Model Questions and Answer (1)

8/6/2019 Model Questions and Answer (1)

1/62

Microwave engg. Model questions

Ques.1. What do you mean by waveguide? Explain the TE and TM modes of rectangular waveguide.

Ans. A waveguide is a special form of transmission line consisting of a hollow, metal tube. The

tube wall provides distributed inductance, while the empty space between the tube walls provide

distributed capacitance.

Waveguides are practical only for signals of extremely high frequency, where the wavelength

approaches the cross-sectional dimensions of the waveguide. Below such frequencies, waveguides are

useless as electrical transmission lines.

When functioning as transmission lines, though, waveguides are considerably simpler than two-

conductor cables -- especially coaxial cables -- in their manufacture and maintenance. With only a single

conductor (the waveguide's shell), there are no concerns with proper conductor-to-conductor spacing,

or of the consistency of the dielectric material, since the only dielectric in a waveguide is air. Moisture is

not as severe a problem in waveguides as it is within coaxial cables, either, and so waveguides are often

spared the necessity of gas filling.

Waveguides may be thought of as conduits for electromagnetic energy, the waveguide itself acting

as nothing more than a director of the energy rather than as a signal conductor in the normal sense of

the word. In a sense, all transmission lines function as conduits of electromagnetic energy when

transporting pulses or high-frequency waves, directing the waves as the banks of a river direct a tidal

wave. However, because waveguides are single-conductor elements, the propagation of electrical


2/62

energy down a waveguide is of a very different nature than the propagation of electrical energy down a

two-conductor transmission line.

All electromagnetic waves consist of electric and magnetic fields propagating in the same direction of

travel, but perpendicular to each other. Along the length of a normal transmission line, both electric and

magnetic fields are perpendicular (transverse) to the direction of wave travel. This is known as the

principal mode, or TEM (Transverse Electric and Magnetic) mode. This mode of wave propagation can

exist only where there are two conductors, and it is the dominant mode of wave propagation where the

cross-sectional dimensions of the transmission line are small compared to the wavelength of the signal.

Ques 2. Explain the propagation of TE waves in rectangular waveguide.

Ans. Propagation of TE Waves in Rectangular Waveguide:-

As we have seen earlier waveguides refer to any structure that can guide electromagnetic (EM) waves

along its axial direction, which include transmission line.

Here we consider waveguide as specifically refers to long metallic structures with only 1 piece of

conductor between the source and load end.

These metallic structures are usually hollow, so that EM fields are confined within the hollow and be

guided along the axial direction.

Applying Maxwells Equations with the proper boundary conditions shows that propagating EM waves

can be supported by the waveguide.

Due to the absence of center conductor, only TE and TM mode exist.


3/62

Ques. 3. What do you mean by phase velocity and group velocity in waveguide?

Ans- Phase Velocity in Waveguide

Since phase velocity vp depends on propagation constant bmn, it too depends on the integer pair (m,n)

hence the property of the TE mode

Fields.

Speed of light in dielectric of (,e)

Group Velocity in Waveguide

The velocity of energy propagation, or the speed that information traveling a waveguide is given by the

Group Velocity vg.

Thus from:

Since vp > c,

The group velocity is thus less than speed of light in vacuum, maintaining the assertion of Relativity

Theory that no mass/energy can travel faster than speed of light.


4/62

Ques.4. Derive the TE modes for rectangular waveguide.

Ans. FOR TE Mode :-

To obtain the TE mode electromagnetic (EM) field pattern, we use the systematic procedure developed

in Chapter 1 Advanced Transmission Line Theory.

We start by solving the pattern function for z-component of the magnetic field and boundary conditions

(1.1)

Problem (1.1) is called Boundary Value Problem (BVP) in mathematics.

Once we know the function of hz(x,y), the EM fields are given by:

(1.2a)

(1.2b)

Expanding the partial differential equation (PDE) of (1.1) in cartesian coordinates:

(1.2)

Using the Separation of Variables Method, we can decompose hz(x,y) into the product of 2 functions

and kc 2 to be the sum of 2 constants:

(1.3a) (1.3b)

Putting these into (1.2), and after some manipulation we obtain 2 ordinary differential equations


5/62

(1.4a) (1.4b)

From elementary calculus, we know that the general solution for (1.4a) and (1.4b) are:

(1.5a)

(1.5b)

Thus hz(x,y) is given by:

(1.6)

A, B, C and D in (1.6) are unknown constants, to be determined by applying the boundary conditions

that the tangential electric field must vanish on the conductive walls of the waveguide. From (1.2b):

Using (1.6) and applying the boundary condition (1.7a):


6/62

Using (1.6) and applying the boundary condition (1.7b):

In the above equations, we can combine the product of AC, lets call it R. Common sense tells us that

R would be different for each pair of integer (m,n), thus we should denote R by: Rmn

From (1.3b), kc and the propagation constant are given by:

Since kc and also depends on the integer pairs (m,n), it is more

appropriate to write these as:

(1.7a)

(1.7b)

With these information, and using (1.2a) and (1.2b), we can write out the complete mathematical

expressions for the EM fields under TE propagation mode for a rectangular waveguide:


7/62

(1.8a)

(1.8b)

(1.8c)

(1.8d)

(1.8e)

Ques.5. Derive TM modes for rectangular waveguide.

Ans. The procedure for obtaining the EM field solution for TM propagation is similar to the TE

procedure.

We start by solving the pattern function for the z-component of the electric field and boundary

conditions (1.1

As in solving TE mode problem, the Separation of Variables Method is used in solving (1.11), and

integer pair (m,n) needs to be introduced in the TM mode solution.

The mathematical expressions for the EM field components thus depends on the integer pair (m,n), and

is denoted by TMmn field.

The derivation details will be omitted here due to space constraint. You can refer to reference [1] for the

procedure.


8/62

The complete expressions for the TMmn field components are show below:

(1.12a)

(1.12b)

(1.12c)

(1.12d)

(1.12e)

Where

(1.13a)

(1.13b)


9/62

Ques.6. Explain circular waveguide.

Ans. For mechanical reasons, a rotating joint must be circular and requires a coaxial line or a section of

circular waveguide.

a.) Transverse electric (TE) and transverse magnetic (TM) waves are propagated in circular waveguides

in almost the same manner as inrectangular waveguides. The field configuration in the circular waveguide

closely follows a sine wave pattern (fig. 65).

b.) The boundary conditions used in the rectangular waveguide also apply to the circular

waveguide. Under these conditions the electric field must be perpendicular to the surface of the

conductor, and the magnetic field parallel to the surface of the conductor. When these boundary

conditions are fulfilled in the circular waveguide, the electric field exists between the center of the

waveguide and the wall, and the magnetic field exists around the inside of the waveguide as shown Figure

82. Field configuration in a circular in figure 82. waveguide.

c.) The dominant mode in the circular waveguide is similar to the dominant mode in the rectangular

waveguide. In the TE mode, the electric field is perpendicular to the direction of propagation. In the TM

mode, the magnetic field is perpendicular to the direction of propagation. The TE mode in figure 83

shows that the electric lines are circular around the center of the waveguide and perpendicular to the

direction of propagation. In the TM mode, the magnetic lines are circular around the center of the

waveguide and perpendicular to the direction of propagation.

The Circular Waveguide Modes;

Figure 7.26 shows a circular waveguide with inner diameter 2 a. We investigate the lossless circular

waveguide with a perfectly conducting wall and free-space inner region. To investigate the TM and TE

modes of the circular cylindric waveguide we derive the fields either from an electric Hertz form ? e or a

magnetic Hertz form ? m exhibiting only a z-component

Circular cylindric waveguide.


10/62

For both cases the Helmholtz equation (3.28) has the following form:

with . With (A.157) we obtain for circular cylindric coordinates

We seek solutions for waves propagating in the positive z-direction and choose the separation

formulation

From this it follows that

We introduce the parameterkc given by

and obtain

Ques.7. What do you understand by striplines. Write advantages and disadvantages of them.

Ans. Stripline is a conductor sandwiched by dielectric between a pair of groundplanes, much like a

coax cable would look after you ran it over with your small-manhood indicating SUV (let's not go

there...) In practice, "classic" stripline is usually made by etching circuitry on a substrate that has a

groundplane on the opposite face, then adhesively attaching a second substrate (which is metalized


11/62

on only one surface) on top to achieve the second ground plane. Stripline is most often a "soft-board"

technology, but using low-temperature co-fired ceramics (LTCC), ceramic stripline circuits are also

possible

Advantages and disadvantages of strip line

y Strip line is a TEM (transverse electromagnetic) transmission line media, like coax. The filling

factor for coax is unity, and "Keff" is equal to ER. This means that it is non-dispersive. Whatever

circuits you can make on micro strip (which is quasi-TEM), you can make better using strip line,

unless you run into fabrication or size constraints. Stripline filters and couplers always offer better

bandwidth than their counterparts in micro strip, and the rolloff of strip line BPFs can be quite

symmetric (unlike micro strip). Strip line has no lower cutoff frequency (like waveguide does).

y Another advantage of stripline is that fantastic isolation between adjacent traces can be achieved

(as opposed to microstrip). The best isolation results when a picket-fence of vias surrounds each

transmission line, spaced at less than 1/4 wavelength. Stripline can be used to route RF signals

across each other quite easily when offset stripline is used.

y Disadvantages of stripline are two: first, it is much harder (and more expensive) to fabricate than

microstrip, some old guys would even say it's a lost art. Lumped-element and active components

either have to be buried between the groundplanes (generally a tricky proposition), or transitions

to microstrip must be employed as needed to get the components onto the top of the board.

y The second disadvantage of stripline is that because of the second groundplane, the strip widths

are much narrower for a given impedance (such as 50 ohms) and board thickness than for

microstrip. A common reaction to problems with microstrip circuits is to attempt to convert them

to stripline. Chances are you'll end up with a board thickness that is four times that of yourmicrostrip board to get equivalent transmission line loss. That means you'll need forty mils thick

stripline to replace ten mil thick microstrip! This is one of the reasons that soft-board

manufacturers offer so many thicknesses.

Ques.8. Define microstriplines. Explian its effective dielectric constant.

Ans. Microstrip transmission lines consist of a conductive strip of width "W" and thickness "t"

and a wider ground plane, separated by a dielectric layer (a.k.a. the "substrate") of thickness "H"as shown in the figure below. Microstrip is by far the most popular microwave transmission line,

especially for microwave integrated circuits and MMICs. The major advantage of microstrip

over stripline is that all active components can be mounted on top of the board. Thedisadvantages are that when high isolation is required such as in a filter or switch, some externalshielding may have to be considered. Given the chance, microstrip circuits can radiate, causing

unintended circuit response. A minor issue with microstrip is that it is dispersive, meaning that


12/62

signals of different frequencies travel at slightly different speeds. Microstrip does not support a

TEM mode, because of its filling factor. For coupled lines, the even and odd modes will not havethe same phase velocity. This property is what causes the asymmetric frequency of microstrip

bandpass filters, for example.Variants of microstrip include embedded microstrip and coatedmicrostrip, both of which add some dielectric above the microstrip conductor.

Effective dielectric constant

Because part of the fields from the microstrip conductor exist in air, the effective dielectricconstant "Keff" is somewhat less than the substrate's dielectric constant (also known as the

relative permittivity). Thanks to Brian KC2PIT for reminding us the term "relative dielectricconstant" is an oxymoron only used my microwave morons!) According to Bahl and Trivedi[1],

the effective dielectric constant eff (a.k.a. Keff) of microstrip is calculated by:

All microstrip equations are approximate. The above equations ignore strip thickness, so we

wouldn't recommend relying on them for critical designs on thick copper boards.

The effective dielectric constant is a seen to be a function of the ratio of the width to the height

of the microstrip line (W/H), as well as the dielectric constant of the substrate material. Becareful, the way it is expressed here it is also a function of H/W! We have a table of "hard"

substrate material properties here, and "soft" substrate material properties here, in case you wantto look up the dielectric constant of a specific material.

Note that there are separate solutions for cases where W/H is less than 1, and when W/H is

greater than or equal to 1. These equations provide a reasonable approximation for eff(effective dielectric constant). This calculation ignores strip thickness and frequency dispersion,

but their effects are usually small.

Ques.9. Give all the characteristics of microstrip lines.

Ans. The different characteristics of microstrip lines are as follows:


13/62

1. Wavelength

Wavelength for any transmission line can be calculated by dividing free space wavelength by the

squareroot of the effective dielectric constant, which is explained above.

2. Characteristic impedance

The characteristic impedance Z0 is also a function of the ratio of the height to the width W/H

(and ratio of width to height H/W) of the transmission line, and also has separate solutionsdepending on the value of W/H. According to Bahl and Trivedi[1], the characteristic impedance

Z0 of microstrip is calculated by:

3. Effect of metal thickness on calculations

Having a finite thickness of metal for the conductor strips tends to increase the capacitance of the

lines, which effects the effand Z0 calculations. We'll add this correction factor at a later date.

4. Effect of cover height on calculations Having a lid in close proximity raises the capacitanceper length, and therefore lowers the impedance. We suggest that if your impedance calculation is

important, to use EDA software to make the final calculation on line widths!

Ques.10. Derive the modes for circular waveguide.

Ans. The Circular Waveguide Modes;

Figure shows a circular waveguide with inner diameter 2 a. We investigate the lossless circular

waveguide with a perfectly conducting wall and free-space inner region. To investigate the TMand TE modes of the circular cylindric waveguide we derive the fields either from an electric

Hertz form ?e or a magnetic Hertz form ?m exhibiting only az-component


14/62

Figure 7.26: Circular cylindric waveguide.

For both cases the Helmholtz equation (3.28) has the following form:

with . With (A.157) we obtain for circular cylindric coordinates

We seek solutions for waves propagating in the positive z-direction and choose the separationformulation


15/62

From this it follows that

We introduce the parameterkc given by

and obtain

Ques.11. Explain the scattering matrix. Write its properties.

Ans.Scattering(S) Parameter-

Scattering parameters orS-parameters (the elements of a scattering matrix orS-matrix) describe the

electrical behavior of linear electrical networks when undergoing various steady state stimuli by electrical

signals.

The S-parameters are members of a family of similar parameters, other examples being: Y-parameters,[1]

Z-parameters,[2] H-parameters, T-parameters or ABCD-parameters.[3][4]They differ from these, in the

sense that S-parameters do not use open or short circuit conditions to characterize a linear electrical

network; instead matched loads are used. These terminations are much easier to use at high signal

frequencies than open-circuit and short-circuit terminations. Moreover, the quantities are measured in

terms of power.

Many electrical properties of networks of components (inductors, capacitors, resistors) may be expressed

using S-parameters, such as gain, return loss, voltage standing wave ratio (VSWR), reflection coefficient

and amplifier stability.


16/62

The term 'scattering' is more common to optical engineering than RF engineering, referring to the effect

observed when a plane electromagnetic wave is incident on an obstruction or passes across dissimilar

dielectric media. In the context of S-parameters, scattering refers to the way in which the traveling

currents and voltages in a transmission line are affected when they meet a discontinuity caused by the

insertion of a network into the transmission line. This is equivalent to the wave meeting an impedance

differing from the line's characteristic impedance.

S-parameters are readily represented in matrix form and obey the rules of matrix algebra.

The S-parameter matrix describing an N-port network will be square of dimension 'N' and will therefore

contain elements. At the test frequency each element or S-parameter is represented by a unitless

complex number that represents magnitude and angle, i.e. amplitude and phase. The complex number

may either be expressed in rectangular form or, more commonly, in polar form. The S-parameter

magnitude may be expressed in linear form or logarithmic form. When expressed in logarithmic form,

magnitude has the "dimensionless unit" of decibels. The S-parameter angle is most frequently expressed

in degrees but occasionally in radians. Any S-parameter may be displayed graphically on a polar diagram

by a dot for one frequency or a locus for a range of frequencies...he following information must be

defined when specifying any S-parameter:

(1)The characteristic impedance (often 50 ).

(2) The allocation of port numbers.(3) Conditions which may affect the network, such as frequency, temperature, control voltage, and bias

current, where applicable.

Ques.12. Explain properties scattering matrix by virtue of different types of networks.

Ans. Properties of S matrix:

1) Reciprocal and non-reciprocal networks:A reciprocal network is one in which the power

losses are the same between any two ports regardless of direction of propagation (scattering parameter S21=S12, S13=S31, etc.) A network is known to be reciprocal if it is passive and

contains only isotropic materials. Examples of reciprocal networks include cables, attenuators,and all passive power splitters and couplers. Anisotropic materials have different electrical

properties (such as relative dielectric constant) depending on which direction a signal propagatesthrough them. One example of an anisotropic material is the class of materials known as ferrites,

from which circulators and isolators are made. Two classic examples of non-reciprocal networksare RF amplifiers and isolators. In both cases, scattering parameter S21 is much different from

S12.A reciprocal network always has a symmetric S-parameter matrix. That means thatS21=S12, S13=S31, etc. All values along the lower-left to upper right diagonal must be equal. A

two-port S-parameter matrix (at a single frequency) is represented by:


17/62

If you are measuring a network that is known to be reciprocal, checking for symmetry across thediagonal of the S-parameter matrix is one simple check to see if the data is valid. Here is an

example of S-parameters of a network that is either a non-reciprocal network, or your technicianhas a drinking problem Although the data shows the part is well matched (S11 and S22

magnitudes are low), and low loss (S21 and S12 magnitudes are high). The magnitudes of S12and S21 are equal, so what is the problem? The phase angles of S12 and S21 are significantly

different. That can't be right.

2) Lossless networks:

For a network to be lossless, all of the power (or energy) that is incident at any one port has to beaccounted for by summing the power output at the other ports with the power reflected at the

incident port. None of the power is converted to heat or radiated within a lossless network. Notethat an active device is not in the same category as a lossless part, since power is added to the

network through its bias connections. Within the S-parameter matrix of a lossless network, thesum of the squares of the magnitudes of any row must total unity (unity is a fancy way of saying"one"). If any of the rows' sum-of-the-squares is less than one, there is a lossy element within the

network, or something is radiating.

3) Passive devices versus active devices:

A passive device contains no source that could add energy to your signal, with one exception.The first law thermodynamics, conservation of energy, implies that a passive device can't

oscillate. An active device is one in which an external energy source is somehow contributing tothe magnitude of one or more responses

Ques.13. Derive S parameters of scattering matrix.

Ans. Scattering Parameters

A scattering matrix (S-parameter matrix) is one way to describe the operation of a linear, time-invariant two-port circuit. A two-port network is defined as any linear device where a signal goes

in one side and comes out the other. The S-parameter matrix is rapidly becoming very popular asa way to characterize connectors and cables for high-speed applications above 1 Gb/s.

The measurement setup associated with S-parameters is as follows (Figure 1).


18/62

From the test equipment, two cables having characteristic impedance Z0 lead to the left and rightsides, respectively, of the device under test (DUT).

Using the first (left-side) cable, inject a sinusoidal signal (in1) of unit amplitude into the DUT.

The test equipment records the amplitude and phase of the signal (out1) reflected back onto thefirst cable from the DUT, and also the amplitude and phase of the signal (out 2) conveyed through

the DUT to the second cable on the other side.

In a separate experiement, using the second (right-side) cable, inject a sinusoidal signal (in 2) of

unit amplitude into the DUT. The test equipment records the amplitude and phase of the signal(out2) reflected from the right side of the DUT, and the amplitude and phase of the signal (out1)

conveyed through the DUT to the other (left) side. The complete S-parameter matrix is acombination of these four basic measurements.

The four elements of an S-parameter matrix may be reported as complex numbers (with real and

imaginary parts) or in logarithmic units (as dB magnitude and phase). An n-port microwavenetwork has n arms into which power can be fed and from which power can be taken. In general,

power can get from any arm (as input) to any other arm (as output).

There are thus n incoming waves and n outgoing waves. We also observe that power can be

reflected by a port, so the input power to a single port can partition between all the ports of thenetwork to form outgoing waves. Associated with each port is the notion of a "reference plane"

at which the wave amplitude and phase is defined. Usually the reference plane associated with acertain port is at the same place with respect to incoming and outgoing waves. The n incoming

wave complex amplitudes are usually designated by the n complex quantities an, and the noutgoing wave complex quantities are designated by the n complex quantities bn. The incoming

wave quantities are assembled into an n-vector A and the outgoing wave quantities into an n-


19/62

vector B. The outgoing waves are expressed in terms of the incoming waves by the matrixequation B = SA where S is an n by n square matrix of complex numbers called the "scattering

matrix". It completely determines the behaviour of the network. In general, the elements of thismatrix, which are termed "s-parameters", are all frequency-dependent.

For example, the matrix equations for a 2-port are

b1 = s11 a1 + s12 a2

b2 = s21 a1 + s22 a2

And the matrix equations for a 3-port are

b1 = s11 a1 + s12 a2 + s13 a3

b2 = s21 a1 + s22 a2 + s23 a3

b3 = s31 a1 + s32 a2 + s33 a3

The wave amplitudes an and bn are obtained from the port current and voltages by the relations a

= (V + ZoI)/(2 sqrt(2Zo)) and b = (V - ZoI)/(2 sqrt(2Zo)). Here, a refers to an if V is Vn and I Infor the nth port. Note the sqrt(2) reduces the peak value to an rms value, and the sqrt(Zo) makes

the amplitude normalised with respect to power, so that the incoming power = aa* and theoutgoing power is bb*. A one-port scattering parameter s is merely the reflection coefficient

gamma, and as we have seen we can relate gamma to the load impedance zL = ZL/Zo by theformula gamma = (zL-1)/(zL+1).

Similarly, given an n by n "Z-matrix" for an n-port network, we obtain the S matrix from theformula S = (Z-I)(Z+I)^-1, by post-multiplying the matrix (Z-I) by the inverse of the matrix

(Z+I). Here, I is the n by n unit matrix. The matrix of z parameters (which has n squaredelements) is the inverse of the matrix of y parameters.

Ques.14. Write a short note on reciprocity in s- matrix.

Ans. Reciprocity

A network will be reciprocal if it is passive and it contains only reciprocal materials that influence the

transmitted signal. For example, attenuators, cables, splitters and combiners are all reciprocal networks

and in each case, or the S-parameter matrix will be equal to its transpose. Networks which

include non-reciprocal materials in the transmission medium such as those containing magnetically biased

ferrite components will be non-reciprocal. An amplifier is another example of a non-reciprocal network.


20/62

An interesting property of 3-port networks, however, is that they cannot be simultaneously reciprocal,

loss-free, and perfectly matched.

A reciprocal network is one in which the power losses are the same between any two ports regardless of

direction of propagation (scattering parameter S21=S12, S13=S31, etc.) A network is known to be

reciprocal if it is passive and contains only isotropic materials. Examples ofreciprocal networks include

cables, attenuators, and all passive power splitters and couplers.

Anisotropic materials have different electrical properties (such as relative dielectric constant) depending

on which direction a signal propagates through them. One example of an anisotropic material is the class

of materials known as ferrites, from which circulators and isolators are made. Two classic examples of

non-reciprocalnetworks are RF amplifiers and isolators. In both cases, scattering parameter S21 is much

different from S12.

A reciprocal network always has a symmetric S-parameter matrix. That means that S21=S12, S13=S31,

etc. All values along the lower-left to upper right diagonal must be equal. A two-port S-parameter matrix

(at a single frequency) is represented by:

Ques.15. Explain briefly about lossless network.

Ans. Lossless networks

A lossless network is one which does not dissipate any power, or : . The sum of the

incident powers at all ports is equal to the sum of the reflected powers at all ports. This implies that the S-

parameter matrix is unitary, that is , where is the conjugate transpose of

and is the identity matrix.

For a network to be lossless, all of the power (or energy) that is incident at any one port has to be

accounted for by summing the power output at the other ports with the power reflected at the incident

port. None of the power is converted to heat or radiated within a lossless network. Note that an active

device is not in the same category as a lossless part, since power is added to the network through its bias

connections.

Within the S-parameter matrix of a lossless network, the sum of the squares of the magnitudes of any row

must total unity (unity is a fancy way of saying "one"). If any of the rows' sum-of-the-squares is less than

one, there is a lossy element within the network, or something is radiating.


21/62

Ques.16. What do you mean by waveguide junction. List various types of junction.

Ans.Waveguide Junctions:

Figure 1: H-type T- junction

Different types of junctions affect the energy in different ways. The T- Junction is the most

simple of the commonly used waveguide junctions. T-junctions are divided into two basic types,the E-TYPE and the H-TYPE.

H-type T-junction

An H-type T-junction is illustrated in the beside figure. It is called an H-type T-junction becausethe long axis of the B arm is parallel to the plane of the magnetic lines of force in the

waveguide. The E-field is fed into arm A and in-phase outputs are obtained from the B and Carms. The reverse is also true.

Figure 2: E-type T- junction


22/62

E-type T-junction

This junction is called an E- type T junction because the junction arm extends from the main

waveguide in the same direction as the E-field in the waveguide. The outputs will be 180 out ofphase with each other.

Magic-T-Hybrid Junction

A simplified version of the magic-T-hybrid junction is shown in the figure. The magic-Tjunction can be described as a dual electromagnetic plane type of T-junction. It is a combination

of the H-type and E-type T.junction therefore. The most common applications of this type of junction are for example as the mixer section for microwave radar receivers or as a part of a

measurement system.

If a signal is fed into the E-plane arm of the magic-T, it will divide into two out-of-phasecomponents (arm B and C). The signal entering the E-arm will not enter the H-plane arm

because of the zero potential existing at the entrance of the H-plane arm. The potential must bezero at this point to satisfy the boundary conditions of the E-plane arm.

Normally a magic-T needs an impedance matching (shown in the figure as matching screws).

Figure 3: Magic-T Hybrid

Ques.17. Explain Magic tee.

Ans. MAGIC-T HYBRID JUNCTION. A simplified version of the magic-T hybrid junction is

shown in figure 3-64. The magic-T is a combination of the H-type and E-type T junctions. The

most common application of this type of junction is as the mixer section for microwave radar


23/62

receiversMagic-T hybrid junction. If a signal is fed into the b arm of the magic-T, it will divide

into two out-of-phase components.

As shown in figure 3-65, view A, these two components will move into the a and c arms. The signalentering the b arm will not enter the d arm because of the zero potential existing at the entrance of the

d arm. The potential must be zero at this point to satisfy the boundary conditions of the b arm.

This absence of potential is illustrated in views B and C where the magnitude of the E field in the

b arm is indicated by the length of the arrows. Since the E lines are at maximum in the center of

the b arm and minimum at the edge where the d arm entrance is located, no potential difference

exists across the mouth of the d arm. Figure 3-65.Magic-T with input to arm b. In summary, when

an input is applied to arm b of the magic-T hybrid junction, the output signals from arms a and c are

180 degrees out of phase with each other, and no output occurs at the d arm. The action that occurs

when a signal is fed into the d arm of the magic-T is illustrated in figure 3-66. As with the H-type T

junction, the signal entering the d arm divides and moves down the a and c arms as outputs that are

in phase with each other and with the input. The shape of the E

fields in motion is shown by the numbered curved slices. As the E field moves down the d arm,

points 2 and 3 are at an equal potential. The energy divides equally into arms a and c, and the E

fields in both arms become identical in shape. Since thepotentials on both sides of the b arm are

equal, no potential difference exists at the entrance to the b arm, resulting in no output.

Ques.18. Describe directional coupler.

Ans.DIRECTION COUPLER:-

Directional couplers are four-port circuits where one port is isolated from the

input port. Directional couplers are passive reciprocal networks All four ports are (ideally) matched, andthe circuit is (ideally) lossless. Directional couplers can be realized in microstrip, stripline, coax and

waveguide. They are used for sampling a signal, sometimes both the incident and reflected waves (this

application is called a reflectometer, which is an important part of a network analyzer). Directional

couplers generally use distributed properties of microwave circuits, the coupling feature is generally a

quarter (or multiple) quarter-wavelengths.


24/62

A directional coupler has four ports, where one is regarded as the input, one is regarded as the "through"

port (where most of the incident signal exits), one is regarded as the coupled port (where a fixed fraction

of the input signal appears, usually expressed in dB), and an isolated port, which is usually terminated. If

the signal is reversed so that it enter the "though" port, most of it exits the "input" port, but the coupled

port is now the port that was previously regarded as the "isolated port". The coupled port is a function of

which port is the incident port.

Ques.20.Write short note on waveguide attenuators.

Ans. Waveguide Attenuators:-Wave guide attenuators are Low Power Fixed Attenuators, Low PowerVariable Attenuators and a range of Precision Variable Attenuators. Fixed Low Power attenuators. All of

the standard fixed attenuators are manufactured from selected waveguide tube.The attenuating element is

manufactured from a metallised glass fibre reinforced PTFE, resistive card vane or an absorptive

composite material. The vane version is supported in the waveguide using two metal rods and is

accurately positioned to give a desired value between 0 and 40dB as required. The composite absorber is

positioned and glued into the tube (the attenuation is based on thelength of the absorber

Variable Attenuators

Based upon the same construction as the Low Power Fixed Attenuators, the metalIised glass fibre

reinforced PTFE resistive card vane is positioned in the Waveguide using a backlash free, spring

controlled piston, precisely fitted in a machined housing to give a high degree of mechanical stability. The

Attenuation is varied by means of a knurled finger-control knob, and a locking screw is provided for

repetitive measurements, or, in the case of the variable precision devices, the attenuation is varied by

means of a standard micrometer drive.


25/62

Ques .21. Explain circulator.

Ans. A waveguide circulator used as an isolator by placing a matched load on port 3. The label

on the permanent magnet indicates the direction of circulation.

A circulator is a passive non-reciprocal three- or four-port device, in which microwave or radio

frequency power entering any port is transmitted to the next port in rotation (only). Thus, to

within a phase-factor, the scattering matrix for an ideal three-port circulator is

When one port of a three-port circulator is terminated in a matched load, it can be used as an

isolator, since a signal can travel in only one direction between the remaining ports. There are

circulators for LF, VHF, UHF, microwave frequencies and for light, the latter being used in

optical fiber networks. Circulators fall into two main classes: 4-port waveguide circulators based

on Faraday rotation of waves propagating in a magnetized material, and 3-port "Y-junction"

circulators based on cancellation of waves propagating over two different paths near a

magnetized material. Waveguide circulators may be of either type, while more compact devicesbased on striplines are of the 3-port type. Sometimes two or more Y-junctions are combined in a

single component to give four or more ports, but these differ in behavior from a true 4-port

circulator.

In radar, circulators are used to route outgoing and incoming signals between the antenna, the

transmitter and the receiver. In a simple system, this function could be performed by a switch

that alternates between connecting the antenna to the transmitter and to the receiver. The use of

chirped pulses and a high dynamic range may lead to temporal overlap of the sent and received

pulses, however, requiring a circulator for this function.

Ques.12. Write down different types of directional couplers.

Ans. Different types of couplers are as follows:-

Forward versus backward wave couplers:-

Waveguide couplers couple in the forward direction (forward-wave couplers); a signal incident on port 1

will couple to port 3 (port 4 is isolated). Microstrip or stripline coupler are "backward wave" couplers. In


26/62

the schematic above, that means for a signal incident on port 1, port 4 is the coupled port (port 3 is

isolated).

COUPLER RULE OF THUMB:-

The coupled port on a microstrip or stripline directional coupler is closest to the input port because it is a

backward wave coupler. On a waveguide broadwall directional coupler, the coupled port is closest to the

output port because it is aforward wave coupler.

The Narda coupler below is made in stripline (you have to cut it apart to know that, but just trust us),

which means it is a backward wave coupler. The input port is on the right, and the port facing up is the

coupled port(the opposite port is terminated with that weird cone-shaped thingy which voids thewarrantee if you remove it. Luckily Narda usually prints an arrow on the coupler to show how to use it,

but the arrow is on the side that is hidden in the photo.

On the waveguide coupler below, the input is on the left, while the coupled port is on the right, pointing

toward your left ear. There is a termination built into the guide opposite the coupled port, although you

can't see it.


27/62

Bethe-hole coupler:-

This is a waveguide directional coupler, using a single hole, and is works over a

narrow band. In waveguide, a two-hole coupler, two waveguides share a broad wall. Holes are 1/4 wave

apart. In the foreword case the coupled signals add, in the reverse they subtract (180 apart) and disappear.

Coupling factor is controlled by hole size. The "holes" are often x-shaped, and...

Bi-directional coupler:-

A directional coupler where the isolated port is not internally terminated. You can use such a

coupler to form a reflectometer, but it is recommended (use the dual-directional coupler you cheapskate!)

Dual-directional coupler:-

Here we have two couplers in series, in opposing directions, with the isolated ports internally terminated.

This component is the basis for the reflectometer.


28/62

Hybrid couplers

A hybrid coupler is a special case, where a 3 dB split is desired between the through path and the coupled

path. There are two types of hybrid couplers, 90 degree couplers (such as Langes or branchlines) and 180

degree hybrids (such as rat-races and magic tees).

Ques.22. What do you mean by Non reciprocal devices?

Ans. Non-reciprocal devices

A non-reciprocal circuit device comprising a first inductance element L1 disposed between a first

input/output port P1 and a second input/output port P2, a first capacitance element Ci parallel-connected

to the first inductance element L1 to constitute a first resonance circuit, a resistance element R parallel-connected to the first parallel resonance circuit, a second inductance element L2 disposed between a

second input/output port P2 of the first resonance circuit and a ground, a second capacitance element Cfa

parallel-connected to the second inductance element L2 to constitute a second resonance circuit, a third

inductance element Lg disposed between the second resonance circuit and the ground, and a third

capacitance element Cfb disposed between a second input/output port P2 of the first resonance circuit and

the ground.

Discription-

This invention relates to a non-reciprocal circuit device having non-reciprocal transmission characteristics

to high-frequency signals, particularly to a non-reciprocal circuit device suitable for mobilecommunications systems such as cellphones, etc.

Non-reciprocal circuit devices such as isolators are used in mobile communications equipments utilizing

frequencies from several hundreds MHz to several tens GHz, such as base stations and terminals of cell

phones, etc. In transmission systems ofmobile communications equipments, for instance, isolators are

disposed between power amplifiers and antennas to prevent unnecessary signals from returning to the

power amplifiers, thereby stabilizing the impedance of the power amplifiers on the loadside. Accordingly,

the isolators are required to have excellent insertion loss characteristics, reflection loss characteristics and

isolationcharacteristics.


29/62

Cell phones have become handling wider frequency bands (wideband), and pluralities of

transmission/receiving systems such as WCDMA, PDC, PHS, GSM, etc. (multi-band, multi-system, etc.)

to adapt to increasing numbers of users. Accordingly,non-reciprocal circuit devices have been getting

required to be operable in wider frequency bands. One of data transmission technologies, which uses a

cell phone network for GSM and TDMA systems, is an enhanced data GSM environment (EDGE). When

twobands of GSM850/900 are used, a frequency passband required for the non-reciprocal circuit device is824-915MHz.

To obtain a wideband, non-reciprocal circuit device, various factors of causing unevenness, such as

inductance generated in lines connecting reactance elements, floating capacitance generated by

interference between electrode patterns, etc.,should be taken into consideration. In the two-port isolator,

however, unnecessary reactance components are connected to the first and second parallel resonance

circuits, resulting in the deviation of the input impedance of the two-port isolator fromthe desired level.

As a result, there appears impedance mismatching between the two-port isolator and the other circuits

connected thereto, leading to deteriorated insertion loss and isolation characteristics.

Accordingly, the first object of the present invention is to provide a non-reciprocal circuit device having a

wide operation frequency band.

The second object of the present invention is to provide a non-reciprocal circuit device with easy input

impedance matching, which has excellent insertion loss characteristics, reflection characteristics and

harmonicssuppression.

Ques.23. Give the characteristics of non reciprocal devices.

Ans. The characteristics of non reciprocal devices are as follows:-

1.A non-reciprocal circuit device comprising a first inductance element L1 disposed between a first

input/output port P1 and a second input/output port P2, a first capacitance element Ci parallel-connected

to said first inductance element L1 to constitute a first resonance circuit, a resistance element R parallel-

connected to said first parallel resonance circuit, a second inductance element L2 disposed between

asecond input/output port P2 of said first resonance circuit and a ground, and a second capacitance

element Cfa parallel-connected to said second inductance element L2 to constitute a second resonance

circuit, and a third inductance element Lg disposedbetween said second resonance circuit and the ground,

and a third capacitance element Cfb disposed between a second input/output port P2 of said first

resonance circuit and the ground.

2. The non-reciprocal circuit device according to claim 1, wherein said first inductance element L1 has

smaller inductance than that of said second inductance element L2.

3. The non-reciprocal circuit device according to claim 1, wherein at least one of the first capacitance

element Ci, the second capacitance element Cfa and the third capacitance element Cfb is constituted by


30/62


31/62

Ans. A highly sensitive low-noise amplifier for ultrahigh-frequency and microwave radiosignals, utilizing as the active element an inductor or capacitor whose reactance is varied

periodically at another microwave or ultra-high frequency. A varactor diode is most commonlyused as the variable reactor. Amplification of weak signal waves occurs through a nonlinear

modulation or signal-mixing process which produces additional signal waves at otherfrequencies. This process may provide negative-resistance amplification for the applied signal

wave and increased power in one or more of the new frequencies which are generated. See alsoVaractor.

There are several possible circuit arrangements for obtaining useful parametric amplification.

The two most common are the up-converter and the negative-resistance amplifier. In both types,the pump frequency is normally much higher than the input-signal frequency. In the up-

converter, a new signal wave is generated at a higher power than the input wave. In the negative-resistance device, negative resistance is obtained for the input signal frequency, causing an

enhancement of signal power at the same frequency. See also Negative-resistance circuits.


32/62

Ques.26. Explain the property of parametric amplifier due to which it is commonly used in receivers with

MASER.

Ans. The most important advantage of the parametric amplifier is its low level of noisegeneration. The parametric amplifier finds its greatest use as the first stage at the input of

microwave receivers where the utmost sensitivity is required. Its noise performance has beenexceeded only by the maser. Maser amplifiers are normally operated under extreme refrigeration

using liquid helium at about 4 K above absolute zero (452F). The parametric amplifier doesnot require such refrigeration but in some cases cooling to very low temperatures has been used

to give improved noise performance that is only slightly poorer than the maser.

We know that :-

Q[charge in a capacitor] = C x V

therefore

V [voltage across a capacitor] = Q/C

Figure 2-45A. - parametric amplifier. CIRCUIT

Figure 2-45B. - parametric amplifier.

The pump signal causes the capacitor in view (A) to vary at a 12-gigahertz rate. The 3-gigahertz

input signal enters via a four-port ferrite circulator, is developed in the signal cavity, and appliedacross the varactor. The nonlinear action of the varactor produces a 9-gigahertz differencefrequency (fp-fs) with an energy-level higher than the original input signal.

The difference (idler) frequency is reapplied to the varactor to increase the gain and to produce

an output signal of the correct frequency. The 9-gigahertz idler frequency recombines with the12-gigahertz pump signal and produces a 3-gigahertz difference signal that has a much larger

amplitude than the original 3-gigahertz input signal. The amplified signal is sent to the ferritecirculator for transfer to the next stage. As with tunnel-diode amplifiers, the circulator improves


33/62

stability by preventing reflection of the signal back into the amplifier. Reflections would beamplified and cause uncontrollable oscillations. The ferrite circulator also serves as an isolator to

prevent source and load impedance changes from affecting gain. Typically, the gain of aparametric amplifier is about 20 dB. The gain can be controlled with a variable attenuator that

changes the amount of pump power applied to the varactor.

Parametric amplifiers are relatively simple in construction. The only component is a varactordiode placed in an arrangement of cavities and waveguides. The most elaborate feature of the

amplifier is the mechanical tuning mechanism. Figure 2-46 illustrates an actual parametricamplifier.

Ques.26. Explain Manley rowe power relation.

Ans. MANLEY ROWE RELATION:-


34/62


35/62


36/62

Ques.27. Describe devise structure of IMPATT diode.

Ans. An IMPATT diode (IMPact ionization Avalanche Transit-Time) is a form of high

power diode used in high-frequency electronics and microwave devices. They are typically madewith silicon carbide owing to their high breakdown fields.

They operate at frequencies between about 3 and 100 GHz or more. A main advantage is theirhigh power capability. These diodes are used in a variety of applications from low

power radar systems to alarms. A major drawback of using IMPATT diodes is the high levelofphase noise they generate. This results from the statistical nature of the avalanche process

Nevertheless these diodes make excellent microwave generators for many applications


37/62

Device structure:-

The IMPATT diode family includes many different junction and metal semiconductor devicesThe first IMPATT oscillation was obtained from a simple silicon p-n junction diode biased into a

reverse avalanche break down and mounted in a microwave cavity. Because of the strongdependence of the ionization coefficient on the electric field, most of the electronhole pairs are

generated in the high field region. The generated electron immediately moves into the N region,while the generated holes drift across the P region. The time required for the hole to reach the

contact constitutes the transit time delay.

The original proposal for a microwave device of the IMPATT type was made by Read andinvolved a structure. The Read diode consists of two regions (i) The Avalanche region (a region

with relatively high doping and high field) in which avalanche multiplication occurs and (ii) thedrift region (a region with essentially intrinsic doping and constant field) in which the generated

holes drift towards the contact. A similar device can be built with the configuration in whichelectrons generated from the avalanche multiplication drift through the intrinsic region.

An IMPATT diode generally is mounted in a microwave package. The diode is mounted with its

highfield region close to a copper heatsink so that the heat generated at the diode junction canbe readily dissipated. Similar microwave packages are used to house other microwave devices.

Ques.28. Explain the principle of working of IMPATT diode.

Ans. Impact ionization

If a free electron with sufficient energy strikes a silicon atom, it can break the covalent bond of

silicon and liberate an electron from the covalent bond. If the electron liberated gains energy bybeing in an electric field and liberates other electrons from other covalent bonds then this process

can cascade very quickly into a chain reaction producing a large number of electrons and a large

current flow. This phenomenon is called impact avalanche.

At breakdown, the n region is punched through and forms the avalanche region of the diode.

The high resistivity region is the drift zone through which the avalanche generated electronsmove toward the anode.

Consider a dc bias VB, just short of that required to cause breakdown, applied to the diode. Let

an AC voltage of sufficiently large magnitude be superimposed on the dc bias, such that duringthe positive cycle of the AC voltage, the diode is driven deep into the avalanche breakdown. At

t=0, the AC voltage is zero, and only a small pre-breakdown current flows through the diode.


38/62

As t increases, the voltage goes above the breakdown voltage and secondary electron-hole pairsare produced by impact ionization. As long as the field in the avalanche region is maintain above

the breakdown field, the electron-hole concentration grows exponentially with t. Similarly thisconcentration decays exponentially with time when the field is reduced below breakdown

voltage during the negative swing of the AC voltage. The holes generated in the avalancheregion disappear in the p+ region and are collected by the cathode. The electrons are injected into

the i

zone where they drift toward the n+ region. Then, the field in the avalanche region reaches itsmaximum value and the population of the electron-hole pairs starts building up. At this time, the

ionization coefficients have their maximum values. The generated electron concentration doesnot follow the electric field instantaneously because it also depends on the number of electron-

hole pairs already present in the avalanche region. Hence, the electron concentration at this pointwill have a small value. Even after the field has passed its maximum value, the electron-hole

concentration continues to grow because the secondary carrier generation rate still remains aboveits average value. For this reason, the electron concentration in the avalanche region attains its

maximum value at, when the field has dropped to its average value. Thus, it is clear that theavalanche region introduces a 90o phase shift between the AC signal and the electron

concentration in this region.


39/62

With a further increase in t, the AC voltage becomes negative, and the field in the avalancheregion drops below its critical value. The electrons in the avalanche region are then injected into

the drift zone which induces a current in the external circuit which has a phase opposite to that ofthe AC voltage. The AC field, therefore, absorbs energy from the drifting electrons as they are

decelerated by the decreasing field. It is clear that an ideal phase shift between the diode currentAnd the AC signal is achieved if the thickness of the drift zone is such that the bunch of electron

is collected at the n+ - anode at the moment the AC voltage goes to zero. This condition isachieved by making the length of the drift region equal to the wavelength of the signal. This

situation produces an additional phase shift of 90o between the AC voltage and the diode current.

Ques.29. Draw the V-I characteristics of IMPATT diode. Also write down its applications.

Ans.

IMPATT V-I CHARACTERISTICS


40/62

Application:-

These IMPATT diodes makes excellent microwave generator for many applications:

y Parametric amplifier

y Parametric up converter

y Parametric down converter

y Negative resistance parametric amplifier

Ques.30. Explain working principle of TRAPATT diode.

Ans. It is derived from the IMPATT diode and is closely related to it.It is a high efficiencymicrowave generator capable of operating from several hundred MHz to several GHz. The basic

operation of the oscillator is a semiconductor pn junction diode reversw biased to current

densities well in excess of those encountered in normal avalanche operation

OPERATION

The basic scheme used to generate these picoseconds-kilovolt signals. A pulse generator withan amplitude larger than the breakdown voltage of the diode is applied to the diode in

the reverse direction. When the pulse is applied to the circuit the diode will first breakdown, i.e., the diode will look like a zener diode, and then if the amplitude of the driving

signal is large enough, so that a large current flows in the circuit, the diode will go intosecond breakdown Second breakdown can be thought of as a change in diode voltage from

the primary breakdown voltage to some much lower value. Since KVL must be maintained

in the circuit this is usually accompanied by an increase in the current flowing through thedevice. Destruction of the device is usually associated with second breakdown. However, if theamount of energy passed through the diode is limited, destruction is avoided. This usually means

narrow pulses,


41/62

The apparent velocity of the plasma should be much larger than the saturation velocity ofelectrons and holes in the semiconductor for proper operation. The plasma is formed by exciting

electrons below the valence band into states above the conduction band. The result is generationof a gaseous conductor (plasma) in picoseconds. Externally this appears as a switch closing in

a time dependent on the plasma formation

Ques.31. Mention advantages and disadvantages of IMPATT diodes.

Ans. Advantages:

1. Its efficiency high as compared to IMPATT diode.

2. It can be used at high frequency from MHz to GHz.


42/62

Disadvantages:

1. It used at high noise figure.

2. It used at upper microwave frequencies is high.

3. It generates strong harmonics due to short duration of current pulse.

Ques.32. Write down the applications of TRAPATT diode.

APPLICATION:

In pulse radar as local oscillator

In radio altimeter.

Air borne and marine radar.

In microwave beacons and landing system.

In low power Doppler radar.

Ques.33. Explain BARITT DIODE. Also write its applications.

Ans. BARITT DIODE (barrier injected transit time device):-


43/62

When no bias voltage is applied, the electric field profile of this structure is determine by built-infield region of pn and np junction.

When the bias voltage is applied the condition is changed and one junction become forward

biased and one become reversed biased.The possibility offered by BARITT diode is restricted as8GHz as compared to IMPATT diode which is offered 100GHz.

But as in compared of TED performance BARITT diode is good as its high power efficiency.

One more advantage of BARITT diode is that it is simple to fabricate using a sophisticatedMaterial. They also have low noise.

APPLICATION:

They are primarily used for amplifier rather than oscillator because of lower

efficiencies.

Ques.34. Write the basic principle of klystron amplifier.

Ans. A klystron is a specialized linear-beam vacuum tube (evacuated electron tube). Klystrons are used as

amplifiers at microwave and radio frequencies to produce both low-power reference signals for

superheterodyne radar receivers and to produce high-power carrier waves for communications and the

driving force for modern particle accelerators.

Klystron amplifiers have the advantage (over the magnetron) of coherently amplifying a reference signal

so its output may be precisely controlled in amplitude, frequency and phase. Many klystrons have a

waveguide for coupling microwave energy into and out of the device, although it is also quite common

for lower power and lower frequency klystrons to use coaxial couplings instead. In some cases a coupling

probe is used to couple the microwave energy from a klystron into a separate external waveguide.

All modern klystrons are amplifiers, since reflex klystrons, which were used as oscillators in the past,

have been surpassed by alternative technologies.

The name klystron comes from the stem form - (klys) of a Greek verb referring to the action of

waves breaking against a shore, and the end of the word electron.


44/62

During the second World War, the Axis powers relied mostly on (then low-powered) klystron technology

for their radar system microwave generation, while the Allies used the far more powerful but frequency-

drifting technology of the cavity magnetron for microwave generation. Klystron tube technologies for

very high-power applications, such as synchrotrons and radar systems, have since been developed.

Klystrons amplify RF signals by converting the kinetic energy in a DC electron beam into radio frequency

power. A beam of electrons is produced by a thermionic cathode (a heated pellet of low work functionmaterial), and accelerated by high-voltage electrodes (typically in the tens of kilovolts). This beam is then

passed through an input cavity. RF energy is fed into the input cavity at, or near, its natural frequency to

produce a voltage which acts on the electron beam. The electric field causes the electrons to bunch:

electrons that pass through during an opposing electric field are accelerated and later electrons are slowed,

causing the previously continuous electron beam to form bunches at the input frequency. To reinforce the

bunching, a klystron may contain additional "buncher" cavities. The RF current carried by the beam will

produce an RF magnetic field, and this will in turn excite a voltage across the gap of subsequent resonant

cavities. In the output cavity, the developed RF energy is coupled out. The spent electron beam, with

reduced energy, is captured in a collector.

Ques.35. Describe the working of two cavity Klystron amplifier.

Ans. Two-cavity klystron amplifier:-


45/62

In the two-chamber klystron, the electron beam is injected into a resonant cavity. The electron beam,

accelerated by a positive potential, is constrained to travel through a cylindrical drift tube in a straight

path by an axial magnetic field. While passing through the first cavity, the electron beam is velocity

modulated by the weak RF signal. In the moving frame of the electron beam, the velocity modulation is

equivalent to a plasma oscillation. Plasma oscillations are rapid oscillations of the electron density in

conducting media such as plasmas or metals.(The frequency only depends weakly on the wavelength). So

in a quarter of one period of the plasma frequency, the velocity modulation is converted to densitymodulation, i.e. bunches of electrons. As the bunched electrons enter the second chamber they induce

standing waves at the same frequency as the input signal. The signal induced in the second chamber is

much stronger than that in the first.

Ques.36. How could you convert a two cavity klystron as an oscillator.

Ans. Two-cavity klystron oscillator:-

The two-cavity amplifier klystron is readily turned into an oscillator klystron by providing a feedback

loop between the input and output cavities. Two-cavity oscillator klystrons have the advantage of being

among the lowest-noise microwave sources available, and for that reason have often been used in theilluminator systems of missile targeting radars. The two-cavity oscillator klystron normally generates

more power than the reflex klystrontypically watts of output rather than milliwatts. Since there is no

reflector, only one high-voltage supply is necessary to cause the tube to oscillate, the voltage must be

adjusted to a particular value. This is because the electron beam must produce the bunched electrons in

the second cavity in order to generate output power. Voltage must be adjusted to vary the velocity of the

electron beam (and thus the frequency) to a suitable level due to the fixed physical separation between the

two cavities. Often several "modes" of oscillation can be observed in a given klystron.

Ques.37. Describe the working principle of reflex klystron.

Ans. In the reflex klystron (also known as a 'Sutton' klystron after its inventor), the electron beam passes

through a single resonant cavity. The electrons are fired into one end of the tube by an electron gun. After

passing through the resonant cavity they are reflected by a negatively charged reflector electrode for

another pass through the cavity, where they are then collected. The electron beam is velocity modulated

when it first passes through the cavity. The formation of electron bunches takes place in the drift space

between the reflector and the cavity. The voltage on the reflector must be adjusted so that the bunching is

at a maximum as the electron beam re-enters the resonant cavity, thus ensuring a maximum of energy is


46/62

transferred from the electron beam to the RF oscillations in the cavity. The voltage should always be

switched on before providing the input to the reflex klystron as the whole function of the reflex klystron

would be destroyed if the supply is provided after the input. The reflector voltage may be varied slightly

from the optimum value, which results in some loss of output power, but also in a variation in frequency.

This effect is used to good advantage for automatic frequency control in receivers, and in frequency

modulation for transmitters. The level of modulation applied for transmission is small enough that the

power output essentially remains constant. At regions far from the optimum voltage, no oscillations areobtained at all. This tube is called a reflex klystron because it repels the input supply or performs the

opposite function of a klystron.

The reflex klystron contains a REFLECTOR PLATE, referred to as the REPELLER, instead of the output

cavity used in other types of klystrons. The electron beam is modulated as it was in the other types of

klystrons by passing it through an oscillating resonant cavity, but here the similarity ends. The feedback

required to maintain oscillations within the cavity is obtained by reversing the beam and sending it back

through the cavity. The electrons in the beam are velocity-modulated before the beam passes through the

cavity the second time and will give up the energy required to maintain oscillations. The electron beam is

turned around by a negatively charged electrode that repels the beam. This negative element is the

repeller mentioned earlier. This type of klystron oscillator is called a reflex klystron because of the reflex

action of the electron beam.


47/62

Ques.38. Explain the effect of modes of operation of reflex klystron at its output .

Ans. The reflex klystron operates in a different mode for each additional cycle that the electrons remain in

the repeller field. Mode 1 is obtained when the repeller voltage produces an electron transit time of 3/4

cycle. Additional modes follow in sequence. Mode 2 has an electron transit time of 1 3/4 cycles; mode 3

has an electron transit time of 2 3/4 cycles; etc. The physical design of the tube limits the number of

modes possible in practical applications. A range of four modes of operation are normally available. Theactual mode used (1 3/4 cycles through 4 3/4 cycles, 2 3/4 cycles through 6 3/4 cycles, etc.) depends upon

the application. The choice of mode is determined by the difference in power available from each mode

and the band of frequencies over which the circuit can be tuned.

OUTPUT POWER. - The variation in output power for different modes of operation can be explained by

examining the factors which limit the amplitude of oscillations. Power and amplitude limitations are

caused by the DEBUNCHING process of the electrons in the repeller field space. Debunching is simply

the spreading out of the electron bunches before they reach electrostatic fields across the cavity grid . The

lower concentration of electrons in the returning bunches provides less power for delivery to the

oscillating cavity. This reduced power from the bunches, in turn, reduces the amplitude of the cavity

oscillations and causes a decrease in output power. In higher modes of operation the electron bunches areformed more slowly. They are more likely to be affected by debunching because of mutual repulsion

between the negatively charged electrons. The long drift time in the higher modes allows more time for

this electron interaction and, as a result, the effects of debunching are more severe. The mutual repulsion

changes the relative velocity between the electrons in the bunches and causes the bunches to spread out.

Figure 2-12. - Electronic tuning and output power of a reflex klystron.


48/62

Electronic tuning does not change the center frequency of the cavity, but does vary the frequency within

the mode of operation. The amount the frequency can be varied above or below the center frequency is

limited by the half-power points of the mode, as shown in figure 2-12. The center frequency can be

changed by one of two methods One method, GRID-GAP TUNING, varies the cavity frequency by

altering the distance between the grids to change the physical size of the cavity. This method varies the

capacitance of the cavity by using a tuning screw to change the distance between the grids mechanically.

The cavity can also be tuned by PADDLES or SLUGS that change the inductance of the cavity.

Ques.39. Write the applications of klystron amplifier.

Ans. Klystrons produce microwave power far in excess of that developed by solid state. In modern

systems, they are used from UHF (hundreds of MHz) up through hundreds of gigahertz (as in the

Extended Interaction Klystrons in the CloudSat satellite). Klystrons can be found at work in radar,

satellite and wideband high-power communication (very common in television broadcasting and EHF

satellite terminals), medicine (radiation oncology), and high-energy physics (particle accelerators and

experimental reactors). At SLAC, for example, klystrons are routinely employed which have outputs in

the range of 50 megawatts (pulse) and 50 kilowatts (time-averaged) at frequencies nearing 3 GHz.

Ques.40. Write the working principle of TWT amplifiers.

Ans. Operation

Power supply arrangements for a typical travelling wave tube


49/62

The power supply arrangements for a typical TWT are shown in above. The voltages andcurrents given are for a 10 W output tube, but the alignment details apply to almost all tubes.

However, manufacturers' data regarding electrode voltages and tube operating conditions should

always be referred to before running any part icular tube.

It is very important that a suitable matched load be connected to the output of the amplifier, asthe power reflected from any mismatch at the output is dissipated in the helix and can burn it out.

For the same reason the antenna must be properly matched.

The beam current is controlled by the grid-cathode voltage. In modern TWTS, the beam

focussing is preset and no adjustment is necessary, but if the focussing is adjustable the tubeshould be run initially at a low beam (collector) current, and the beam focussing magnets

adjusted for minimum helix current. The helix voltage should also be set for minimum helixcurrent. With the tube running at its specified collector current, RF drive can be applied. The

collector current will hardly change, but the helix voltage should be set for maximum outputconsistent with not exceeding the tube voltage or helix current ratings. If the focussing is

adjustable this should be readjusted for minimum helix current, since the RF drive will defocus

the beam slightly. As the helix is fragile and will not dissipate more than a certain power withoutdamage, the helix current should be metered, and a current trip incorporated to cut the powersupplies to the tube if the helix current becomes excessive. The EHT supplies to the tube should

be well smoothed, since ripple will phase-modulate the output and give a rough note.

If the collector dissipates more than about 100 W it may be necessary to use a blower to cool thecollector end of the tube. Typical efficiency of the TWTA is about 10 per cent, though some

modern tubts may reach 40 per cent.

The transfer characteristic of a travelling wave tube amplifier


50/62

The transfer characteristic is essentially linear, which permits the tube to be used to amplify ssb-one of its great advantages in an amateur context. As the input is increased, however, the

amplifier saturates. There is no harm to the tube in operating at saturated output power, exceptthat amplification is no longer linear, although if appreciable harmonic power is generated this

may be reflected at the output transition and damage the helix through over dissipation. Theoutput from the amplifier can also be amplitude-modulated by a signal on the grid, but the

attendant phase modulation is quite high; this method is not normally used to produce a greatdepth of modulation, other than to operate the TWT in the pulsed mode. This is because at some

voltages between maximum and minimum output, beam interception by the helix occurs, whichcauses excessive helix dissipation unless the transitions are rapid. Phase modulation is obtained

by varying the helix voltage over a small range. Typically, plus or minus 100V from 2 kVnominal helix voltage will give 2 rad phase shift, with 1-2 dB reduction in output, which occurs

because the gain is very sensitive to cathode-helix voltage.

Ques.41. What do you mean by helix structure. Explain its utility to be used in TWT.

Ans. The essential principle of operation of a TWT lies in the interaction between an electronbeam and an RF signal. The velocity, v, of an electron beam is given by:

An anode voltage of 5 kV gives an electron velocity of 4.2 x 10*7 mso*-1. The signal wouldnormally travel at c, the velocity of light (3x10*8 ms*-1), which is much faster than any

'reasonable' electron beam (relativistic effects mean that the electron mass actually increases asits velocity approaches c, so that achieving electron velocities approaching c is a complicated

business), If, however, the signal can be slowed down to the same velocity as the electron beam,it is possible to obtain amplification of the signal by virtue of its interaction with the beam. This

is usually achieved using the helix electrode, which is simply a spiral of wire around the electronbeam,


51/62

The geometery of the helix

Without the helix, the signal would travel at a velocity c. With the helix, the axial signal velocity

is approximately c x (p /2a) where a, p are shown above, so the signal is slowed by the factorp/2a. Note that this is independent of signal frequency. The signal travelling along the helix is

known as a slow wave, and the helix is referred to as a slow-wave structure, The condition forequal slow-wave and electron-beam velocities is therefore approximately

The interaction between the beam and the slow wave takes the form of 'velocity modulation' of

the beam (ie some electrons are accelerated and some retarded) forming electron bunches withinthe beam. The beam current therefore becomes modulated by the RF signal, and the bunches

react with the RF fields associated with the slow wave travelling down the helix, resulting in anet transfer of energy from the beam to the signal, and consequent amplification. Since there areno resonant structures involved in this interaction, amplification is obtained over a wide

bandwidth. In fact the principal factors which limit bandwidth are the input/output couplingarrangements.

It should also be mentioned that it is possible to construct an oscillator, utilizing the so-called

backward wave, whose energy travels in the reverse direction to the electron beam. These tubesare known as backward wave oscillators (BWOs) and have the advantage of a very wide tunable

range (an octave or more). They have been used extensively in swept frequency sources(sweepers), but are rapidly being displaced by Gunn diodes and, more recently, transistor

sources.


52/62

Ques.42. Explain construction and operation of magnetron.

Ans. All cavity magnetrons consist of a hot cathode with a high (continuous or pulsed) negative

potential by a high-voltage, direct-current power supply. The cathode is built into the center ofan 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) positiveouter 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 alongtheir 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 tobunch 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 theextracted 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.

The sizes of the cavities determine the resonant frequency, and thereby the frequency of emitted

microwaves. However, the frequency is not precisely controllable. The operating frequency varies with

changes in load impedance, with changes in the supply current, and with the temperature of the tube.[5]

This is not a problem in uses such as heating, or in some forms of radar where the receiver can be

synchronized with an imprecise magnetron frequency. Where precise frequencies are needed, other

devices such as the klystron are used.

The magnetron is a self-oscillating device requiring no external elements other than a power supply. A

well-defined threshold anode voltage must be applied before oscillation will build up; this voltage is a

function of the dimensions of the resonant cavity, and the applied magnetic field. In pulsed applications

there is a delay of several cycles before the oscillator achieves full peak power, and the build-up of anode

voltage must be coordinated with the build-up of oscillator output.[5]

The magnetron is a fairly efficient device. In a microwave oven, for instance, a 1.1 kilowatt input will

generally create about 700 watt of microwave power, an efficiency of around 65%. (The high-voltage and

the properties of the cathode determine the power of a magnetron.) Large S-band magnetrons can produce

up to 2.5 megawatts peak power with an average power of 3.75 kW.[5] Large magnetrons can be water

cooled. The magnetron remains in widespread use in roles which require high power, but where precise

frequency control is unimportant.

Ques.43. Write the application of magnetron.

Ans. In radar devices the waveguide is connected to an antenna. The magnetron is operated with very

short pulses of applied voltage, resulting in a short pulse of high power microwave energy being radiated.

As in all radar systems, the radiation reflected off a target is analyzed to produce a radar map on a screen.


53/62

Several characteristics of the magnetron's power output conspire to make radar use of the device

somewhat problematic. The first of these factors is the magnetron's inherent instability in its transmitter

frequency. This instability is noted not only as a frequency shift from one pulse to the next, but also a

frequency shift within an individual transmitter pulse. The second factor is that the energy of thetransmitted pulse is spread over a wide frequency spectrum, which makes necessary its receiver to have a

corresponding wide selectivity. This wide selectivity permits ambient electrical noise to be accepted into

the receiver, thus obscuring somewhat the received radar echoes, thereby reducing overall radar

performance. The third factor, depending on application, is the radiation hazard caused by the use of high

power electromagnetic radiation. In some applications, for example marine radar mounted on a

recreational vessel, a radar with a magnetron output of 2 to 4 kilowatts is often found mounted very near

an area occupied by crew or passengers. In practical use, these factors have been overcome, or merely

accepted, and there are today thousands of magnetron aviation and marine radar units in service. Recent

advances in aviation weather avoidance radar and in marine radar have successfully implemented

semiconductor transmitters that eliminate the magnetron entirely.

Heating:-In microwave ovens the waveguide leads to a radio frequency-transparent port into the cooking

chamber.

Lighting:-In microwave-excited lighting systems, such as a sulfur lamp, a magnetron provides the

microwave field that is passed through a waveguide to the lighting cavity containing the light-emitting

substance (e.g., sulfur, metal halides, etc.)

Ques.44. Write the advantages of TWT over its disadvantages.

Ans. The problem unfortunately still remains that TWTS are very difficult to acquire.

Nevertheless, their high power output, high gain, and ease of operation make them the ideal wayto run power at frequencies above about 4 GHz, where conventional tubes like the 2C39A run

out of steam, and they represent practically the only way to run high-power SSB. It is relativelyeasy to generate the 1 mW or so of SSB at microwave frequencies required to drive most TWTS

[3], which will produce over 1 W of power. This, in conjunction with high-gain antennas,permits advantage to be taken of tropospheric scatter as a reliable propagation mode.

Ques.45. Explain transfer characteristics of TWT.

Ans. If the collector dissipates more than about 100 W it may be necessary to use a blower tocool the collector end of the tube. Typical efficiency of the TWTA is about 10 per cent, though

some modern tubts may reach 40 per cent.


54/62

The transfer characteristic of a travelling wave tube amplifier

The transfer characteristic is essentially linear, which permits the tube to be used to amplify ssb-

one of its great advantages in an amateur context. As the input is increased, however, theamplifier saturates. There is no harm to the tube in operating at saturated output power, except

that amplification is no longer linear, although if appreciable harmonic power is generated thismay be reflected at the output transition and damage the helix through overdissipation.

The output from the amplifier can also be amplitude-modulated by a signal on the grid, but the

attendant phase modulation is quite high; this method is not normally used to produce a greatdepth of modulation, other than to operate the TWT in the pulsed mode. This is because at some

voltages between maximum and minimum output, beam interception by the helix occurs, whichcauses excessive helix dissipation unless the transitions are rapid. Phase modulation is obtained

by varying the helix voltage over a small range. Typically, plus or minus 100V from 2 kVnominal helix voltage will give 2 rad phase shift, with 1-2 dB reduction in output, which occurs

because the gain is very sensitive to cathode-helix voltage.

It

Documents

Model Questions and Answer (1)