Final Year ECE PPG Institute of Technology

Department of ECE
EC6701 RF AND MICROWAVE ENGINEERING LTPC3003

OBJECTIVES:
 To inculcate understanding of the basics required for circuit representation of RF
networks.
 To deal with the issues in the design of microwave amplifier.
 To instill knowledge on the properties of various microwave components.
 To deal with the microwave generation and microwave measurement techniques
UNIT I TWO PORT NETWORK THEORY 9
Review of Low frequency parameters: Impedance, Admittance, Hybrid and ABCD
parameters, Different types of interconnection of Two port networks, High Frequency
parameters, Formulation of S parameters, Properties of S parameters, Reciprocal and
lossless Network, Transmission matrix, RF behavior of Resistors, Capacitors and
Inductors.

UNIT II RF AMPLIFIERS AND MATCHING NETWORKS 9

Characteristics of Amplifiers, Amplifier power relations, Stability considerations,
Stabilization Methods, Noise Figure, Constant VSWR, Broadband, High power and
Multistage Amplifiers, Impedance matching using discrete components, Two
component matching Networks, Frequency response and quality factor, T and Pi
Matching Networks, Microstrip Line Matching Networks.

UNIT III PASSIVE AND ACTIVE MICROWAVE DEVICES 9

Terminations, Attenuators, Phase shifters, Directional couplers, Hybrid Junctions,
Power dividers, Circulator, Isolator, Impedance matching devices: Tuning screw, Stub
and quarter wave transformers. Crystal and Schottkey diode detector and mixers, PIN
diode switch, Gunn diode oscillator, IMPATT diode oscillator and amplifier, Varactor
diode, Introduction to MIC.

UNIT IV MICROWAVE GENERATION 9

Review of conventional vacuum Triodes, Tetrodes and Pentodes, High frequency
effects in vacuum Tubes, Theory and application of Two cavity Klystron Amplifier,
Reflex Klystron oscillator, Traveling wave tube amplifier, Magnetron oscillator using
Cylindrical, Linear, Coaxial Voltage tunable Magnetrons, Backward wave Crossed field
amplifier and oscillator.

UNIT V MICROWAVE MEASUREMENTS 9

Measuring Instruments : Principle of operation and application of VSWR meter, Power
meter, Spectrum analyzer, Network analyzer, Measurement of Impedance, Frequency,
Power, VSWR, Q-factor, Dielectric constant, Scattering coefficients, Attenuation, S-
parameters.

TOTAL: 45 PERIODS
TEXT BOOKS:
1. Reinhold Ludwig and Gene Bogdanov, “RF Circuit Design: Theory and Applications”,
Pearson Education Inc., 2011
2. Robert E Colin, “Foundations for Microwave Engineering”, John Wiley & Sons Inc,
2005

REFERENCES:
1. David M. Pozar, “Microwave Engineering”, Wiley India (P) Ltd, New Delhi, 2008.
2. Thomas H Lee, “Planar Microwave Engineering: A Practical Guide to Theory,
Measurements and Circuits”, Cambridge University Press, 2004.
3. Mathew M Radmanesh, “RF and Microwave Electronics”, Prentice Hall, 2000.
4. Annapurna Das and Sisir K Das, “Microwave Engineering”, Tata Mc Graw Hill
Publishing Company Ltd, New Delhi, 2005

1
 Final Year ECE PPG Institute of Technology
Department of ECE
TWO MARKS

UNIT - I
1) Define scattering matrix.
Scattering matrix is a square matrix which gives all the combinations of power
relationships between the various input and output port of a microwave junction.

2) What are scattering coefficients?

The elements of scattering matrix are called scattering coefficients or
scattering parameters.

3) Why the S-parameters are used in microwaves?

The H, Y, Z and ABCD parameters are difficult at microwave frequencies due to
following reasons.
 Equipment is not readily available to measure total voltage and total current at the ports
of the networks.
 Short circuit and open circuit are difficult to achieve over a wide range of frequencies.
 Presence of active devices makes the circuit unstable for short (or) open circuit.
Therefore, microwave circuits are analyses using scattering (or) S parameters which
linearly relate the reflected wave’s amplitude with those of incident waves.

4) Write the properties of [S] matrix.

 [s] is always a square matrix of order (nxn) .
 Under perfect matched conditions, the diagonal elements of [s] are zero.

5) What is the zero property of S-matrix?

It states that, “for a passive lossless N-port network, the sum of the products of
each term of any row or any column multiplied by the complex conjugate of the
corresponding terms of any other row or column is zero”.

6) Write the unitary property for a lossless junction.

For any lossless network the sum of the products of each term of any one row or
of any column of the S-matrix multiplied by its complex conjugate is unity.

7) Mention the many forms of wire.

Wire in a circuit can takes on many forms,
I. Wire wound resistors
II. Wire wound inductors
III. Leaded capacitors
IV. Elements-to- element interconnection applications

8) Write about the skin effect in a wire.

As frequency increases, the electrical signal propagates less and less in the
inside of the conductor. The current density increases near the outside perimeter of the
wire and causes higher impedance for the signal. This will act as resistance of the wire.

R=ρl/A
Where,
A-Effective cross-sectional area.
When area (A) decreases, the resistance of the wire will be increases.

9) Give a short note on straight-wire Inductance in wire.

In the wire medium, surrounding any current carrying conductor, there exists a
magnetic field. If the current (I) is AC, this magnetic field is alternately expanding and
contracting. This produces an induced voltage in the wire that opposes any change in
the current flow. This opposition to change is called “self inductance”.

2
Final Year ECE PPG Institute of Technology
Department of ECE
10) Name the types of resistors.
Types of resistors:
o Carbon composition resistors, which have a high capacitance due to carbon
granules parasitic capacitance.
o Wire wound resistors, which have high lead inductance.
o Metal film resistors of temperature-stable materials.
o Thin-film chip resistors of aluminum or beryllium-based materials.

11) Define Quality-factor (Q) of Capacitor.

It is defined as “the measure of the ability of an element to store energy, equal to
2 times the average energy stored divided by the energy dissipated per cycle".

12) Write the applications of inductors.

Inductors have a variety of applications in RF circuits such as,
1. Resonance circuits
2. Filters
3. Phase shifters
4. Delay networks
5. RF chokes

UNIT – II

1) Write the function of matching networks?

Matching networks can help stabilize the amplifier by keeping the source and
load impedances in the appropriate range.

2) What is function of input and output matching networks?

Input and output matching networks are needed to reduce undesired reflections
and improve the power flow capabilities.

3)What are the parameters used to evaluate the performance of an

amplifier?
Key parameters of amplifier, to evaluate the performance are
i. Gain and gain flatness(in dB)
ii. Operating frequency and bandwidth (in Hz)
iii. Output power (in dB)
iv. Power supply requirements (in V and A)
v. Input and output reflection coefficients (VSWR)
vi. Noise figure (in dB)

4)Define transducer power gain.

Transducer power gain is nothing but the gain of the amplifier when placed between
source and
load.

¿ PL
G=Power Delivered ¿the load the source ¿=
Available power ¿ PA
5) Define unilateral power gain.
It is the amplifier power gain, when feedback effect of amplifier is
neglected i.e.S12=0.

6) What is available Power Gain (GA) at Load?

The available power gain for load side matching is given

¿ PN
G=Power Available ¿ the Network the source ¿=
Power Available ¿ PA

3
 Final Year ECE PPG Institute of Technology
Department of ECE
7) Define Operating Power Gain.
The operating power gain is defined as “the ratio of power delivered to the load to
the power supplied to the amplifier”.
¿ PL
G=Power delivered ¿ the load the amplifier ¿=
Power supplied ¿ P¿

8)Write a short note on feedback of RF circuit.

i. If |β|>1, then the magnitude of the return voltage wave increases called positive
feedback, which causes instability (oscillator).
ii. If |β|<1, then the return voltage wave is totally avoided (amplifier). It’s called as
negative feedback.

9) Define unconditional stability.

Unconditional stability refers to the situation where the amplifier remains stable
for any passive source and load at the selected frequency and bias conditions.

10) Define noise figure.

Noise figure F is defined as “the ratio of the input SNR to the output SNR”.

Input SNR
F=
Output SNR
11) What are the properties of scattering matrix for a lossless junction?
1. The product of any column of the S-matrix with conjugate of this column
equals unity.
2. The product of any column of the scattering matrix with the complex
conjugate of any other column is zero.

12) What is transmission matrix?

When a number of microwave devices are connected in cascade. Each junction is
represented by a transmission matrix which gives the output quantities in terms of input
quantities.

UNIT – III

1) Enumerate the basic advantage of microwaves.

 Fewer repeaters are necessary for amplification.

 Minimal cross talk exists between voice channels.
 Increased reliability and less maintenance are important factors.
 Increased bandwidth availability.

2) Write the applications of microwaves.

a. Microwave becomes a very powerful tool in microwave radio spectroscopy for

analysis.
b. Microwave landing system (MLS), used to guide aircraft to land safety at airports.
c. Special microwave equipment known as diathermy machines are used in
medicine for heating body muscles and tissues without hurting the skin.
d. Microwave ovens are a common appliance in most kitchens today.

3) What is waveguide & its uses?

A waveguide is a hollow metal tube designed to carry microwave energy from
one place to another.
Some uses of waveguide tees:
It is used to connect a branch or section of the waveguide in series or
parallel with the power in a waveguide system.

4
 Final Year ECE PPG Institute of Technology
Department of ECE
4) What is H-plane Tee & E-Plane tee?
An H-plane tee is a waveguide tee in which the axis of its side arm is parallel to
the H-field of the main guide.
An E-plane tee is a waveguide tee in which the axis of its side arm is parallel to
the E-field of the main guide.

5) Define difference arm.

In E-plane tee, the power out of port 3 is proportional to the difference between
instantaneous powers entering from port 1 and port 2. Therefore, this third port is called
as difference arm.

6) What is sum arm?

In a H=plane tee, if two input waves are fed into port1 and port2 of the collinear
arm, the output wave at port3will be in phase and additive. Because of this, the third
port is called as sum arm.

7) Write the applications of magic tee.

A magic tee has several applications,
i. Measurement of impedance
ii. As duplexer
iii. As mixer
iv. As an isolator

8) Define coupling factor(C).

The coupling factor of a directional coupler is defined as the ratio of the incident
power ‘pi’ to the forward power ‘pf’ measured in dB
Pi
Coupling factor (dB)=10 log
Pf
The coupling factor is a measure of how much of the incident power is being sampled.

9) Define directivity of directional coupler.

The directivity of a directional coupler is defined as the ratio of forward power ‘p’
to the back power ‘p’ expressed in Db.
Pf
Directivity (dB)=10 log
Pb
10) Write the characteristics of a three port tee junction.
a) A short circuit may always be placed in one of the arms of a three port junction in
such a way that no power can be transferred through the other two arms.
b) If the junction is symmetric about of its arms, a short circuit can always be placed in
that arm so that no reflections occur in power transmission between the other two arms.
c) It is impossible for a general three port junction of arbitrary to present matched
impedances at all three arms.

11)Write the properties of ferrites.

Properties of ferrites:
 Ferrites possess strong magnetic properties.
 Ferrites are most suitable for use in microwave device in order to reduce the
reflected power.
 Ferrites possess high resistivity, hence they can be used up to 100 GHz
 Ferrites also exhibit non-reciprocal property.

12)Define 4-port circulator.

A 4-port circulator which is a non-reciprocal component very similar to the 3-port
circulator. All the four ports are matched and transmission of power takes place in
cyclic order only, that is, from port 1 to port2, port 2 to port 3, port 3 to port 4 and from
port 4 to port 1.

UNIT - IV
5
Final Year ECE PPG Institute of Technology
Department of ECE
1)Name the two configuration of klystron
There are two basic configurations of Klystron tubes
a. Reflex Klystron – It is used as low power microwave oscillator
b. Two cavity (or) Multicavity Klystron – It is used as low power microwave
amplifier.

2)Define bunching.
The electrons passing the first cavity gap at zeros of the gap voltage pass through
with unchanged velocity, those passing through the +ive half cycles of gap voltage
undergo an increase in velocity, those passing through the –ive half cycles of gap
voltage undergo an decrease in velocity, As a result of these, electron bunch together
in drift space. This is called bunching.

3)Define reflex klystron.

The reflex klystron is an oscillator with a built in feedback mechanism. It uses the cavity
for bunching and for the output cavity.

4)Give the comparison between TWTA and klystron amplifier.

Comparison between TWTA and klystron amplifier is,
Sl.N
o Klystron amplifier TWTA
1. Linear beam or ‘O’ type device. Linear beamor ‘O’ type device.
Uses cavities for input and output
2.
circuits Uses non – resonant wave circuit.
Narrow band device due to use of Wide band device because use of
3.
resonant cavities non – resonant wave circuit

5)Write short notes on negative resistance magnetron.

Negative – resistance magnetrons ordinarily operate at frequencies below the
microwave region. This type of magnetron uses a static negative resistance between
two anode segments but has low efficiency and is useful only at low frequencies.

6)Write short notes on

 Coaxial magnetron
 Voltage – tunable magnetron
Coaxial magnetron:
The coaxial magnetron is composed of an anode resonator structure surrounded by an
inner – single, high-Q cavity operating in the TE011.
Voltage tunable magnetron:
The voltage tunable magnetron is a broadband oscillator with frequency changed by
varying the applied voltage between the anode and sole.

7)State the power output and efficiency of magnetron.

 A magnetron can deliver a peak power output of up to 40MW with the
dc voltage of 50KV at 10GHz.
 The average power output is 800KW.
 The magnetron possesses a very high efficiency ranging from 40 to
70%.
 Magnetrons are commercially available for peak power output from
3KW and higher.

8)Define bolometer.
A bolometer is a power sensor whose resistance changes with temperature as it
6
Final Year ECE PPG Institute of Technology
Department of ECE
absorbs microwave power. The types of bolometer are, the barrater and the thermistor.

9)What is calorimetric direct & indirect heating method?

 In the calorimetric direct heating method, the rate of production of heat can be
measured by observing the rise in the temperature of the dissipating medium.
 In the calorimetric indirect heating method, heat is transferred to another medium
before measurement.
10)Distinguish between thermistor and barrater?

Sl.No Barrater Thermistor

Barrater has a positive Thermistor has negative
1. temperature temperature
coefficient, i.e., resistance
increases coefficient.
with temperature.

2. They are less sensitive. They are more sensitive.

Thermistor need more bias
3. They need less bias current. current.
Barrater are usually operated Thermistor are operated at
4. at 100 ohm
100 ohm to 200ohm.

11)What do you meant by thermocouple sensor?

A thermocouple sensor is a junction of two dissimilar metals or semiconductors.
It generates an emf when two ends are heated up differently by absorption of
microwaves in a thin film tantalum – nitride resistive load deposited on a Si substrate
which forms one electrode of the thermocouple. This emf is proportional to the incident
microwave power to be measured.

12) Define wavelength.

The term microwave refers to electromagnetic energy having a frequency higher
than 1 gigahertz (billions of cycles per second), corresponding to wavelength shorter
than 30 centimeters.

UNIT - V

1).List the different types of Impedence measurement methods?

1.Slotted line method
2.Reflectometer method
3.Reactor discontructer method

2).How do you measure microwave frequency?

1.Wavemeter method
2.Slotted line method
3.Downconversion method

3).What is a wavemeter?

7
Final Year ECE PPG Institute of Technology
Department of ECE
It is a device used for frequency measurement in microwave.It has cylindrical
cavity with a variable short circuit termination .It changes the resonant frequency of
cavity by changing cavitylength.
4).Define dielectric constant?
It is defined by the ratio of permittivity of medium to permittivity of freespace.
xr=x/xo=((10^-9)/36p)

5).How the S-parameter of a microwave circuit measured?

S-parameters are conveniently measured using the deschamps method which
utilizes the measured value of complex input reflection coefficient under a number of a
reactive terminations.

6). List the methods for measuring dielectric constants?

1.Waveguide method
2.cavity pertubaration method

7).What is radiation pattern?

Radiation pattern is a representation of radiation characteristics of an antenna
which is a function of elevation angle azimuth angle for a constant radial distance and
frequency.

8). What is radiation efficiency?

Radiation efficiency is defined as the ratio of total power radiated to total power
accepted at its input .

9). How do you measure the polarization?

The polarization of an antenna is measured using transmitting mode and probing
the polarization by a dipole antenna in the which the dipole is rotated in the plane of
polarization and the received voltage pattern is recorded.

10). What is spectrum analyzer?

Spectrum analyzer is a broad band super heterodyne receiver which is used to
display a wave in frequency domain additionally, power measurements, side bands can
also be observed.

11).List the types of spectrum analyzer

 Real time spectrum analyzer
 Swept tuned frequency spectrum analyzer

12). List some application of spectrum analyzer.

 Identifying frequency terms and their power levels
 Measuring harmonic distortion in a wave
 Determine type of wave modulation
 Signal to noise ratio
 For identifying wave distortion

8
 Final Year ECE PPG Institute of Technology
Department of ECE
SIXTEEN MARKS
UNIT – I
1. List and explain the properties of S parameters.

9
Final Year ECE PPG Institute of Technology
Department of ECE

2. Formulate scattering matrix for a n- port microwave network.

10
Final Year ECE PPG Institute of Technology
Department of ECE

3. Give the [ABCD] matrix for a two port network & derive its [S] matrix

11
Final Year ECE PPG Institute of Technology
Department of ECE

12
Final Year ECE PPG Institute of Technology
Department of ECE

4. For the delta shown below

S= [
0 . 1∠0 0 0. 8 ∠90 0
]
0 . 8∠ 900 0 .2 ∠00 determine whether the
network is reciprocal or lossless. If a short circuit is placed on port 2 . What will
be the return loss at port 1.
Solution:
Since [S] is symmetrical, the network is reciprocal.

|S11|2 +|S 21|2=0. 12 +0 . 82 =0 . 65≠1

The network is not lossless.

V− + + + −
1 =S 11 V 1 +S 12 V 2 =S11 V 1 −S 12 V 2

V− + + + −
2 =S 21 V 1 +S22 V 2 =S 21 V 1 −S 22 V 2

S 21
V−
2= V +1
1+ S22

V V S S
  1 S11  S12 2 S11  12 21
V1 V1 1  S 22

( j 0 . 8)( j0 . 8)
0 . 1− =0 . 633
= 1+0 .2

The return lossless is

R.L. = −20log|Γ|=3.97dB .

13
Final Year ECE PPG Institute of Technology
Department of ECE
5. Derive the relation between Z, Y, ABCD parameters with S parameters.

14
Final Year ECE PPG Institute of Technology
Department of ECE
6. Two transmission lines of characteristic impedance Z1 and Z2 are joined at
plane pp’.
Express S parameters in terms of impedances.

15
Final Year ECE PPG Institute of Technology
Department of ECE

UNIT - II

1. Explain the following:

i.Impedance matching networks.

 Maximum power is delivered when the load is matched the line and the power
loss in the feed line is minimized
 Impedance matching sensitive receiver components improves the signal to noise
ratio of the system
 Impedance matching in a power distribution network will reduce amplitude and
phase errors
 Complexity
 Bandwidth
 Implementation
 Adjustability
ii.Microstrip line matching networks.
 In strip lines and microstrip lines, realization of stubs are difficult since short
circuiting of lines are difficult.
 But lumped inductance and capacitance can be used for impedance matching.
 An abrupt change in width or shape of microstrip will form an additional fringing
electric field from the open-circuited portions of the strip.
 The effect of this fringing field can be modeled as shunt capacitance or
inductance at the junction.
 Various lumped matching elements are
a. A spiral shunt inductor
b. A spiral series inductor
c. An open circuited stub
d. An inter digital capacitor.

2. What is a matching network? Why is this required?

 Impedance matching is the practice of designing the input impedance of an
electrical load or the output impedance of its corresponding signal source to maximize
the power transfer or minimize signal reflection from the load.
 In the case of a complex source impedance ZS and load impedance ZL,
 maximum power transfer is obtained when

where the asterisk indicates the complex conjugate of the variable.

Minimum reflection is obtained when

 The design of impedance matching networks is an important part of microwave

engineering.
 A microwave system comprises of transmission lines and devices, each has
their own characteristics impedances.
 In order to transfer maximum power to the subsequent circuits, impedance
matching is must otherwise reflection from load will occur and power will be wasted,
reducing transmission efficiency.
16
Final Year ECE PPG Institute of Technology
Department of ECE
 In a mismatch impedance, the reactive component should be cancelled and the
real part should be transformed to Z0 .
 The simplest type of matching networks is two component networks or L-section
networks.
 These networks use two reactive components (L and C) to transform the load
impedance (ZL) to desired input impedance (Zin).
 L-section utilize purely reactive components such that no power is dissipated in
the matching network.
 Smith Charts are an extremely useful manner by which to design L-section
matching networks.

3. Design an L-section matching network to match a series RC load with an

impedance ZL = 200-j100W, to a 100W line, at a frequency of 500 MHz?
Solution ( Use Smith chart)
1. Because the normalized load impedance ZL= 2-j1 inside the 1+jx circle,
so case 1 network is select.
2. jB close to ZL, so ZL ® YL.
3. Move YL to 1+jx admittance circle, jB =j 0.3, where YL ® 0.4+j 0.5.
4. Then YL ® ZL, ZL ® 1+j 1.2. So jX =j 1.2.
5. Impedance identity method derives jB =j 0.29 and jX =j 1.22.

B XZ0
C= =0 .92 pF; L= =38 . 8 nH
2 π fZ 0 2 πf
6. Solution 2 uses jB =-j 0.7, where YL ® 0.4-j 0.5.

7. Then YL ® ZL, ZL ® 1-j 1.2. So jX =-j 1.2.

17
Final Year ECE PPG Institute of Technology
Department of ECE
8. Impedance identity method derives jB =-j 0.69 and jX =-j 1.22.

−1 −Z 0
C= =2 . 61 pF; L= =46 . 1 nH
2 π fXZ 0 2 π fB

4. An L section LC network is employed for matching a series RC circuit (R=100

Ω, C=6.366 pF) to a 50 Ω transmission line at 500 MHz. Find the values of L and C.
Show the matching process on the Smith chart and plot the reflection coefficient
from 0 to 1 GHz.

18
Final Year ECE PPG Institute of Technology
Department of ECE

C=1.84 pF and L= 19.49 nH

Single Shunt Stub Tuner Design Procedure

1. Locate normalized load impedance and draw VSWR circle (normalized load
admittance point is 180o from the normalized impedance point).

19
Final Year ECE PPG Institute of Technology
Department of ECE
2. From the normalized load admittance point, rotate CW (toward generator) on the
VSWR circle until it intersects the r = 1 circle. This rotation distance is the length d of
the terminated section of t-tline. The nomalized admittance at this point is 1 + jb.
3. Beginning at the stub end (rightmost Smith chart point is the admittance of a short-
circuit, leftmost Smith chart point is the admittance of an open-circuit), rotate CW
(toward generator) until the point at 0 - jb is reached. This rotation distance is the stub
length l.

20
Final Year ECE PPG Institute of Technology
Department of ECE

5.A certain two-port network is measured and the following scattering matrix is

[ ]
o o
obtained:
[ S ] = 0 .1 ∠0 o 0 . 8 ∠90o
0. 8 ∠90 0. 2 ∠0

From the data , determine whether the network is reciprocal or lossless. If a

short circuit is placed on port 2, what will be the resulting return loss at port 1?
Solution:
Since [S] is symmetry, the network is reciprocal. To be lossless, the S parameters must
satisfy

}
n
∑ S ki S¿kj = 10 for i= j
for i≠ j
k =1

For i=j

|S11|2 + |S12|2 = (0.1)2 + (0.8)2 = 0.65

Since the summation is not equal to 1, thus it is not a lossless network.
Reflected power at port 1 when port 2 is shorted can be calculated as follow and the
fact that a2= -b2 for port 2 being short circuited, thus
b1=S11a1 + S12a2 = S11a1 - S12b2 (1)

b2=S21a1 + S22a2 = S21a1 - S22b2 (2)

Short at port 2

a
- 21

2
a2=
Final Year ECE PPG Institute of Technology
Department of ECE

From (2) we have

S 21 (3)
b 2= a
1+S 22 1
Dividing (1) by a1 and substitute the result in (3) ,we have

b1 b2 S12 S 21 ( j0 . 8 ) ( j 0 .8 )
ρ= =S11 −S12 =S11 − =0 .1− =0. 633
a1 a1 1+ S 22 1+ 0. 2
−20 log ρ =−20 log ( 0 .633 )=3 . 97 dB

6. Determine if the network is reciprocal, and lossless? If port 2 terminated with a

matched load, what is the return loss at port 1? If port 2terminated with a short
circuit, what is the return loss seen at port 1?

[ S]=
[0.85∠45°
0.15∠0° 0.85∠−45°
0.2∠ 0° ]
Solution:

Since [S] is not symmetric, the network is not reciprocal.

|S11|2 +|S 21|2=(0 .15 )2 +(0 .85 )2 =0 .745≠1

So the network is not lossless.

When port 2 terminated with a matched load, G=S11=0.15

RL=−20log|Γ|=−20 log(0.15)=16.5 dB
When port 2 terminated with a short circuit,
V− + + + −
1 =S 11 V 1 + S 12 V 2 = S11 V 1 − S 12 V 2
V− + + + −
2 =S 21 V 1 + S22 V 2 =S 21 V 1 − S 22 V 2
The second equation gives
− + S21
V 2 =V 1
1+ S22
Dividing the first equation by V +1 and using the above result
V−
1 V−
2 S12 S 21
Γ= +
= S 11− S12
+
= S 11−
V1 V1 1+ S 22
( 0. 85 ∠−45 ° )( 0 . 85 ∠ 45 ° )
¿ 0 . 15−
1+0 . 2
¿−0 . 452
So RL=− 20 log|Γ|=−20 log ( 0 . 452)=6 . 9 dB

22
Final Year ECE PPG Institute of Technology
Department of ECE

7. Determine if the network is reciprocal, and lossless ?

Solution
D
Port 2
Port 1 G
Z0
Z0
S

G D
Port 1  g mVgs Port 2
Vgs Rds

S S

( Y −Y 11 )( Y 0 + Y 22 )+Y 12 Y 21 ( Y 0 −0 )( Y 0 +1 / R ds )+0⋅g m Y 0 +1/ Rds

S11 = 0 = = =1
( Y 0 +Y 11 )( Y 0 +Y 22 )−Y 12 Y 21 ( Y 0 +0 )(Y 0 +1/ Rds )−0⋅g m Y 0 +1/ Rds
−2 Y 12 Y 0 −2⋅0
S12= = =0
( Y 0 +Y 11 )( Y 0 + Y 22 )−Y 12 Y 21 ( Y 0 +0 )( Y 0 +1 / R ds )−0⋅gm
−2 Y 21 Y 0
S21=
( Y 0 +Y 11 )( Y 0 +Y 22 )−Y 12 Y 21
−2⋅g m Y 0 −2⋅g m Z 0 R ds
¿ = =−2⋅gm
( Y 0 +0 )( Y 0 +1 / R ds )−0⋅gm Y 0 +1/ Rds Z 0 + Rds
( Y +Y )( Y 0 −Y 22 )+Y 12 Y 21
S22= 0 11
( Y 0 +Y 11 )( Y 0 +Y 22 )−Y 12 Y 21
( Y +0 )( Y 0 −1/ Rds )+0⋅gm Y 0−1 / R ds Rds −Z 0
¿ 0 = =
( Y 0 +0 )( Y 0 +1 / R ds )−0⋅gm Y 0 +1/ Rds Rds + Z 0

[ ]
1 0
∵ [ S ]= Z 0 R ds R ds −Z 0
−2⋅gm
Z 0 + Rds R ds + Z 0
∴ Since the network is neither reciprocal nor lossless,
[ S ] should be neither symmetric nor unitary .

UNIT – III

1)Discuss the structure and principle of operation of;

i.Isolator
ii.Circulator
Isolator:
 Isolator is a non – reciprocal ferrite transmission device.Isolators is generally
used to improve the frequency stability of microwave generators.
 Isolators transmits electromagnetic wave only in one direction , the reflected
wave is attenuated .Thus microwave generating active devices are isolated.
 An ideal isolator completely absorbs power of propagation in one direction and
provides lossless transmission in the opposite direction.
 The Faraday rotation isolator provides 1 dB insertion loss in forward transmission
and about 20 to 30 dB isolation in reverse direction.
 Isolators can be produced by inserting a ferrite rod along the axis of a rectangular
waveguide.
23
Final Year ECE PPG Institute of Technology
Department of ECE

 Let the incident wave has E in X-direction when it propagates through ferrite rod,
it is rotated by 45°
 Reflected wave from load travels in reverse direction and is again rotated by 45°
by ferrite rod.
 Reflected E appearing at resistive value -1 is in Y –direction and it is completely
attenuated.
 Measured in terms of two basic parameters.
i) Insertion Loss (IL):
Insertion loss is defined as
IL (dB) = 10 log P1/ P2
Where,
P1 is Power launched at input port
P2 is Power received at output port

ii) Isolation (Is):

Isolation is defined as

P´1
Is (dB) = 10 log ´
P2

Where P 1́ is the Power at input port

P2́ is the Power launched from the output port.

Application:

 In Klystrons and Magnetrons to improve the frequency stability.

Circulator:

24
Final Year ECE PPG Institute of Technology
Department of ECE
 A Microwave circulator is a multiport device in which power is circulated from n th
port to (n+1) th port only in one direction.
 A four port circulator is most commonly used.
 It is non-reciprocal component. All the port is matched and transmission of power
takes place in cyclic order only.
 An ideal circulator is perfectly lossless.

Principle of Operation:

 Working of circulator is based on principle of Faraday rotation.

 All the ports 1,2,3,4 are oriented such that the E-field of transmitted signal
couples to these ports successively after going through a rotation of 45° in clockwise
direction.

Four-Port Circulator:

 Power entering port-1 travels along the magnetized ferrite.

 The direction of the E field vector gets rotated by 45°.
 Therefore power entered at port-1 appears at port-2. The power cannot be
coupled to port-4 because port-2 and 4 are 90° out of phase. Similarly, port-3 is
coupled to port-4 and port-4 to port-1

Applications of circulator:

 Isolation of transmitter and receivers connected to same antenna eg. In radar

system.
 Isolation of input and output in two terminal amplifying devices eg. parametric
amplifiers.

2. Explain how directional coupler can be used to measure reflected power.

 Directivity is a measure of how well the coupler isolates two opposite-travelling

(forward and reverse) signals.
 In the case of measuring reflection coefficient (return loss) of a device under test,
directivity is a crucial parameter in the uncertainty of the result.
 Figure 1 shows how the reflection signal, Er, is degraded by the undesired portion
of the incident signal D2. And since the undesired signal, D 2, combines with the
reflected signal as a phasor, the error in the measured signal Em2 can only be
compensated or corrected on a broadband basis using vector analyzers.
 Because the reverse-coupled signal is very small, it adds a negligible amount of
uncertainty when measuring large reflections. But as the reflected signal becomes
smaller, the reverse-coupled signal becomes more significant.
Applications
 Directional couplers are general purpose tools used in RF and microwave signal
routing for isolating, separating or combining signals. They find use in a variety of
measurement applications:
 Power monitoring
 Source leveling
 Isolation of signal sources
 Swept transmission and reflection measurements

25
Final Year ECE PPG Institute of Technology
Department of ECE

3. Explain the properties of H-plane Tee and give reasons why it is called shunt
Tee.
Shunt tee:

 A waveguide tee in which the axis of its side arm is “shunting” the E-field or
parallel to the H-field of the main guide.

 If two input waves are fed into port 1 and port 2 of the collinear arm, the output
wave at port 3 will be in phase and additive.

 If the input is fed into port 3, the wave will split equally into port 1 and port 2 in
phase and in the same magnitude.

 Therefore the S matrix of H-plane tee is similar to E-plane tee except

By using above equation, it can be rewritten as

26
Final Year ECE PPG Institute of Technology
Department of ECE

27
Final Year ECE PPG Institute of Technology
Department of ECE

4. Derive the equation for the scattering matrix of magic tee.

28
Final Year ECE PPG Institute of Technology
Department of ECE

We get,

By using the above equation, we get

The Scattering matrix becomes,

29
Final Year ECE PPG Institute of Technology
Department of ECE

5. What is a hybrid ring? With the help of a neat diagram explain its working
principle.

30
Final Year ECE PPG Institute of Technology
Department of ECE
6. Write a note on

 Microwave Frequencies.
 Characteristics.
 Applications.

Microwave Frequencies:

31
Final Year ECE PPG Institute of Technology
Department of ECE

UNIT - IV

1) Explain the operation of two cavity klystron amplifier.

Two Cavity Klystron Amplifiers:

Principle

• Velocity modulated tube

• High velocity electron beam is generated by an electron gun and sent down
along a gas tube through an input cavity (BUNCHER), drift space (FIELD FREE) and
an output cavity (CATCHER) to a collector electrode anode.
• The anode is kept positive to receive the electrons, while the output is taken from
the tube via resonant cavities with the aid of coupling loops
• Two grids of the buncher cavity are separated by a small gap A while the two
grids of the catcher cavity are separated by a small gap B.
OPERATION

• The input buncher cavity is excited by the RF signal, (the signal to be amplified)
which will produce an alternating voltage of signal frequency across the gap A.
32
Final Year ECE PPG Institute of Technology
Department of ECE
• This voltage generated at the gap A is responsible to produce bunching of
electrons or velocity modulation of the electron beam.

Applications:
 As power output tubes
1. in UHF TV transmitters
2. in troposphere scatter transmitters
3. satellite communication ground station
4. radar transmitters
 As power oscillator (5 – 50 GHz), if used as a klystron oscillator

2) Explain the operation of TWT.

TWT (Traveling Wave Tube):

 Traveling Wave Tube (TWT) is the most versatile microwave RF power

amplifiers.
 The main virtue of the TWT is its extremely wide band width of operation.
Basic structure

 The basic structure of a TWT consists of a cathode and filament heater plus an
anode that is biased positively to accelerate the electron beam forward and to focus it
into a narrow beam.
 The electrons are attracted by a positive plate called the collector, which has
given a high dc voltage.
 The length of the tube is usually many wavelengths at the operating frequency.
 Surrounding the tube are either permanent magnets or electromagnets that keep
the electrons tightly focused into a narrow beam.
Advantages
1. TWT has extremely wide bandwidth. Hence, it can be made to amplify signals
from UHF to hundreds of gigahertz.
2. Most of the TWT’s have a frequency range of approximately 2:1 in the desired
segment of the microwave region to be amplified.
3. The TWT’s can be used in both continuous and pulsed modes of operation with
power levels up to several thousands watts.

Applications of TWT
1. Low noise RF amplifier in broad band microwave receivers.
2. Repeater amplifier in wide band communication links and long distance
telephony.
33
Final Year ECE PPG Institute of Technology
Department of ECE
3. Due to long tube life (50,000 hours against ¼th for other types), TWT is power
output tube in communication satellite.
4. Continuous wave high power TWT’s are used in troposcatter links (due to larger
power and larger bandwidths).
5. Used in Air borne and ship borne pulsed high power radars.

3. Explain the types & working principles of magnetron?

 Magnetrons provide microwave oscillations of very high frequency.

Types of magnetrons
1. Negative resistance type
2. Cyclotron frequency type
3. Cavity type
Negative resistance Magnetrons
 Make use of negative resistance between two anode segments but have
low efficiency and are useful only at low frequencies (< 500 MHz).
Cyclotron frequency Magnetrons
 Depend upon synchronization between an alternating component of
electric and periodic oscillation of electrons in a direction parallel to this field.
 Useful only for frequencies greater than 100 MHz.
Cavity Magnetrons
 Depend upon the interaction of electrons with a rotating electromagnetic
field of constant angular velocity.
 Provide oscillations of very high peak power and hence are useful in radar
applications

 Each cavity in the anode acts as an inductor having only one turn and the slot
connecting the cavity and the interaction space acts as a capacitor.
 These two form a parallel resonant circuit and its resonant frequency depends on
the value of L of the cavity and the C of the slot.
 The frequency of the microwaves generated by the magnetron oscillator depends
on the frequency of the RF oscillations existing in the resonant cavities.
 Magnetron is a cross field device as the electric field between the anode and the
cathode is radial whereas the magnetic field produced by a permanent magnet is axial.
 A high DC potential can be applied between the cathode and anode which
produces the radial electric field.
 Depending on the relative strengths of the electric and magnetic fields, the
electrons emitted from the cathode and moving towards the anode will traverse through
the interaction space
 In the absence of magnetic field (B = 0), the electron travel straight from the
cathode to the anode due to the radial electric field force acting on it,
Working
34
Final Year ECE PPG Institute of Technology
Department of ECE
 The RF Oscillations of transient nature produced when the HT is switched on,
are sufficient to produce the oscillations in the cavities, these oscillations are
maintained in the cavities reentrant feedback which results in the production of
microwaves.
 Reentrant feedback takes place as a result of interaction of the electrons with
the electric field of the RF oscillations existing in the cavities.
 The cavity oscillations produce electric fields which fringe out into the interaction
space from the slots in the anode structure, as shown in Fig (iv).
 Energy is transferred from the radial dc field to the RF field by the interaction of
the electrons with the fringing RF field.
 This electron travels in a longest path from cathode to the anode as indicated by
‘a’ in Fig (iv), transferring the energy to the RF field are called as favoured electrons
and are responsible for bunching effect and give up most of its energy before it finally
terminates on the anode surface.
 An electron ‘b’ is accelerated by the RF field and instead of imparting energy to
the oscillations, takes energy from oscillations resulting in increased velocity, such
electrons are called unfavoured electrons which do not participate in the bunching
process and cause back heating.
 Every time an electron approaches the anode “in phase” with the RF signal, it
completes a cycle. This corresponds to a phase shift 2p.
 For a dominant mode, the adjacent poles have a phase difference of p radians,
this called the p - mode.
Applications of Magnetron
 Pulsed radar is the single most important application with large pulse powers.
 Voltage tunable magnetrons are used in sweep oscillators in telemetry and in
missile applications.
 Fixed frequency, CW magnetrons are used for industrial heating and microwave
ovens.

4) Explain briefly about cavity magnetron.

Cavity magnetron
A cavity magnetron is a high-powered vacuum tube that generates coherent
microwaves. They are commonly found in microwave ovens, as well as various radar
applications.

Construction and operation

All cavity magnetrons consist of a hot filament (cathode) kept at, or pulsed to, a
high negative potential by a high-voltage, direct-current power supply. The cathode is
built into the center of an evacuated, lobed, circular chamber. A magnetic field parallel
to the filament is imposed by a permanent magnet. The magnetic field causes the
electrons, attracted to the (relatively) positive outer part of the chamber, to spiral
outward in a circular path rather than moving directly to this anode. Spaced around the
rim of the chamber are cylindrical cavities. The cavities are open along their length and
connect the common cavity space. As electrons sweep past these openings, they
induce a resonant, high-frequency radio field in the cavity, which in turn causes the
electrons to bunch into groups. A portion of this field is extracted with a short antenna
that is connected to a waveguide (a metal tube usually of rectangular cross section).
The waveguide directs the extracted RF energy to the load, which may be a cooking
chamber in a microwave oven or a high-gain antenna in the case of radar.
A cross-sectional diagram of a resonant cavity magnetron. Magnetic field is
perpendicular to the plane of the diagram.
35
Final Year ECE PPG Institute of Technology
Department of ECE
The sizes of the cavities determine the resonant frequency, and thereby the
frequency of emitted microwaves. However, the frequency is not precisely controllable.
This is not a problem in many uses such as heating or some forms of radar where the
receiver can be synchronized with an imprecise output. Where precise frequencies are
needed, other devices such as the Klystron are used. The voltage applied and the
properties of the cathode determine the power of the device. The magnetron is a fairly
efficient device. In a microwave oven, for instance, a 1,100 Watt input will generally
create about 700 Watts of microwave energy, an efficiency of around 65%. Modern,
solid-state, microwave sources at this frequency typically operate at around 25 to 30%
efficiency and are used primarily because they can generate a wide range of
frequencies. Thus, the magnetron remains in widespread use in roles which require
high power, but where precise frequency control is unimportant.

5) Explain the operation of Triodes.

A triode is an electronic amplifying vacuum tube (or valve in British English)
consisting of three electrodes inside an evacuated glass envelope: a heated filament or
cathode, a grid, and a plate (anode).
In the triode, electrons are released into the tube from the metal cathode by
heating it, a process called thermionic emission. The cathode is heated red hot by a
separate current flowing through a thin metal filament. In a few triodes, the filament
itself is the cathode, while in most the filament heats a separate cathode electrode.
Virtually all the air is removed from the tube, so the electrons can move freely. The
negative electrons are attracted to the positively charged plate (anode), and flow
through the spaces between the grid wires to it, creating a current through the tube
from cathode to plate.
The magnitude of this current can be controlled by a voltage applied between the
cathode and the grid. The grid acts like a gate for the electrons. A more negative
voltage on the grid will repel some of the electrons, so fewer get through to the plate,
reducing the plate current. A positive voltage on the grid will attract more electrons from
the cathode, so more reach the plate, increasing the plate current. Therefore, a low
power varying (AC) signal applied to the grid can control a much more powerful plate
current, resulting in amplification. Variation in the grid voltage will cause identical
proportional variations in the plate current. By placing a suitable load resistance in the
plate circuit, the varying current will cause a varying voltage across the resistance
which can be much larger than the input voltage variations, resulting in voltage gain.
The triode is a normally "on" device; and current flows to the plate with zero
voltage on the grid. The plate current is progressively reduced as the grid is made more
negative with respect to the cathode. Usually a constant DC voltage ("bias") is applied
to the grid to set the DC current through the tube, and the varying signal voltage is
superimposed on it. A sufficiently negative voltage on the grid, usually around 3-5 volts
in small tubes such as the 6AV6, but as much as –130 volts in early audio power
devices such as the '45, will prevent any electrons from getting through to the plate,
turning off the plate current. This is called the "cutoff voltage". Since below cutoff the
plate current ceases to respond to the grid voltage, the voltage on the grid must remain
above the cutoff voltage for faithful (linear) amplification.
The triode is very similar in operation to the n-channel JFET; it is normally on,
and progressively switched off as the grid/gate is pulled increasingly negative of the
source/cathode. Cutoff voltage is equivalent to the JFET's pinch-off voltage (V p)or
VGS(off); the point at which current stops flowing entirely.

6) Explain about different typical Magnetron Parameters

36
Final Year ECE PPG Institute of Technology
Department of ECE
The following is a discussion and explanation of typical magnetron specification
parameters.
Thermal Drift
At the time high voltage is first applied to a magnetron, the thermal equilibrium of
the device is suddenly altered. The anode vanes being to heat at the tips due to
electron bombardment and the entire anode/cathode
structure undergoes a transient change in thermal profile. During the time required for
each part of the magnetron to stabilize at its normal operating temperature, the output
frequency of the magnetron will "drift." The curve of output frequency vs. time during
the period following initial turn on is called the "Thermal Drift" curve. Generally
speaking, the
maximum drift occurs during the first few minutes after turn on, and slowly approaches
equilibrium over a period ranging from 10 to 30 minutes depending upon the structure
mass, power output, type of cooling and basic magnetron design. Thermal drift curves
across a variety of magnetron types operating at the same frequency and output power
may differ radically from each other. Each type is usually designed for a particular
purpose and subtle differences in the internal magnetron configuration can produce
radical differences in the thermal drift curve.It should be noted that a thermal drift effect
will occur not only at initial turn-on, but whenever the peak or average input power to
the magnetron is changed, e.g., a change of pulse duration, PRF or duty. Figure 7
shows typical thermal drift curves for a particular magnetron plotted as a function of
duty. The dotted line indicates the effect of a change in duty from .001 to .0005 after
thermal equilibrium has been initially achieved.

Temperature Coefficient
After the thermal drift period has expired and a stable operating frequency has
been achieved, changes to ambient conditions which cause a corresponding change in
the magnetron temperature will produce a change in the output frequency. In this
content ambient changes include cooling air temperature or pressure in air cooled
magnetrons; mounting plate temperature in heat sink cooled magnetrons; and flow rate
or temperature in liquid cooled magnetrons.The change in magnetron output frequency
for each degree change in body temperature, as measured at a specified point on the
outside surface of the magnetron body, is defined as the Temperature Coefficient for
the magnetron and is usually expressed in MHz/oC. For most magnetrons the
temperature coefficient is a negative (frequency decreases as temperature increases)
and is essentially constant over the operating range of the magnetron.When estimating
magnetron frequency change due to temperature coefficient, keep in mind that the
temperature coefficient relates magnetron frequency to body temperature and there is
not necessarily a 1:1 relation between body temperature and, for example,ambient air
temperature. In addition, for airborne systems, the cooling effect of lower air
temperature at altitude may offset by a corresponding reduction in air density.

Pushing Figure

37
Final Year ECE PPG Institute of Technology
Department of ECE
The pushing figure of a magnetron is defined as the change in magnetron
frequency due to a change in the peak cathode current.Referring back to the earlier
theory discussion, we noted that the resonant frequency of a vane resonator is
determined by its mechanical dimensions plus the reactive effect of any perturbation.
The presence of electrons in the vicinity of the vane tips affects the equivalent
capacitance of the resonator by an amount proportional to the density of the electrons
and, since electron density is similarly related to peak pulse current, changes in pulse
current level will produce changes in output frequency. The pushing figure expressed in
MHz/Amp is represented by the slope of a frequency vs. peak current curve plotted for
a particular magnetron type From the curve of Figure 8, it can be seen that the slope is
not a constant over the full range of operating current. It is therefore meaningless to talk
about a specific value for the pushing figure unless one also specifies the range of peak
current over which it applies It should be noted that since power output is proportional
to peak current in a magnetron, the pushing figure at peak current levels well below the
normal operating point of the magnetron are usually unimportant because the power
output at these current levels is low.

The primary importance of a low pushing figure near the magnetron operating
point is that the pushing figure will determine intrapulse FM, and thereby will affect the
spectral quality of the transmitting pulse.The Pulling Figure is defined as the maximum
change in output frequency that results when an external, fixed amplitude mismatch,
located in the output waveguide, is moved through a distance of one half wavelength
relative to the magnetron. Stated somewhat less formally, the pulling figure is a
measure of a magnetron's ability to maintain a constant output frequency against
changes in load mismatch.During the design of a magnetron, the degree to which the
output waveguide is electrically coupled to the internal resonator structure is selected to
optimize certain performance parameters. Strong coupling increases output power and
efficiency but also increases time jitter and sensitivity to changes to load mismatch.
Generally, the coupling is chosen
to obtain the best compromise between efficiency and stability.Depending upon the
phase relation between incident and reflected power at the output port of a magnetron,
reflected power will appear as a reactance across the coupling transformer and
effectively change the degree of coupling. Therefore, using a fixed mismatch and

38
Final Year ECE PPG Institute of Technology
Department of ECE
varying its distance from the magnetron output port will cause the magnetron frequency
to shift and the output power to vary concurrently.To standardize the measurement
values, pulling figure is normally measured using a fixed 1.5:1 VSWR; however, in very
high power magnetrons a 1.3:1 VSWR is often used. When referring to the pulling
figure of a magnetron one should always indicate the VSWR value used in the
measurement.

UNIT V
1. Explain the principles of microwave power measurements.
Power Measurement

 Power is defined as the quantity of energy dissipated or stored per unit time.

 Microwave power is divided into three categories – low power (less than 10mW),
medium power (from 10mW to 10W) and high power (greater than 10w).

 The general measurement technique for average power is to attach a properly

calibrated sensor to the transmission line port at which the unknown power is to be
measured.

 The output from the sensor is connected to an appropriate power meter. The RF
power to the sensor is turned off and the power meter zeroed. This operation is often
referred to as “zero setting” or “zeroing.” Power is then turned on.

 The sensor, reacting to the new input level, sends a signal to the power meter
and the new meter reading is observed.

 There are three popular devices for sensing and measuring average power at RF
and microwave frequencies.

 Each of the methods uses a different kind of device to convert the RF power to a
measurable DC or low frequency signal.

 The devices are the diode detector, the bolometer and the thermocouple.

 Diode Detector

 The low-barrier Schottky (LBS) diode technology which made it possible to

construct diodes with metal-semiconductor junctions for microwave frequencies that
was very rugged and consistent from diode to diode.
 These diodes, introduced as power sensors in 1974, were able to detect and
measure power as low as −70 dBm (100 pW) at frequencies up to 18 GHz.

 Bolometer Sensor:
 Bolometers are power sensors that operate by changing resistance due to a
change in temperature.
 The change in temperature results from converting RF or microwave energy into
heat within the bolometric element.
 There are two principle types of bolometers, barretters and thermistors.
39
Final Year ECE PPG Institute of Technology
Department of ECE
 A barretter is a thin wire that has a positive temperature coefficient of resistance.
 Thermistors are semiconductors with a negative temperature coefficient.
 Thermistor elements are mounted in either coaxial or waveguide structures so
they are compatible with common transmission line systems used at microwave and RF
frequencies.
 Power meters are constructed from balanced bridge circuits.
 The principal parts of the power meter are two self-balancing bridges, the meter-
logic section, and the auto-zero circuit.

2. Discuss slotted line method of VSWR measurement.

VSWR Measurements:
In a microwave network, if load impedance and line impedance are not matched,
signal fed from the source is reflected again towards source causing standing wave
pattern in the network. Voltage Standing Wave Ratio is a measure used for finding the
magnitude of ration of reflected signals maximum and minimum amplitudes.
Vmax
S=
Vmin

For analyzing standing wave pattern and to find S slotted line carriage is used in
laboratory.
Low VSWR Measurements: (S<20)

Procedure:

1) Microwave Source is energized with 1 KHz square wave signal as carrier.

2) Tunable passive components are so adjusted to get reading across the VSWR
meter in 30 dB scale.
3) Detector (Tunable probe detector) is adjusted to get maximum power across the
VSWR meter.
4) Slotted line carriage is moved from the load towards source to find the standing
wave minimum position.
5) By adjusting the gain control knob of VSWR meter and attenuator the reading
across the VSWR meter is made as 1 or 0 dB known as normalization.
6) Again the slotted carriage is moved towards source to find the next minimum
position. The reading shown at this point in the VSWR meter is the ratio of magnitude
of reflected signals minimum and maximum voltages ( S = Vmax / Vmin ).
7) VSWR meter has three different scales with different ranges as specified below.

a) NORMAL SWR Scale 1 ---- 1 – 4

b) NORMAL SWR Scale 2 ----3.2 –10
c) EXPANDED SWR Scale
3 ----1 to 1.33

8) If the device under test (DUT) is having the range of VSWR 1 – 4, reading is
40
 Final Year ECE PPG Institute of Technology
Department of ECE
taken from the first scale from the top (NORMAL SWR Scale 1 – 1 – 4).
9) If the device under test (DUT) is having the range of VSWR 3.2 – 10, reading is
taken from the second scale from the top (NORMAL SWR Scale 2 (3.2 – 10).
10) If the device under test (DUT) is having the range of VSWR 1 – 1.33, reading is
taken from the third scale from the top (EXPANDED SWR Scale 3 (1 – 1.33).

11) If the device under test (DUT) is having the range of VSWR 10 – 40, a 20 dB
range is selected in the VSWR meter and reading is taken from the first scale from the
top (NORMAL SWR Scale 1 – 1 – 4) which is then multiplied by 10 for getting the
actual reading.

Possible Errors in Measurements:

1. Detector may not work square law region for both Vmax. and Vmin.
2. Depth of the probe in the slotted line carriage is made as minimum. If not, it may
cause reflections in addition to the load reflections.
3. For the device having low VSWR, connector used for measurement must have
proper matching with line impedance.
4. If the geometrical shape of the slotted line is not proper, Vmax. (or) Vmin. Value
will not constant across the slotted line.
5. If the microwave signal is not properly modulated by a 1 KHz square wave, then
signal becomes frequency modulated thereby it causes error in the Vmin. value
measured. The value becomes lower than the actual.
6. Residual VSWR of slotted line carriage may cause error in the measurements.

3. Describe how the frequency of a given microwave source can be measured.

 Microwave frequency can be measured by a number of different mechanical and
electronic techniques.

 Mechanical techniques -Slotted Line Method (Indirect Method)

 The standing waves setup in a transmission line or a waveguide produce minima

every half wavelength apart.

Set-up for the measurement of frequency

 These minima are detected and the distance between them is measured. From
which the wavelength and frequency can be calculated by

41
Final Year ECE PPG Institute of Technology
Department of ECE

Resonant Cavity Method (Direct Method)

 The most commonly used type of microwave frequency meter is wave meters.
 It consists of a cylindrical or coaxial resonant cavity. The size of the cavity can be
altered by adjustable plunger.
 The cavity is designed in such a way that for a given position of the plunger, the
cavity is resonant only at one frequency in the specified range.

WAVEMETER

 The cavity is coupled to the waveguide through an iris in the narrow wall of the
waveguide.
 If the frequency of the wave passing through the waveguide is different from the
resonance frequency of the cavity, the transmission is not affected.
 If these two frequencies coincide then the wave passing through the waveguide
is attenuated due to power loss. It will be indicated as a dip in the meter.

42
Final Year ECE PPG Institute of Technology
Department of ECE

Electronic Technique Counter Method

 An accurate measurement of microwave frequency can be measured here.
 The input signal is divided into two equal signals by a resistive power divider.
 These two parts of the signal are fed to 2 mixers.
 The mixer 1 is used in the input PLL (Phase Locked Loop) and the mixer 2 is
used to determine the harmonic number.
 The frequency f1 of the input PLL is also fed to the direct counter circuits. The
input PLL consists of a voltage controlled oscillator (VCO), mixer, an IF amplifier, a
phase detector and a gain control block.
 The VCO searches over its range until an IF signal equal to 20MHz is found.
 Phase lock occurs when the phase detector output sets the VCO frequency f1
such that
f x = nf1 - IF1

where IF1 = 20 MHz at the phase lock and fx is the unknown frequency to be
measured
 The f1 is translated to a frequency f2 so that

f 2 = f1 = f 0

where f0 = 20 MHz offset frequency.

 This is done by a frequency translation unit (FTU). The frequency f2 drives the
second sampler and produces a second output. IF2 is given as

 By mixing IF2 with IF1 and rejecting 20 MHz and higher frequencies, nf0 is
obtained. Counting the number of zero crossing for the period of f0, determines the
harmonic number n of the phase lock loop.
 The input frequency is then calculated by presetting into IFref counter, measuring
f1 and extending gate time according to number n.

4. Explain in detail about the Measurement of Cavity Q?

43
Final Year ECE PPG Institute of Technology
Department of ECE

5) Explain in detail about any two Measurement systems.

Many different and ingenious ways of measuring attenuation have been
developed over the years, and most methods in use today embody the following
principles:
(1) Power ratio
(2) Voltage ratio
(3) AF substitution
(4) IF substitution
(5) RF substitution

44
Final Year ECE PPG Institute of Technology
Department of ECE

Power ratio method

` The power ratio method of measuring attenuation is perhaps one of the easiest
to configure. Figure represents a simple power ratio configuration. First, the power
sensor is connected directly to the matching attenuator and the power meter indication
noted P1. Next, the device under test is inserted between the matching pad and power
sensor and the power meter indication again noted P2. Insertion loss is then calculated
using
L(dB) = 10 log10P1/P2

This simple method has some limitations:

(1) Amplitude stability and drift of the signal generator

(2) Power linearity of the power sensor
(3) Zero carry over
(4) Range switching and resolution

Voltage ratio method

Figure represents a simple voltage attenuation measurement system, where a
DVM (digital voltmeter) is used to measure the potential difference across a feed
through termination, first when it is connected directly to a matching attenuator, V1,and
then when the device under test has been inserted, V2. Insertion loss may be
calculated from:
L(dB) = 20 log10V1/V2

45
Final Year ECE PPG Institute of Technology
Department of ECE
This simple system is limited by the frequency response and resolution of the
DVM as well as variations in the output of the signal generator. The voltage coefficient
of the device under test and resolution of the DVM will determine the range, typically
40–50 dB from dc to 100 kHz. A major contribution to the measurement uncertainty
is the linearity of the DVM used, which may be typically 0.01 dB/10 dB for a good
quality eight digit DVM. This may be measured using an inductive voltage divider, and
corrections made.

6) Explain in detail about Network analyser.

Network analyser block diagram
Figure 1 shows the general schematic of an S-parameter measurement system
whilst Figure 2 is the block diagram of a modern microwave vector network analyser.
The schematic diagram shown in Figure 2 is a RF system which has an
integrated source and a tuned receiver based on samplers (labelled S). The system
can be configured with a three-channel or four-channel receiver and consequently the
test set can be either a transmission/reflection type or capable of full S-
parameters.There are two basic types of test set that are used with network analysers
for transmission/reflection (TR) test sets. The RF power always comes out of test port 1
and test port 2 is always connected to a receiver.

Figure 1

46
Final Year ECE PPG Institute of Technology
Department of ECE

Figure 2

To measure reverse transmission or output reflection the device must be

disconnected, turned around and reconnected again. TR-based network analysers offer
only response and one-port calibration so measurement accuracy is not as good as the
one that can be achieved using S-parameter test sets. An S-parameter test set allows
both forward and reverse measurements without reconnection and allows
characterisation of all four S-parameters. RF power can come out of either test port 1 or
test port 2 and either test port can be connected to
a receiver. The internals rearrangement is carried out by switches inside the test
set.These are usually solid-state switches which are fast and do not wear out. Although
it is possible to configure an S-parameter test set with only three samplers or mixers
the architecture provides fewer choices for calibration as does a four receiver
architecture.The display and processor section allows in current systems is usually an
in-built,full-featured PC that not only the reflection and transmission data to be
formatted in many ways to allow for easy display, comparison and interpretation but
also supports algorithms for calibration, data storage and various other features.

47