ECE481-MICROWAVE AND OPTICAL COMMUNICATION LABORATORY MANUAL

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KALASALINGAM UNIVERSITY

(Kalasalingam Academy of Research and Education)

Anand Nagar, krishnankoil.

Department of Electronics and Communication Engineering

Even semester (2011-2012)

Lab manual

for

Microwave & Optical Communication Laboratory

ECE 481

IV year/VIII semester/ECE A,B&C

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ECE 481-Microwave & Optical Communication Laboratory List of Experiments Cycle-I

1. Characteristics of Reflex Klystron

2. Characteristics of Gunn Diode Oscillator

3. Radiation Pattern of Horn Antenna

4. Setting up of a Fiber Optic Analog Link

5. Time Division Multiplexing of Optical Signals

6. Characteristics of Directional Coupler

Cycle-II 7. Numerical Aperture of a Fiber

8. Microwave Magic Tee

9. Setting up of a Fiber optic Digital Link

10. Frequency Measurement.

11. VSWR and Impedance measurement

12. Measurement of Attenuation

13. Characteristics of LASER diode.

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Ex.No:1 Characteristics of Reflex Klystron

Aim:

To obtain the mode characteristics of reflex klystron

Components Required:

Klystron power supply, Reflex Klystron oscillator, Isolator, Frequency meter, Variable

attenuator, Detector mount with crystal diode, CRO and Waveguide stands. Theory:

The Reflex Klystron makes the use of velocity modulation to transform

continuous electron beam energy into microwave power. Electrons emitted from the

cathode are accelerated and passed through the positive resonator towards negative

reflector, which retards and, finally, reflects the electrons and the electron turn back

through the resonator. Suppose an RF- Field exists between the resonators, the

electrons traveling forward will be accelerated or retarded, 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 into bunches. As the electron

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

bunches passes the grid such that the electrons are slowed down by the voltage then energy

will be delivered to the resonator and the Klystron will oscillate. Fig.1.2 shows the

relationship between output power, frequency and reflector voltages.

The frequency is primarily determined by the dimensions of the 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.

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Block Diagram

Klystron Power

Supply SKPS-610

Multi Meter

Klystron Isolator Frequency meter Variable Detector Mount XF-455 mount

XI-621 Attenuator XD-451

Fig.1.1. Block diagram of Reflex Klystron

Table1.1 Observation in Reflex Klystron

Sl. No. Repeller Voltage in Volts Reading in the CRO

VSWR Mounts- 115

CRO

Fig 1.2 Model Graph

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Procedure:

1. Check the Klystron power supply by observing all the knobs are in the initial

Position. Before connecting the Reflex klystron to the power supply, switch on the

power supply keeping the front panel in “Beam Off” position. Wait for few minutes

and then change the switch to “Beam On position. The meter on the power supply

should indicate 280V which can be adjusted by beam voltage control. Bring back the

switch to “Beam Off” position and switch off the supply. Now, connect the klystron

leads to the socket output of the klystron power supply.

2. Switch ON the power supply and wait for few minutes. Turn the modulation

Switch to Internal Modulation position.

3. Set the variable attenuator to maximum attenuation.

4. Connect a CRO to the output of the diode detector.

5. Switch ON the beam voltage and check the beam current on the meter of the

power supply. The rated values are Beam voltage: 290V, Beam current: 20 to 25 mA 6. Obtain square waveform in the CRO. If there is no waveform, then decrease the

attenuation and / or beam voltage to get some waveform.

7. Keep the knob at the repeller voltage mode and for various values of Repeller

voltage, the corresponding reading in the CRO is noted.

8. Adjust the rotatable knob (micrometer type) of the frequency meter to get a DIP in

the CRO reading. The corresponding frequency of oscillation is read from the

frequency meter.

9. A graph is drawn between Repeller voltage Vs Detector output. Result:

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Ex.No: 2 Characteristics of Gunn diode Oscillator Aim:

To obtain the characteristics of Gunn Diode Oscillator


1. Gunn Power Supply GS-610

2. Gunn Oscillator XG-11

3. Isolator XI-621

4. Variable Attenuator

5. Frequency Meter XF-710

6. Matched Termination XL-400

7. Detector Mount with crystal diode

8. Oscilloscope

9. BNC Cable

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

travels 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.

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

Oscillator frequency is determined by cavity dimensions. 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 i.e. the output ratio between ON and OFF state. Gunn

diode has no junctions. InP, CdTe or GaAs materials can be used to

fabricate Gunn diode. These semi conducting materials have four energy bands and hence

suitable for establishing negative resistance characteristics which is the source for producing

sustained oscillations.

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Gunn Power CRO/ Supply Multimeter

Gunn oscillator Variable Frequency Diode Detector XG -11 Isolator XI -621 Attenuator Meter

Fig.2.1. Block diagram of Gunn Diode Oscillator

Table2.1 Observation in Gunn Diode Oscillator

Sl. No. Gunn Bias Voltage (v) Gunn Current (A) Nature of output waveform

Threshold voltage

I (mA)

Volts (V)

V-I CHARACTERISTICS OF GUNN DIODE OSCILLATOR

Fig2.2 Model graph

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Procedure:

1. Set the components as shown in Fig.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. Set the micrometer of Gunn Oscillator for required frequency of operation.

4. Switch ON the Gunn Power Supply.

5. Measure the Gunn Diode Current corresponding to the various Gunn bias voltage

through the digital panel meter and meter switch. Do not exceed the bias voltage

above 10V.

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

7. At one particular value of bias voltage, the current starts to decrease. This voltage

is called Peak voltage. At another value of bias voltage, the current again starts to

increase. The voltage is called as valley voltage.

8. Plot the voltage and current readings on the graph. Result:

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Ex.No:3 Radiation Pattern of Horn Antenna

Aim:

To measure the gain of the pyramidal horn antenna .


1. Klystron power supply

2. Reflex klystron oscillator

3. Isolator

4. Frequency meter

5. Variable attenuator

6. Diode detector

7. Horn antennas

8. Multimeter and Waveguide stands

Theory:

The transmitted power (PT) of an antenna of gain G1 and the received power (PR) of an antenna of gain G2 are related by the equation,

PR / PT = (λ0/4пS)2 G1 G2

Where S is the separation between the two antennas and λ0 is the free space wavelength. If two similar horn antennas are used then G1G2 (=G) and the equation reduces to

PR / PT= (λ0/4пS)2 G2 λ0 can be calculated by using the formula

(1/λg)2 = (1/λ0)2 _ (1/2a)2 where λg is twice the distance of separation between successive minima. Smin, the minimum distance of separation between the antennas is given by Smin =2d2/ λ0, where d(=12.5cm) is the larger dimension of the transmitting antenna.

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BLOCK DIAGRAM:

Fig.3.1.Block diagram of Horn antenna

Table3.1 Observation in Horn Antenna Transmitting power PT Receiving power PR Distance of

Gain Sl. No. (mW = Voltage*Current) (mW = Voltage(CRO) * separation ( S

(dB) Current) in cm)

Table3.2 Observation in Horn Antenna

Angle in Degrees

Sl. No.

Receiving Gain in dB

Power(Pr)(=Voltage(CRO) * Current)

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Procedure:

1. Before connecting the two antennas, obtain the square waveform in the CRO

using Setup1of the Characteristics of Reflex Klystron..

2. Note down the multimeter/CRO reading (PT).

3. Note down the frequency of oscillation (f) from the frequency meter.

4. From the value of frequency (f) calculate λ0 (=c/f) and then λg.

5. Connect the two horns H1 and H2 between the frequency meter and diode

detector.

6. Keep the distance between the two horns greater than Smin (so that the antenna

under test is in the far field of transmitting antenna) and note down the distance

of separation ‘S’ between the two horns.

7. Note the corresponding multimeter/CRO reading (PR) in the multimeter/CRO

connected to the diode detector mount without any tuning.

8. Calculate the gain using the formula Gain in dB =10*log10 (PR/PT *4пS/ λ0)

RADIATION PATTERN MEASUREMENT

1. By using angle measurement platform, for various degrees of orientation of

Horn antennas, the corresponding gain is calculated.

2. A graph is plotted with Gain Vs Angle in the Polar Plot. This gives the

radiation pattern of the horn antenna.

Result

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Ex.No:4 Setting up of a Fiber Optic Analog Link

Aim:

1. To set up an 850nm fiber optic analog link and to observe the linear

relationship between the input signal and the received signal.

2. To measure the bandwidth of the optical link.


1. Optical Fiber Trainer Kit

2. Two channel, 20MHz Oscilloscope

3. Function Generator, 1Hz-10MHz

Theory:

An analog fibre optic link is to be set up in this experiment. The preparation of

the optical fibre for coupling light into it and the coupling of the fibre to the LED and

detector are given in table 1. The LED used is 850nm LED. The fibre is a multimode

fibre with a core diameter of 1000µm.The detector is a simple PIN detector. The LED

optical power output is directly proportional to the current driving the LED.

Similarly, for the PIN diode, the current is proportional to the amount of light falling

on the detector. Thus, even though the LED and the PIN diode are non-linear

devices, the current in the PIN diode is directly proportional to the driving current of

the LED.

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Fig 4.1: Layout diagram

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Sl. No.

Identification

name

Function Location

1

P11ANALOG IN

Used to feed in analog sinusoidal 1V p-p signal

Transmitter Block

2 P32

PD1 O/P

PIN Detector signal monitoring post Optical Rx1

3 P31 Received signal with amplification Optical Rx1

4 GAIN Gain adjustment potentiometer Optical Rx1

5 SW8 Analog/Digital selection switch

should be set to ANALOG position)

6 LED1

850nm

850nm LED source Optical Tx1

7 PD1 Pin detector Optical Tx1

8 I/O1,I/O2,I/O3 Input /Output BNCs and posts

i.for feeding signal to the experimentor from generator or

ii.to observe signal from the experimentor on the oscilloscope

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Procedure: SETTING UP THE ANALOG LINK

1. Set the switch SW8 to the ANALOG position. Switch the power ON. 2. Feed a 1Vp-p sinusoidal signal at 1KHz from a function generator to the

ANALOG IN pot p11 using the following procedure: i. Connect a BNC-BNC cable from the function generator to the BNC socket

I/O3. ii. Connect the signal post I/O3 to the ANALOG IN post P11 using a patch cord. With this signal from the function generator is fed through to the ANALOG IN signal post P11 from the I/O3 BNC socket.

3. Connect one end of the 1m fiber to the LED source LED1 in the optical TX1 block. Observe the light output (red tinge) at the other end of the fiber. 4. Connect the other end of the fiber to the detector PD1 in the optical RX1 block.

The PIN detector output signal is available at P32 in the optical RX1 block. Vary the input signal driving the LED and observe the received signal at the PIN detector.

5. Plot the received signal peak-to-peak amplitude with respect to the input signal peak-to-peak amplitude. 6. Repeat the above steps using the 3m fiber instead of the 1m fibre. Plot the

received signal amplitude at the PIN detector as a function of input signal amplitude.

7. Observe that the relationship between the input electrical signal and the output electrical signal is linear. Thus the fibre optic link is a linear element.

TO MEASURE THE BANDWIDTH OF THE LINK 8. Apply a 2Vp-p sinusoidal signal at P11 and observe the output at P31. Adjust gain such that no clipping takes place. Vary the frequency of the input signal from 100Hz to 5MHz and measure the amplitude of the received signal. 9. Plot the received signal amplitude as a function of frequency [using a

logarithmic scale for frequency]. 10. Note the frequency range for which the response is flat.

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Fig 4.2: Block diagram

Result

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Ex.No:5 Time Division Multiplexing of Optical Signals

Aim:

To set up the multiplexer and de-multiplexer using optical fibre trainer, and to observe the

Simultaneous transmission of several channels using Time Division Multiplexing.


1. Optical Fibre Trainer Kit (OFT)



Theory:

OFT is as much a synchronous Time Division Multiplexing (TDM) unit as a fibre

optic communication unit. The basic multiplexer has twelve 64 kbps channels which are

time multiplexed. The multiplexed data stream is Manchester coded and the resulting TTL

bit-stream drives the LEDs. At the receiver, the TTL signal is fed to a Manchester decoder

which recovers the clock and the data.

TDM is also the basis of time-switching used today in telecom switches. While

multiplexing, say the voice signal from port 1, V1 is transmitted before V2, the voice signal

from port 2. But at the receiver, the first received signal can be fed to port 2, and the later

signal to port 1, resulting in switching between the two ports.

If an asynchronous low bit rate signal is to be inserted in a synchronous MUX, the

simplest technique is to sample the input signal using a submultiples of the MUX output

c lock. However, this gives rise to jitter in the received signal. Procedure:

1. Set the jumpers , switches and short the shorting links, as given in the table.

2. During power ON, both even and odd marker patterns at the marker generator. and

marker reference blocks will be set automatically as follows:

Even marker: 0 0 0 0 0 0 0 0 & Odd marker: X 1 X X X X X X

3. Turn ON at least one of the switches SW0-SW7 in the 8-bit data transmit block.

This ensures that the multiplexer is correctly aligned.

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Fig 5.1 layout Diagram

4. Set up a 850nm digital link by connecting LED1 in the optical TX1 block and PD1 in

the optical RX1 block using 1m optical fiber.

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Fig5.2: Transmitter of TDM-Block diagram

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Fig5.3: Receiver of TDM-Block diagram

5. Adjust the gain control until, the LEDs L0-L7 in 8-bit-data receiver block light up

corresponding to the ON positions of SW0-SW7. When the TDM is working, the

LEDs L8&L9 in the marker detection block will be OFF without any flicker .Toggle

SW0 and observe the toggling of L0. The digital link and the TDM MUX-DEMUX are

now set up.

6. Signal inputs given at the voice coder block. OFT is now being used in the loop-

back mode. The data and voice channels multiplexed on the transmit side are

demultiplexed on the receive side of the trainer.

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7. Feed a sinusoidal signal of 1KHz and 1Vp-p at post B of S1 and display it on channel

1 of the oscilloscope. Trigger the scope on channel 1. Observe the received signal at voice 1

signal post P23 on channel 2 of the scope. Vary the frequency of the input signal and observe

the received signal. Note the lower frequency cut off when the output voltage falls to 0.7Vp-p (

3dB below 1Vp-p).

8. The signal is being digitized by a CODEC at 64bits/sec, multiplexed and transmitted on

the fiber link. The received optical signal is converted to a TTL

signal and demultiplexed to obtain the transmitted signal back. The signal at P23

is the reconstructed version of the signal. The frequency response obtained is that

of the CODEC used to digitize and reconstruct the voice signal. Observe the

received signal closely on the oscilloscope. Observe that it is a step approximation

of the original signal.

9. The multiplexer also multiplexes the TTL signals controlled by switches SW0-SW7.

At the receiver, the received signal I demultiplexed and the switch inputs are

displayed at the LEDs L0-L7 respectively.

10. OFT also provides for directly feeding in two low-frequency TTL signals

instead of the static switch settings at SW7 & SW6. Now insert the TTL signal at

both P1 and P2 using two function generators.

11. Now insert the TTL signal at both P1 and P2. Observe the outputs at P21 and

P22 on channel 1 and channel 2 of the oscilloscope.

Result:

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Ex.No:6 Characteristics of Directional Coupler

Aim: To measure the Coupling Factor & Directivity of the Directional Coupler.

Components Required: Gunn power supply, Gunn oscillator, Pin modulator, Directional coupler, Variable

attenuator, Matched termination, Detector mount, CRO and Waveguide stands.

Theory:

The Directional coupler is a four port wave guide junction. It consists of a primary waveguide

and a secondary wave guide. When all ports are terminated in their characteristics impedances, there is

free transmission of power, without reflection , between port 1 and port 2 , and there is no transmission of

power between port 1 and 3 or between port 2 and 4 because no coupling exists between these two pairs of

ports. The degree of coupling between port 1 and 2 and between port 2 and 3 depends on the structure of

the coupler. The characteristics of a directional coupler can be expressed in terms of its coupling factor and

its directivity. Assuming wave is propagating from port 1 to 2 in the primary line, the coupling factor and

directivity are defined as Coupling factor (dB) = 10 log 10 (P1 / P4)

Directivity (dB) = 10 log 10 (P4 / P3)

Where P1 = power input to port 1

P3 = power output from port 3P4= power output from port 4

Primary

Secondary

Wave Guide

Wave Guide

Port 1

Port 3

Port 2

Port 4

DIRECTIONAL COUPLER

Fig: 6.1

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.Procedure:

1. Set up the components and equipments as shown in the figure.

2. Energize the microwave source for particular frequency of operation

3. Remove the multihole directional coupler and connect the detector mount of the frequency meter.

Tune the detector for maximum output.

4.Set any reference level of power on VSWR meter with the help of variable attenuator, gain control knob of VSWR meter, and note down the reading(reference level let x)

5. Insert the directional coupler as shown in second fig 3 with detector to the auxiliary port 3 and matched termination to port 2,without changing the position of variable attenuator and gain control knob of VSWR meter.

6. Note down the reading on VSWR meter on the scale with the help of range-db switch if required. Let it be Y.

7. Calculate coupling factor which will be X-Y=C(db)

8.Now carefully disconnect the detector from the auxiliary port 3 and match termination from port 2 without disturbing the set-up

9.Connect the matched termination to the auxiliary port 3 and detector to port 2 and measure the reading on VSWR meter. Suppose it is Z.

10.Compute insertion loss X-Z in db

Gunn power supply

Matched termination

Gunn oscillator

Pin modulator

Isolator

Variable attenuator

Directional coupler

VSWR meter

Detector mount

Fig:6.2

BLOCK DIAGRAM

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11.Repeat the steps from 1 to 4.

12.Connect the directional coupler in the reverse direction.i.e,port 2 to frequency meter side.Matched termination to port 1 and detector mount to port 3.Without disturbing the position of the variable attenuator and gain control knob of VSWR meter.

13.Measure and note down the reading on VSWR meter. let it be Yd.X-Yd gives isolation I(dB).

14.Compute the directivity as Y-Yd=I-C.

15.Repeat the same for other frequencies

Result:

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Ex.No:7 Microwave Magic Tee Aim:

To measure the basic parameter Input VSWR,Isolation for Magic Tee. .


Microwave Source.Isolator,Variable attenuator, Frequency meter, slotted line,

tunable probe, magic tee, matched termination, waveguide stand, detector

mount,VSWR meter .

. Theory:

Wave-guide tees are three port components. They are used to connect a branch or section of the wave

guide in series are parallel with the main wave guide transmission line for providing means of splitting

and also of combining power n a wave guide system. The two basic types of T-junctions are E-plane

(series) T and H-plane (shunt) T. these are named according to the axis of the side arm which is

parallel to the E-field or the H-field in the collinear arms.The device magic Tee is a combination of the

E and H plane tee.The basic parameters to bemeasured formagic Tee are a)Input VSWR:Value of SWR

corresponding to each port, as a load to the line while other ports are terminated in matched load.

b)Isolation The isolation between E and H arms is defined as the ratio of poer supplied by the

generator connected to the E-arm to the power detected at H-arm when side arms

terminated in matched load. Hence Isolation(db)=10 logP4/P3

BLOCK DIAGRAM

GUNN POWER SUPPLY

VARIABLE ATTENUATOR

GUNN OSCILLATOR

PIN MODULATOR

ISOLATOR MATCHED TERMINATION

DETECTOR

MOUNT

E,H AND MAGIC

TEE

dDsoD

MATCHED TERMINATION

CRO

DETECTOR MOUNT

Fig:7.1

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Procedure

VSWR Measurement of the Ports

1. Set up the components and equipments as shown in the fig.

2.Energize the microwave source for particular frequency of operation

3.Measure the VSWR of E-Arms similar to the measure ment of SWR for low and medium value.

4.Connect another arm to slotted line and terminate the other port with matched termination termination.

Measure the VSWR as above.

Measurement of Isolation

1.Remove the tunable probe and Magic Tee from the slotted line and connect the detector

mount to slotted line.

2. With the help of variable attenuator and gain control knob of VSWR meter,set any power

level in the VSWR meter. Let it P3

3. Without disturbing the position of variable attenuator and gain control knob, carefully place

the magic tee after slotted line keeping H arm connected to slotted line, detector to E-arm and matched termination to arm 1&2. Let it beP4

4.Determine the Isolation.

Result:

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Ex.No:8 Numerical Aperture of a Fiber

Aim:

To measure the numerical aperture of the given optical fiber.


Transmitter trainer kit, Receiver trainer kit, Fiber optic cable, Patch cords

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 cone of acceptance of

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

refracted out of the fiber.

Procedure:

1.Insert one end of the fibre into the Numerical Aperture Measurement unit as shown in

figure.Adjust the fibre such that its tip is 10 mm from the screen.

2.Gently tighten the screw to hold the fibre firmly in place.

3.Connect the other end of the fibre to LED2 through the simplex connector. The fibre will

project a ircular patch of red light onto the screen. Let ‘d ‘ be the distance between the fibre –

tip and the screen. Now measure the diameter of the circular patch of red light in two

perpendicular directions (BC and DE in figure).The mean radius of the circular

patch is given by

X = (DE + BC) /4

4. Carefully measure the distance‘d’ between the tip of the fibre and the illuminated screen

(OA in figure). The Numerical Aperture of the fibre is given by

NA = Sin θ = X/Square root (d2 + X2)

5. Repeat steps 3 to 6 for different values of d. Compute the average value of Numerical Aperture.

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Block Diagram

Result:

Fig:8.1

Fig:8.2

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Ex.No:9 Setting up of a Fiber Optic Digital Link

Aim:

To set up 850nm and 650 nm digital links and to measure the maximum

bit rates supportable on these links.

Components required:

1. Optical Fibre Trainer Kit (OFT)



Theory:

The OFT can be used to set up two fibre optic digital links,one at a

wavelength of 650nm and the other at 850nm. LED1,in the optical TX1 block, is an

850nm LED and LED2, in the optical TX2 block, is a 650nm LED.

PD1, in the optical RX1 block, is a PIN detector which gives a current

proportional to the optical power falling on the detector. The received signal is

amplified a nd converted to a TTL signal using a comparator. The gain control plays a

crucial role in this conversion.

PD2, in the optical RX2 block, is another receiver which directly gives out a

TTL signal. Both the PIN detectors can receive 650 nm as well as 850 nm signals,

though their sensitivity is lower at 650 nm.

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l

Fig.9.1:Layout Diagram

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Table 9.1:Interface Details

sl.no Identification Name

Function Location

1 SWB Analog/Digital selection switch(should be set to DIGITAL position)

2 LED1 850nm 850nm LED source Optical Tx1 Block

3 LED2 650nm 6 50nm LED source Optical Tx2 Block

4 PD2 Optical receiver with TTL output Optical Rx2 Block

5 PD1 Pin detector Optical Rx1 Block

6 P31 Pin detector signal after gain Optical Rx1 Block

7 JP2 PD1/PD2 receiver selec

posts B& A1 should be shorted to

select PD1

8 GAIN Gain control potentiometer Optical Rx1 Block

9 S6 coded data Manchester coded data shorting link

post A :coder output post B: Input to Tx1/Tx2/Electrical posts A&B should be shorted

Manchester block

10 S26 coded data Received Manchester coded data shorting link

PostA:Receiver output(Rx1/Rx2)

PostB:Input to decoder&clock recovery block

Posts A&B should be shorted

Decoder&clock recovery block

11 I/O1,I/O2,I/O3 Input /Output BNCs and postsfor feeding in and observing signals

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Procedure:

SETTING UP A DIGITAL LINK OF 85Onm

1. Set the switch SW8 to the digital position.

2. Connect a 1m optical fibre between LED1 and the PIN diode PD1. Remove the

shorting plugs of the coded data shorting links, S6 in the Manchester coder

block and S26 in the decoder & clock recovery block. Ensure that the shorting

plug of jumper JP2 is across the oposts B & A1 [for PD1 receiver selection]

3. Feed a TTL signal of about 20 KHz from the function generator to post B of

S6.Observe the received analog signal at the amplifier post P31 on c

hannel1of the oscilloscope.

4. Compare this signal with signal at post A of S26 and observe that the signal at

S26 is the inverted version of the signal at P31.

5. The received signal at P31 changes with gain but that of S26 does not, this is

because the P31 signal is fed to the comparator-cum-inverter circuit to give

the signal at S26. The comparator reference voltage is 0.55V and unless the

signal amplitude is greater than 0.55V, the comparator output is high. Verify

this.

6. Set the gain such that the signal at P31 is about 2V. Observe the input signal

from the function generator on channel 1 and the received TTL signal at post A

of S26 on channel 2. Vary the frequency of the input signal and observe the

output response.

7. Calculate the maximum bit rate that can be transferred on this digital link.

8. Repeat the above steps using the 3m fibre.

SETTING UP A DIGITAL LINK OF 65Onm

1. Use the 1m fibre and insert it into LED2. Observe the light output at the other

end of the fibre. The output is a bright red signal. This is because the light

output around 650 nm is the visible range.

2. The other end of the fibre should now be inserted into PD1.

3. Repeat the steps 3-7 using this new link.

4. Use the 3m fibre and set up the 650 nm digital link between LED2 and PD1

and repeat the steps from 3-7.

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Fig 9.2: Block diagram

Result:

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Ex.No:10 Frequency Measurement

Aim:

To demonstrate the relationship between Frequency(f), Wavelength in free space(λ0) and Guide Wavelength(λg).


1. Klystron power supply 2. Reflex klystron tube 3. Isolator 4. Variable Attenuator 5. Frequency meter 6. Detector Mount with crystal diode 7. Slotted Section with Tunable probe 8. Waveguide stands and CRO

Theory:

Microwave frequency is measured using a commercially available frequency counter and cavity wave meter. The frequency also can be computed from measured guide wavelength in a voltage standing wave pattern along a short circuited line by using a slotted line.

Three methods of frequency measurement are

1. Wavemeter method 2. Slotted line method 3. Down conversion method

Procedure:

1. Connect the microwave components as shown in above figure. 2. Obtain the square waveform in the CRO. 3. Replace the detector Mount by a slotted section with Tunable probe. 4. Connect the CRO to the tunable probe mounted on the slotted section and obtain the square

waveform. 5. Move the probe in the slot and measure the distance between two adjacent minima. The distance

between these minima is λg/2.

6. Calculate λ0 from the equation by substituting the value of λg. 7. Place the probe at the position of maxima in the slotted line. Adjust the rotatable knob of the

frequency meter to get a dip in the CRO. From the frequency meter reading, find out the frequency from the calibration chart. Calculate λ0 from the relation λ0=c/f where c is the velocity of electromagnetic waves in free.

8. Compare the values of λ0 obtained by using step 5 and 6.

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9. Repeat the experiment at 2 different operating frequencies of the reflex klystron. The frequency 10. of the Oscillations can be changed by tuning the turning screw fixed on the body of the klystron

clockwise . For every frequency setting, the repeller voltage is to be adjusted for maximum output.

Block Diagram

Klystron Variable Frequency With Mount Attenuator Meter

Fig 10.1. Measurement of frequency

Table 10.1 Measurement of frequency

Result:

Position of first minima(P1)

Position of second minima(P2)

λg/2=

P1~ P2

Lenth of cavity Microwave frequency

(GHz) MSR VSR P1, cm

MSR VSR P2, cm

PSR HSR PSR+HSR

mm

Klystron Power supply

Isolator CRO Diode detector

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Ex.No:11 VSWR AND Impedance Measurement

Aim:

To measure the standing wave ratio of an unknown load and determine its impedance .


1. Klystron power supply 2. Reflex klystron oscillator 3. Isolator 4. Variable Attenuator 5. Frequency meter 6. Detector Mount with crystal diode 7. Movable chart 8. Waveguide stands and CRO

Theory: VSWR Measurement: VSWR and the magnitude of voltage reflection coefficient X are very important parameters which determine the degree of impedance matching. These parameters are also used for the measurement of load impedance by the slotted line method. When a load ZL≠Z0 is connected to a transmission line, standing waves are produced. By inserting a slotted line system in the line, standing waves can be traced by moving the carriage with a tunable probe detector along the line. VSWR can be measured by detecting Vmax and Vmin in the VSWR meter : S = Vmax/Vmin. Low value of VSWR (S<20) can be measured directly from the VSWR meter. For high VSWR (S>20), the difference of power at voltage maximum and voltage minimum is large, so it would be difficult may exceed 20µA. Therefore, VSWR measurement with a VSWR meter calibrated on a square-law basis (I=kV2) will be inaccurate. Hence double minimum method is used where measurements are carried out at two positions around a voltage minimum point. Impedance measurement:

Impedance is a complex quantity, both amplitude and phase of the test signal are required to be measured. The following techniques are commonly used for impedance measurement. 1. Slotted line method 2. Impedance measurement of Reactive Discontinuity 3. Impedance measurement by Reflectometer.

Procedure: 1. Arrange the apparatus as shown in above figure. 2. After obtaining the oscillation, measure λg. 3. By noting down Vmax and Vmin by adjusting the slotted line, measure VSWR by using the formula

VSWR= (Vmax/Vmin).

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4. To find the distance of the first minima from the load, note down the position of any minima (PL) in the slotted line when the load is connected.Now remove the load and connect a fixed short at the slotted line. Now move the probe towards the short till you obtain the minima position again.Note down this position (PS). The distance between these two readings (PL~PS ) will give the value of X.

5. Draw the circle of radius equal to VSWR taking unity as the centre in the smart chart. 6. Move a distance (X/ λg.) towards the load side on the outer circumference of the smith chart and

mark it. 7. Draw a line joining the centre of the VSWR circle with this point. 8. The above line will cut the VSWR circle at a point. 9. Read the value of impedance corresponding to this point. This will give the normalized impedance. 10. Compare this value with the impedance calculated using the formula.

Block Diagram

Klystron Isolator Variable Frequency Slotted line Unknown load With mount Attenuator Meter

Fig 11.1 Measurement of VSWR and Impedance

Table 11.1 Measurement of VSWR

Load Position of first minima(P1)

cm

Position of second minima(P2)

cm

λg=2(P1~P2)

cm

Vmin

Volts

Vmax

Volts

VSWR

=V(Pmax/Pmin)

= Vmax/Vmin

Klystron power supply

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Position of minimum with load (PL) cm

Position of minimum with shorting plate (PS) cm

X=PL~PS

λg

Normalized Load ZL in Ω

Theoretical Practical Theoretical Practical

Table 11.2 Measurement of Impedance

Result:

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Ex.No:12 Measurement of Attenuation

Aim:

To measure the attenuation of a given microwave component.


1. Microwave Source 2. Fixed / Variable Attenuator 3. Waveguide detector mount 4. VSWR meter 5. CRO 6. Test Component 7. Waveguide stands.

Block Diagram

XM 251 XA 621 XA 520 XD451

SETUP 1

XM 251 XA 621 XA 520 Test XD451 component

SETUP 2


CRO

CRO

Test


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Theory:

Measurement of attenuation or calibration of a given attenuator can be effected by three methods.

1. Direct or power measuring method. 2. Substitution method. 3. Impedance measuring method. When a device or network is inserted in the transmission line, part Pr of the input signal power Pi is reflected from the input terminal and the remaining part Pi-Pr which actually enters the network is attenuated due to the non-zero loss of the network. The output signal power P0 is therefore less than Pi. Insertion Loss = Reflection Loss + Attenuation Loss

Where by definition,

Insertion Loss (dB) = 10 log P0 / Pi

Attenuation Loss (dB) = 10 log (P0 / Pi Pr )

For perfect matching, Pr = 0, and the insertion loss and the attenuation loss become the same.

Procedure:

1. Arrange the apparatus as shown in setup 1. 2. Obtain the microwave oscillation as per the procedure in basic setup. 3. Measure the peak to peak voltage V1 of the square wave in the CRO. 4. Insert the test component as shown in setup 2. 5. Now measure the peak to peak voltage V2 of the square wave in the CRO. 6. The attenuation of a given component is calculated using the formula

Attenuation A= 10 log10(V1/V2) db 7. If VSWR meter is used, then the differences in decibel reading for the two setups will give the

attenuation of the component.

Result:

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Ex.No:13 Characteristics of LaserDiode

Aim:

To study the voltage-current (V-I) characteristics of LASER

diode.


1. Fiber link - E Kit 2. Glass Fiber Cable with ST connector 3. Patch cords 4. Voltmeter 5. Ammeter 6. Power supply

Theory

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

optical signal with the help of electrical to optical conversion device as LED or LASER

diode. After this optical signal is transmitted through optical fiber, it is retrieved in its

original electrical form with the help of optical to electrical conversion device such as

photo detector. All the LASER diodes distinguish themselves in offering high output

power coupled in to the important peak wavelength of emission, conversion

efficiency, optical rise and fall times which put the limitation on operating frequency,

maximum forward current through LASER diode and typical forward voltage across

LASER diode. An important feature of LASER diodes is their ability to respond to

direct, high speed modulation.

V-I Characteristics of LASER diode:

The voltage-current (V-I) properties of GaAlAs semiconductor laser is

similar to that of silicon diodes. When a forward voltage is applied to the laser,

current starts to pass at a certain threshold voltage. This is called as the

Threshold Voltage; the threshold voltage of the GaAlAs semiconductor

laser is approx. 1.2 V, which is considerably higher than that of silicon

diodes (approx. 0.6 V) in general. Since the reverse breakdown voltage is far

lower (absolute maximum rating = 2V) than that of silicon diodes (more than

30V), care must be taken not to apply a reverse voltage exceeding this

maximum rating.

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Fig.13.1: Voltage-current characteristics of LASER diode at different temperatures

Fig.13.2: Voltage-current characteristics of LASER diode –Block Diagram.

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Procedure:

1. Confirm that the power switch is in OFF position and then connect it to the kit.

2. Make the jumper settings and connection as shown in the block diagram.

3. Insert the jumper connecting wires (provided along with the kit) in jumper

JP1, JP2 and JP3 at positions shown in the diagram.

4. Connect the ammeter and voltmeter with the jumper wires connected to JP1,

JP2 and JP3 as shown in the block diagram.

5. Keep the potentiometer P5 in anticlockwise rotation is used to control

intensity of laser diode.

6. Connect external signal generator to ANALOG IN post of analog buffer and

apply sine wave frequency of 1MHz, 1Vp-p signal precisely.

7. Then connect ANALOG OUT post to ANALOG IN post of transmitter.

8. Then switch ON the power supply. To get the V-I characteristics of LASER

diode, rotate Pr5 slowly and measure forward current and corresponding

forward voltage at JP1,JP2 and JP3 respectively.

9. Take number of such readings for various current values and plot V-I

characteristics graph for the LASER diode. . When a forward voltage is applied

to the laser, current starts to pass at a certain threshold voltage. This is called

as the Threshold Voltage.

Result:

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ECE481-MICROWAVE AND OPTICAL COMMUNICATION LABORATORY MANUAL

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