Microwave Ok

Course Name: MICROWAVE LABORATORY

Microwave & Optical Communication Lab

Exp. No. :

Date :

MODE CHARACTERISTICS OF REFLEX KLYSTRONAIM:To study the mode characteristics of reflex klystron.

EQUIPMENTS:1.Klystron power supply

2. Reflex Klystron with mount

3. Isolator

4.Variable Attenuator

5.Frequency meter

6.Waveguide-detector mount with detector

7.Micro ammeter, VSWR meter

8. Wave guide stand and accessories.PROCEDURE:

a) Carrier Wave operation

1Assemble the equipment as shown in figure with microammeter as indicating meter

2Fire the Klystron correctly for CW operation with optimum beam voltage

3Adjust the repeller voltage to maximum negative value and decrease it in steps of 1V and record o/p power and frequency in table. The frequency is measured by tuning the frequency meter to have a dip in the o/p each time. The frequency meter should be detuned each time while measuring power

5Plot power/frequency versus repeller voltage to get mode curves-figure

6Compute various parameters from the graph

b) Modulated source (square wave operation)

1Assemble the equipment as shown in figure with VSWR meter as indicating meter

2Set the mod-selector switch to AM-MOD position. Keep AM-MOD and AM-FRE, knob at the mid position

3Fire the Klystron correctly for square wave operation with optimum beam voltage

4Adjust the modulation voltage and repeller voltage to obtain maximum reading in the VSWR meter and read it on the power scale

5Also adjust the klystron tuning plunger for a maximum in VSWR meter.

6Repeat steps 3 to 6 of CW procedure

RESULT:

Figure: Mode Characteristics of reflex klystron

K Kly

Figure: Setup for study of Klystron tube

Table: Mode Characteristics of Reflex Klystron

Sl noRepeller voltage,( V)Microammeter reading ( A )Frequency,( GHz)

Exp. No. :

Date :

GUNN DIODE CHARACTERISTICS

AIM:

To study the Gunn oscillator as a source of microwave power and hence to study the a) I-V characteristics

b) To find the threshold voltage.

EQUIPMENTS:1Gunn Oscillator

2Gunn Oscillator power supply

3PIN diode modulator

4Isolator

5Frequency meter

6Attenuator

7Detector with tunable mount.PROCEDURE:1Set the equipment as shown in figure.

2Set the Gunn oscillator micrometer tuning screw at suitable frequency (~9GHz). Adjust attenuator for suitable power level

3Change Gunn biasing in steps of 0.5V and record corresponding currents in the table (read current in the panel meter of the Gunn power supply)

4Draw the current voltage characteristic.

5Set the Gunn bias voltage above Vo and with zero attenuation. Record power and frequency for this Gunn diode biasing

6Reduce the Gunn biasing in steps of 0.5V and record corresponding power and frequency

7Plot power/frequency versus bias characteristics

8With Gunn oscillator micrometer screw in the middle of its range, set Gunn biasing for maximum power o/p and record in table this power and frequency of oscillation

9Move micrometer screw to one extreme till power falls to a low but readable value

10 Move micrometer screw in the opposite direction in steps of 0.5mm and record in table, the power and corresponding frequency till the screw reaches another extreme

11Plot power versus frequency characteristics and frequency versus micrometer reading curve-the calibration curve

RESULT:

Figure: I-V Characteristics of Gunn oscillator

K Kly

Figure: Setup for the study of Gunn DiodeMicrometer Reading = Frequency =

Table: I-V Characteristics

Sl noBias Voltage (V)Current (mA)Threshold Voltage (V)

Exp. No. :

Date :

FREQUENCY AND WAVELENGTH MEASUREMENT

AIM: To determine the frequency and wavelength in a rectangular waveguide and to verify the relation between 0 , g, and c

EQUIPMENTS :Klystron oscillaltor, isolator, frequency meter, variable attenuator, standing wave detector, movable short terminator, Klystron power supply and VSWR meter

PROCEDURE:Frequency measurement:1. Set up the microwave bench as shown in figure

2. Set variable attenuator at minimum position

3. Set VSWR meter at 40dB

4. Switch ON klystron power supply. Modulate with 1kHz square wave

5. Adjust the reflector voltage to get maximum deflection in the VSWR meter.

6. Maximize the deflection with tubing probe in the VSWR meter

Frequency measurement using frequency meter

7. Tune the frequency meter until a dip is observed in the VSWR meter. Also tune the frequency meter to obtain minimum deflection. Note frequency directly from frequency meter.

Wavelength measurement:

8. Replace the termination with variable short. Detune the frequency meter.

9. Move the probe along slotted line. The deflection in the VSWR meter will vary.

10. Move probe to minimum deflection point.

11. Move the probe to next minimum position and record the probe position. Calculate

guide wavelength as twice the distance

12. Calculate guide wavelength as twice the distance between minimum positions.

13. Calculate frequency using the expression f = c

where a is the inner dimension of waveguide broader side

RESULT

K Kly

Figure: Setup for frequency & wavelength measurementsExp. No. :

Date :

ATTENUATOR CHARACTERISTICS

AIM:

To study the attenuator characteristics by

a) Measuring the insertion loss and

b) Plotting o/p power versus micrometer reading for the given variable attenuator.

EQUIPMENTS: Standard microwave test bench and the given test attenuator.PROCEDURE:1Assemble the equipment as shown in figure with VSWR as indicating meter

2Fire the Klystron correctly for AM-MOD operation with optimum beam voltage

3Adjust for maximum power o/p (~30dB). Note the o/p power in VSWR meter (P0)

4Insert the test attenuator between frequency meter and detector mount without disturbing the bench.

5Move the micrometer screw to fully released position. Note the o/p power in VSWR meter (P1). Calculate the insertion loss as (P0-P1)

6Move the micrometer screw in the opposite direction in steps of 0.5mm and note down the corresponding power (P2) from the VSWR meter

7Find out the attenuation (P1-P2) value for different micrometer readings and plot the graph

K Kly

Figure: Setup for studying attenuator characteristics

Table: Attenuator Characteristics

Sl noMicrometer reading (mm)o/p Power (dB) P2P0-P2 (dB)

Calculation:

Frequency of operation =

Input power P0 =

Output power P1 =

Insertion loss = P0-P1 =

RESULT:Exp. No. :

Date :

ISOLATOR AND CIRCULATOR

AIM:

To study the operation of isolator and circulator and hence to measure

a) Insertion loss

b) Isolation

EQUIPMENTS:Standard microwave test bench and the given test isolator and circulator.PROCEDURE:1Assemble the equipment as shown in figure with VSWR as indicating meter


3Adjust for maximum power o/p (~30dB). Note the o/p power in VSWR meter (P0)

4Insert the test isolator/circulator in the forward direction (with arrow towards the detector) between frequency meter and detector mount without disturbing the bench. The third port of the circulator should be terminated with a matched load.

5Note the o/p power in VSWR meter (P1). Calculate the insertion loss as (P0-P1)

6For measurement of isolation, connect the isolator/circulator in the reverse direction.

7Note the o/p power in VSWR meter (P2). Calculate isolation as P0-P2

8The same should be repeated for the other ports of the given circulator

RESULT:

Figure: Circulator and Isolator

K Kly

Figure: Setup for studying isolator & circulator Table: Insertion loss

i/p portPower at i/p (db)o/p portPower at o/p (db)Insertion loss (db)

Port 1Port 2

Port 2Port 3

Port 3Port 1

Table: Isolation

i/p portPower at i/p (db)o/p portPower at o/p (db)Isolation loss (db)

Port 1Port 3

Port 2Port 1

Port 3Port 2

Exp. No. :

Date :

DIRECTIONAL COUPLERAIM:To measure the coupling factor, insertion loss and directivityEQUIPMENTS:Standard microwave test bench and the directional coupler under test and matched load.PROCEDURE:1Assemble the equipment as shown in figure with VSWR as indicating meter


3Adjust for maximum power o/p (~30dB). Note the o/p power in VSWR meter (Pi)

4Insert the test directional coupler in the forward direction between frequency meter and detector mount without disturbing the bench and connect the detector to the auxiliary arm with the main arm o/p terminated in matched load

5 Note the auxiliary arm o/p power in VSWR meter (Pao), ie., the power coupled to the auxiliary arm. Also calculate the coupling factor (C) as Pi-Pao.

6Now carefully interchange the detector of the auxiliary line o/p and matched load of the main line without disturbing the setup.

7Note the main arm o/p power in VSWR meter (Po), and calculate insertion loss (L) as Pi-Po

8Restore the original arrangement, with the exception that the directional coupler under test be in the reverse direction.

9Note the auxiliary arm o/p power in VSWR meter (Pd), it is assumed that Pi is same as before. The directivity (D) is calculated as Pao-Pd

10 Compute the isolation (I) as Pi-Pd and also check I=C+DRESULT

Figure: Directional coupler

K Kly

Figure: Setup for studying isolator & circulator

Calculation:


Table: Directional coupler

Sl noPi (db)Pao (db)Po (db)Pd (db)C (dB)L (dB)D (dB)I = C+D (dB)

1

2

Exp. No. :

Date :

E-PLANE TEE & H-PLANE TEEAIM:

To study the properties of E- and H-plane tee junctions and to determine isolations and coupling coefficients.

EQUIPMENTS:

Standard microwave test bench and the E- and H-plane tee junctions under test and matched load

PROCEDURE:1Assemble the equipment as shown in figure 3.1 with VSWR as indicating meter


3Adjust for maximum power o/p (~30dB). Note the o/p power in VSWR meter (Pi)

4Insert the test E-plane tee junction between frequency meter and detector mount without disturbing the bench. and connect the detector to one of the ports and terminate the other in a matched load.

5Determine the isolation in decibels by noting the o/p level in the VSWR meter (db scale)

6Interchange the position of the detector and matched load and determine the isolation in decibels by noting the change in the o/p level

7 Repeat steps 4, 5 and 6 for other orientations of the tee as well as for H-plane tee also.

8 Calculate the coupling coefficient from C=10 (-/20), where is the attenuation. Substitute the isolation calculated in each case as and hence calculate coupling coefficient.

RESULT:

K Kly

Figure: Setup for studying E, H-plane

Calculation:


Table: Insertion loss

i/p portPower at i/p (dB)o/p portPower at o/p (dB)Insertion loss (dB)

Port 1

Port 2I12 =

Port 3I13 =

Port 2Port 1I21 =

Port 3I23 =

Port 3Port 1I31 =

Port 2I32 =

1 = (I12+I21)/2 =

2 = (I23+I32)/2 =

3 = (I13+I31)/2

Hence the corresponding coupling factors are

C1 =

C2 =

C3 =

Exp. No. :

Date :

LOW VSWR MEASUREMENTAIM:

To study the method of measuring VSWR at the input of the component under test or unknown load when VSWR < 10

EQUIPMENTS:Standard microwave test bench and slotted section slide screw tuner and components under test.

PROCEDURE: 1Assemble the equipment as shown in figure with microammeter as indicating meter

2Terminate the load end with the given slide screw tuner followed by the matched load.


4Tune the detector by adjusting the stub length for maximum meter deflection. Also set the slide screw tuner for the desired VSWR

5Set the probe position for maximum reading on microammeter. That is at voltage maximum of the standing wave. Record the meter reading in the table

6Move the probe along the slotted line and adjust it at minimum position, read and record in table.

7 Calculate the VSWR as (Imax/Imin)1/2.

8Repeat the same for different components.

If a VSWR meter is used in place of microammeter, set the klystron for square wave modulation and follow the following steps

9Adjust the gain of the VSWR meter amplifier to give full scale deflection to read 1.00 on the VSWR meter at the voltage maximum (step 5)

10Move the probe along the slotted line and adjust it at minimum position, note the VSWR meter reading. This gives the VSWR directly when the maximum is set for 1.00

Note 1: A slide screw tuner can be used between the slotted section and the component for generating any desired VSWR between 1 and 10.

Note 2: A matched load has a VSWR close to unity. Hence to measure the VSWR of this matched load, the slide screw tuner setup should be removed and only the matched load is connected. Then the measurement procedure is same as before, but since the VSWR is close to unity, VSWR reading is taken after the VSWR meter scale is set in the 1 to range. If the reading is less than 1.3, then it is changed over to EXPANDED SWR scale when the slotted line probe is in the field minimum position. This reading gives the VSWR of the matched load which will be close to unity ( do the experiment for

1) open ended wave guide 2) a blank flange 3) a terminated fixed attenuator

Note 3: Unloaded Q of a Cavity resonator. Connect the given test cavity in place of slide screw tuner and matched load. Measure the VSWR for different frequencies and compute the Q of the cavity as explained in appendix.

RESULT:

Standing wave patterns

K Kly

Figure: Setup for studying VSWRCalculation:

Table: Low VSWR measurement

Sl no

Maximum reading

[I max (A)]Minimum reading

[I min (A)]S=

[ In VSWR meter set the maximum reading as 1, hence minimum reading directly gives the VSWR]

Table: Low VSWR measurement ( VSWR meter)

Sl no

S=

Calculation : Q of a resonant cavity

f1 =

f2=

fo=

Q = (f2=f1)/fo =

Exp. No. :

Date :

HIGH VSWR MEASUREMENTAIM:

To study the method of measuring VSWR at the input of the component under test or unknown load when VSWR > 10

EQUIPMENTS:Standard microwave test bench and slotted section slide screw tuner and components under test.

PROCEDURE: 1Assemble the equipment as shown in figure with microammeter as indicating meter

2Terminate the load end with the given slide screw tuner followed by the matched load.


4Tune the detector by adjusting the stub length for maximum meter deflection. Also set the slide screw tuner for the desired VSWR (>10)

5 Measure and record minimum meter reading with probe carriage at the minimum of standing wave pattern

6Move the probe carriage by a distance less than /4 towards generator and record its position (x1) in table where meter reading is double of the minimum reading set in step 5

7Move the probe carriage towards load by a distance less than /2 to a position when meter reading is again doubled. Record its position (x2) in table.

8Compute the SWR using S= g/(x2-x1)

If a VSWR meter is used in place of microammeter, set the klystron for square wave modulation and follow the following steps

9Set the minimum meter reading of step 5 as 3db in the VSWR meter

10Move the probe carriage by a distance less than /4 towards generator and record its position (x1) in table where meter reading is 0.00db

11Move the probe carriage towards load by a distance less than /2 to a position when meter reading is again 0.00db. Record its position (x2) in table.

12Compute the SWR using S= g/(x2-x1)

The guide wave length g is measured as twice the distance between two successive minima

The experiment can be repeated for different frequencies

RESULT:

X1 X2

Figure: Double minima method for VSWR measurement

K Kly

Figure: Setup for studying High VSWR Calculation:

Guide Wave length =


Table: High VSWR measurement

Sl no

Minimum reading

[I min (A)]X1(cm)X2 (cm)S= g/(x2-x1)

Exp. No. :

Date :

IMPEDANCE MEASUREMENTAIM: To measure the unknown impedance of the given device

EQUIPMENTS:

Standard microwave test bench, slotted section, iris backed with a matched load (a flat plate with round hole) as unknown impedance and blank flange.

PROCEDURE:1Assemble the equipment as shown in figure with VSWR meter as indicating meter


3Adjust for maximum power o/p (~30dB).

4Terminate the slotted section with the blank flange and accurately measure the position of the voltage minima (S1). (See figure) .Also calculate guide wave length.

4Connect the device under test in place of blank flange and measure VSWR and the new positions of the minima (X1). The VSWR can be used to calculate magnitude and the shift in position of the minima for calculating the phase of the reflection coefficient. Note down lmin = S1-X1. Also determine the VSWR value S and g.

5Compute the unknown load impedance of the given device from the lmin, S and g analytically as ZL=Zo (1-jStanlmin)/ (S-jtanlmin) or by using smith chart.

RESULT:

K Kly

Figure: Setup for Impedance measurementCalculation:

Table: Impedace measurement

Sl. noS1 (cm)S2 (cm)X1(cm)g (cm)lmin (cm)

Guide wave length =

VSWR, S =

Zmin =

= 2/g =

ZL=Zo (1-jStanlmin)/ (S-jtanlmin) =

Exp. No. :

Date :

DIELECTRIC CONSTANT MEASUREMENTAIM:

To study the two-point method for measuring dielectric constant of the given low loss solid dielectric (Teflon)

EQUIPMENTS:Standard microwave test bench and slotted section samples of given dielectric and blank flange. PROCEDURE1Assemble the equipment as shown in figure with microammeter as indicating meter. Terminate the load end with the short circuit.


3Tune the detector by adjusting the stub length for maximum meter deflection.

4With no sample in the shorted wave guide, measure and record in the table the positions of standing wave minima, starting from any arbitrary plane d =0 as shown in figure. Compute guide wavelength. The position of the first minima is taken as d1min

5 Using frequency meter determine the frequency of operation and compute the free space wavelength (verify the value by using g and c=2a)

6Remove the short circuit, insert gently the dielectric sample and replace the short-circuit in such a manner that it touches the end of the sample

7Measure and record in table the position of the standing wave voltage minima from the same reference plane as chosen in step 5. The position of the first minima (with dielectric is taken as d2min)

8Find out the VSWR.

9 Measure and record waveguide dimensions. (For necessary theory refer appendix)

10Calculate the dielectric constant (detailed description of the calculations are shown in appendix)

RESULT:

Figure: Setup for measuring the dielectric constant

Calculations:

Waveguide dimension , a=

b=

Cut of wavelength c (=2a) =


Guide wave length g (without dielectric) =

Guide wavelength d (with dielectric) =

Table: Dielectric constant

Sl noSampleThickness t1 (cm)d 1min (cm)d2min (cm)tan (1+t1))/

( t1)tan (dt1)/ (d t1)Possible values of X

X1 =

X2 =

X3=

X1 =

X2 =

X3=

X1 =

X2 =

X3=

Exp. No. :

Date :

HORN ANTENNAAIM:To study the E-plane and H-plane radiation pattern of a pyramidal horn antenna and compute

a) Beam width

b) Directional gain of the antenna.

EQUIPMENTS:Standard microwave test bench, horn antennae, waveguide twist etc.

.PROCEDURE:1Assemble the equipment as shown in figure with horns properly aligned and distance between antennas equal to double of rmin > 2D2/o . E vector must be parallel to the ground

2Fire the Klystron correctly for CW operation with optimum beam voltage micrometer as indicating meter

3Adjust for maximum power o/p (~30dB). Record this power received in the table

4Rotate the receiving antenna in steps of 10deg. On both the sides and enter it in the table

5Plot the E plane radiation pattern and determine the half power beam width E. compare it with the theoretical value (E.=53o/b)

6Repeat the procedure after replacing the waveguide twists with a straight wave guide and compute H (H.=80o/a)

7Compute the directional gain of the antenna using G= 13326/ E H8Compare it with the theoretical value of G=2ab/ o2

Figure: Setup for studying Horn antenna

Figure: Antenna radiation pattern, rectangular plot

Table: Antenna field pattern

Sl noE-plane settingH-plane setting

Angular setting (deg)o/p current (A)Angular settingo/p current (A)

LeftrightLeftright

155

21010

31515

42020

52525

63030

73535

84040

94545

105050

115555

126060

136565

147070

157575

168080

178585

189090

Calculations:

Waveguide dimension, a =

b =


Free space wavelength =

E.=53o/b =

H.=80o/a =

G= 13326/ E H =

G=2ab/ o2 =

RESULT:Course Name: MICROWAVE LABORATORY

Instructions to the students:

1General:

All laws and rules of the SASTRA University apply to the use of this lab. In particular, uniform and attendance are compulsory. It is the responsibility of any user of University facilities to know and abide by all University regulations.

2Lab Reports:

Lab reports are required of individual students, and are due two weeks after the corresponding experiment has been completed. Students are encouraged to keep a lab notebook to record original data, equipment layout, and notes about the experiment. Reports should be neat and clearly organized, and should include original data sheets. Graphs should be neatly drawn, either using a computer graphics package, or by hand. Each graph axis of a graph must include a title and units. 3Care of Equipment:

Please be very careful with the microwave test equipment, as it is very delicate, and expensive to repair or replace. If you suspect something is not operating correctly, report it to the lab technician. Be especially careful when using connectors to avoid breaking pins and cross-threading. If at any time you are uncertain about lab safety, please ask the lab-in-charge before proceeding.Make sure that all equipment you used has been turned off and returned to the place you

obtained it from.

4Microwave Radiation Hazards

Excessive exposure to electromagnetic fields, including microwave radiation, can be harmful. Although the power levels used in Microwave Instructional Lab are very low and should not present a health risk, it is still prudent to,

be aware of the recommended safe power limits

be aware of the power densities with which you will be working

use good work habits to minimize exposure to radiated fields

The question of what is a "safe" radiation level is controversial; like highway speed limits, all we can say with total certainty is that less is safer. Microwave radiation is nonionizing, so the main biological effect is induced heating, which may occur relatively deep inside the body to affect sensitive organs. Health risks increase according to the power density and the duration of the exposure. The eye is the most sensitive organ, and studies have shown that cataracts can develop from exposures as short as 1.5 hours to power densities of 150 mW/cm2. Thus, using a safety factor of more than 10, the current US safety standard, C95.1-1991, recommends a maximum exposure power density of 10 m W/cm2, at frequencies above 10 GHz, with lower levels at lower frequencies. By comparison, the power density from the sun on a clear day is about 100 mW/cm2, but most of this power is beyond the microwave spectrum, and so does not enter deeply into the body. The sources used in the Microwave Instructional Laboratory, such as klystron generators and Gunn diodes, have power outputs in the 10 - 15 m W range. In most cases, there is little danger of being exposed to radiation at these power levels because our experiments use coaxial lines or waveguide, which provide a high degree of shielding. It is possible, however, to encounter power densities near the US recommended limit at the end of an open-ended coaxial cable or waveguide. Such power densities exist only right at the open end of the coax line or waveguide, due to the 1/r2 decrease of radiated power with distance. For example, at a distance of 10 cm from a waveguide flange with an

input power of 20 mW, the Friis formula gives the power density as,

which is seen to be far below the recommend safety limits.

Even though there should be little danger from microwave radiation hazards in the lab, the following work habits are recommended whenever working with RF or microwave equipment:

Never look into the open end of a waveguide or transmission line that is connected to other equipment.

Do not place any part of your body against the open end of a waveguide or transmission line.

Turn off the microwave power source when assembling or disassembling componentsAppendix- A

Microwave Test Bench

A standard microwave experimental setup is shown in the following figure. The explanation of the important components is also follows

Figure: Setup for a Microwave test bench

1Reflex Klystron and Gunn Diode Oscillator.

Measurement of any electrical parameter requires a signal source. For microwave measurements we need a microwave source which generates signal at microwave frequencies. In the laboratory these microwave signals are produced either by a Reflex Klystron which is an electron tube or a Gunn diode oscillator which based on microwave semiconductor. In Klystron the kinetic energy of electrons are converted into microwaves by velocity modulation of electron beam inside the tube. Important parts of a Reflex Klystron are cathode, cavity resonator, and repeller.

The Reflex Klystron uses two types of regulated voltages, a high positive dc voltage called beam voltage for the accelerating grid and a high negative dc voltage for the repeller. Both the voltages can be varied. These voltages are produced separately in a regulated power supply unit. Microwave frequencies are of the order of GHz. It cannot be measured by a CRO which has a frequency range of few MHz. Therefore the output of Reflex Klystron has to be amplitude modulated and this is done by superimposing 1KHz square wave modulating signal on the dc repeller voltage. The power output from a Reflex Klystron used in the laboratory produces a few milliwatts of power.

Another microwave source is Gunn diode oscillator which needs a low voltage of order of few volts. Gunn diode oscillations are produced when two terminal bulk device made of GaAs or InP is subjected to high electric fields. At these high electric fields, the devices exhibit negative differential conductivity and so they produce oscillations at microwave frequencies when they are located in in a microwave cavity resonator. As in the case Klystron the microwave signals have to be amplitude modulated. In a Gunn diode oscillator this is done either by superimposing a 1 KHz square wave signal on the dc bias source or by a PIN modulator externally.

2Ferrite Isolator.

An isolator is a two port device which produces minimum attenuation for propagating signals in one direction and very high attenuation in the opposite direction. This property is exhibited when a microwave ferrite material in a waveguide is subjected to a steady magnetic field. When the isolator is inserted between the microwave source and the load, almost all power is transmitted to the load, but the reflected signal from the load is not fedback to the source. This eliminates the undesirable variations in source output or frequency due to changing loads. Thus the ferrite isolator effectively isolates the source and the load for the reflected signal. The amount of isolation provided by an isolator is of the order of 20-30 dB in the backward direction. The insertion loss is about 0.5 dB.

3Frequency Meter.

A frequency meter is of absorption type resonator is a variable cavity resonator and is weakly coupled into the mainline. This is tuned to resonance by adjusting the length of the cavity using a micrometer if it is an indirect type or a drum if it is direct type. When the frequency meter is adjusted to resonance, it throws a weak short circuit across its point of connection with the main guide and hence a dip in the meter reading results in the VSWR meter. In indirect type frequency meter the frequency is found by a calibration chart which gives the relation between micrometer reading and microwave frequencies. In direct type frequency meter the frequencies are given on the drum.

4Variable Attenuator.

Microwave attenuators or pads are used to control the level of signal power from the microwave source to the load to the required level. In the flap type attenuator a resistive card enters the waveguide through a slot in the broad wall, thereby intercepting and absorbing a portion of the electromagnetic wave. The amount of attenuation is varied from zero to a maximum value of about 30dB.

5Slotted Line.

A slotted line is a piece of transmission line like rectangular waveguide with a slot in that. The width of the slot is small enough to avoid any distortion in the original field configuration inside the waveguide. However, the slot is big enough to accommodate a coaxial E-field probe mounted on a carriage which could be moved along the slot at a constant depth to achieve uniform coupling coefficient between the electric field inside the waveguide and the probe in all positions. The length of the slot will be about 15cm, enough to cover 2-3 wavelengths to ensure the possibility of finding several maxima and minima of standing wave pattern. A scale is fixed to the guide and a Vernier scale is fixed to the probe carriage for length measurements. The probe fixed in the probe carriage generally can moved up and down by screw mechanism to the control depth of penetration. If the penetration is too deep, it may function as obstacle and disturb the field lines inside the guide. If the too small, then it provides loose coupling to the electric field and the voltage developed in the probe may be too small for measurement.

6Diode Detector.

The crystal diode detector used in the slotted line a microwave crystal rectifier consisting of a tungsten wire that is pointed to diameters of about 2.5microns. This is pressed against a n-type or p-type silicon chip. The above metal-semiconductor combination when suitably processed results in a rectifying contact and hence can be used for detecting the microwaves. At power levels below 0.01mw (-20dB) the crystal behaves like a square law detector and as a linear detector at 0.1mw (-10dB)

The current-voltage relation of a point contact diode such as 1N23B used as a detector element in the probe is of the form

Id = I0(eaV 1)

where I0 and a are diode constants. At low power level, the voltage applied to the detector will be very low and therefore approximating we get

Id = I0 ( aV + a2V2/2 + )

If we assume the voltage applied to a microwave detector is V = V0cost, the above equation becomes

Id = I0 { a V0cost+ (a2V2/4)( 1+ cos2t)}

The ac components are filtered out by the by-pass capacitors thus giving

Id = I0a2V02/4 (or) Id = kV0 where k = I0a2/4

This equation shows that for small signals picked up by the probe from the field in the waveguide, the crystal detector follows square-law and the detector is called a square-law detector. Corresponding to voltage maximum and voltage minimum on the line, the detector current will be

Idmax = kVmax2 and Idmin = kVmin2

Therefore the standing wave ratio becomes

S = Vmax/Vmin = (Idmax/Idmin)

So the VSWR in a slotted line is given by the square root of the ratio of maximum diode current to minimum diode current.

The output of the detector could either be applied to an ammeter or to a VSWR meter to measure the VSWR.

7VSWR Meter

The VSWR meter is a high gain voltage amplifier tuned to a fixed frequency of 1 KHz. The amplifier has a variable gain and the maximum gain possible is about 60dB which could be altered in steps of 10dB. There is also a course gain control and a fine gain control for continuous variation of gain. There are three scales for measurement of VSWR and each scale can be selected through a switch. One scale has the VSWR range from 1 to and another one from 3 to 10. For measurement of VSWR less than 1.5, there is an expanded scale which can measure VSWR from 1 to 1.3, and this ensures accuracy of measurement when VSWR is low. There is also dB scale and dB expanded scale which will be useful for measurement off high VSWR. The klystron should be operated in AM-MOD (square modulated) mode when it is used with the VSWR Meter.

Appendix-B

Two point method of measuring dielectric constant.

In this method a rectangular waveguide terminated by a metal plate act as a short circuit is connected to the o/p end of the slotted section. The voltage minimum (d1min) is located. The dielectric under test is shaped so as to fit into the waveguide. The sample thickness t1 fitted into the waveguide end and is backed by a short circuit metal plate. The new voltage minimum (d2min) is located by using the impedance relation

Zd tan (d d1min) = -Zo tan ( d2min) ---- 1

The following transcendental equation can be derived

tan (d t1)/ d t1 = tan ( (1+t1))/ t1

where 1=d1min-d2min is the shift in the position of minimum; =2/g and d denote the phase constants in the empty waveguide and the dielectric, respectively. The equation is solve for d t1 and the dielectric constant of the sample is calculated from the relation

d=2/o[rr-( o(2a))]1/2

where r =1 and a width of the guide

The solution of eqn.1 gives multiple roots. The measurement is repeated with a second ample of thickness t2 and d is determined. The correct solution is the one common to both the measurements.

Appendix- C:

Determination of Cavity Q by VSWR measurement.

Cavities such as those incorporated within a tube normally have only one coupling such as those incorporated within a tube normally have only one coupling connection, that through which power is extracted in operation. For such a cavity, the half power frequencies are determined by impedance measurement. This is because the VSWR set up in the line that feeds the resonator is uniquely related to the equivalent cavity impedance in which the line is terminated. Half power frequencies may be found directly from VSWR measurements. The circuit used for VSWR measurement can be used for this purpose. Terminate the bench with the single cavity resonator. Note down the VSWR value for various frequency values and plot frequency versus S value graph. From the graph note down fo, f1 and f2 and hence calculate Q as Q=(f2-f1)/fo

Exp. No. :

Date :

STUDY OF LOSSES IN OPTICAL FIBER

AIM: To measure propagation loss and bending loss for two plastic fibers.

EQUIPMENTS: Link-B kit with power supply, patch chords, 20MHz dual channel oscilloscope,

1MHz function generator, 1&3m fiber cable.

PROCEDURE:1. Make the connections. Connect the power supply cables with proper polarities to link-B kit. While connecting this ensures that the power supply is OFF.

2. Keep the SW9 towards TX1 position for SFH756

3. Keep jumpers and SW8 positions as shown in fig.

4. Keep intensity control pot P2 towards minimum position.

5. Switch ON power supply.

6. Feed about 2Vp-p sinusoidal signal of 1 kHz from the function generator to the IN post of analog buffer.

7. Connect the output post OUT of analog buffer to the post TX IN of transmitter.

8. Slightly unscrew the cap of SFH756 V (660nm).Do not remove the cap from the connector. Once the cap is loosened, insert the 1m fiber in to the cap. Now tighten the cap by unscrewing it back

9. Connect the other end of the fiber to the detector SFH350V (phototransistor

detector) very carefully as per the instructions above.

10. Observe the detected signal at post ANALOG OUT on oscilloscope. Adjust

intensity control pot P2 optical power control potentiometer so that you receive

signal of 2V p-p.

11. Measure the peak value of received signal at ANALOG OUT terminal. Let this value be V1.

12. Now replace 1m fiber with 3m fiber between same Led and detector. Do not disturb any settings. Again take peak voltage reading and let it be V2.

13. If is the attenuation of the fiber then we have P1/P2 =V1/V2= e[-(L1+L2)] where

Where = nepers/meter,

L1= Fiber length for V1

L2 = Fiber length for V2

This is for peak wave length of 660nm.

14. Switch OFF the power supply.

15. Keep SW9 towards TX1 position for SFH 756.

16. Set the jumpers to form simple analog link using LED SFH 450 V at 950nm and

phototransistor SFH 350V with 1m fiber cable.

17. Switch ON power supply.

18. Repeat the same procedure as above for this link to get at 950 nm and compare

the values.

MEASUREMENT OF BENDING LOSS:1. Set up the 660 nm analog link using 1m fiber as per the above procedure.

2. Bend the fiber in a loop and measure the amplitude of received signal.

3. Keep reducing the diameter of bend to about 2 cm and take corresponding out voltage

readings (do not reduce loop diameter less than 1cm).

4. Plot the graph of received signal amplitude versus loop diameter repeat the procedure for second transmitter.

RESULT:Exp. No. :

Date :

STUDY OF NUMERICAL APERTURE OF OPTICAL FIBER

AIM:

To measure the numerical aperture of optical fiber provided with the kit using

650nm wavelength LED and find the V number.

EQUIPMENTS:FCL-01, 1m fiber cable, ruler, power supply, NA JIG.

PROCEDURE:1. Make connections as shown in fig. Connect the power supply cables with proper polarity to FCL -01 kit. While connecting this, ensure that power supply is OFF.

2. Slightly unscrew the cap of LED SFH 756V (660 nm).Do not remove the cap from the connector. Once the cap is loosened, insert the fiber in to the cap. Now tighten the cap by screwing it back.

3. Keep the jumpers JP1, JP2 and JP4 on FCL 01 as shown in figure.

4. Keep the switch S2 in VI position.

5. Switch ON the power supply.

6. Insert the other end of the fiber in to the numerical aperture measurement JIG. Hold the white sheet facing the fiber. Adjust the fiber such that its cut face is perpendicular to the axis of fiber.

7. Keep the distance of about 10mm between fiber tip and screen. Gently tighten the screw and thus fix the fiber in the place.

8. Observe the bright red spot on the screen by varying intensity pot P3 and bias pot P4.

9. Measure exactly the distance d and also the vertical and horizontal diameters MR and PN as indicated in the figure

10. Mean radius is calculate using the formulae r = (MR+PN)/4

11. Find the numerical aperture of the fiber using the formulae

NA= sinmax = r / d2+r2 where max is the maximum angle at which light

incident is properly transmitted through the fiber.

12. V- number can be calculated using the formulae:

V=2* NA *a /

where a is the core radius of the fiber, NA is the Numerical aperture, is the wavelength .

RESULT:Exp. No. :

Date :

SETTING UP A FIBER OPTIC ANALOG LINK

AIM:

To study fiber optic analog link

EQUIPMENTS:FCL-01 and FCL-02, FCL-01 with power cable, 1m fiber cable, power supply, 20MHz dual channel oscilloscope, patch chords.

PROCEDURE:1. Make the connections. Connect the power supply cables with proper polarities to

FCL-01 and FCL-02kit.While connecting this ensure that the power supply is OFF.

2. Connect function generator FG-01 to FCL-01 using power cable.

3. Keep the jumpers JP1, JP2, JP3 and JP4 on FCL-01 as shown in figure

4. Keep the jumpers JP1&JP2 on FCL-02 as shown in figure.

5. Keep switch S2 in TX IN position on FCL-01.

6. Switch ON the power supply

7. Connect 1 KHz 2Vp-p signal from FG-01 as a constant signal to the IN post of analog buffer on FCL-01.

8. Connect the output of analog buffer post OUT to post TX IN

9. Slightly unscrew the cap of LED SFH756B (660 nm).Do not remove the cap from the connector. Once the cap is loosened insert the fiber in to the cap. Now tighten the cap by screwing it back.

10. Now rotate the optical power control pot P3 in FCL-01 in anti clock wise direction. This ensures maximum current flow through LED.

11. Slightly unscrew the cap of photodiode SFH250V.Do not remove the cap from the connector. Once the cap is loosened, insert the other end of the fiber in to the cap. Now tighten the cap by screwing it back.

12. Keep switch SW1 to ANALOG OUT position in FCL02

13. Observe the output signal from the detector at OUT post on oscilloscope by adjusting optical power control pot P3 on FCL-01 and you should get the original transmitted signal

14. To measure the analog bandwidth of the link keeps the same connections and varies the frequency of input signal from 100 Hz onwards. Measure the amplitude of the received signal for each frequency read.

15. Plot a graph of amplitude-frequency. Measure the frequency range for which response is flat.

16. Perform the above procedure again for all combinations of transmitter and receiver.

RESULT:Exp. No. :

Date :

STUDY OF CHARACTERISTICS OF FIBER OPTIC LED AND PHOTODETECTOR:

AIM:

To study the VI characteristics of fiber optic LED and plot the graph of forward current versus output optical energy and also to study the photo detector response.

EQUIPMENTS: FCL-01 & FCL-02, 1meter Fiber cable, Patch chords, Jumper to crocodile wires, Power Supply, Voltmeter, Current meter, 20 MHz Dual Channel Oscilloscope.

PROCEDURE:

1. Make connections as shown in figure. Connect the power supply with proper polarity to FCL-01 & FCL-02 Kits. While connecting this, ensure that the power supply is off.

2. Slightly unscrew the cap of LED SFH 756V (660 nm) .Dont remove the cap from the connector. Once the cap is loosened, insert the 1meter fiber into the cap. Now tighten the cap by screwing it back.

3. Slightly unscrew the cap of Photo detector SFH 250V. Dont remove the cap from the connector. Once the cap is loosened, insert the other end of the fiber into the cap. Now tighten the cap by screwing it back.

4. Keep the jumpers JP1, JP2, JP3 & JP4 on FCL-01 as shown in figure.

5. Keep jumpers JP1 &JP2 on FCL-02 as shown in figure.

6. Keep switch S2 in VI position on FCL-01

7. Connect voltmeter and current meter as per the polarities shown in figure.

8. Switch on the power supply.

9. Keep the potentiometer P3 in its maximum position.P3 is used to control current flowing through LED.

10. . Keep potentiometer P4 in its fully clockwise rotation.P4 is used to control bias voltage of LED.

11. To get the VI characteristics of LED, rotate P3 slowly and measure the forward current and corresponding forward voltage.

12. For each reading taken above, find out power, which is the product of I and V. This is the electrical power supplied to the LED. Data sheets for the Led specify optical power coupled into plastic fiber when forward current was 10 mA as 200 W. This means that the electrical power at 10 mA current is converted into 200 W of optical energy. Hence the efficiency of LED comes out to be approximately 1.15%.

13. With this efficiency assumed, find out optical power coupled into plastic optical fiber for each of the reading. Plot the graph of forward current versus output optical power of the LED.

14. Similarly measure the current at the detector.

15. Plot the graph of receiver current versus output optical power of the LED.

16. Perform the above procedure again for all the combinations of the Transmitter & Receiver.

RESULT:

Exp. No. :

Date :

VI CHARACTERISTICS OF FIBER OPTIC LED & DETECTORVf(V)If (mA)Pi(mW)Po(W)V (V)I (A)

Vf= Forward voltage of LED SFH 756

If=Forward Current of LED SHF 756

Pi =V* I (Electrical Power)

Po=Pi* 1.15 % (Optical power of LED 756)

V=Output Voltage of SFH 250

I=Output Current of SFH 250

Klystron Power Supply

Klystron Mount + Tube

Frequency meter

Variable attenuator

Isolator

Detector mount

Micro ammeter

VSWR Meter

Microwave Ok

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