JUMPER SETTING DIAGRAM: 1 VEC/2011-12/ODD/ECE/VII/EC-2405 FIBER OPTIC CABLE P 2 D R I V E R FIBER OPTIC TRANSMITTER SFH 756V TX1 SCREW P1 BIAS INTENSITY A OA=d r = (MR+PN)/4 N M P O R SCREEN ILLUMINATED CIRCULAR PATCH +5V JP 5 +12V JP 6 JP 8 TX SW 8 VI TX 1 SW 9 TX 2
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JUMPER SETTING DIAGRAM:
1 VEC/2011-12/ODD/ECE/VII/EC-2405
FIBER OPTIC CABLE
P2
DRIVER
FIBER OPTIC TRANSMITTER
SFH 756V TX1
SCREW
P1
BIAS
INTENSITY
A OA=dr = (MR+PN)/4
N
M
PO
RSCREEN
ILLUMINATED CIRCULAR PATCH
+5V
JP 5
+12V
JP 6
JP 8 TX
SW 8
VITX 1
SW 9TX 2
1. STUDY OF NUMERICAL APERTURE OF OPTICAL
FIBERAIM:
To measure the numerical aperture of the plastic fiber provided with
kit using 660 nm wavelength LED.
APPARATUS REQUIRED:
S.No. Name of the Equipments Quantity
1. Link – B Advance Fiber Optic
Communication Trainer Kit1
2. Power Supply 1
3. Fiber Optic Cable (Plastic) 1 meter
4. Numerical Aperture measurement
Jig 1
5. Steel Ruler 1
6. Patch Chords Required
THEORY:
Numerical aperture refers to the maximum angle at the light incident
on the fiber end is totally internal 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 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 is the measure of the power launching efficiently of an
optical fiber. When N.A. is small, then the light available from various
2 VEC/2011-12/ODD/ECE/VII/EC-2405
directions from the source, only a portion of light is accepted by an optical
fiber and the remaining is rejected.
OBSERVATION:
d
(mm)
MR
(mm)
PN
(mm)
r
(mm)NA
r = (MR+PN)
4
NA = sin θ max =
3 VEC/2011-12/ODD/ECE/VII/EC-2405
PROCEDURE:
1. Make connections as shown in figure. Connect the power supply cables
with proper polarity to Link – B Kit. While connecting this, ensure that
the power supply is OFF.
2. Keep Intensity control pot P2 towards minimum position.
3. Keep Bias control pot P1 fully clockwise position.
4. Switch ON the power supply.
5. Slightly unscrew the cap of SFH 756V (660) nm. Do not remove the cap
from the connector. Once the cap is loosened, insert the 1 Meter Fiber
into the cap. Now tighten the cap by screwing it back.
6. 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.
7. Keep the distance of about 5mm between the fiber tip and the screen.
Gently tighten the screw and thus fix the fiber in the place.
8. Increase the intensity pot P2 to get bright red light circular patch.
9. Now observe the illuminated circular patch of light on the screen.
10.Measure exactly the distance d and also the vertical and horizontal
diameters MR and PN as indicated in the Figure.
11.Mean radius is calculated using the following formula
r = (MR+PN)/4.
12.Find the numerical aperture of the fiber using the formula
NA = sin θ max = 4 VEC/2011-12/ODD/ECE/VII/EC-
2405
Where θ max is the maximum angle at which the light incident is
properly transmitted through the fiber.
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RESULT :
Thus Numerical aperture of the plastic fiber provided with kit using
660 nm wavelength LED was measured as ________.
7 VEC/2011-12/ODD/ECE/VII/EC-2405
REVIEW QUESTIONS:
1. Define Numerical Aperture.
2. What is V number?
3. What is the need for cladding?
4. Define the refractive index of a medium.
5. State snell’s law.
8 VEC/2011-12/ODD/ECE/VII/EC-2405
9 VEC/2011-12/ODD/ECE/VII/EC-2405
2. FIBER OPTIC COMMUNICATION LINK
AIM:
To obtain the transmitted analog or digital signal in the fiber optic
receiver using optical fiber/glass fiber.
APPARATUS REQUIRED:
S.No. Name of the Equipments Quantity
1. Link – A Fiber Optic Trainer Kit 1
2. Fiber Optic Cable (Plastic) 1 meter
3. Power Supply 1
4. 20 MHz Dual Channel Oscilloscope 1
5. Probe, Patch Chords Required
THEORY:
ANALOG LINK
Fiber Optic Links can be used for transmission for digital as well as
analog signals. Basically a fiber optic link contains three main elements, a
transmitter, an optical fiber & a receiver. The transmitter module takes the
input signal in electrical form & then transforms it into optical (light) energy
containing 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.
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11 VEC/2011-12/ODD/ECE/VII/EC-2405
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TRANSMITTER:
Fiber optic transmitters are typically composed of a buffer, driver &
optical source. The buffer electronics provides both an electrical connection
& isolation between the transmitter & the electrical system supplying the
data. The driver electronics 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 SFH 756V supplied with the kit
operates inside the visible light spectrum. It’s optical output is centered at
near visible wavelength of 660 nm. The emission spectrum is broad, so a
dark red glow can usually be seen when the LED is on. The LED SFH 450V
supplied wit the kit operates outside the visible light spectrum. It’s optical
output is centered at near infrared wavelength of 950 nm.
RECEIVER:
The function of the receiver is to convert the optical energy into
electrical form which is then conditioned to reproduce the transmitted
electrical signal in its original form. The detector SFH250V used in the kit
has a diode type output. The parameters usually considered in the case of
detector are it’s responsivity at peak wavelength & response time. SFH250V
has responsivity of about 4µA per 10µW of incident optical energy at 950
nm and it has rise & fall time of 0.01µsec. PIN photodiode is normally
reverse biased. When optical signal falls on the diode, reverse current start to
flow, thus diode acts as closed switch and in the absence of light intensity, it
acts as an open switch. Since PIN diode usually has low responsivity, a trans
impedance amplifier is used to convert this reverse current into voltage. This
13 VEC/2011-12/ODD/ECE/VII/EC-2405
voltage is then amplified with the help of another amplifier circuit. This
voltage is the duplication of the transmitted electrical signal.MODEL GRAPH:
FIBER OPTIC COMMUNICATION - ANALOG LINK:
FIBER OPTIC COMMUNICATION – DIGITAL LINK:
14 VEC/2011-12/ODD/ECE/VII/EC-2405
Am
plitu
de
Time Period
INPUTVoltage (V)
Time (ms)
Am
plitu
de
Time Period
OUTPUTVoltage (V)
Time (ms)
Am
plitu
de
Time Period
INPUT
Voltage (V)
Time (ms)
Am
plitu
de
Time Period
OUTPUT
Voltage (V)
Time (ms)
DIGITAL LINK
In the experiment no. 1, we have seen how analog signal can be
transmitted and received using LED, fiber and detector. The same LED,
fiber and detector can be configured for the digital applications to transmit
binary data over fiber. Thus basic elements of the link remains same even
for digital applications.
TRANSMITTER:
LED digital, DC coupled transmitters are one of the most popular
variety due to their case of fabrication. Standard TTL gate is used to drive a
NPN transistor, which modulates the LED SFH450V OR SFH756V source
(Turns it ON and OFF).
RECEIVER:
There are various methods of configure detectors to extract digital
data.
Usually detectors are of linear nature. Photodector SFH551V has TTL
type output. Usually it consist of PIN photo diode, transimpedance
amplifier and level shifter.
15 VEC/2011-12/ODD/ECE/VII/EC-2405
OBSERVATION:
ANALOG LINK:
AMPLITUDE
(volts)
TIME
(seconds)
INPUT
OUTPUT
DIGITAL LINK:
AMPLITUDE
(volts)
TIME
(seconds)
INPUT
OUTPUT
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17 VEC/2011-12/ODD/ECE/VII/EC-2405
PROCEDURE:
FIBER OPTIC COMMUNICATION - ANALOG LINK:
1. Slightly unscrew the cap of LED SFH 756V TX1 (660 nm) from kit.
Do not remove the cap from the connector. Once the cap is loosened,
insert the fiber into the cap and assure that the fiber is properly fixed.
Now tight the cap by screwing it back. Keep INTENSITY pot P3 at
minimum position i.e. fully anticlockwise.
2. Make the connections and Jumper settings as shown in Figure.
Connect the power supply cables with proper polarity to kit. While
connecting this, ensure that the power supply is OFF.
3. Switch on the power supply.
4. Select the frequency range of Function Generator with the help of
Range Selection Switch SW1, frequency can be varied with Pot P2.
Adjust the voltage LEVEL of the Sine Wave with Pot P1 as per
following setting FREQUENCY: 1 KHz, LEVEL: 2Vp-p.
5. Connect SINE post of the Function Generator section to IN post of
Analog Buffer Section.
6. Connect OUT post of the Analog Buffer Section to TX IN post of
Analog Buffer Section.
7. Connect the other end of the fiber to detector SFH 250V (RX 1) in kit
very carefully as per the instructions in step 1.
8. Check the output signal of the Analog Buffer at its OUT post in Kit.
It should be same as that of the applied input signal.
9. Observe the output signal from the detector at ANALOG OUT post
on CRO by adjusting INTENSITY (Optical Power Control) Pot P3 in
18 VEC/2011-12/ODD/ECE/VII/EC-2405
kit and you should get the reproduction of the original transmitted
signal.
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FIBER OPTIC COMMUNICATION – DIGITAL LINK:
1. Slightly unscrew the cap of LED SFH 756V TX1 (660 nm) from kit.
Do not remove the cap from the connector. Once the cap is loosened,
insert the fiber into the cap and assure that the fiber is properly fixed.
Now tight the cap by screwing it back.
2. Make the connections and Jumper settings as shown in Figure.
Connect the power supply cables with proper polarity to kit. While
connecting this, ensure that the power supply is OFF. Now Switch on
the power supply
3. Feed the Onboard Square (TTL) signal of about 1 KHz to IN post of
Digital Buffer Section and observe the signal at OUT post. It should
be same as that of the input signal.
4. Connect OUT post of the Digital Buffer section to TX IN post of
TRANSMITTER.
5. Connect the other end of the fiber to detector SFH 551V RX 2
(Digital Detector) in kit very carefully as per the instructions in step 1.
6. Observe the output signal from the detector at TTL OUT post on
CRO. The Transmitted signal and received signal are same. Vary the
frequency of the input signal and observe the output response.
20 VEC/2011-12/ODD/ECE/VII/EC-2405
RESULT:
Thus the transmitted analog or digital signal in the fiber optic receiver
using optical fiber (plastic) was done and has been verified.
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REVIEW QUESTIONS:
1. What is the optical frequency range?
2. Name the basic principle used in optical fiber communication.
3. Give four advantages of optical fiber communication.
4. Based on the modes, what are the classifications of fiber?
5. Based on the refractive index profile, how the fiber is classified.
22 VEC/2011-12/ODD/ECE/VII/EC-2405
BLOCK DIAGRAM:
JUMPER SETTING DIAGRAM:
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YELLOWWHITE
V
JP16JP17
1
2
3
4
1
2
3+
_
+_
BLACK RED
+5V
+9V
SFH 756V ANODE
Emitter of Q3 (2N2907)
Cathode of SFH 756V
Collector of Q1 (2N3904)
Cathode of SFH 450V
FIBER OPTIC CABLE
Pr10
DRIVER
FIBER OPTIC TRANSMITTER
SFH 756V
FIBER OPTIC RECEIVER
ANALOG OUT
SFH350V
DETECTOR
Pr10 Terminal
JP18
GND
Emitter of Q1 (2N3904)
1 2 3
Amplifier Output
JP20
Digital Buffer Output
Base of Q1 (2N3904)
1 2 3
3. V-I CHARACTERISTICS OF FIBER OPTIC LED &
PHOTO DETECTORAIM:
To study the characteristics of fiber optic LED and plot the graph of
forward current Vs output optical energy and also to the study the photo
detector response
APPARATUS REQUIRED:
S.No.Name of the
EquipmentsRange Quantity
1.
Link – B Fiber Optic
Communication
Trainer Kit
- 1
2. Power Supply - 1
3.Fiber Optic Cable
(Plastic)- 1 meter
4. Voltmeter (0-5)V, (0-10)V Each 1
5. Ammeter (0-50)mA 1
6.Connecting Wires,
Patch Chords- Required
24 VEC/2011-12/ODD/ECE/VII/EC-2405
MODEL GRAPH:
I vs. V Characteristics of SFH 756 V
CHARACTERISTICS OF OPTICAL POWER OF LED vs. If:
25 VEC/2011-12/ODD/ECE/VII/EC-2405
I (amp)(I vs. V)
V (volt)
PO (µW)(PO vs. If)
If (mA)
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 retrived in
its orginial electrical form with the help O/E conversion device as
photodetector.
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
important peak wavelength of emission, conversion efficiency usually
specified in terms of power launched in optical fiber peak wavelength of
emission,optical raise and fall times which put the limitation on operating
frequency, maximum forward current through LED and typical forward
voltage across LED.
Photodetectors usually comes in variety of forms
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 limitation on the operating frequency,
wavelength sensitivity and responsivity.
26 VEC/2011-12/ODD/ECE/VII/EC-2405
OBSERVATION:
CHARACTERISTICS OF LED:
LED voltage
Vd
(V)
LED Current
Id
(mA)
Optical power Of LED
Pd
(dBm)
Optical power Of LED
Pd
(mW)
TO CONVERT dBm TO Mw
CHARACTERISTICS OF DETECTOR: R= 100 Ω
LED Current
Id
(mA)
Optical power Of
LED
Pd
(mW)
Detector voltage
Vp
(V)
Detector Current
Ip
(mA)
27 VEC/2011-12/ODD/ECE/VII/EC-2405
PROCEDURE:
1. Confirm that the power switch is on OFF position.
2. Make the jumper settings as shown in the jumper diagram.
3. Insert the jumper connecting wires (provided along with the kit) in
jumper JP17 and JP16 at positions shown in figure.
4. Connect the ammeter and volt-meter with the jumper wires connected
to JP17 and JP16 as shown in figure.
5. Keep the potentiometer Pr10 in its maximum position (anti-
clockwise rotation) and Pr9 in its minimum position (clockwise
rotation). Pr10 is used to control current flowing through the LED and
Pr9 is used to vary the amplitude of the received signal at
phototransistor.
6. To get the VI characteristics of LED, rotate Pr10 slowly and measure
forward current and corresponding forward voltage. Take number of
such readings for various current values and plot VI characteristics
graph for the LED.
7. For each reading taken above, find out the power which is product of I
and V. This is the electrical power supplied to the LED.
28 VEC/2011-12/ODD/ECE/VII/EC-2405
8. With this efficiency assumed, find out optical power coupled into
plastic Optical Fiber for each of the reading in step7. Plot the graph of
forward current vs. output optical power of the LED.
9. In our experimental kit, when Pr9 is at its minimum position, 100
ohms of resistance is in series of emitter and ground of
phototransistor.
10.Connect the 1m optical Fiber Cable supplied with the kit between
LED SFH 756V (660nm) and phototransistor SFH 350V (Analog
Detector).
29 VEC/2011-12/ODD/ECE/VII/EC-2405
11.From the transfer characteristics obtained in step 8, launched known
optical energy into plastic fiber and measure output voltage at
ANALOG OUTPUT TERMINAL. Find out the current flowing
through phototransistor with this voltage value and 100 ohms of
resistance.
12.Repeat step11 for various launched Optical energy values and plot the
graph for the responsivity of phototransistor. Find out the portion
where detector response is linear.
30 VEC/2011-12/ODD/ECE/VII/EC-2405
RESULT :
Thus the characteristics of fiber optic LED and photo detector was
studied and has been verified .
31 VEC/2011-12/ODD/ECE/VII/EC-2405
REVIEW QUESTIONS:
1. Name two optical Sources.
2. What is the function of optical source?
3. Give the advantages of LED.
4. Define internal quantum efficiency for LED.
5. What is population inversion?
6. Define the modulation bandwidth of LED.
32 VEC/2011-12/ODD/ECE/VII/EC-2405
33 VEC/2011-12/ODD/ECE/VII/EC-2405
4. V-I CHARACERISTICS OF GUNN DIODE
AIM :
To study the V-I Characteristics of Gunn Diode.
COMPONENTS REQUIRED:
i. Gunn power Supply
ii. Gunn oscillator
iii. PIN modulator
iv. Isolator
v. Frequency Meter
vi. Variable Attenuator
vii. Detector Mount
viii. CRO
ix. Bayonet Neill Concelman(BNC) Connector
x. Threaded Neill Concelman(TNC) Connector
xi. Cooling Fan
xii. Waveguide Stand, Screw & Net
THEORY :
Gunn diodes are negative resistance device which are normally used
as low power oscillator at microwave frequencies in transmitter and as local
oscillator in receiver front end. J.B. Gunn in 1963 discovered microwave
oscillation. At low electric field in the material most of the electron will be
located in the lower central valley. At high electric field most of the electron
will be transferred in to the higher frequency satellite L and X valleys.
34 VEC/2011-12/ODD/ECE/VII/EC-2405
OBSERVATION :
MODEL GRAPH:
35 VEC/2011-12/ODD/ECE/VII/EC-2405
S.NoVoltage
(V)
Current
(mA)
I (amp)
THRESHOLD VOLTAGE
V (volt)
PROCEDURE:
1. Set the components as shown in block diagram.
2. Keep the control knobs of Gunn power supply (GPS) as below.
Meter Switch – off
Gunn bias knob – Fully anticlockwise
PIN Mod. Amp knob – Mid position
PIN Mod. Freq. knob – Mid position
3. Switch ON the Gunn power supply, VSWR meter and Cooling fan.
Set Gunn bias Voltage at 7.5V.
4. Set the micrometer of Gunn oscillator for required frequency of
operation.
5. Measure the operating frequency using frequency meter.
6. Measure the Gunn Diode Current corresponding to the various
Gunn bias voltage. Do not exceed the bias voltage above 10 volts.
7. Plot the voltage Vs Current and measure the threshold voltage
which corresponds to maximum current.
NOTE:
Do not keep gun bias knob position at threshold position for more
than 10-15 seconds reading should be obtained as fast as possible.
Otherwise due to excessive heating, Gunn diode may burn
RESULT:
Thus the V-I characteristics of Gunn Diode was studied.
Threshold voltage, Vth = Volts
36 VEC/2011-12/ODD/ECE/VII/EC-2405
37 VEC/2011-12/ODD/ECE/VII/EC-2405
REVIEW QUESTIONS:
1. Define Attenuation.
2. What are the types of attenuator?
3. Why isolators are called uniline?
4. Define Gunn Effect.
5. What is negative resistance in Gunn diode?
6. Name the semiconductor used in Gunn diode
7. What is transferred electron effect?
38 VEC/2011-12/ODD/ECE/VII/EC-2405
39 VEC/2011-12/ODD/ECE/VII/EC-2405
5. FREQUENCY AND WAVELENGTH
MEASUREMENT
AIM :
To determine the frequency and wavelength in a rectangular
waveguide working in TE10 mode.
COMPONENTS REQUIRED:
i. Klystron power Supply
ii. Klystron tube with mount
iii. Isolator
iv. Frequency Meter
v. Variable Attenuator
vi. Detector Mount
vii. CRO
viii. Bayonet Neill Concelman(BNC) Connector
ix. Cooling Fan
x. Waveguide Stand, Screw & Net
THEORY:
For dominant TE10 mode in rectangular waveguide λ0, λg and λc are
related as below
1/ λ0 2 = 1/ λg
2 + 1/ λc 2
Where, λ0 = free space wavelength
λg = guide wavelength
λc = cutoff wavelength
40 VEC/2011-12/ODD/ECE/VII/EC-2405
CALCULATION:
Guided Wavelength λg = 2d = cm.
Cut off Wavelength λc = 2a = cm.
Where a = 22.8 mm (Broader Dimension of the rectangular
waveguide)
λ = [ (1/ λg)2+(1/λc)2 ] -1/2 cm
f = c/λ GHz.
Where c = 3×10 10 cm.
OBSERVATION:
Frequency (GHz) Probe Position (cm)Successive Difference
(cm)
d1= d2 - d1
d2= d3 - d2
d3= Avg (d ) =
41 VEC/2011-12/ODD/ECE/VII/EC-2405
For TE10 mode , λc = 2a,
a = broader dimension of waveguide
The following relationship can be proved,
c = f λ
c = velocity of light
f = frequency of oscillation
INITIAL ADJUSTMENTS:
1. Keep the variable attenuator in the minimum attenuation position.
2. Keep the control knob of klystron power supply as below, before
switching ON the device.
Beam voltage = OFF
Mod-switch = AM
Beam voltage knob = Fully anticlockwise
Repeller voltage knob = Fully anticlockwise
AM frequency & Amplitude knob = mid position
FM frequency & Amplitude knob = minimum position
PROCEDURE:
1. Set the components as shown in Block diagram.
2. Keep the control Knobs of klystron Power supply as mentioned in
the basic set up.
3. Switch ON the Klystron power supply and set the beam voltage at
250 volts.
4. Adjust the repeller Voltage (120V) to get maximum output in
CRO.
42 VEC/2011-12/ODD/ECE/VII/EC-2405
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5. Tune the frequency meter knob to get a dip on CRO and note down
the frequency of oscillation directly. Detune the frequency meter.
6. Move the probe along the slotted line to a minimum output
voltage.
7. Record the probe position and let it be d1.
8. Move the probe to a next minimum position and note it as d2.
9. Calculate the Wavelength and Frequency.
10.Verify the calculated Frequency with the Frequency obtained from
Frequency meter.
RESULT:
44 VEC/2011-12/ODD/ECE/VII/EC-2405
Thus the Frequency and Wavelength in a rectangular Waveguide was
determined.
Freq = GHz , Wavelength= cm.
45 VEC/2011-12/ODD/ECE/VII/EC-2405
REVIEW QUESTIONS:
1. Give the frequency range for X, J, S- band.
2. What is an isolator?
3. Why TE01 cannot be considered as the dominant mode in
rectangular waveguide?
4. Why S-matrix is used in microwave analysis?
5. What are standing waves?
6. What is a dominant mode?
46 VEC/2011-12/ODD/ECE/VII/EC-2405
47 VEC/2011-12/ODD/ECE/VII/EC-2405
6. DETERMINATION OF TERMINATED IMPEDANCE
AIM:
To measure the impedance of the unknown microwave component
COMPONENTS REQUIRED:
i. Gunn power Supply
ii. Gunn oscillator
iii. PIN modulator
iv. Isolator
v. Frequency Meter
vi. Variable Attenuator
vii. Slotted line section
viii. Slide screw tuner(SST)
ix. Tunable probe
x. VSWR
xi. CRO
THEORY:
The impedance at any point of a transmission line can be written in the form
R + jX. For comparison SWR can be calculated as
S = (1 + ρ) (1 – ρ)
ρ = reflection coefficient = [Z – λg] / [Z + λg]
48 VEC/2011-12/ODD/ECE/VII/EC-2405
OBSERVATION: Operating Frequency = ____________GHz
WITH LOAD WITHOUT LOAD
λg = 2(d2-d1) dmin=(d1-do)/ λgSWR
do
(cm)
d1
(cm)
d2
(cm)
Unknown impedance= Zo × normalized impedance
Where Zo= 50 ohms
Where fc=c/2a, fo = Frequency of oscillation, η=377
, ,
The normalized impedance was calculated using smith chart
From smith chart ΦL=_________
Practical:
ZL/Zo=___________
Theoretical:
ZL/Zo=___________
Z is the impedance at any point. The measurement is performed in the
following way.
49 VEC/2011-12/ODD/ECE/VII/EC-2405
The unknown device is connected to the slotted line and the SWR value and
position of one minima is determined.Then unknown device is replaced by movable
short to the slotted line. Two successive minima positions are noted. The twice of
the difference between minima position will be guide wavelength. One of the
minima is used as reference minima and minima position obtained from the
unknown load. Let it be do. Take a smith chart, taking ‘1’ as centre, draw a circle of
radius equal to SWR value. Mark a point on circumference of chart towards load
side at a distance equal to λg. Join the centre with this point. Find the point where it
cuts the drawn circle. The co-ordination of this point will show the normalized
impedance of the load.
INITIAL SETUP IN VSWR METER:
1. Set input selector switch in 200 Ohms.
2. Keep meter selector in Normal.
3. Select the range as 50db or 40db or 30db and then vary the gain knob
(fine and coarse) to get minimum attenuation. (VSWR = 1).
PROCEDURE:
1. Set the components as shown in block diagram.
2. Keep the control knobs of Gunn power supply (GPS) as below.
Meter Switch – off
Gunn bias knob – Fully anticlockwise
PIN bias knob - Fully anticlockwise
PIN mode frequency – Any position
50 VEC/2011-12/ODD/ECE/VII/EC-2405
3. Switch on the GPS. Set the Gunn bias voltage at 7.5V
4. Set the micrometer of Gunn oscillator for required frequency of operation.
51 VEC/2011-12/ODD/ECE/VII/EC-2405
5. Measure the operating frequency using frequency meter.
6. Then remove the CRO and connect the VSWR meter to slotted line
section.
7. Initial setup in VSWR meter has to be done without load.
8. Keep the depth of SST to around 3 to 4 mm and lock it.
9. Move the probe along the slotted line to get maximum deflection.
10.Adjust the gain control knob and variable attenuator until the meter
indicates ‘1’ on the normal db SWR scale.
11. Move the probe to next minimum position and note down the SWR value
on the scale. Also note down two successive minimum position. Let it be
‘d1’ and ‘d2’.
12. Find out the normalized impedance using Smith chart.
SWR MEASUREMENT:
A. If the reading at the minimum is lower than 3 on the top scale, set
RANGE Switch to next higher range and read the indication on the
second SWR or (3 to 10) scale of SWR.
B. If the range switch is changed by two steps used top SWR scale,
however all indication on this scale must be multiplied by 10.
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RESULT:
Thus the impedance of the unknown microwave component was measured.
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REVIEW QUESTIONS:
1. What is a VSWR meter?
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2. Give two differences between transmission line and waveguide.
3. Name two advantages of rectangular waveguide over circular waveguide
4. What is the function of an Isolator?
5. What is the basic principle used in Isolator?
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7. MEASUREMENT OF PROPAGATION LOSS AND BENDING
LOSS IN THE FIBER
AIM:
To Measure propagation loss and bending loss in the fiber.
APPARATUS REQUIRED:
S.No. Name of the Equipments Quantity
7.Link – D Fiber Optic
Communication Trainer Kit1
8. Power Supply 1
9. Fiber Optic Cable (Plastic) 1,4 meter
10. Patch Chords Required
THEORY:
Losses are introduced in fiber due to various reasons. As light propagates
from one end of fiber to another end, part of it is absorbed in the material exhibiting
absorption loss. Also part of the light is reflected back or in some other direction
from the impurity particles present in the material contributing to the loss of the
signal at the other end of the fiber. In general terms it is known as propagation loss.
Plastic fibers have higher loss of the order of 180 dB/Km. Whenever the condition
for angle of incidence of the incident light is violated the losses are introduced due
to refraction of light. This occurs when fiber is subjected to bending. Lower the
radius of
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curvature more is the loss. Another losses are due to the coupling of fiber at LED &
photo detector ends.
Although fibers are good at bending, each time the fiber is bent, a little light
lost.
PROCEDURE:
FOR PROPAGATION LOSS:
1. Make jumper connections as shown in jumper block diagram. Connect the
power supply cables with proper polarity to Link – D Kit. While connecting this,
ensure that the power supply is OFF.
2. Connect the AMP O/P as a constant signal to the TX I/P using a patch cord.
3. You will measure the light output using the SIGNAL STRENGTH section of the
kit. The loss will be larger for a longer piece of fiber, so you will measure the
loss of the long piece of fiber. In order to measure the loss in the fiber you first
need a reference of how much light goes in to the piece of fiber from the LIGHT
TRANSMITTER. You will use the short piece of fiber to measure this
reference.
4. Switch on the power supply. Connect the short piece of fiber to between the
transmitter TX and the receiver RX2 of the kit. Adjust the transmitter level until
the signal strength reads 6. This will be your reference value. Now connect the
long piece of fiber instead of the short piece. What reading do you get? Loss in
optical fiber systems is usually measured in dBs. Loss of fiber itself is measured
in dBs per meter.
Subtract the length of the short fiber from the length of the long fiber to get the
difference in the fiber lengths (4m-1m). The extra length of three
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OBSERVATION FOR PROPAGATION LOSS:
S.No: Length of the Fiber Signal Strength
1. 1 m
2. 4 m
FORMULA:
POWER = 10 log (P2/P1) dB
Where P2 : Reference reading by 1 meter fiber
P1 : Reading obtained after replacing the fiber.
Fiber loss (dB/m) = Power /Difference in fiber length
= (dB/m)
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meters is what created the extra loss you measured. Then take the signal strength
reading you obtained for the loss of the long fiber and convert it to dB using the
Equation. Finally divide the dB reading by the length to get the loss in dB per
meter.
The reason for converting to dB per meter is that now in order to find the loss
of any length of fiber you just have to multiply the dB per meter by the length of
the fiber. For e.g. If you have a 10 meter long piece of fiber the loss will be
0.6 dB per meter * 10 meters = 6dB
FOR BENDING LOSS:
1. Make jumper connections as shown in jumper block diagram. Connect the
power supply cables with proper polarity to Link – D Kit. While connecting this,
ensure that the power supply is OFF.
2. Connect the AMP O/P as a constant signal to the TX I/P using a patch cord.
You will measure the light output using the SIGNAL STRENGTH section of the
kit.
3. Switch ON the power supply. Connect the long piece of fiber to between the
light transmitter TX and the photo detector receiver RX2 so there are no sharp
bends in the fiber between them.
4. Adjust the transmitter power so that the SIGNAL STRENGTH reading is 6.
Now take the portion of the fiber and loop it to match the bends as shown in a
diagram. As you match each bends write down the reading
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OBSERVATION FOR BENDING LOSS:
S.No: Bending Diameter in cm Signal Strength
1.
2.
3.
4.
5.
BEND vs SIGNAL STRENGTH:
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NO BEND
BEND 1
BEND 2
BEND 3
BEND 4
SIGNAL STRENGTH
from SIGNAL STRENGTH indicator. What happens as bends the fibers? Don’t
bend the fiber too tightly or it may not come back to shape.
5. If you were designing the fiber optic communications system, you would need
to known the relationship between the size of the bend and the light loss from
the bend.
RESULT:
Thus the fiber propagation and bending loss was studied.
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REVIEW QUESTIONS:
1. When bending loss (or) Radiative loss occurs?
2. Name the two types of bending loss.
3. What is Intermodal Dispersion?
4. What is Intramodal Dispersion?
5. What is bandwidth – distance product?
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8. MEASUREMENT OF BIT ERROR RATE
AIM:
To Measure bit error rate.
APPARATUS REQUIRED:
S.No. Name of the Equipments Quantity
11.Link – B Advance Fiber Optic
Communication Trainer Kit1
12. Power Supply 1
13. Fiber Optic Cable (Plastic) 1 meter
14.20 MHz Dual channel
Oscilloscope1
15. Probes, Patch Chords Required
THEORY:
BIT ERROR RATE:
In telecommunication transmission, the bit error rate (BER) is a Ratio of bits
that have errors relative to the total number of bits received in a transmission. The
BER is an indication of how often a packet of other data unit has to be retransmitted
because of an error. Too high a BER may indicate that a slower data rate would
actually improve overall transmission time for a given amount of transmitted data
since the BER might be reduced, lowering the number of packets that had to be
resent.
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OBSERVATION:
Tb = 320Kbits
S.No: Error Counter Eb BER
1.
2.
3.
4.
5.
BER = Eb/Tb
Where Eb – Errored bits
Tb – Total bits Transmitted in a period of time t seconds.
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Measuring Bit Error Rate:
A BERT (bit error rate tester) is a procedure or device that measures the BER
for a given transmission. The BER, or quality of the digital link, is calculated from
the number of bits received in error divided by the number of bits transmitted.
BER = (Bits in error) / (Total bits transmitted)
PROCEDURE:
1. Make connections as shown in figure. Connect the power supply cables with
proper polarity to Link – B Kit. While connecting this, ensure that the power
supply is OFF.
2. Keep PRBS switch SW7 as shown in figure to generate PRBS signal.
3. Keep switch SW8 towards TX position.
4. Keep switch SW9 towards TX1 position.
5. Keep the switch SW10 at fiber optic receiver output to TTL position.
6. Select PRBS generator clock at 32 KHz by keeping jumper JP4 at 32K position.
7. Keep Jumper JP5 towards +5V position.
8. Keep Jumper JP6 shorted.
9. Keep Jumper JP8 towards Pulse position.
10.Switch ON the power supply.
11.Connect the post DATA OUT of PRBS Generator to the IN post of digital
buffer.
12.Connect OUT post of digital buffer to TX IN post.
13.Slightly unscrew the cap of SFH 756V (660) nm. Do not remove the cap from
the connector. Once the cap is loosened, insert the one Meter Fiber into the cap.
Now tighten the cap by screwing it back.
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14.Slightly unscrew the cap of RX1 Photo Transistor with TTL logic output SFH
551V. Do not remove the cap from the connector. Once the cap is loosened,
insert the other end of fiber into the cap. Now tighten the cap by screwing it
back.
15.Connect detected signal TTL OUT to Bit Error Rate event counter DATA IN
post & post IN of Noise Source.
16.Connect post OUT of Noise Source to post RXDATA IN of Bit Error Rate
event counter.
17.Connect post CLK OUT of PRBS Generator to post CLK IN of Bit Error Rate
event counter.
18.Press Switch SW 11 to start counter.
19.Vary pot P3 for Noise Level to observe effect of noise level on the error count.
20.Observe the Error Count LED’s for the error count in received signal in time 10
seconds as shown in figure.
RESULT:
Thus Bit error rate for given Sequence was measured.
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REVIEW QUESTIONS:
1. Define bit error rate (BER)
2. What are the typical error rates for optical fiber telecommunication
systems?
3. Why transimpedance amplifier is commonly used in optical
communication receiver?
4. What devices are used as pre-amplifiers for Giga bits/sec. data rate?
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9. STUDY OF EYE PATTERN
AIM:
To Study eye pattern using fiber optic link.
APPARATUS REQUIRED:
S.No. Name of the Equipments Quantity
1.Link – B Advance Fiber Optic
Communication Trainer Kit1
2. Power Supply 1
3. Fiber Optic Cable (Plastic) 1 meter
4.20 MHz Dual channel
Oscilloscope1
5. Probes, Patch Chords Required
THEORY:
The eye-pattern technique is a simple but powerful measurement method for
assessing the data-handling ability of a digital transmission system. This method
has been used extensively for evaluating the performance of wire systems and can
also be applied to optical fiber data links. The eye-pattern measurements are made
in the time domain and allow the effects of waveform distortion to be shown
immediately on an oscilloscope.
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MODEL GRAPH:
EYE PATTERN
INTERPRETATION OF EYE PATTERN:
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Noise Margin
Sampling periodof receiving signal
Sensitivity to Timing end
Distortion of zero crossing
NoiseMargin
Distortion at Sampling Time
Time interval over Which received signal is Sampled
To measure system performance with the eye-pattern method, a variety of
word patterns should be provided. A convenient approach is to generate a random
data signal, because this is the characteristic of data streams found in practice. This
type of signal generates ones and zeros at a uniform rate but in a random manner. A
variety of pseudorandom pattern generators are available for this purpose.
A pseudorandom bit sequence comprises four different 2-bit-long
combinations, eight different 3-bit-long combinations, sixteen different 4-bit-long
combinations, and so on (that is, sequences of different N-bit-long combinations)
up to a limit set by the instrument. After this limit has been generated, the data
sequence will repeat.
PROCEDURE:
1. Make connections as shown in figure. Connect the power supply cables with
proper polarity to Link – B Kit. While connecting this, ensure that the power
supply is OFF.
2. Keep switch SW7 as shown in figure to generate PRBS signal.
3. Keep switch SW8 towards TX position.
4. Keep switch SW9 towards TX1 position.
5. Keep the switch SW10 to EYE PATTERN position.
6. Select PRBS generator clock at 32 KHz by keeping jumper JP4 at 32K position.
7. Keep Jumper JP5 towards +5V position.
8. Keep Jumper JP6 shorted.
9. Keep Jumper JP8 towards TTL position.
10.Switch ON the power supply.
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OBSERVATION:
PRBS Generator
Frequency
Noise Margin
(V)
Sampling period of
Received Signal
(sec)
32 KHz
64 KHz
128 KHz
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11.Connect the post DATA OUT of PRBS Generator to the IN post of
digital buffer.
12.Connect OUT post of digital buffer to TX IN post.
13.Slightly unscrew the cap of SFH 756V (660) nm. Do not remove the cap
from the connector. Once the cap is loosened, insert the one Meter Fiber
into the cap. Now tighten the cap by screwing it back.
14.Slightly unscrew the cap of RX1 Photo Transistor with TTL logic output
SFH 551V. Do not remove the cap from the connector. Once the cap is
loosened, insert the other end of fiber into the cap. Now tighten the cap
by screwing it back.
15.Connect CLK OUT of PRBS Generator to EXT.TRG. Of Oscilloscope.
16.Connect detected signal TTL OUT to vertical channel Y input of
oscilloscope. Then observe EYE PATTERN by selecting EXT.TRG.
KNOB on oscilloscope as shown in figure. Observe the Eye Pattern for
different clock frequencies. As clock frequency increases the EYE
opening becomes smaller.
RESULT :
Thus Eye Pattern was studied using Fiber optic Link.
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REVIEW QUESTIONS:
1. What is the significance of eye pattern?
2. What is intersymbol interference?
3. How the performance of analog receiver and digital receiver is
measured?
4. What does pseudorandom means?
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10. MEASUREMENT OF RADIATION PATTERN OF
HORN ANTENNA
AIM:
To measure the radiation pattern of a waveguide Horn Antenna.
COMPONENTS REQUIRED:
i. Klystron Power Supply
ii. Klystron Mount with Tube 2K25
iii. Isolator
iv. Variable Attenuator
v. Frequency Meter
vi. Two pyramidal Horn Antenna
vii. Tunable Detector Mount
viii. VSWR Meter,CRO
ix. Bayonet Neill Concelman(BNC) Connector
x. Cooling Fan
xi. Radiation pattern Twin Table
xii. Waveguide Stand, Screw & Net
THEORY:
Horn antenna is an opened out waveguide. A waveguide is capable of
radiating radiation into open space provided the same is excited at one end
and opened at the other end. The radiation is much greater through
waveguide than transmission line. In waveguide, a small portion of the
incident wave is radiated and large portion is reflected back by the open
circuit. To minimize the reflections of the guided wave, the region between
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MODEL GRAPH: (Polar port)
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θ
0
–3
–G
Main lobe of antenna With Gain G dB
Lobe of omni directional antenna
Side Lobe
Back Lobe
the waveguide at the throat and free space at the aperture could be given a
gradual exponential taper.
HPBW of E direction, θE = 56λ/h degree.
HPBW of H direction, θH = 67λ/w degree
The horn antenna is most useful for broadband signals. The horn
antenna serves as a feed element for large radio astronomy, communication
dishes and satellite tracking throughout the world. As it is widely used at
micro frequencies, it may be considered as an aperture antenna.
FORMAULA:
r ≥2D2/λo, λo = C/f ( for rectangular horn antenna)
Where
r– Distance between transmitter and receiver horn antenna.
D– Size of the broad wall of horn antenna.(10.1 cm)
λo– Free space wavelength. C = 3×108 m/s. (Velocity of light)
f – Frequency of oscillation in GHz.
INITIAL SETUP IN VSWR METER:
1. Set input selector switch in 200 Ohms.
2. Keep meter selector in Normal.
3. Select the range as 50db or 40db or 30db and then vary the gain
knob (fine and coarse) to get minimum attenuation. (VSWR = 1).
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OBSERVATION:
Operating Frequency = GHz.
Right Left
Angle θ
(Degree)
Power
(dB)
Relative Power
(dB)
Angle θ
(Degree)
Power
(dB)
Relative Power
(dB)
CALCULATION:-
λo = C/f =
r = 2D2/λo = cm
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INITIAL ADJUSTMENTS IN KLYSTRON POWER SUPPLY:
1. Keep the variable attenuator in the minimum attenuation position.
2. Keep the control knob of klystron power supply as below, before
switching ON the device.
Beam voltage = OFF
Mod-switch = AM
Beam voltage knob = Fully anticlockwise
Repeller voltage knob = Fully anticlockwise
AM freq. & Amp. Knob = Around mid position
FM freq. & Amp. knob = minimum position
PROCEDURE:
1. Set the components as shown in Block diagram.
2. Keep the control Knobs of klystron Power supply as mentioned in
the basic set up.
3. Replace the transmitting horn by detector mount or keep the
transmitting and receiving antenna at close position.
4. Switch ON the VSWR meter, CRO, cooling fan & Klystron power
supply and set the beam voltage at 250 volts.
5. Adjust the repeller Voltage (120V) to get maximum output in
CRO.
6. Tune the frequency meter knob to get a ‘dip’ on CRO and note
down the frequency of oscillation directly. Detune the frequency
meter.
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7. Using the formula r = 2D2/λo, Calculate the distance between
antennas and keeping the axis of both horns in same line.
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8. Then remove the CRO and connect the VSWR meter to Tunable
Detector mount.
9. Obtain full scale deflection (0dB) on normal dB scale (0-10dB)
and change the appropriate range dB position to get the deflection
on scale (do not touch the gain control knob)
10.Note the range dB position and deflection of VSWR meter.
11.Tune the receiving horn to the left in 100 steps up to 400 and note
down the corresponding VSWR dB reading in the normal dB
range. (When necessary, change the range switch to next higher
range and add 10dB to observed value.)
12.Repeat the above step but this time turn the receiving horn to the
right and note down the readings.
13.Plot a relative power pattern i.e. Output vs. angle.
14.From the diagram determine 3dB–width (beam width) of the horn
antenna.
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RESULT:
Thus the radiation pattern of the pyramidal horn antenna was
measured.
HPBW =
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REVIEW QUESTIONS:
1. State Hygen’s principle.
2. Define an antenna.
3. Name the types of horn Antenna.
4. Define 3dB Beamwidth
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1
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1. MEASUREMENT OF RADIATION OF
PARABOLIC REFLECTORAIM:
To measure the radiation pattern of a Parabolic Reflector.
COMPONENTS REQUIRED:
i. Klystron Power Supply
ii. Klystron Mount with Tube 2K25
iii. Isolator
iv. Variable Attenuator
v. Frequency Meter
vi. One Horn Antenna
vii. Parabolic Reflector
viii. Tunable Detector Mount
ix. VSWR Meter, CRO
x. Bayonet Neill Concelman (BNC) Connector
xi. Cooling Fan
xii. Radiation pattern Twin Table
xiii. Waveguide Stand, Screw & Net
THEORY:
.To improve the overall radiation characteristic of the reflector
antenna, the parabolic structure is frequently used. Basically a parabola is a
locus of a point which moves in such a way that the distance if the point
from fixed point called focus plus the distance from the straight line called
directrix is constant. When the beam of parallel rays is incident on a
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MODEL GRAPH: (Polar port)
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θ
0
–3
–G
Main lobe of antenna With Gain G dB
Lobe of omni directional antenna
Side Lobe
Back Lobe
Parabolic reflector, then the radiations focus at a focal point. This principle
is used in the receiving antenna.
FORMAULA:
r ≥2D2/λo, λo = C/f ( for rectangular horn antenna)
Where
r– Distance between transmitter and receiver horn antenna.
D– Size of the broad wall of horn antenna.(10.1 cm)
λo– Free space wavelength. C = 3×108 m/s. (Velocity of light)
f – Frequency of oscillation in GHz.
INITIAL SETUP IN VSWR METER:
1. Set input selector switch in 200 Ohms.
2. Keep meter selector in Normal.
3. Select the range as 50db or 40db or 30db and then vary the gain
knob (fine and coarse) to get minimum attenuation. (VSWR = 1).
INITIAL ADJUSTMENTS IN KLYSTRON POWER SUPPLY:
1. Keep the variable attenuator in the minimum attenuation position.
2. Keep the control knob of klystron power supply as below, before
switching ON the device.
Beam voltage = OFF
Mod-switch = AM
Beam voltage knob = Fully anticlockwise
Repeller voltage knob = Fully anticlockwise
AM freq. & Amp. Knob = Around mid position
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FM freq. & Amp. Knob = minimum position
OBSERVATION:
Operating Frequency = GHz.
Right Left
Angle θ
(Degree)
Power
(dB)
Relative Power
(dB)
Angle θ
(Degree)
Power
(dB)
Relative Power
(dB)
CALCULATION:-
λo = C/f =
r = 2D2/λo = cm
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PROCEDURE:
1. Set the components as shown in Block diagram.
2. Keep the control Knobs of klystron Power supply as mentioned in
the basic set up.
3. Replace the transmitting horn by detector mount or keep the
transmitting and receiving antenna at close position.
4. Switch ON the VSWR meter, CRO, cooling fan & Klystron power
supply and set the beam voltage at 250 volts.
5. Adjust the repeller Voltage (120V) to get maximum output in
CRO.
6. Tune the frequency meter knob to get a ‘dip’ on CRO and note
down the frequency of oscillation directly. Detune the frequency
meter.
7. Using the formula r = 2D2/λo, Calculate the distance between
antennas and keeping the axis of antennas in same line.
8. Then remove the CRO and connect the VSWR meter to Tunable
Detector mount.
9. Obtain full scale deflection (0dB) on normal dB scale (0-10dB)
and change the appropriate range dB position to get the deflection
on scale (do not touch the gain control knob)
10.Note the range dB position and deflection of VSWR meter.
11.Tune the receiving parabolic reflector to the left in 100 steps up to
400 and note down the corresponding VSWR dB reading in the
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normal dB range. (When necessary, change the range switch to
next higher range and add 10dB to observed value.)
12.Repeat the above step but this time turn the receiving parabolic
reflector to the right and note down the readings.
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13.Plot a relative power pattern i.e. Output vs. angle.
14.From the diagram determine 3dB–width (beam width) of the
Parabolic Reflector.
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RESULT:
Thus the radiation pattern of the parabolic reflector was measured.
HPBW =
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REVIEW QUESTIONS:
1. Define Radiation Pattern.
2. Define Front to Back Ratio.
3. Define Radiation Resistance.
4. Give the relation between Gain & Directivity.
5. Name the types of Parabolic Reflectors.
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12. VSWR MEASUREMENT
AIM:
To determine the Standing Wave Ratio and Reflection co-efficient.
COMPONENTS REQUIRED:
i. Gunn Power Supply
ii. Gunn Oscillator
iii. PIN Modulator
iv. Isolator
v. Variable Attenuator
vi. Frequency Meter
vii. Slide Screw Tuner
viii. Tunable Detector Mount
ix. VSWR Meter, CRO
x. Bayonet Neill Concelman(BNC) Connector
xi. Threaded Neill Concelman(TNC) Connector
xii. Cooling Fan
xiii. Waveguide Stand, Screw & Net
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 to the load and the reflected wave which
propagates towards the generator. The reflected wave is set up 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
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OBSERVATION:
Frequency of Oscillation = ________GHz.
No. of Threads VSWR (S)Reflection Co-efficient
K=(S-1)/(S+1)
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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 minimum (or maximum) 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 = Emax/Emin
= |Ei| + |Er|/|Ei| – |Er|
Where Ei = Incident Voltage
Er = Reflected Voltage
Reflection Coefficient, ρ is
ρ = Er/Ei = (ZL–ZO) / (ZL+ZO)
Where ZL is the load impedance, Zo is characteristics impedance.
The above equation gives following equation
(ρ) = (S–1)/(S+1)
INITIAL SETUP IN VSWR METER:
1. Set input selector switch in 200 Ohms.
2. Keep meter selector in Normal.
3. Select the range as 50db or 40db or 30db and then vary the gain
knob (fine and coarse) to get minimum attenuation. (VSWR = 1).
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PROCEDURE:
1. Setup the equipments as shown in block diagram.
2. Keep the control knobs of Gunn power supply (GPS) as below.
Meter Switch – off
Gunn bias knob – Fully anticlockwise
PIN Mod. Amp knob – Mid position
PIN Mod. Freq. knob – Mid position
3. Switch ON the Gunn power supply, VSWR meter and Cooling fan.
Set Gunn bias Voltage at 7.5V.
4. Tune the frequency meter to get a ‘dip’ on the CRO. Measure the
operating frequency using frequency meter and detune the frequency
meter.
5. Then remove the CRO and connect the VSWR meter to Tunable
Detector mount.
6. If necessary change the range dB-switch, Variable attenuator position
and gain control knob to get deflection in the scale of VSWR meter.
7. Adjust the VSWR meter gain control knob or variable attenuator until
the meter indicates 1.0 on normal VSWR Scale.
8. Set the depth of S.S Tuner to around 3-4 mm. Read the VSWR on
scale and record it.
9. Repeat the above step for change of S.S. Tuner probe depth and
record the corresponding SWR.
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10.If the reading at the minimum is lower than 3 on the top scale, set
RANGE Switch to next higher range and read the indication on the
second SWR or (3 to 10) scale of SWR.
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11.If the range switch is changed by two steps used top SWR scale,
however all indication on this scale must be multiplied by 10.
12.Using the formula, K=S-1/S+1, find the reflection co-efficient.
110 VEC/2011-12/ODD/ECE/VII/EC-2405
RESULT:
Thus the Standing Wave Ratio was measured and Reflection
Co-efficient was verified.
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REVIEW QUESTIONS:
1. What should be the value of S for Low VSWR measurement and High
VSWR measurement?
2. What is the value of VSWR for a perfectly matched system?
3. Give two limitations of VSWR measurement
112 VEC/2011-12/ODD/ECE/VII/EC-2405
DIRECTIONAL COUPLER:
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PORT 1 PORT 2
PORT 3
PORT 4
13. MULTIHOLE DIRECTIONAL COUPLER
AIM:
To measure coupling factor, insertion loss, isolation and directivity of
Multihole directional coupler.
COMPONENTS REQUIRED:
i. Gunn Power Supply
ii. Gunn Oscillator
iii. PIN Modulator
iv. Isolator
v. Variable Attenuator
vi. Frequency Meter
vii. Multihole Directional Coupler
viii. Tunable Detector Mount
ix. Matched Termination
x. VSWR Meter, CRO
xi. Bayonet Neill Concelman(BNC) Connector
xii. Threaded Neill Concelman(TNC) Connector
xiii. Cooling Fan
xiv. Waveguide Stand, Screw & Net
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, electromagnetically coupled to each
114 VEC/2011-12/ODD/ECE/VII/EC-2405
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other. The power entering, in the main-arm gets 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)
The coupling factor is defined as
Coupling (dB) = 10 log 10 [P1/P3] where port 2 is terminated.
Isolation (dB) = 10 log 10 [P2/P3] where P1 is matched.
With built-in termination and power entering at Port 1, the directivity
of the coupler is a measure of separation between incident wave and the
reflected wave. Directivity is measured indirectly as follows:
Hence Directivity D (dB) = Isolation – Coupling
= 10 log 10 [P2/P1]
Insertion loss = 10 log 10 [P1/P2]
INITIAL SETUP IN VSWR METER:
1. Set input selector switch in 200 Ohms.
2. Keep meter selector in Normal.
3. Select the range as 50db or 40db or 30db and then vary the gain
knob (fine and coarse) to get minimum attenuation. (VSWR = 1).
PROCEDURE:
1. Setup the equipments as shown in block diagram.
2. Keep the control knobs of Gunn power supply (GPS) as below.
Meter Switch – off
Gunn bias knob – Fully anticlockwise
PIN Mod. Amp knob – Mid position
PIN Mod. Freq. knob – Mid position
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3. Switch ON the Gunn power supply, VSWR meter and Cooling fan.
Set Gunn bias Voltage at 7.5V.
OBSERVATION:
Frequency of Oscillation = GHz.
X = dB (Without Directional Coupler)
Z = dB (o/p at port 3, Terminate at port 2, I/p at port 1)
Y = dB (o/p at port 2, Terminate at port 3, I/p at port 1)
Yd = dB (o/p at port 1, Terminate at port 3, I/p at
port 2)
Coupling Factor, C = X–Y = dB
Insertion loss, = X–Z = dB
Isolation I = X–Yd = dB
Directivity D = Y–Yd = dB
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4. Tune the frequency meter to get a ‘dip’ on the CRO. Measure the
operating frequency using frequency meter and detune the frequency
meter.
5. Then remove the CRO and connect the VSWR meter to Tunable
Detector mount.
6. Remove the multihole directional coupler and connect the detector
mount of the frequency meter.
7. 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)
8. Insert the directional coupler as shown in block diagram 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).
9. Note down the reading on VSWR meter on the scale with the help of
range-dB switch if required. (Let it be Y).
10.Calculate coupling factor which will be X–Y=C(dB)
11.Now carefully disconnect the detector from the auxiliary port 3 and
match termination from port 2 without disturbing the set-up.
12.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.
13.Compute insertion loss X–Z in dB.
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14.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 variable
attenuator and gain control knob of VSWR meter.)
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15.Measure and note down the reading on VSWR meter. Let it be Yd. X–
Yd gives Isolation I (dB).
16.Compute the directivity as Y–Yd = I – C
120 VEC/2011-12/ODD/ECE/VII/EC-2405
RESULT:
Thus the measuring of
Coupling Factor, C = dB
Insertion loss, = dB
Isolation I = dB
Directivity D = dB
Of Multihole directional coupler were calculated.
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REVIEW QUESTIONS:
1. Define coupling factor & Directivity.
2. Name the four types of Directional Coupler.
3. Define Directional coupler.
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14. POWER MEASUREMENT
AIM:
To draw the Attenuation characteristics by measuring power using
power meter.
COMPONENTS REQUIRED:
i. Microwave source
ii. Microwave power meter
iii. Waveguide to Coax Adapter
iv. Variable Attenuator
v. Thermocouple mount
vi. Waveguide Stand, Screw & Net
THEORY:
The output power level of a system or component is frequently the
critical factor in the design and ultimately the purchase and performance of
almost all radio frequency and microwave equipment. The convenient unit
for power measurement is dBm. The formula for dBm is the ratio of one
power level P to the reference level where Pref is always one milliwatt. dBm
is used as a measure of absolute power.
Positive dBm means “dB above one milliwatt” & negative dBm is
interpreted as “dB below one milliwatt”. The advantages of the term dBm is
that it uses compact numbers and allows the use of addition instead of
multiplication when cascading gains or losses in a transmission system.
124 VEC/2011-12/ODD/ECE/VII/EC-2405
MODEL GRAPH:
OBSERVATION:
Attenuator (Probe depth)
(mm)
Power meter reading
(dBm)
125 VEC/2011-12/ODD/ECE/VII/EC-2405
Pow
er (d
Bm
)
Attenuation (mm)
PROCEDURE:
1. Give the connections as shown in the block diagram.
2. The menu switch in the power meter is used to select the different
menu options like measurement units, averaging time etc. press enter
after each settings.
3. To determine the attenuation Characteristics, slightly increase the
prove depth in the Variable Attenuator and note down the
corresponding power in the power meter.
4. A graph is plotted between Attenuation and power.
RESULT:
126 VEC/2011-12/ODD/ECE/VII/EC-2405
Thus the attenuator characteristic by measuring power using power
meter was drawn.
127 VEC/2011-12/ODD/ECE/VII/EC-2405
REVIEW QUESTIONS:
1. Name the classifications of power measurement.
2. What is a Bolometer? Name the types.
3. What is the advantage of thermistor in microwave
powermeasurement?
128 VEC/2011-12/ODD/ECE/VII/EC-2405
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BLOCK DIAGRAM:
130 VEC/2011-12/ODD/ECE/VII/EC-2405
KLYSTRON POWERSUPPLY
KLYSTRON MOUNT
WITHTUBE
ISOLATOR VARIABLE ATTENUATOR
FREQUENCY METER
TUNABLE DETECTOR
MOUNT
CRO
2K 25
15. MODE CHARACTERISTICS OF REFLEX
KLYSTRONAIM :
To Study the Mode characteristics of the reflex klystron tube
COMPONENTS REQUIRED:
i. Klystron power Supply
ii. Klystron tube with mount
iii. Isolator
iv. Frequency Meter
v. Variable Attenuator
vi. Detector Mount
vii. CRO
viii. Bayonet Neill Concelman(BNC) Connector
ix. Cooling Fan
x. Waveguide Stand, Screw & Net
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 travelling forward will be
accelerated of retarded, as the voltage at the resonator changes in amplitude.
The accelerated electrons leave at the reduced velocity. The electrons
131 VEC/2011-12/ODD/ECE/VII/EC-2405
OBSERVATION:
Beam Voltage:________V
Beam Current:________mA
S.No. Negative Repeller Voltage(V)
Frequency(GHz)
Output Voltage(mV)
132 VEC/2011-12/ODD/ECE/VII/EC-2405
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.
INITIAL ADJUSTMENTS:
1. Keep the variable attenuator in the minimum attenuation position.
2. Keep the control knob of klystron power supply as below, before
switching ON the device.
Beam voltage = OFF
Mod-switch = AM
Beam voltage knob = Fully anticlockwise
Repeller voltage knob = Fully anticlockwise
AM frequency & Amplitude knob = mid position
FM frequency & Amplitude knob = minimum position
133 VEC/2011-12/ODD/ECE/VII/EC-2405
Repeller Voltage (V)
Out
put V
olta
ge (V
)
Repeller Voltage (V)
Freq
uenc
y C
hang
e (M
Hz)
MODEL GRAPH:
134 VEC/2011-12/ODD/ECE/VII/EC-2405
PROCEDURE:
1. Connect the components as shown in Block diagram.
2. Keep the control Knobs of klystron Power supply as mentioned in
the basic set up.
3. Switch ON the Klystron power supply and set the beam voltage at
250 volts.
4. Check & measure the beam current whether it is less than 30mA.
5. By changing repeller Voltage from –10V to –180V to get
maximum output in CRO and measure the corresponding output
voltage.
6. Tune the frequency meter to get a dip on CRO and note down the
corresponding frequency of oscillation directly. Detune the
frequency meter.
7. Get two readings below and above the mode.
8. Plot the Negative repeller voltage Vs ouput voltage readings on the
graph.
135 VEC/2011-12/ODD/ECE/VII/EC-2405
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RESULT:
Thus the mode characteristics of Reflex Klystron was studied.
138 VEC/2011-12/ODD/ECE/VII/EC-2405
BLOCK DIAGRAM:
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MICROWAVE SOURCE ISOLATOR
VARIABLE ATTENUATOR
FREQUENCY METER
MATCHED LOAD
ISOLATORSLOTTED
LINE SECTION
TUNABLE PROBE
VSWR METER
16. S–PARAMETER MEASUREMENT OF ISOLATOR
AIM :
To measure the S–parameter of isolator
COMPONENTS REQUIRED:
i. Microwave source (Gunn, Klystron)
ii. Isolator
iii. Frequency Meter
iv. Variable Attenuator
v. Slotted line section with tunable probe
vi. Detector Mount
vii. CRO / VSWR
viii. Bayonet Neill Concelman(BNC) Connector
ix. Cooling Fan
x. Waveguide Stand, Screw & Net
THEORY:
ISOLATOR:
An isolator is a two-port device that transfers energy from input to
output with little attenuation and from output to input with very high
attenuation
The isolator can be derived form a three-port circulator by simply
placing a matched load (reflection less termination) on one port.
The important isolator parameters are:
A. Insertion loss:
140 VEC/2011-12/ODD/ECE/VII/EC-2405
Insertion loss is the ratio of power detected at the output port to the
power supplied by source to the input port, measured with other ports
terminated in the matched load. It is expressed in dB.
READINGS:
Input Power at Output Powers at VSWRPort 1 Port 2
Port 1
Port 2
S PARAMETERS :
or ----------1.1
---------- 1.2
---------2.1
Where Pi = power output at port i, Pj = power input at port j
The S matrix of Isolator
141 VEC/2011-12/ODD/ECE/VII/EC-2405
B. Isolation:
Isolation is the ratio of power applied to the output to that measured at
the input. This ratio is expressed in dB. The isolation of a circulator is
measured with the third port terminated in a matched load.
C. Input VSWR:
The input VSWR of an isolator or circulator is the ratio of voltage
maximum to voltage minimum of the standing wave existing is the
line with all parts except the test port are matched
PROCEDURE:
A. MEASUREMENT OF INSERTION LOSS, ISOLATION AND
SCATTERING PARAMETERS
1. Set up the components and equipment and connect the detector mount
to the slotted section as shown in figure. The output of the detector
mount should be connected with VSWR meter.
2. Energize the microwave source for maximum output for a particular
frequency of operation. Tune the detector mount for maximum output
in the VSWR Meter.
3. Set reference level 0dB of power in 30dB range in VSWR meter with
the help of variable attenuator and gain control knob of VSWR meter.
4. Carefully remove the detector mount from slotted line without
disturbing the position of variable attenuator and gain control knob.
5. Insert the isolator between slotted line and detector mount. Keeping
input port (1) to slotted line and detector at its output port (2).
142 VEC/2011-12/ODD/ECE/VII/EC-2405
6. Record the reading in the VSWR meter. If necessary change range-db
switch to high or lower position and taking 10db change for one step
change of switch position.
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7. Similarly measure the power through port (2) terminating port (1) in
matched termination. Note the readings in Table.
Insertion loss = 10 log10 (P1/P2) = 10 log10 (I1/I2)
Isolation = 10 log10 (P1/P3) = 10 log10 (I1/I3)
B. INPUT VSWR MEASUREMENT:
1. Set up the components and equipment as shown in the fig.2 with input
port 1 of isolator towards slotted line and matched termination on
other ports of it.
2. Energize the microwave source for particular operation of frequency.
3. With the help of slotted line, probe and VSWR meter find out SWR
of port of the isolator as described earlier for low and medium SWR
measurements.
4. The above procedure can be repeated for other ports or for other
frequencies.
144 VEC/2011-12/ODD/ECE/VII/EC-2405
RESULT:Thus the S-parameters of isolator were measured.
145 VEC/2011-12/ODD/ECE/VII/EC-2405
BENCH SET-UP:
FIG.1 SET UP FOR MEASUREMENT OF INSERTION LOSS, ISOLATION & SCATTERING PARAMETERS
146 VEC/2011-12/ODD/ECE/VII/EC-2405
Circulator1 2 3
Detector mount
Microwave Source
(RKO/GO)
Isolator Variable Attenuator
Frequency Meter
Slotted line VSWR meter
Detector mount
Circulator1 2 3
Matched Termination
Matched Termination
Detector mount
VSWR meter
VSWR meter
17. S–PARAMETER MEASUREMENT OF CIRCULATOR
AIM :
To study the operation of ferrite circulator and hence measure
i. Insertion loss ii. Isolation iii. And to determining S parameters
COMPONENT REQUIRED:
i. Microwave Source (RKO/GO)
ii. Isolator
iii. Variable Attenuator
iv. Frequency meter
v. Slotted line with Tunable probe
vi. detector mount
vii. VSWR meter
viii. Circulator and
ix. Matched Terminations-2.
THEORY: The circulator is a multi port junction that permits transmission in
certain ways. The wave incident at nth port can be coupled to (n+1) th port
only.
FIG. Y JUNCTION CIRCULATOR
147 VEC/2011-12/ODD/ECE/VII/EC-2405
BENCH SET-UP:
FIG.2 SET UP FOR MEASUREMENT OF VSWR OF CIRCULATOR
148 VEC/2011-12/ODD/ECE/VII/EC-2405
Microwave Source
(RKO/GO)
IsolatorVariable
AttenuatorFrequency
MeterSlotted line
V.S.W.R meter
TunableProbe
Matched Termination
Circulator1 2
3
Matched Termination
Following are the basic parameters of isolator and circulator.
INSERTION LOSS:
The ratio of the power supplied by a source to the input port to the
power detected by a detector in the coupling arm. i.e output arm with other
port terminated in the matched load, is defined as INSERTION LOSS or
FORWARD LOSS.
ISOLATION:
It is the ratio of the power fed to input arm to the power detected by a
detector at not coupled port with other port terminated with the matched
port.
INPUT VSWR:
The input of circulator is the ratio of voltage maximum to voltage
minimum of the standing wave existing on the line, when one port of it
terminates the line and others have matched termination.
PROCEDURE:
A. MEASUREMENT OF INSERTION LOSS, ISOLATION AND
SCATTERING PARAMETERS
1. Set up the components and equipment and connect the detector mount
to the slotted section as shown in fig.1. The output of the detector
mount should be connected with VSWR meter.
2. Energize the microwave source for maximum output for a particular
frequency of operation. Tune the detector mount for maximum output
in the VSWR Meter.
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3. Set reference level 0dB of power in 30dB range in VSWR meter with
the help of variable attenuator and gain control knob of VSWR meter.
4. Carefully remove the detector mount from slotted line without
disturbing the position of variable attenuator and gain control knob.
5. Insert the circulator between slotted line and detector mount. Keeping
input port (1) to slotted line and detector at its output port (2). A
matched termination should be placed at third port (3).
6. Record the reading in the VSWR meter. If necessary change range-db
switch to high or lower position and taking 10db change for one step
change of switch position.
7. Similarly measure the power through port (3) terminating port (2) in
matched termination. Note the readings in Table.
Insertion loss = 10 log10 (P1/P2) = 10 log10 (I1/I2)
Isolation = 10 log10 (P1/P3) = 10 log10 (I1/I3)
8. Repeat the steps 5 to 7 by feeding power at ports 2 and 3. Note the
reading.
B. INPUT VSWR MEASUREMENT:
1. Set up the components and equipment as shown in the fig.2 with input
port 1 of circulator towards slotted line and matched termination on
other ports of it.
2. Energize the microwave source for particular operation of frequency.
151 VEC/2011-12/ODD/ECE/VII/EC-2405
3. With the help of slotted line, probe and VSWR meter find out SWR
of port 1 of the circulator as described earlier for low and medium
SWR measurements.
4. The above procedure can be repeated for other ports or for other
frequencies.
READINGS:TABLE-1
Input Power at Output Powers at VSWRPort 1 Port 2 Port 3
Port 1
Port 2
Port 3
S PARAMETERS :
Or ----------1.1
---------- 1.2
---------2.1
Where Pi = power output at port i, Pj = power input at port j
152 VEC/2011-12/ODD/ECE/VII/EC-2405
The S matrix of circulator
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RESULT:Thus the S-parameters of circulator were measured
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REVIEW QUESTIONS:
1. Verify Carlin’s Theorem.
2. Construct 4 port circulator with 3 port circulators.
3. What are the basic properties of ferrites which make them useful at
Microwave frequencies?
4. How does a circulator is differ from Magic Tee?
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Microwave Source
Isolator Variable Attenuator
Frequency Meter
Slotted Line
Detector
Tunable Probe
VSWR Meter
CRO
2
Tee Junctions
1
4 3
Matched Load
Detector Mount
Matched Load
BLOCK DIAGRAM:
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18. S-MATRIX OF E-PLANE, H-PLANE AND MAGIC
TEEAIM :
To measure the S-matrix of E-plane, H-plane and Magic Tee.
COMPONENTS REQUIRED:
i. Microwave source (Gunn, Klystron)
ii. Isolator
iii. Frequency Meter
iv. Variable Attenuator
v. Slotted line section with tunable probe
vi. E-plane Tee, H-plane tee, Magic tee
vii. Detector Mount
viii. CRO / VSWR
ix. Bayonet Neill Concelman (BNC) Connector
x. Cooling Fan
xi. Waveguide Stand, Screw & Net
THEORY:
The device magic tee is a combination of the E and H plane Tee. Arm
3, the H-arm forms an H-plane Tee and arm 4, E-arm forms an E-plane Tee
combination of arm1 and 2 as side of collinear arms. If the power is fed in
arm3 (H-arm), the electric field divides equally between arm 1 and 2 with
the same phase and no electric field exits in arm4. If power is fed in arm 4
(E-arm), it divides equally in to arm 1 and 2 but out of phase with no power
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READINGS:TABLE-1
Input Power at
Output Powers at VSWRPort 1 Port 2 Port 3 Port 4Port 1
Port 2
Port 3
Port 4
S PARAMETERS :
Or ----------1.1
---------- 1.2
---------2.1
Where Pi = power output at port i, Pj = power input at port j
to arm 3, further , if the power is fed in arm1 and 2 simultaneously it is
added in arm 3(H-arm) and it is subtracted in E-arm i.e. arm 4 159 VEC/2011-12/ODD/ECE/VII/EC-
2405
The basic parameters to be measured for magic Tee are defined
below.
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 the power
supplied by the generator connected to the E-arm (port 4) to the power
detected at H-arm (port 3) when side arms 1 and 2 terminated in matched
load
Hence Isolation (dB) = 10 log10 [P4/P3]
Similarly, Isolation between other parts may also be defined.
C. Coupling Factor:
It is defined as Cij = 10-α/20
Where α is attenuation / isolation in dB when i is input arm and j is
output arm.
Thus α = 10 log10 [P4/P3]
Where P3 is the power delivered to arm i and P4 is power detected at j
arm.
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The S matrix of Magic Tee
The S matrix of E-Plane and H-Plane Tee
PROCEDURE:
A. VSWR measurement of the ports:
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1. Connect the components as shown in Block diagram. Keeping E-
arm towards slotted line and matched termination to other ports.
2. Energize the microwave source for particular frequency of operation. Detune the frequency meter
3. Measure the VSWR of E-arms as described in measurement of SWR for low and medium value.
4. Connect another arm to slotted line and terminate the other port
with matched termination. Measure the VSWR as above.
Similarly, VSWR of any port can be measured.
B. Measurement of Isolation and coupling factor:
1. Remove the tunable probe and Magic tee from the slotted line and
connect the detector mount to slotted line.
2. Energize the microwave source for particular frequency of
operation and tune the detector mount for maximum output.
3. With the help of variable attenuator and gain control knob of VSWR meter, set any power level in the VSWR meter and note down. Let it be P3.
4. 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 and 2. Note down the reading of VSWR meter let it be P4.
5. Determine the isolation between port 3 and 4 as P3–P4 in dB.6. Determine the coupling coefficient from equation given in the
theory part.7. The same experiment may be repeated for other ports also.
8. Repeat the above experiment for other frequencies.
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RESULT:
Thus the S-parameters of E-plane, H-plane and magic Tee was
measured.
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BLOCKDIAGRAM:
165 VEC/2011-12/ODD/ECE/VII/EC-2405
He-Ne LASER SOURCE
LASER SOURCE LASER HOLDER ASSEMBLY
LASER TO FIBER COUPLER HOLDER
ASSEMBLY
FIBER HOLDER ASSEMBLY
SCREEN
19. MODE OBSERVATION IN FIBER OPTIC CABLE
AIM:
Observation of lower order Linearly Polarized (LP) modes.
APPARATUS REQUIRED:
S.No. Name of the Equipments Specification Quantity
6. Laser source (633nm-1mW) 2mW 1
7. Source to Fiber Coupler 1
8. Single Mode Fiber SMF 9/125µm 1 meter
9. Fiber Holding Stand 1
10. Opaque Screen 1
11. Multimode Fiber 62.5/125µm 1meter
THEORY:
The central spot carries 95% of the intensity for laser beams with
Gaussian profile.
I=I0e-z(r/w) 2
Where e=2.718 beam of natural algorithm accepted definition of a
radius of a Gaussian beam is the distance at which beam intensity has
dropped to 1/e2=0.135 times its peak value I0. This radius called spot size.
The spot diameter is W.
Spot diameter (d) micron=focal length of the Lanes (f) mm X Laser
Beam full divergence angle (DA) mrad.
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OBSERVATION:
Single mode Fiber:
a = 4.5µm (core radius)
NA = 0.11
V = 4.91
From figure only 4 LP modes propagate.
Total number of modes = V2/2
No. of modes = 12
Multimode Fiber:
a = 31.25µm (core radius)
NA = 0.11
V = 34.12
Total number of modes = V2/2 = (34.12)2/2
No. of modes = 582.11
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In order to achieve maximum coupling efficiency, the fiber core
diameter has to be bigger than the spot diameter.
If NArays ≤ NAfiber and spot diameter (w) ≤ fiber core diameter (d), then
all of the laser light will be coupled into the fiber. 90% coupling efficiency
into the single mode fiber from the Ne-Ne lasers is achievable. For
beginners, coupling efficiency of 50% is considered to be a good result.
Operation Principle of Laser to Fiber Source Coupler
The source coupler is comprised of two base plates. One of the base
plates contains a focusing lens and a female connector receptacle. The other
base plate is attached onto the laser. An O-ring is sandwiched between the
base plates. Threaded screws interconnect the two base plats. A screwdriver
to alter the angular orientation of one base plate relative to the other can then
adjust the screws.
For small tilt angles, the resolution of the coupler Δz is determined by
Δz–fΔx /L. where Δx is the resolution of the screws and L is the lever arm.
For 80TPI (threads per inch) screws, a lens with 1mm focal length, and
20mm lever arm Δz = 1mm 2 micron/20mm = 0.1micron.
The number of modes propagating through the fiber depends on V-
number. If the fiber whose V-number is less than 2.405, it allows to
propagate single mode through it, so it is called as Single Mode Fiber. This
time you will start with a fiber, which has V-number slightly greater than
2.405. Such a fiber is Multimode fiber, but the number of allowed modes is
small enough so that they may be individually identified when the output of
the fiber is examined.
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LOWER ORDER LINEARLY POLARISED MODES OF
OPTICAL FIBER
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0 1 2 3 4 5
LP01
LP11 LP21
LP02
V=number
When V-number is less than 2.405, then only a single mode may propagate
in the fiber wave-guide. This mode is HE11 mode or LP01 – Linearly
Polarized mode.
When V-number>2.405, other modes may propagate in the fiber. The
first LP mode, which comes in at V=2.405, is the LP11 mode, the next lowest
mode in the weakly guiding approximation.
When V is slightly greater than 2.405 i.e. V=4.91 then 4 Linearly
Polarized modes will propagate through fiber.
LP02: Degenerated twice: 2 modes
LP11: 4 times degenerated: 4 modes
LP02: Degenerated twice: 2 modes
LP21: 4 times degenerated: 4 modes
Total 12 modes can propagate through fiber. This number is identical
to that given by formula: Ma=V2/2=12
The electromagnetic field distributions of these modes are as shown
figure. We have a fiber with the proper V-number; varying the position and
angle at which a tightly focused beam of the proper wavelengths is projected
onto the fiber core can selectively launch these modes.
PROCEDURE:
1. Keep Optical Bread board onto original and flat table surface, so
that is will not toggle.
2. Fix the pre-fitted cylindrical head of the He-Ne laser source on to
the surface of the breadboard from the bottom side with the help of
Allen screws provided with it. Confirm the rigid ness of the mount.
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3. Fix the laser to the fiber coupler mount on to the breadboard with
the base plate orientation of it towards He-Ne laser exit.
IRRADIANCE PATTERN OF SOME LOWER ORDER LINEARLY
POLARIZED MODE
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LP01
LP11
LP02 LP21
4. Turn on the He-Ne laser and locate the beam spot on to the central
portion of the laser-fiber coupling lens assembly by adjusting the
vertical and horizontal travel arrangement provided with the
mount. Tighten the screws of the vertical and horizontal slots.
5. Now look for the back reflection of the He-Ne spot from the rod
lens of the coupler. In case if you found the back spot, away from
the exit of the cylindrical laser head of the laser, adjust the back-
reflected spot going back in exit hole by slowly moving the four
screws provided for the laser mount.
6. Confirm the central alignment of the laser beam at the exit of the
laser fiber coupler by putting a white card sheet and zooming the
spot on to it. In case the spot is found off-center, adjust it to the
center by slightly moving the screws of the laser mount.
7. Put the multimode optical patch cord on to the laser-fiber coupler
exit and fix the other end of the fiber in the fiber holding stand by
moving the grub screws provided with the holder.
8. You will see the bright laser-beam spot coming out of the fiber.
Adjust the height of exit tip of the fiber to about 50mm. Min. from
the white sheet of the paper.
9. Now you will see a bright round shape circular spot with laser
speckle pattern on to the screen. If multimode pattern can be
refined screws provided with laser-fiber coupler. Slightly adjusting
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or moving the screws on the laser mount can also view the change
in pattern of this multimode spot.
10.Once you observe the multimode pattern, change multimode fiber
optic patch cord with single mode fiber patch cord.
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11.As soon as you place the single mode patch cord, you will see the
blur pattern of the various single mode patterns on to the screen.
That is, single circular two lobes, three lobes and four lobes
patterns can be very well observed by slightly adjusting the Allen
screws of the laser-fiber coupler.
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RESULT:
Thus the lower order linearly polarized modes were observed.