I MSC AE
SUBJECT NAME:MICRO WAVE AND RADAR NAVIGATION SYSYTEMS
MICROWAVE AND RADAR SYSTEMS
UNIT – I
Section A
1. What is wave guide?
A hollow conducting metallic tube of uniform cross section is used for
propagating EM waves that are guided along the surfaces of the tube is
called waveguide.
2. Mention the applications of waveguide
The wave guides are employed for transmission of energy at very high
frequencies where the attenuation caused by wave guide is smaller.
3. Why is rectangular or circular form used as wave guide?
Waveguides usually take the form of rectangular or circular cylinders
because of its simpler forms in use and less expensive to manufacture.
4. Define a wave?
If a physical phenomenon that occurs at one place at a given time is
reproduced at other places at later times, the time delay being proportional to
the space separation from the first location, then the group of phenomena
constitutes a wave.
Section B
1. Write a short note about Rectangular Waveguides...
Rectangular waveguides are th one of the earliest type of the transmission lines.
They are used in many applications. A lot of components such as isolators,
detectors, attenuators, couplers and slotted lines are available for various standard
waveguide bands between 1 GHz to above 220 GHz.
A rectangular waveguide supports TM and TE modes but not TEM waves because
we cannot define a unique voltage since there is only one conductor in a
rectangular waveguide. The shape of a rectangular waveguide is as shown below.
A material with permittivity e and permeability m fills the inside of the conductor.
A rectangular waveguide cannot propagate
below
some
certain
frequency.
This
frequency is called the cut-off frequency.
Here, we will discuss TM mode rectangular
waveguides and TE mode rectangular
waveguides separately. Let’s start with the
TM mode.
TM Modes
Consider the shape of the rectangular waveguide above with dimensions a and b
(assume a>b) and the parameters e and m. For TM waves Hz = 0 and Ez should be
solved from equation for TM mode;
Ñ2xy Ez0 + h2 Ez0 = 0
Since Ez(x,y,z) = Ez0(x,y)e-gz, we get the following equation,
If we use the method of separation of variables, that is Ez0(x,y)=X(x).Y(y) we get,
Since the right side contains x terms only and the left side contains y terms only,
they are both equal to a constant. Calling that constant as kx2, we get;
where ky2=h2-kx2
Now, we should solve for X and Y from the preceding equations. Also we have the
boundary conditions of;
Ez0(0,y)=0
Ez0(a,y)=0
Ez0(x,0)=0
Ez0(x,b)=0
From all these, we conclude that
X(x) is in the form of sin kxx, where kx=mp/a, m=1,2,3,…
Y(y) is in the form of sin kyy, where ky=np/b, n=1,2,3,…
So the solution for Ez0(x,y) is
(V/m)
From ky2=h2-kx2, we have;
For TM waves, we have
From these equations, we get
where
Here, m and n represent possible modes and it is designated as the TM mn mode. m
denotes the number of half cycle variations of the fields in the x-direction and n
denotes the number of half cycle variations of the fields in the y-direction.
When we observe the above equations we see that for TM modes in rectangular
waveguides, neither m nor n can be zero. This is because of the fact that the field
expressions are identically zero if either m or n is zero. Therefore, the lowest mode
for rectangular waveguide TM mode is TM11 .
Here, the cut-off wave number is
and therefore,
The cut-off frequency is at the point where g vanishes. Therefore,
Since l=u/f, we have the cut-off wavelength,
At a given operating frequency f, only those frequencies, which have f c<f will
propagate. The modes with f<fc will lead to an imaginary b which means that the
field components will decay exponentially and will not propagate. Such modes are
called cut-off or evanescent modes.
The mode with the lowest cut-off frequency is called the dominant mode. Since
TM modes for rectangular waveguides start from TM11 mode, the dominant
frequency is
The wave impedance is defined as the ratio of the transverse electric and magnetic
fields. Therefore, we get from the expressions for Ex and Hy (see the equations
above);
The guide wavelength is defined as the distance between two equal phase planes
along the waveguide and it is equal to
which is thus greater than l, the wavelength of a plane wave in the filling medium.
The phase velocity is
which is greater than the speed of light (plane wave) in the filling material.
Attenuation for propagating modes results when there are losses in the dielectric
and in the imperfectly conducting guide walls. The attenuation constant due to the
losses in the dielectric can be found as follows:
TE Modes
Consider again the rectangular waveguide below with dimensions a and b (assume
a>b) and the parameters e and m.
For TE waves Ez = 0 and Hz should be
solved from equation for TE mode;
Ñ2xy Hz + h2 Hz = 0
Since Hz(x,y,z) = Hz0(x,y)e-gz, we get the
following equation,
If we use the method of separation of variables, that is Hz0(x,y)=X(x).Y(y) we get,
Since the right side contains x terms only and the left side contains y terms only,
they are both equal to a constant. Calling that constant as kx2, we get;
where ky2=h2-kx2
Here, we must solve for X and Y from the preceding equations. Also we have the
following boundary conditions:
at x=0
at x=a
at y=0
at y=b
From all these, we get
(A/m)
From ky2=h2-kx2, we have;
For TE waves, we have
From these equations, we obtain
where
As explained before, m and n represent possible modes and it is shown as the TE mn
mode. m denotes the number of half cycle variations of the fields in the x-direction
and n denotes the number of half cycle variations of the fields in the y-direction.
Here, the cut-off wave number is
and therefore,
The cut-off frequency is at the point where g vanishes. Therefore,
Since l=u/f, we have the cut-off wavelength,
At a given operating frequency f, only those frequencies, which have f>f c will
propagate. The modes with f<fc will not propagate.
The mode with the lowest cut-off frequency is called the dominant mode. Since
TE10 mode is the minimum possible mode that gives nonzero field expressions for
rectangular waveguides, it is the dominant mode of a rectangular waveguide with
a>b and so the dominant frequency is
The wave impedance is defined as the ratio of the transverse electric and magnetic
fields. Therefore, we get from the expressions for Ex and Hy (see the equations
above);
The guide wavelength is defined as the distance between two equal phase planes
along the waveguide and it is equal to
which is thus greater than l, the wavelength of a plane wave in the filling medium.
The phase velocity is
which is greater than the speed of the plane wave in the filling material.
The attenuation constant due to the losses in the dielectric is obtained as follows:
After some manipulation, we get
Example:
Consier a length of air-filled copper X-band waveguide, with dimensions
a=2.286cm, b=1.016cm. Find the cut-off frequencies of the first four propagating
modes.
Solution:
From the formula for the cut-off frequency
2. Write a short note about Gauss's law
This article is about Gauss's law concerning the electric field. For an analogous law
concerning the magnetic field, see Gauss's law for magnetism. For an analogous
law concerning the gravitational field, see Gauss's law for gravity. For Gauss's
theorem, a general theorem relevant to all of these laws, see Divergence theorem.
In physics, Gauss's law, also known as Gauss's flux theorem, is a law relating the
distribution of electric charge to the resulting electric field. Gauss's law states that:
The electric flux through any closed surface is proportional to the enclosed electric
charge.
The law was formulated by Carl Friedrich Gauss in 1835, but was not published
until 1867.[1] It is one of the four Maxwell's equations, which form the basis of
classical electrodynamics. Gauss's law can be used to derive Coulomb's law,[2] and
vice versa.
Gauss's law may be expressed in its integral form:
where the left-hand side of the equation is a surface integral denoting the electric
flux through a closed surface S, and the right-hand side of the equation is the total
charge enclosed by S divided by the electric constant.
Gauss's law also has a differential form:
where ∇ · E is the divergence of the electric field, and ρ is the charge density.
The integral and differential forms forms are related by the divergence theorem,
also called Gauss's theorem. Each of these forms can also be expressed two ways:
In terms of a relation between the electric field E and the total electric charge, or in
terms of the electric displacement field D and the free electric charge.
Gauss's law has a close mathematical similarity with a number of laws in other
areas of physics. See, for example, Gauss's law for magnetism and Gauss's law for
gravity. In fact, any "inverse-square law" can be formulated in a way similar to
Gauss's law: For example, Gauss's law itself is essentially equivalent to the
inverse-square Coulomb's law, and Gauss's law for gravity is essentially equivalent
to the inverse-square Newton's law of gravity.
Gauss's law can be used to demonstrate that there is no electric field inside a
Faraday cage with no electric charges. Gauss's law is something of an electrical
analogue of Ampère's law, which deals with magnetism.
In terms of total charge
Integral form
For a volume V with surface S, Gauss's law states that
where ΦE,S is the electric flux through S, Q is total charge inside V, and ε0 is the
electric constant.
The electric flux is given by a surface integral over S:
where E is the electric field, dA is a vector representing an infinitesimal element of
area,and · represents the scalar product.
Applying the integral form
If the electric field is known everywhere, Gauss's law makes it quite easy, in
principle, to find the distribution of electric charge: The charge in any given region
can be deduced by integrating the electric field to find the flux.
However, much more often, it is the reverse problem that needs to be solved: The
electric charge distribution is known, and the electric field needs to be computed.
This is much more difficult, since if you know the total flux through a given
surface, that gives almost no information about the electric field, which (for all you
know) could go in and out of the surface in arbitrarily complicated patterns.
An exception is if there is some symmetry in the situation, which mandates that the
electric field passes through the surface in a uniform way. Then, if the total flux is
known, the field itself can be deduced at every point. Common examples of
symmetries which lend themselves to Gauss's law include cylindrical symmetry,
planar symmetry, and spherical symmetry. See the article Gaussian surface for
examples where these symmetries are exploited to compute electric fields.
Differential form
In differential form, Gauss's law states:
where: ∇ · denotes divergence, E is the electric field, and ρ is the total electric
charge density (including both free and bound charge), and ε0 is the electric
constant.
This is mathematically equivalent to the integral form, because of the divergence
theorem.
Equivalence of integral and differential forms
Beginning with the integral form of Gauss's law, Q can be rewritten in terms of
charge density so that
where dτ is an infinitesimal element of volume.
It follows from the divergence theorem that
Thus
and so
Thus the integral and differential forms are equivalent.
In terms of free charge
Free versus bound charge
The electric charge that arises in the simplest textbook situations would be
classified as "free charge"—for example, the charge which is transferred in static
electricity, or the charge on a capacitor plate. In contrast, "bound charge" arises
only in the context of dielectric (polarizable) materials. (All materials are
polarizable to some extent.) When such materials are placed in an external electric
field, the electrons remain bound to their respective atoms, but shift a microscopic
distance in response to the field, so that they're more on one side of the atom than
the other. All these microscopic displacements add up to give a macroscopic net
charge distribution, and this constitutes the "bound charge".
Although microscopically, all charge is fundamentally the same, there are often
practical reasons for wanting to treat bound charge differently from free charge.
The result is that the more "fundamental" Gauss's law, in terms of E, is sometimes
put into the equivalent form below, which is in terms of D and the free charge
only.
Integral form
This formulation of Gauss's law states that, for any volume V in space, with surface
S, the following equation holds:
where ΦD,S is the flux of the electric displacement field D through S, and Qfree is the
free charge contained in V. The flux ΦD,S is defined analogously to the flux ΦE,S of
the electric field E through S. Specifically, it is given by the surface integral
Differential form
The differential form of Gauss's law, involving free charge only, states:
where ∇ · D is the divergence of the electric displacement field, and ρfree is the free
electric charge density.
The differential form and integral form are mathematically equivalent. The proof
primarily involves the divergence theorem.
Equivalence of total and free charge statements
In linear materials
In homogeneous, isotropic, nondispersive, linear materials, there is a nice, simple
relationship between E and D:
where ε is the permittivity of the material. Under these circumstances, there is yet
another pair of equivalent formulations of Gauss's law:
3. write a short note about FARADAY’S LAW
Electromagnetic induction
Electromagnetic induction is the production of voltage across a conductor
situated in a changing magnetic field or a conductor moving through a stationary
magnetic field.
Michael Faraday is generally credited with having discovered the induction
phenomenon in 1831
Technical details
Faraday found that the electromotive force (EMF) produced around a closed path
is proportional to the rate of change of the magnetic flux through any surface
bounded by that path.
In practice, this means that an electrical current will be induced in any closed
circuit when the magnetic flux through a surface bounded by the conductor
changes. This applies whether the field itself changes in strength or the conductor
is moved through it.
Electromagnetic induction underlies the operation of generators, all electric
motors, transformers, induction motors, synchronous motors, solenoids, and most
other electrical machines.
Faraday's law of electromagnetic induction states that:
,
Thus:
is the electromotive force (emf) in volts
ΦB is the magnetic flux in webers
For the common but special case of a coil of wire, composed of N loops with the
same area, Faraday's law of electromagnetic induction states that
where
is the electromotive force (emf) in volts
N is the number of turns of wire
Section C
1. Write a short note about Electromagnetic wave equation
The electromagnetic wave equation is a second-order partial differential equation
that describes the propagation of electromagnetic waves through a medium or in a
vacuum. The homogeneous form of the equation, written in terms of either the
electric field E or the magnetic field B, takes the form:
where c is the speed of light in the medium. In a vacuum, c = c0 = 299,792,458
meters per second, which is the speed of light in free space.[1]
The electromagnetic wave equation derives from Maxwell's equations.
It should also be noted that in most older literature, B is called the "magnetic flux
density" or "magnetic induction".
Speed of propagation
In vacuum
If the wave propagation is in vacuum, then
metres per second
is the speed of light in vacuum, a defined value that sets the standard of length, the
metre. The magnetic constant μ0 and the vacuum permittivity
are important
physical constants that play a key role in electromagnetic theory. Their values (also
a matter of definition) in SI units taken from NIST are tabulated below:
Symbol Name
speed
vacuum
Numerical Value
of
light
in
SI Unit of
Measure
metres
second
per
Type
defined
farads
electric constant
per derived;
metre
henries
magnetic constant
metre
characteristic
ohms
impedance of vacuum
per
defined
derived;
μ0c0
In a material medium
The speed of light in a linear, isotropic, and non-dispersive material medium is
where
is the refractive index of the medium,
is the magnetic permeability of the
medium, and is the electric permittivity of the medium.
The origin of the electromagnetic wave equation
Conservation of charge
Conservation of charge requires that the time rate of change of the total charge
enclosed within a volume V must equal the net current flowing into the surface S
enclosing the volume:
where j is the current density (in Amperes per square meter) flowing through the
surface and ρ is the charge density (in coulombs per cubic meter) at each point in
the volume.
From the divergence theorem, this relationship can be converted from integral
form to differential form:
Ampère's circuital law prior to Maxwell's correction
In its original form, Ampère's circuital law relates the magnetic field B to the
current density j:
where S is an open surface terminated in the curve C. This integral form can be
converted to differential form, using Stokes' theorem:
Inconsistency between Ampère's circuital law and the law of conservation of
charge
Taking the divergence of both sides of Ampère's circuital law gives:
The divergence of the curl of any vector field, including the magnetic field B, is
always equal to zero:
Combining these two equations implies that
Because
is nonzero constant, it follows that
However, the law of conservation of charge tells that
Hence, as in the case of Kirchhoff's circuit laws, Ampère's circuital law would
appear only to hold in situations involving constant charge density. This would rule
out the situation that occurs in the plates of a charging or a discharging capacitor.
Maxwell's correction to Ampère's circuital law
Maxwell conceived of displacement current in connection with linear polarization
of a dielectric medium. The concept has since been extended to apply to the
vacuum. The justification of this virtual extension of displacement current is as
follows:
Gauss's law in integral form states:
where S is a closed surface enclosing the volume V. This integral form can be
converted to differential form using the divergence theorem:
Taking the time derivative of both sides and reversing the order of differentiation
on the left-hand side gives:
This last result, along with Ampère's circuital law and the conservation of charge
equation, suggests that there are actually two origins of the magnetic field: the
current density j, as Ampère had already established, and the so-called
displacement current:
So the corrected form of Ampère's circuital law becomes:
Maxwell's hypothesis that light is an electromagnetic wave
A postcard from Maxwell to Peter Tait.
In his 1864 paper entitled A Dynamical Theory of the Electromagnetic Field,
Maxwell utilized the correction to Ampère's circuital law that he had made in part
III of his 1861 paper On Physical Lines of Force. In PART VI of his 1864 paper
which is entitled 'ELECTROMAGNETIC THEORY OF LIGHT'[2], Maxwell
combined displacement current with some of the other equations of
electromagnetism and he obtained a wave equation with a speed equal to the speed
of light. He commented:
The agreement of the results seems to show that light and magnetism are
affections of the same substance, and that light is an electromagnetic
disturbance propagated through the field according to electromagnetic
laws.[3]
Maxwell's derivation of the electromagnetic wave equation has been replaced in
modern physics by a much less cumbersome method involving combining the
corrected version of Ampère's circuital law with Faraday's law of induction.
To obtain the electromagnetic wave equation in a vacuum using the modern
method, we begin with the modern 'Heaviside' form of Maxwell's equations. In a
vacuum and charge free space, these equations are:
Taking the curl of the curl equations gives:
By using the vector identity
where
is any vector function of space, it turns into the wave equations:
where
meters per second
is the speed of light in free space.
UNIT – II
Section A
1. What is Esaki or tunnel diodes
These have a region of operation showing negative resistance caused by quantum
tunneling, thus allowing amplification of signals and very simple bistable circuits.
These diodes are also the type most resistant to nuclear radiation.
2. What is Gunn diodes
These are similar to tunnel diodes in that they are made of materials such as GaAs
or InP that exhibit a region of negative differential resistance. With appropriate
biasing, dipole domains form and travel across the diode, allowing high frequency
microwave oscillators to be built.
3. What is Laser diodes
When an LED-like structure is contained in a resonant cavity formed by polishing
the parallel end faces, a laser can be formed. Laser diodes are commonly used in
optical storage devices and for high speed optical communication.
4. What is PIN diodes
A PIN diode has a central un-doped, or intrinsic, layer, forming a ptype/intrinsic/n-type structure. They are used as radio frequency switches
and attenuators. They are also used as large volume ionizing radiation
detectors and as photodetectors. PIN diodes are also used in power
electronics, as their central layer can withstand high voltages. Furthermore,
the PIN structure can be found in many power semiconductor devices, such
as IGBTs, power MOSFETs, and thyristors.
Section B
1. Write a short note about Two-cavity klystron amplifier
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 density modulation, 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.
2. Write a short note about 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 the
illuminator systems of missile targeting radars. The two-cavity oscillator klystron
normally generates more power than the reflex klystron—typically 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
3. Write a short note about Reflex klystron
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 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 are obtained at all. This tube is called a reflex klystron because it repels
the input supply or performs the opposite function of a [Klystron].
There are often several regions of reflector voltage where the reflex klystron will
oscillate; these are referred to as modes. The electronic tuning range of the reflex
klystron is usually referred to as the variation in frequency between half power
points—the points in the oscillating mode where the power output is half the
maximum output in the mode. It should be noted that the frequency of oscillation is
dependent on the reflector voltage, and varying this provides a crude method of
frequency modulating the oscillation frequency, albeit with accompanying
amplitude modulation as well.
Modern semiconductor technology has effectively replaced the reflex klystron in
most applications.
4. Write a short note about Multicavity klystron
The very large klystrons as used in the storage ring of the Australian Synchrotron
to maintain the energy of the electron beam
In all modern klystrons, the number of cavities exceeds two. A larger number of
cavities may be used to increase the gain of the klystron, or to increase the
bandwidth.
5. Write a short note about Schottky diodes
Schottky diodes are constructed from a metal to semiconductor contact.
They have a lower forward voltage drop than p-n junction diodes. Their
forward voltage drop at forward currents of about 1 mA is in the range
0.15 V to 0.45 V, which makes them useful in voltage clamping applications
and prevention of transistor saturation. They can also be used as low loss
rectifiers although their reverse leakage current is generally higher than that
of other diodes. Schottky diodes are majority carrier devices and so do not
suffer from minority carrier storage problems that slow down many other
diodes — so they have a faster “reverse recovery” than p-n junction diodes.
They also tend to have much lower junction capacitance than p-n diodes
which provides for high switching speeds and their use in high-speed
circuitry and RF devices such as switched-mode power supply, mixers and
detectors.
Shockley diode equation
The Shockley ideal diode equation or the diode law (named after transistor coinventor William Bradford Shockley, not to be confused with tetrode inventor
Walter H. Schottky) is the I–V characteristic of an ideal diode in either forward or
reverse bias (or no bias). The equation is:
where
I is the diode current,
IS is the reverse bias saturation current,
VD is the voltage across the diode,
VT is the thermal voltage,
and n is the emission coefficient, also known as the ideality factor. The emission
coefficient n varies from about 1 to 2 depending on the fabrication process and
semiconductor material and in many cases is assumed to be approximately equal to
1 (thus the notation n is omitted).
The thermal voltage VT is approximately 25.85 mV at 300 K, a temperature close
to “room temperature” commonly used in device simulation software. At any
temperature it is a known constant defined by:
where
q is the magnitude of charge on an electron (the elementary charge),
k is Boltzmann’s constant,
T is the absolute temperature of the p-n junction in kelvins
The Shockley ideal diode equation or the diode law is derived with the assumption
that the only processes giving rise to current in the diode are drift (due to electrical
field), diffusion, and thermal recombination-generation. It also assumes that the
recombination-generation (R-G) current in the depletion region is insignificant.
This means that the Shockley equation doesn’t account for the processes involved
in reverse breakdown and photon-assisted R-G. Additionally, it doesn’t describe
the “leveling off” of the I–V curve at high forward bias due to internal resistance.
Under reverse bias voltages (see Figure 5) the exponential in the diode equation is
negligible, and the current is a constant (negative) reverse current value of −IS.
The reverse breakdown region is not modeled by the Shockley diode equation.
For even rather small forward bias voltages (see Figure 5) the exponential is very
large because the thermal voltage is very small, so the subtracted ‘1’ in the diode
equation is negligible and the forward diode current is often approximated as
The use of the diode equation in circuit problems is illustrated in the article on
diode modeling.
Small-signal behaviour
For circuit design, a small-signal model of the diode behavior often proves useful.
A specific example of diode modeling is discussed in the article on small-signal
circuits.
6. Write a short note about Avalanche diodes
Diodes that conduct in the reverse direction when the reverse bias voltage exceeds
the breakdown voltage. These are electrically very similar to Zener diodes, and are
often mistakenly called Zener diodes, but break down by a different mechanism,
the avalanche effect. This occurs when the reverse electric field across the p-n
junction causes a wave of ionization, reminiscent of an avalanche, leading to a
large current. Avalanche diodes are designed to break down at a well-defined
reverse voltage without being destroyed. The difference between the avalanche
diode (which has a reverse breakdown above about 6.2 V) and the Zener is that the
channel length of the former exceeds the “mean free path” of the electrons, so there
are collisions between them on the way out. The only practical difference is that
the two types have temperature coefficients of opposite polarities.
Sectiion C
1. write a short note about Diodes
Esaki or tunnel diodes
These have a region of operation showing negative resistance caused by quantum
tunneling, thus allowing amplification of signals and very simple bistable circuits.
These diodes are also the type most resistant to nuclear radiation.
Gunn diodes
These are similar to tunnel diodes in that they are made of materials such as GaAs
or InP that exhibit a region of negative differential resistance. With appropriate
biasing, dipole domains form and travel across the diode, allowing high frequency
microwave oscillators to be built.
Light-emitting diodes (LEDs)
In a diode formed from a direct band-gap semiconductor, such as gallium arsenide,
carriers that cross the junction emit photons when they recombine with the
majority carrier on the other side. Depending on the material, wavelengths (or
colors) from the infrared to the near ultraviolet may be produced. The forward
potential of these diodes depends on the wavelength of the emitted photons: 1.2 V
corresponds to red, 2.4 V to violet. The first LEDs were red and yellow, and
higher-frequency diodes have been developed over time. All LEDs produce
incoherent, narrow-spectrum light; “white” LEDs are actually combinations of
three LEDs of a different color, or a blue LED with a yellow scintillator coating.
LEDs can also be used as low-efficiency photodiodes in signal applications. An
LED may be paired with a photodiode or phototransistor in the same package, to
form an opto-isolator.
Laser diodes
When an LED-like structure is contained in a resonant cavity formed by polishing
the parallel end faces, a laser can be formed. Laser diodes are commonly used in
optical storage devices and for high speed optical communication.
Photodiodes
All semiconductors are subject to optical charge carrier generation. This is
typically an undesired effect, so most semiconductors are packaged in light
blocking material. Photodiodes are intended to sense light(photodetector), so they
are packaged in materials that allow light to pass, and are usually PIN (the kind of
diode most sensitive to light). A photodiode can be used in solar cells, in
photometry, or in optical communications. Multiple photodiodes may be packaged
in a single device, either as a linear array or as a two-dimensional array. These
arrays should not be confused with charge-coupled devices.
PIN diodes
A PIN diode has a central un-doped, or intrinsic, layer, forming a ptype/intrinsic/n-type structure. They are used as radio frequency switches
and attenuators. They are also used as large volume ionizing radiation
detectors and as photodetectors. PIN diodes are also used in power
electronics, as their central layer can withstand high voltages. Furthermore,
the PIN structure can be found in many power semiconductor devices, such
as IGBTs, power MOSFETs, and thyristors.
Varicap or varactor diodes
These are used as voltage-controlled capacitors. These are important in PLL
(phase-locked loop) and FLL (frequency-locked loop) circuits, allowing
tuning circuits, such as those in television receivers, to lock quickly,
replacing older designs that took a long time to warm up and lock. A PLL is
faster than an FLL, but prone to integer harmonic locking (if one attempts to
lock to a broadband signal). They also enabled tunable oscillators in early
discrete tuning of radios, where a cheap and stable, but fixed-frequency,
crystal oscillator provided the reference frequency for a voltage-controlled
oscillator.
UNIT IV RADAR
Introduction – Block diagram – Classification – Radar range equation – Factors
affecting the range of a radar receivers – Line pulse modulator – PPI (Plane
Position Indicator) – Moving Target Indicator (MTI) – FM CW RadarApplications.
SECTION A
1. DEFINE PPI?
PULSE POSITION RADAR
2. CLASSIFY THE RADAR?
 CW RADAR
 PULSED RADAR
3. RADAR STANDS FOR RADIO FREQUENCY DETECTING AND
RANGING
SECTION B
1. EXPLAIN THE CW RADAR?
* The simple pulse radar described above was actually preceded in the prewar
timeframe by simpler systems that consisted of a transmitter sending a signal to a
receiver on a continuous basis. For example, the transmitter could be placed on one
side of the mouth of a harbor and the receiver placed on the other, with a ship
entering the harbor breaking the radio beam and announcing its presence. The
transmitter and receiver could also be placed on the same side of the harbor, with
the passing ship bouncing the radio beam back to the receiver. These simple
"continuous wave (CW)" systems weren't really radars since they couldn't
realistically give a range to a target, they could only detect that something was
there. They are better described as "CW alarm systems", and they are really
something of a footnote in the history of radar.
However, although a simple CW alarm can't determine range, a direct variation on
it can be used to determine velocities. Suppose a continuous radio signal at a given
frequency is focused on an aircraft approaching head-on; the velocity of the
aircraft will actually increase the frequency of the echo. If the aircraft is going
directly away, it will decrease the frequency of the echo return. This is known as
the "Doppler shift". The same effect is observed with sound waves when an aircraft
is flying overhead: as it approaches, the sound of the aircraft has a high pitch, in
other words a high audio frequency, and when it is going away it has a low pitch,
or a low audio frequency.
Derivation of the Doppler shift is a bit beyond this document but can be found in
any simple physics text. For a radar, which generally tracks targets moving much
slower than the speed of light, the shift in frequency, or "Doppler frequency" is
roughly:
doppler_frequency = 2 * target_velocity / radar_wavelength
For example, if:


The radar frequency is 100 MHz, corresponding to a wavelength of 3
meters.
The target velocity is 1,000 KPH, corresponding to 278 meters per second.
-- then the Doppler frequency is:
2 * 278 / 3 = 185 Hz
This is the shift up in frequency if the target is approaching and the shift down in
frequency if the target is moving away. Incidentally, the Doppler shift for radio
emissions transmitted from the target itself is only half this, 92.5 Hz; the value is
doubled for radar because the target is moving as the radar arrives, causing a shift,
and as the return is reflected, causing a second shift.
It would be possible to imagine a simple continuous-wave radar Doppler velocity
meter, consisting of a transmitter unit and a receiver unit. Since this device is only
intended to measure velocity, the two units can share the same antenna, with the
receiver blocking out the signal of the transmitter with a "band rejection filter" that
blocks signals at the transmit frequency but allows all others to pass. The
transmitter would send a radio beam at a target. A calibrated dial on the receiver
could then be used to adjust a variable filter until the Doppler-shifted echo was
received and lit up an indicator light. The velocity could be read off the dial. Of
course, this scheme only gives the velocity of the target on a straight-line radial to
the detector. At any angle off the radial the perceived velocity is smaller, until at
90 degrees it falls to zero.
A more practical device would feature a receiver that had sets of fixed filters in a
range of frequencies above and below the transmitter frequency. Each filter would
be connected to a simple circuit that activated a light on the front panel of the
receiver, with each light marked with a particular range of velocities. If the
Doppler shifted echo passed through a specific filter, it would turn on the
appropriate light to give the target velocity. The radars used by police to catch
speeders actually operate more or less along such a principle, though their
implementation is much more sophisticated and they will actually give speed
readings.
The essential point here is that, although timing radio pulses can be used to obtain
ranges to targets, it is also useful to obtain information from the Doppler shift to
learn about the velocities of targets. The combination of these two approaches in
"pulse Doppler" radars is an important theme in modern radars.
2.EXPLAIN THE FUNCTIONAL BLOCK DIAGRAM?
Functional block diagram
In the interrogator on the ground:
The secondary radar unit needs a synchronous impulse of the (analogous) primary
radar unit to the synchronization of the indication.


The chosen mode is encoded in the Coder. (By the different modes different
questions can be defined to the airplane.)
The transmitter modulates these impulses with the RF frequency. Because
another frequency than on the replay path is used on the interrogation path,
an expensive duplexer can be renounced.

The antenna is usually mounted on the antenna of the primary radar unit
and turns synchronously to the deflection on the monitor therefore.
In the aircrafts transponder:
A receiving antenna and a transponder are in the airplane.




The receiver amplifies and demodulates the interrogation impulses.
The decoder decodes the question according to the desired information and
induces the coder to prepare the suitable answer.
The coder encodes the answer.
The transmitter amplifies the replay impulses and modulates these with the
RF reply-frequency.
Again in the interrogator on the ground:



The receiver amplifies and demodulates the replay impulses. Jamming or
interfering signals are filtered out as well as possible at this.
From the informations „Mode” and „Code” the decoder decodes the answer.
The monitor of the primary radar represents the additional interrogator
information. Perhaps additional numbers must be shown on an extra display.
3.EXPLAIN SIMPLE PULSED RADAR?
SIMPLE PULSE RADAR SYSTEM
* The best way to explain radar is to imagine standing on one side of a canyon, and
shouting in the direction of the distant wall of the canyon. After a few moments, an
echo will come back. The length of time it takes an echo to come back is directly
related to how far away the distant canyon wall is. Double the distance, and the
length of time doubles as well. Given that the speed of sound is about 1,200 KPH
(745 MPH) at sea level, then timing the echo with a stopwatch will give the
distance to the remote canyon wall. If it takes four seconds for the echo to come
back, then since sound travels about 330 meters (1,080 feet) in a second, the
distance is about 660 meters (2,160 feet).
Radar uses exactly the same principle, but it times echoes of radio or microwave
pulses and not sound. Like a wireless telegraphy set, a simple radar has a
transmitter and a receiver, with the transmitter sending out pulses, short bursts, of
EM radiation and the receiver picking them up.
In the case of the radar, the receiver is picking up echoes from a distant target, with
the echoes timed to determine the distance to the target. Early radars simply used
an oscilloscope to perform the timing, with the detected return signal fed into the
oscilloscope as a "video" signal, and showing up as a peak or "blip" on the display.
An oscilloscope measures an electrical signal on an electronic beam that moves or
"sweeps" from one side of a display to the other at a certain rate. The rate is
determined by a "timebase" circuit in the oscilloscope. For example, the sweep rate
might push the sweep from one side of the display to the other in a millisecond
(thousandth of a second). If the display were marked into ten intervals, that would
mean the sweep would pass through each interval in 0.1 milliseconds. While this
would be shorter than the human eye could follow, the sweep is normally
generated repeatedly, allowing the eye to see it.
Since EM radiation propagates at 300,000,000 meters per second, or 300,000
meters per millisecond, then each 0.1 millisecond interval would correspond to
30,000 meters, or 30 kilometers (18.6 miles). If the sweep on the scope is
"triggered" to start when the radar transmitter sends out the radio pulse, and the
sweep displays a blip on the sixth interval on the display, then the pulse has
traveled a total of 180 kilometers (112 miles). Since this is the round-trip distance
for the pulse, that means that the target is 90 kilometers (56 miles) away. The
trigger signal provides synchronization, so it can be regarded as a type of "synch"
or "sync" signal. The sweep is called a "range sweep" and the output of the display
is called a "range trace".
The display scheme described here is known as an "A scope", and allows the user
to determine the range to a target. A simple representation of an A-scope is shown
below, along with a graph plotting the travel of the pulse with respect to time:
The amplitude of the return also gives some indication of the size of the target,
though the relation between return amplitude and target size is not straightforward,
as discussed later.
* It would also be nice to know what the direction to the target is, in terms of its
"altitude (vertical direction)" and "azimuth (left to right direction)".
This is a bit trickier to describe, but no more complicated in the end. Some early
radars, like the famous British "Chain Home" sets that helped win the Battle of
Britain, simply transmitted radio waves from high towers in a flood over their field
of view, and used a directional receiver antenna to determine the direction of the
echo. Chain Home actually used a scheme where the power of the echo was
compared at separated receiver antennas to give the direction, which astoundingly
actually worked reasonably well. Other such "floodlight" radars used directional
receiver antennas that could be steered to identify the direction of the echo.
Incidentally, a radar that uses receive and transmit antennas sited in different
locations is known as a "bistatic", or in the more general case "multistatic", radar.
Floodlight radars were quickly abandoned. They spread their radio energy over a
wide area, meaning that any echo was faint and so range was limited. The next step
was to make a radar with a steerable transmitter antenna. For example, two
directional antennas, one for the transmitter and the other for the receiver, could be
ganged together on a steerable mount and pointed like a searchlight, an
arrangement that is sometimes called "quasi-monostatic". The transmitter antenna
generated a narrow beam, and if the beam hit a target, an echo would be picked up
by the receiving antenna on the same mount. The direction of the antennas
naturally gave the direction to the target, at least to an accuracy limited by the
width in degrees of the beam, while the distance to the target was given by the
trace on the A-scope.
Of course, it was realized early on that it would be more economical and less
physically cumbersome to use one antenna for both transmit and receive instead of
separate antennas; it was possible to do so in theory because a radar transmits a
pulse and then waits for an echo, meaning it doesn't transmit and receive at the
same time. The problem in practice was that the receiver was designed to listen for
a faint echo, while the transmitter was designed to send out a powerful pulse. If the
receiver was directly linked to the transmitter when a pulse was sent out, the
transmit pulse would fry the receiver.
The solution to this problem was the "duplexer", a circuit element that protected
the receiver, effectively becoming an open connection while the transmit pulse was
being sent, and then closing again immediately afterward so that the receiver could
pick up the echo. This was done with certain types of gas-filled tubes, with the
output pulse ionizing the gas and making the tube nonconductive, and the tube
recovering quickly after the end of the pulse. More sophisticated duplexer schemes
would be developed later. The receiver was also generally fitted with a "limiter"
circuit that blocked out any signals above a certain power level. This prevented,
say, transmissions from another nearby radar from destroying the receiver.
* After this evolution of steps, the result is a simple, workable radar. It has a
single, steerable antenna that can be pointed like a searchlight. The antenna
repeatedly sends out a radio pulse and picks up any echoes reflected from a target.
An A-scope display gives the interval from the time the pulse is sent out and the
time the echo is received, allowing the operator to determine the distance to the
target.
The transmitter emits pulses on a regular interval, typically a few dozen or a few
hundred times a second, with the A scope trace triggered each time the transmitter
sends out a pulse to display the receiver output. The number of pulses sent out each
second is known as the "pulse repetition rate" or more generally as the "pulse
repetition frequency (PRF)", measured in hertz.
The width of a radar pulse is an important but tricky consideration. The longer the
pulse, the more energy sent out, improving sensitivity and increasing range.
Unfortunately, the longer the pulse, the harder it is to precisely estimate range. For
example, a pulse that last 2 microseconds is 600 meters (2,000 feet) long, and in
that case there is no real way to determine the range to an accuracy of better than
600 meters, and there is also no way to track a target that is closer than 600 meters.
In addition, a long pulse makes it hard to pick out two targets that are close
together, since they show up as a single echo.
PRF is another tricky consideration. The higher the PRF, the more energy is
pumped out, again improving sensitivity and range. The problem is that with a
simple radar it makes no sense to send out pulses at a rate faster than echoes come
back, since if the radar sends a pulse and then gets back an echo from an earlier
pulse, the operator is likely to be confused by the "ghost echo". This is usually not
too much of a problem, since a little quick calculation shows that even a PRF of
1,000 gives enough time to get an echo back from 150 kilometers (95 miles) away
before the next pulse goes out. However, as mentioned propagation of radar waves
can be freakishly affected by atmospheric conditions that create ducting or other
unusual phenomena, and sometimes radars can get back echoes from well beyond
their design range.
This can be confusing, because a pulse will be sent out and a return will be
received very quickly, indicating that the target is close. In reality, the target is
distant and the return is from the previous pulse. This is called a "second time
around" return. Given a PRF of 1,000, then a target 210 kilometers (130 miles)
away will appear to be only 60 kilometers (37 miles) away. Similar confusions
could be caused by returns that arrive from long ranges after more than one
additional pulse, resulting in "multiple time around" returns. Of course, a simple
pulse radar also has "blind ranges" or "blind zones": if our example radar is trying
to spot a target exactly 150, 300, or 450 kilometers away, the return will arrive
when the next pulse is being sent out and the radar will never spot it.
To deal with such "range ambiguities", radars were designed so they could be
switched between different PRFs. Switching from one PRF to another would not
affect a "first time around" echo, since the delay from pulse output to pulse
reception would remain the same, but the switch would make a ghost return from a
current pulse jump on the display. Suppose our radar could be switched from a
PRF of 1,000 Hz to 1,250 Hz, and is trying to track a target 210 kilometers away.
At 1,000 Hz, the maximum range is 150 kilometers and the target appears to be 60
kilometers away, but at 1,250 Hz the maximum range is 120 kilometers (75 miles)
and the target return jumps to a perceived range of 90 kilometers (56 miles). The
fact that the target range jumps when PRF is changed reveals the range ambiguity;
adding perceived range to the maximum range for each PRF setting gives the
actual range.
* Incidentally, the power of the transmitter pulse is given as "peak power", usually
in kilowatts or megawatts. This may be an impressive value, but it's only the power
that goes into the pulse itself. Suppose we have a pulse width of 2 microseconds
with a peak power of 150 kilowatts. If we have a PRF of 500, then the time from
pulse to pulse, or "pulse period", is 1/500 = 2 milliseconds, or two thousandths of a
second. This means that the average power transmitted by our radar is only:
150 kW * ( 2 microseconds / 2 milliseconds) = 0.15 kW = 150 watts
-- which is about as much as the power draw of a bright old-fashioned household
incandescent light bulb.
* One of the first US early warning radars, the "SCR-27O", provides a good
example of such a simple radar. It was a VHF radar, actually operating at what by
modern standards at a low frequency / long wavelength of 100 MHz / 3 meters. It
had a steerable rectangular "flyswatter" style dipole array of 4 x 8 dipoles, and
featured a pulse width of 10 to 25 microseconds, a PRF of 621 Hz, and a peak
power of 100 kW.
SECTION C
1. EXPLAIN THE RADAR?
RADAR
Radar is an object detection system that uses electromagnetic waves to
identify the range, altitude, direction, or speed of both moving and fixed objects
such as aircraft, ships, motor vehicles, weather formations, and terrain. The term
RADAR was coined in 1941 as an acronym for radio detection and ranging. Radar
was originally called RDF (Radio Direction Finder, now used as a totally
different device) in the United Kingdom.
A radar system has a transmitter that emits microwaves or radio waves.
These waves are in phase when emitted, and when they come into contact with an
object are scattered in all directions. The signal is thus partly reflected back and it
has a slight change of wavelength (and thus frequency) if the target is moving. The
receiver is usually, but not always, in the same location as the transmitter.
Although the signal returned is usually very weak, the signal can be amplified
through use of electronic techniques in the receiver and in the antenna
configuration. This enables radar to detect objects at ranges where other emissions,
such as sound or visible light, would be too weak to detect.
Radar is used in meteorological detection of precipitation, measuring ocean
surface waves, air traffic control, police detection of speeding traffic, and by the
military.
Principle
The radar dish, or antenna, transmits pulses of radio waves or microwaves
which bounce off any object in their path. The object returns a tiny part of the
wave's energy to a dish or antenna which is usually located at the same site as the
transmitter. The time it takes for the reflected waves to return to the dish enables a
computer to calculate how far away the object is, its radial velocity and other
characteristics.
Radar equation
The power Pr returning to the receiving antenna is given by the radar equation:
where







Pt = transmitter power
Gt = gain of the transmitting antenna
Ar = effective aperture (area) of the receiving antenna
σ = radar cross section, or scattering coefficient, of the target
F = pattern propagation factor
Rt = distance from the transmitter to the target
Rr = distance from the target to the receiver.
In the common case where the transmitter and the receiver are at the same location,
Rt = Rr and the term Rt² Rr² can be replaced by R4, where R is the range. This yields:
This shows that the received power declines as the fourth power of the range,
which means that the reflected power from distant targets is very, very small.
2. Explain About Pulsed Radar?
Basic concept
A Doppler radar is a radar that produces a velocity measurement as one of
its outputs. Doppler radars may be Coherent Pulsed, Continuous Wave, or
Frequency Modulated. A continuous wave (CW) doppler radar is a special case
that only provides a velocity output. The advantage of combining doppler
processing to pulse radars is to provide accurate velocity information. This velocity
is called Range-Rate. It describes the rate that a target moves towards or away
from the radar. A target with no range-rate reflects a frequency near the transmitter
frequency, and cannot be detected. Due to the low pulse repetition frequency
(PRF) of most coherent pulsed radars, which maximizes the coverage in range, the
amount of doppler processing is limited. The doppler processor can only process
velocities up to ±1/2 the PRF of the radar. This was not a problem for weather
radars.
Doppler Effect
This is most often demonstrated by the change in the sound wave of a
passing train. The sound of the train whistle will become "higher" in pitch as it
approaches and "lower" in pitch as it moves away. This is explained as follows: the
number of sound waves reaching the ear in a given amount of time (this is called
the frequency) determines the tone, or pitch, perceived. The tone remains the same
as long as you and the train are not moving relative to each other. As the train
moves closer to you the number of sound waves reaching your ear in a given
amount of time increases. Thus, the pitch increases. As the train moves away from
you the opposite happens.
Continuous Wave RADAR(CW RADAR)
Continuous-wave radar system is a radar system where a known stable
frequency continuous wave radio energy is transmitted and then received from any
reflecting objects. The return frequencies are shifted away from the transmitted
frequency based on the Doppler effect if they are moving.
The main advantage of the CW radars is that they are not pulsed and simple
to manufacture. They have no minimum or maximum range and maximize power
on a target because they are always broadcasting.
They also have the disadvantage of only detecting moving targets, as
stationary targets (along the line of sight) will not cause a Doppler shift and the
reflected signals will be filtered out. CW radars also have a disadvantage because
they cannot measure range. Range is normally measured by timing the delay
between a pulse being sent and received, but as CW radars are always
broadcasting, there is no delay to measure. .
3.DRAW THE BLOCKDIAGRAM OF PPI RADAR?
BASIC RADAR SYSTEMS
BASIC RADAR SYSTEMS
Radar systems like other complex electronic systems are composed of several major
subsystems and many individual circuits. Although modern radar systems are quite
complicated, you can easily understand the concept by basic diagram of pulsed radar
FUNDAMENTAL RADAR SYSTEM
Since most radars used today are some variation of the pulse-radar system, this
section discusses those used in a pulse radar. All other types of radars use some
variation of these units. Refer to the block diagram in figure 1-4.
Modulator You can see on the block diagram that the heart of the radar system is
the modulator. It generates all the necessary timing pulses (triggers) for use in the
radar and its associated systems. Its function is to ensure that all subsystems of
the radar system operate in a definite time relationship with one another and
that the intervals between pulses, as well as the pulses themselves, are of the proper
length.
Transmitter The transmitter generates powerful pulses of electromagnetic energy at
precise intervals. The required power is obtained by using a highpower microwave
oscillator (such as a magnetron) or a microwave amplifier (such as a klystron) that
is supplied by a low- power RF source.
4. EXPLAIN THE BASIC PRINCIPLE OF RADAR?
Radar Technology
The basic principle of operation of primary radar is simple to understand.
However, the theory can be quite complex. An understanding of the theory is
essential in order to be able to specify and operate primary radar systems correctly.
The implementation and operation of primary radars systems involve a wide range
of disciplines such as building works, heavy mechanical and electrical engineering,
high power microwave engineering, and advanced high speed signal and data
processing techniques. Some laws of nature have a greater importance here.
Radar measurement of range, or distance, is made possible because of the
properties of radiated electromagnetic energy.
1. Reflection of electromagnetic waves
The electromagnetic waves are reflected if they meet an electrically leading
surface. If these reflected waves are received again at the place of their
origin, then that means an obstacle is in the propagation direction.
2. Electromagnetic energy travels through air at a constant speed, at
approximately the speed of light,
o 300,000 kilometers per second or
o 186,000 statute miles per second or
o 162,000 nautical miles per second.
This constant speed allows the determination of the distance between the
reflecting objects (airplanes, ships or cars) and the radar site by measuring
the running time of the transmitted pulses.
3. This energy normally travels through space in a straight line, and will vary
only slightly because of atmospheric and weather conditions. By using of
special radar antennas this energy can be focused into a desired direction.
Thus the direction (in azimuth and elevation of the reflecting objects can be
measured.
These principles can basically be implemented in a radar system, and allow the
determination of the distance, the direction and the height of the reflecting object.
(The effects atmosphere and weather have on the transmitted energy will be
discussed later; however, for this discussion on determining range and direction,
these effects will be temporarily ignored.)
5.EXPLAIN THE DIRECTION FINDING OF RADAR?
Direction-determination
The angular determination of the target is determined by the directivity of the
antenna. Directivity, sometimes known as the directive gain, is the ability of the
antenna to concentrate the transmitted energy in a particular direction. An antenna
with high directivity is also called a directive antenna. By measuring the direction
in which the antenna is pointing when the echo is received, both the azimuth and
elevation angles from the radar to the object or target can be determined. The
accuracy of angular measurement is determined by the directivity, which is a
function of the size of the antenna.
Radar units usually work with very high frequencies. Reasons for
this are:



quasi-optically propagation of these waves.
High resolution (the smaller the wavelength, the smaller the objects the radar
is able to detect).
Higher the frequency, smaller the antenna size at the same gain.
The True Bearing (referenced to true north) of a radar target is the angle between
true north and a line pointed directly at the target. This angle is measured in the
horizontal plane and in a clockwise direction from true north. (The bearing angle to
the radar target may also be measured in a clockwise direction from the centerline
of your own ship or aircraft and is referred to as the relative bearing.)
Figure 2: Variation of echo signal strength
Figure 2: Variation of echo signal strength
The antennas of most radar systems are designed to radiate energy in a onedirectional lobe or beam that can be moved in bearing simply by moving the
antenna. As you can see in the Figure 2, the shape of the beam is such that the echo
signal strength varies in amplitude as the antenna beam moves across the target. In
actual practice, search radar antennas move continuously; the point of maximum
echo, determined by the detection circuitry or visually by the operator, is when the
beam points direct at the target. Weapons-control and guidance radar systems are
positioned to the point of maximum signal return and maintained at that position
either manually or by automatic tracking circuits.
In order to have an exact determination of the bearing angle, a survey of the north
direction is necessary. Therefore, older radar sets must expensively be surveyed
either with a compass or with help of known trigonometrically points. More
modern radar sets take on this task and with help of the GPS satellites determine
the northdirection independently.
6.EXPLAIN THE RADAR RANGE EQUATION?
Transmitted Power
Not every transmitting vacuum tube is equally good. Minimal production
tolerances can influence the obtainable transmit power and therefore also the
theoretical attainable range.
Remember: the most important feature of this equation is the fourth-root
dependence!
Other then the transmit power we assume all other factors are constant.
Calling all of them the coefficient k, so the maximum range equation becomes:
Now it is easy to see that:
In order to double the range, the transmitted power would have to be increased by
16-fold!
We can explain how such deviations change the maximum range values: if e.g. the
transmitted power of the russian Radar “Spoon Rest” is permited to fluctuate from
160 kW to 250 kW, will the maximum range values be correct for distances
between 250 to 270 km?
From the relationship shown, we can state that
250km(160 kW)· 1,118 = 279.5 km(250 kW) so the maximum range value would be
correct between 250 and 270 km!
In practice results were reached between 180 kW and 240 kW since the transmitted
power of the planar vacuum tube was frequency dependent.
The inversion of this argument is also permissible: if the transmit power is reduced
by 1/16 (e.g. failure into one of sixteen transmitter modules), then the change on
the maximum range of the radar station is negligible in the practice < 2%.
Sensitivity of the Receiver
While evaluating the minimal received power we follow a different procedure:
It's also under the 4th root, but in the denominator.
Well, a reduction of the minimal received power of the receiver gets an
increase of the maximum range.
For every receiver there is a certain receiving power as of which the receiver can
work at all. This smallest workable received power is frequently often called MDS
- Minimum Discernible Signal in radar technology. Typical radar values of the
MDS echo lie in the range of -104 dBm to -110 dBm.
Antenna Gain
The antenna gain is squared under the 4th root (Remember: the same antenna is
used during transmission and reception).
If one quadruples the antenna gain, it will double the maximum range.
Here is a concrete example from VHF- radar technology: Sometimes the P-12
(yagi antennae array: G = 69) was mounted at the antenna of the P-14 (same
frequency, parabolic dish antenna: G = 900). This combination was often
mentioned jocularly to “P–13”. In accordance with our radar equation the
maximum range should increase:
(Please note the fourth root was simplified against the square in the numerator and
in the denominator at once.)
It would be beautiful, if the maximum range could be tripled so simply. But bigger
antennas use much longer supply cables. Losses on the incoming feeding lines and
losses due to the mis-adjustment of the antenna give away half of what is invested.
Nevertheless: 1.6 times the maximum range isn't degraded either. But there are
more disturbances now: too many ambiguous targets (overreaches).
Section A
1. The outermost layer of the optical fiber cable sheath or jacket
2. Define a fiber optic system.
An optical communications system is an electronic communication system
that uses light as the carrier of information. Optical fiber communication systems
use glass or plastic fibers to contain light waves and guide them in a manner
similar to the way electromagnetic waves are guided through a waveguide.
3.Define refractive index.
The refractive index is defined as the as the ratio of the velocity of propagation of
light ray in free space to the velocity of propagation of a light ray in a given
material.
Mathematically, the refractive index is
n = c/µ
where c = speed of light in free space
µ = speed of light in a given material
4. Define critical angle.
Critical angle is defined as the minimum angle of incidence at which alight ray
may strike the interface of two media and result in an angle of refraction of 90°or
greater.
5.Define single mode and multi mode propagation.
If there is only one path for light to take down the cable, it is called single mode. If
there is more than one path ,it is called multimode.
6. Define acceptance angle.
It defines the maximum angle in which external light rays may strike their/fiber
interface and still propagate down the fiber with a response that is no greater than
10 dB below the maximum value.
7. Define numerical aperture.
Numerical aperture is mathematically defined as the sine of the maximum angle a
light ray entering the fiber can have in respect to the axis of the fiber and still
propagate down the cable by internal reflection.
Section B
1. EXPLAIN THE REFRACTIVE INDEX?
Refraction of light
As a light ray passes from one transparent medium to another, it changes direction;
this phenomenon is called refraction of light. How much that light ray changes its
direction depends on the refractive index of the mediums.
Refractive Index
Refractive index is the speed of light in a vacuum (abbreviated c,
c=299,792.458km/second) divided by the speed of light in a material (abbreviated
v). Refractive index measures how much a material refracts light. Refractive index
of a material, abbreviated as n, is defined as
n=c/v
In 1621, a Dutch physicist named Willebrord Snell derived the relationship
between the different angles of light as it passes from one transparent medium to
another. When light passes from one transparent material to another, it bends
according to Snell's law which is defined as:
n1sin(θ1) = n2sin(θ2)
where:
n1 is the refractive index of the medium the light is leaving
θ1 is the incident angle between the light beam and the normal (normal is 90° to the
interface between two materials)
n2 is the refractive index of the material the light is entering
θ2 is the refractive angle between the light ray and the normal
Note:
For the case of θ1 = 0° (i.e., a ray perpendicular to the interface) the solution is θ2 =
0° regardless of the values of n1 and n2. That means a ray entering a medium
perpendicular to the surface is never bent.
The above is also valid for light going from a dense (higher n) to a less dense
(lower n) material; the symmetry of Snell's law shows that the same ray paths are
applicable in opposite direction.
2.EXPLAIN THE NUMERICAL APECTURE?
Optical Fiber’s Numerical Aperture (NA)
Multimode optical fiber will only propagate light that enters the fiber within a
certain cone, known as the acceptance cone of the fiber. The half-angle of this cone
is called the acceptance angle, θmax. For step-index multimode fiber, the
acceptance angle is determined only by the indices of refraction:
Where
n is the refractive index of the medium light is traveling before entering the fiber
nf is the refractive index of the fiber core
nc is the refractive index of the cladding
How to Calculate Number of Modes in a Fiber?
Modes are sometimes characterized by numbers. Single mode fibers carry only the
lowest-order mode, assigned the number 0. Multimode fibers also carry higherorder modes. The number of modes that can propagate in a fiber depends on the
fiber’s numerical aperture (or acceptance angle) as well as on its core diameter and
the wavelength of the light. For a step-index multimode fiber, the number of such
modes, Nm, is approximated by
Where
D- is the core diameter
λ -is the operating wavelength
NA- is the numerical aperture (or acceptance angle)
3.DRAW THE STURCTURE OF THE FIBER OPTIC CABLE? The
Structure of an Optical Fiber
Typical optical fibers are composed of core, cladding and buffer coating.
The core is the inner part of the fiber, which guides light. The cladding surrounds
the core completely. The refractive index of the core is higher than that of the
cladding, so light in the core that strikes the boundary with the cladding at an angle
shallower than critical angle will be reflected back into the core by total internal
reflection.
For the most common optical glass fiber types, which includes 1550nm single
mode fibers and 850nm or 1300nm multimode fibers, the core diameter ranges
from 8 ~ 62.5 µm. The most common cladding diameter is 125 µm. The material
of buffer coating usually is soft or hard plastic such as acrylic, nylon and with
diameter ranges from 250 µm to 900 µm. Buffer coating provides mechanical
protection and bending flexibility for the fiber.
4. EXPLAIN THE TOTAL INTERNAL REFLECTION?
Total Internal Reflection
When a light ray crosses an interface into a medium with a higher refractive index,
it bends towards the normal. Conversely, light traveling cross an interface from a
higher refractive index medium to a lower refractive index medium will bend away
from the normal.
This has an interesting implication: at some angle, known as the critical angle θc,
light traveling from a higher refractive index medium to a lower refractive index
medium will be refracted at 90°; in other words, refracted along the interface.
If the light hits the interface at any angle larger than this critical angle, it will not
pass through to the second medium at all. Instead, all of it will be reflected back
into the first medium, a process known as total internal reflection.
The critical angle can be calculated from Snell's law, putting in an angle of 90° for
the angle of the refracted ray θ2. This gives θ1:
Since
θ2 = 90°
So
sin(θ2) = 1
Then
θc = θ1 = arcsin(n2/n1)
For example, with light trying to emerge from glass with n1=1.5 into air (n2 =1),
the critical angle θc is arcsin(1/1.5), or 41.8°.
For any angle of incidence larger than the critical angle, Snell's law will not be able
to be solved for the angle of refraction, because it will show that the refracted
angle has a sine larger than 1, which is not possible. In that case all the light is
totally reflected off the interface, obeying the law of reflection.
5.Describe The Advantages Of Fiber Optic Systems?
Advantages of Fiber Optics:
 Less expensive - Several miles of optical cable can be made cheaper than
equivalent lengths of copper wire. This saves your provider (cable TV,
Internet) and you money.
 Thinner - Optical fibers can be drawn to smaller diameters than copper
wire.
 Higher carrying capacity - Because optical fibers are thinner than copper
wires, more fibers can be bundled into a given-diameter cable than copper
wires. This allows more phone lines to go over the same cable or more
channels to come through the cable into your cable TV box.
 Less signal degradation - The loss of signal in optical fiber is less than in
copper wire.
 Light signals - Unlike electrical signals in copper wires, light signals from
one fiber do not interfere with those of other fibers in the same cable. This
means clearer phone conversations or TV reception.
 Low power - Because signals in optical fibers degrade less, lower-power
transmitters can be used instead of the high-voltage electrical transmitters
needed for copper wires. Again, this saves your provider and you money.
 Digital signals - Optical fibers are ideally suited for carrying digital
information, which is especially useful in computer networks.
 Non-flammable - Because no electricity is passed through optical fibers,
there is no fire hazard.
 Lightweight - An optical cable weighs less than a comparable copper wire
cable. Fiber-optic cables take up less space in the ground.
 Flexible - Because fiber optics are so flexible and can transmit and receive
light, they are used in many flexible digital cameras for the following
purposes:
o Medical imaging - in bronchoscopes, endoscopes, laparoscopes
o Mechanical imaging - inspecting mechanical welds in pipes and
engines (in airplanes, rockets, space shuttles, cars)
o Plumbing - to inspect sewer lines
 Because of these advantages, you see fiber optics in many industries, most
notably telecommunications and computer networks. For example, if you
telephone Europe from the United States (or vice versa) and the signal is
bounced off a communications satellite, you often hear an echo on the line.
But with transatlantic fiber-optic cables, you have a direct connection with
no echoes.
6.Explain The Fiber Optic Communication Model?
Above the figure shows the basic fiber optic communication model
It consists of
 Source- voice,audio,digital data or encoded data
 Transmitter- it consists of modulator &transmitter(here intensity modulation
is used)
 Channel-fiber optic cable
 Receiver-it consists of demodulator
 Destination-user end
7..Explain multimode fiber?
An optical fiber (or fibre) is a glass or plastic solid rode that
carries light along its length with the help of the total internal
reflection.
It consists of core and cladding. The refractive index of the
core is greater then the cladding.. They can be either single
mode or multi-mode fibers.
Multi-mode Fiber:
Fiber with large core diameter (greater than 10 micrometers)
may be analyzed by geometric optics. Such fiber is called
multi-mode fiber, from the electromagnetic analysis In a stepindex multi-mode fiber, rays of light are guided along the fiber
core by total internal reflection. Rays that meet the corecladding boundary at a high angle (measured relative to a line
normal to the boundary), greater than the critical angle for this
boundary, are completely reflected. The critical angle
(minimum angle for total internal reflection) is determined by
the difference in index of refraction between the core and
cladding materials. Rays that meet the boundary at a low angle
are refracted from the core into the cladding, and do not
convey light and hence information along the fiber. The
critical angle determines the acceptance angle of the fiber,
often reported as a numerical aperture. A high numerical
aperture allows light to propagate down the fiber in rays both
close to the axis and at various angles, allowing efficient
coupling of light into the fiber. However, this high numerical
aperture increases the amount of dispersion as rays at different
angles have different path lengths and therefore take different
times to traverse the fiber. A low numerical aperture may
therefore be desirable.
In graded-index fiber, the index of refraction in the core
decreases continuously between the axis and the cladding. This
causes light rays to bend smoothly as they approach the
cladding, rather than reflecting abruptly from the corecladding boundary. The resulting curved paths reduce multipath dispersion because high angle rays pass more through the
lower-index periphery of the core, rather than the high-index
center. The index profile is chosen to minimize the difference in
axial propagation speeds of the various rays in the fiber. This
ideal index profile is very close to a parabolic relationship
between the index and the distance from the axis.
Single mode Fiber:
The most common type of single-mode fiber has a core
diameter of 8-10 micrometers and is designed for use in the
near infrared. The mode structure depends on the wavelength of
the light used, so that this fiber actually supports a small
number of additional modes at visible wavelengths. Multimode fiber, by comparison, is manufactured with core
diameters as small as 50 micrometers and as large as hundreds
of micrometres. The normalized frequency V for this fiber
should be less than the first zero of the Bessel function J 0
(approximately 2.405). Single mode fiber has the least
dispersion and hence are used for longer distances.
8.Discuss the quantum efficiency of Led?
Quantum Efficiency of LED:
9. Explain The Modulation Of LED ?
10.. Explain The Optical Fiber Modes And Configuration?
SECTION C
1. Explain avalanche photodiode?
2.Explain the elements of optical fiber communication link?
3.Explain the single mode fiber optic cable?
4. Discuss the Losses of Optical Fiber?
Absorption Loss
Scattering Loss
Bending Losses
5.EXPLAIN LED?
LED structures
6.Explain LASER DIODES?
Laser Diodes
7. EXPLAIN PIN DIODE?