequencies, i.e, the non-reciprocal property.

When two circular polarized waves one rotating clockwise and other anti clockwise is made

to propagate through ferrite, the material reacts differently to the two rotating fields, thereby


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presenting different effective permeabilities to both the waves. Isolator and circulator are two

microwave devices that make use of Faraday rotation principle.

An isolator is a two - ports device that transfers energy from input to output with little

attenuation and from output to input with very high attenuation. This is generally used

between the source and rest of the setup to avoid over loading of the source due to reflected

power

Insertion Loss:

Insertion Loss is the ratio of power supplied by a source to port 1(V1) to the power

detected in port 2 (V2) expressed in decibels and is given by.

20log (V1/V2) dB

Isolation:

Isolation is the ratio of power supplied by a source to port 2(V1) to the power detected in port

1 (V3) expressed in decibels and is given by

20log (V1/V3) dB

Since isolator is a two port device, the scattering matrix of isolator is a 2 by 2 matrix and is

given by

11 12

21 22

s sS

s s

Where 11s and 22s are reflection coefficients of port 1 and port 2 respectively.

BLOCK DIAGRAM:

Figure 13.1: Set up to measure input voltage.


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Figure 13.2: Set up to measure isolator characteristics.

PROCEDURE:

1. Connect the bench as for the block diagram.

2. Keep the Control. Knobs of Klystron power supply as below

a. Beam voltage switch - OFF

b. Mod switch - AM

c. Beam Voltage knob - Fully anticlockwise

d. Repeller Voltage - Fully clockwise

e. AM - Amplitude - Around fully clockwise

f. AM - Frequency knob - Around Mid position.

3. Adjust the beam voltage to 300 volts and beam current to less than 30 mA

4. Rotate the repellar voltage to get the perfect square wave and note the amplitude of

wave as V1 without test isolator.

5. Connect the isolator port 1 to input side and output voltage V2 is measured at port 2

using CRO.

6. Now interchange the ports i.e. connect port 2 of Isolator to input side and output

voltage V3 is measured at port-1 using CRO.

7. Calculate insertion loss = 20log (V1/V2) dB.

8. Calculate Isolation = 20log (V1/V3) dB.

9. Now calculate scattering coefficients 12s and 21s of isolator by using relevant

expressions.

10. To calculate 11s and 22s , connect input to port 1 and port 2 terminated by matched

load. Now measure Max. voltage and Min. voltage and calculate standing wave ratio

and reflection coefficients 11s and 22s


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OBSERVATIONS:

Voltage (v)

Insertion loss (dB) Isolation (dB)V1 V2 V3

CALCULATIONS:

Insertion Loss =1

2

20logv

v = dB

Isolation =1

3

20logv

v = dB

Scattering parameter 12s =3

1

v

v=

Scattering parameter 21s =2

1

v

v=

Standing wave ratio S= max

min

V

V=

Reflection coefficients11s , 22s =

1

1

S

S

=

Scattering matrix=

PRECAUTIONS:




4.

RESULT:

VIVA QUESTIONS:

1. What is the property of isolator?

2. What are the applications of isolator?

3. What is the scattering matrix for isolator?

4. What is meant by insertion loss?4. How circulator can be used as an Isolator?


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14. ENCODING METHODS FOR FIBER OPTIC DIGITAL TRANSMISSION

AIM: To study the following encoding methods used in optical fiber digital transmission.

1. Baseband or Non Return to Zero(NRZ) Transmission.

2. Return to Zero Coding (RZ).

3. Non Return to Zero Inverted (NRZI).

4. Biphase Coding.

5. Manchester Coding.

APPARATUS:

THEORY: A modulation code is defined as a rule by which a serial train of binary data(comprising ones and zeroes) is converted to a signal for transmission. Some of the

commonly used codes are listed for study in this experiment. There are a few others which

are outside the scope of this experiment.

In serial data transmission a symbol is a signal level (low or high) that is held for

length of time (symbol width or symbol time). The capacity of a channel is the symbol rate

(inverse of symbol time). This is the symbol per second or baud. Channel capacity has the

units of symbols per second or baud. Some modulation codes require several symbols per bit

of data. For examples, self clocking codes require two symbols per bit data. The various

codes are described below.

Baseband or Non Return to Zero(NRZ) : This is a level type code and is one that widely used

in serial data transmission. A 00 is low level and a 1 is a high level.

Return to Zero(RZ): This is an impulse type code where 1 is represented by a high level that

returns to zero. Its advantage is power conservation as transmission takes place only for a 1.

Non return to Zero Inverted: It is an edge type code where a 0 is represented by an edge and

a 1 is represented by an no edge.

S.No. Name of the item Specifications Quantity1 Fiber optical Transmitter kit(FT2106) =660nm 1

2 Fiber optical Receiver kit (FT2106) =660nm 1

3 Cathode Ray Oscilloscope. 30MHz 1

4 Optical fiber cable PMMA Type 1

5 Connecting patch cords --- ---


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Biphase: This is an edge type invertible self cocking code in which each bit cell starts with an

edge and a 1 is represented by no edge.

Manchester: This is a level type code in which a 1 bit cell is initially high and then has a

high to low transition in the middle of the bit cell. A 0 bit cell is initially low and has a low

to high transition in the middle of the bit cell.

BLOCKDAIGRAM:

Figure 14.1: Block diagram for fiber optic digital link


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Figure: 14.2 Fiber Optic Digital Transmitters

Figure: 14.3 Fiber Optic Digital Receivers


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PROCEDURE:

1. Connect the one end of the cable 1 to the LED port of the FT 2106-TX and other end

to the Fo port of FT 2106-RX. While connecting the cable please note that minimum

force should be applied. At the same time ensure that the connector is not loosely

coupled the receptacle.

2. Connect NRZ encoder output to Vin on the transmitter side . Also connect it to

channel 1of a dual trace oscilloscope. Connect Vo on the receiver side to channel 2 of

the oscilloscope.

3. Set Rin to 200 ohms using a DMM to measure the resistance.

4. Now turn the power on for transmitter and receiver units. The NRZ waveform should

appear on channel 1. It should be a 5KHz squire wave.

5. Adjust Rth until the waveform on channel 2, is almost identical to the NRZ.

6. Next connect to Vo to NRZ input of the decoder on the receiver side and connect the

oscilloscope channel 2 to Vout. Reset both the transmitter and receiver systems once.

Observe the decoded Vout and compare with the NRZ encoder output. Read the serial

code 1100 (this is repeated cyclically )

7. Repeat Step 6 for other waveforms one after the other , connecting the appropriate

jumper on the transmitter and receiver sides and resetting system each time. The

oscilloscope probes shall remain on the NRZ output(as this is baseband test signals

for other codes) and Vout. Match the received signals with expected waveform.

MODEL WAVEFORMS:


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Microwave & Optical Communication Lab Electronics & Communication EngineeringOBSERVATIONS:

a) Clock:Amplitude: Volts

Time Period: msec

Frequency:b) NRZ Signal:

Amplitude: volts

c) RZ Signal:

Amplitude: volts

d) NRZI Signal:Amplitude: volts

e) Biphase Signal:Amplitude: volts

f) Manchester:Amplitude: volts

RESULT:

VIVA QUESTIONS:

1. What is mean by NRZ encoding?

2.

What is mean by RZ encoding?3. What is mean by Manchester encoding?

4. What is the band width for NRZ encoding?

5. What is the bandwidth for RZ encoding?

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