Microwave Link Repeater

  • The microwave repeaters act as a link between two long distance communication terminals. The different subsystems of the repeater station (Block diagram of Microwave Link Repeater) are shown in below Figure.
    Block Diagram of Microwave Repeater Station
    Here the signal will be received and retransmitted in the desired direction by the repeater.

    The Antenna: Two antennas are provided, one is meant for east direction and other is meant for west direction. The antennas are parabolic reflectors. Hoghorn antennas are referred to obtain broader bandwidth along with low noise level.

    The Circulator: In microwave repeater the signal received from one direction will be amplified and then transmitted to the other direction. Here same antenna will be used for receiving a signal from one direction as well as to transmit the signals received from the other direction. So there should be some circuitry that prevents the power being transmitted from the front end of mixer stage.

    Here this purpose is accomplished by the use of circulator. That is the circulator connects the antenna to both the receiver and transmitter sections without causing any interference in these two sections due to the other. Along with this circulator, some receiver protection circuits are also used for protecting the receiver.

    The Mixer Stage: The frequencies of the order of 3 GHz and 6 GHz are used for transmission. So the amplification at these frequencies is difficult in repeaters. The signal was down converted to around 70 MHz. Amplification can now be achieved very easily at low frequencies. In order to get this low frequency signal the incoming and local oscillator signals are mixed in the mixer stage. The output IF signal is now passed through a band pass filter whose centre frequency is tuned to 70 MHz and which has 12 MHz bandwidth. Thus channelization is achieved in microwave repeater circuits.

    IF Amplifier: It is a low noise amplifier. It provides required gain by adequate number of I.F. amplifier. IF amplifier receives AGC signals and hence maintains gain within the limits. The active device used is a low noise broad band transistor.

    Amplitude Limiter: This stage avoids the unwanted amplitude by the amplitude modulation and thus prevents the noise amplification.

    Transmitter Mixer: Transmission is done at high frequencies. This mixer stage converts the frequency of the amplified signal to the required transmitting frequency. The output is applied to a transmitting band pass filter. This stage allows required signal to next stage of power amplification.

    Power Amplifier: The power amplification depends on bandwidth of the link. For low power amplification usually reflex klystron oscillator itself is sufficient. For high power amplification either push-pull disk seal triode type or TWT power amplifiers are used. Travelling wave tube amplifiers provide high power amplification at high frequencies than the semiconductor devices. That is why normally TWT amplifiers are used.

    Microwave Source: The source that generates the microwave frequency is a VHF transistor crystal oscillator. The oscillator output frequency is multiplied using a varactor multiplier. The signal from this source reaches power splitter after multiplication.

    Power Splitter: This stage splits the power and diverts the 75% of power to the transmitter mixer and the remaining is to mixer stage. The mixer is also supplied to shift oscillator.

    Mixer and Shift Oscillator: The output of the mixer is passed through a band pass filter and is given to a receiver mixer. The function of this circuit is to provide the receiver mixer with a frequency which is higher than the incoming signal by 70MHz. Then only the IF of 70MHz is obtained. The band pass filter removes unwanted frequencies form output of balanced mixer.

    The different blocks of Microwave Link Repeater block diagram shown in the above figure are for west reception direction and transmission to east direction. Similarly the identical blocks are required for west transmission and east reception.


    Horn Antenna - Working, Advantages and Applications

  • Horn Antenna - Working, Advantages and Applications

    The dipole is a opened out transmission line which has the total length of 1/4 and which fully couples the electric field into the space. In a similar manner the wave guide also has the capability to radiate the energy into the space when it is suitably excited at one end and opened out at the other end. This type of arrangement is known as t torn Antenna. Thus we can say that the horn antenna is an opened out wave guide which couples the electric field fully into the space. It is used to opened out wave guide to transmit or receive microwave frequency signals.

    The horn has the shapes as shown in below Figure with proper impedance matching all the energy fed at the input end of the wave guide will be radiated fully and hence causes more improvement in directivity. The disadvantage is that the diffraction is reduced.

    Types of Horn Antennas

    1. Sectoral horn flared in 'H' plane.
    2. Sectoral horn flared in 'E' plane.
    3. Pyramidal horn
    4. Circular or conical horn
    5. Bi-conical horn
    Horn Antenna and its types
    H plane horn (sectoral horn) is the horn which is flared out in horizontal direction. The E plane horn is also a sectoral horn which is flared out in vertical direction only. The pyramidal horn is the horn which is flared out in both the directions. The circular horn is similar to a H plane or a E plane horn but it is now flared out from a circular wave guide. So the horn antenna can be effectively feed from a wave guide.

    Radiation from Horn Antenna (Working)

    When a wave guide is suitably excited at one end and opened at the other end a small portion of the energy will be radiated. The reason is mismatch at the end of the wave guide with space. If the mouth of the waveguide is opened out, the draw back of mismatch can be avoided. This opening of the wave guide results in an electro-magnetic horn.

    When the wave guide is terminated by a horn, the abrupt discontinuity is replaced by a gradual transformation and impedance matching is correct. Hence all the energy travels forward and will be radiated. The shape of a radiated field depends on the flare angle of the horn.

    The pyramidal horn and the conical horn give pencil like beams that have pronounced directivity in both vertical and horizontal planes. Fan shaped beams result for sectoral horns. The biconical horn produces a pomcake shaped beam in vertical direction but uniform in horizontal plane.

    Advantages of Horn Antenna

     1. Good directivity
    2. Adequate band width
    3. Simple mechanical construction
    4. Convenience to employ with wave guides

    Applications of Horn Antenna 

    1. As primary radiator for paraboloid reflectors
    2. For satellite tracking purposes
    3. At communication stations


    Block Diagram of Microwave Transmitter and Receiver


    The block diagram shows the equipment of a microwave transmitter station on earth.
    Block Diagram of Microwave Transmitter
    The signal to be transmitter must be at uplink frequency. The converter multiply the signal frequency to uplink frequency after it is encoded and modulated properly.

    After upconverting the frequency, it is applied to power dividers. The output of power divider goes to high power amplifier. Normally travelling wave tube amplifiers or multicavity klystron amplifiers are used. These tubes require good amount of cooling.

    Here the modulation is performed at 70 MHz intermediate frequency and is then upconverted to a uplink frequency of 6 GHz. The output of several high power amplifiers are combined in a power combining amplifier and the output then passes through band pass filter and circulators. Frequency stability and power control are necessary to avoid interferences. The manufacturing is high and it increases as transmitted power increases.

    Block Diagram of Microwave Receiver
    The first stage of the terminal station receiver is the front end converter which is usually a double converter to convert the down link frequency signal of the order of GHz into an intermediate frequency of 70 MHz.

    Now this intermediate frequency signal is first passed through a chain of bandpass filters and amplifier combinations to improve signal strength. Thus the IF signal is demodulated to get the original baseband signal. The FM used here is a phase-locked loop (PLL) type of FM demodulator.

    Now the signal is amplified after it is given to de-emphasis network. A 5.5 MHz sound trap is provided in the circuit to get the sound IF. Now this sound IF is given to FM detector to get the original audio signal.


    Waveguides in Microwave Engineering

  • Electromagnetic waves can be transmitted to the load using transmission lines of the open wire type or cables. As the frequency of working increases beyond 300 MHz. it becomes difficult to use transmission lines or cables. The reason is the magnitude of losses in the line and the associated dielectrics used in forming the line or the cable.

    At frequencies above 3000 MHz we use a metallic tube for the transmission of electromagnetic waves. As a matter of fact the open wire transmission line or the co-axial cable, used as transmission path can be called as a 'waveguide', because the line or the cable guides the electromagnetic wave from the generator to the load.

    However this name WAVEGUIDE in Microwave Engineering is used only to specially made hollow metal pipes that carry electromagnetic waves at microwave frequencies. Hence we can say that a WAVEGUIDE is a hollow conducting metallic structure (tubular) used for the transmission of electromagnetic waves. The electromagnetic waves are transmitted by successive reflections from the inner walls of the structure.

    Types of Propagation of Electromagnetic Waves at Microwave Frequencies

    There are three types of propagation possible for the electromagnetic waves at microwave frequencies. They are:

    The transverse electric or TE wave
    The transverse magnetic or TM wave
    The transverse electromagnetic or TEM wave

    The transverse electric waves are the simplest form of electromagnetic wave that can be propagated in a media. It has no component of electric field in the direction of propagation. The waves in which the electric field is wholly transverse are called transverse electric or TE waves.

    The transverse magnetic waves have no component of magnetic field in the direction of propagation. The waves in which the magnetic field is wholly transverse are called transverse magnetic or TM waves.

    The waves in which both the electric and magnetic field components are transverse to the direction to propagation are called transverse electromagnetic or TEM waves.

    The fields in the TM, TE and TEM waves using vector notation are shown in Figure.

    Let the direction of propagation of wave be 'z' direction as shown in Figure. The magnetic field is always transverse to the direction of propagation in TM waves. Therefore = = 0. Hence the electric field has components and y with since no electric field exists in the direction of magnetic field as represented in figure.

    In the transverse electric waves x = z = 0 and the electric field is made wholly transverse to the direction of propagation ‘z'. The magnetic field has components Hx; Hz and Hy = 0. This is shown in Figure.

    In the TEM wave the magnetic field is wholly along y axis and no field exists in the ‘z’ direction. This is shown in figure.

    The following are the equations that represent the TM, TE and TEM waves:

    γĤy = (σ + jωƐ) Êx --------------------------- (1)

    Hy/x(σ + jωƐ) Ê ---------------------------(2)

    γ Êz + ∂Êz/x = jωμĤy  -------------------------------(3)

    Equations (1) to (3) represent TM wave.

    γĤy + Êz/x = -(σ + jωƐ) Êy  ------------------------------(4)

    γ Êy = - jωμĤZ ---------------------------- (5)

    Êz/x = - jωμĤz -------------------------------(6) 

    Equations (4) to (6) represent TE wave 0

    γĤZ= (σ + jωƐ) Êx ---------------------------- (7)

    ∂Ĥy/∂x = 0 -----------------------------(8)

     γ Êy = jωμĤy ----------------------------- (9)

    Equations (7) to (9) represent TEM waves.

    In these equations 'γ' is the propagation constant is the electric field vector, H is the magnetic field vector m is the permeability s is conductivity, q is the dielectric constant and w is the radiant frequency.

    Types of Waveguides - Important Characteristics of Waveguides :

    The physical structure of a Waveguides in Microwave Engineering can be anything that is supported by electromagnetic waves. Waveguides can be of rectangular, circular, elliptical, single ridged or double ridged. However the irregular shapes are complicated for analysis. Therefore popularly rectangular and circular wave guides are used. Therefore we classify waveguides into only two principle types.

    There are two types of waveguides that are commonly used. They are :
    (a) Rectangular waveguides and
    (b) Circular waveguides

    The important characteristics of waveguides are given below :

    1. Waveguides are hollow metal tubes and used for transmission of very high frequencies with as low attenuation as is possible.

    2. At ultra high and microwave frequencies, waveguides provide a practical alternative to transmission line for transmission of electrical energy.

    3. Skin effect is not experienced in waveguides.

    4. Any configuration of electric field and magnetic field that exist inside a waveguide must be a solution of Maxwell equations. Further these fields must satisfy the boundary condition imposed by the walls of the guide.

    5. As the walls of a waveguide are perfect conductors, there can be no tangential component of electric field at walls.

    6. Many different field configurations can be found inside a waveguide which satisfy the boundary conditions and solutions of Maxwell equations. Each such configuration is termed as a 'mode'.

    7. The various possible field configurations or modes that can exist in a waveguide belong to one or the other of the two fundamental types. They are transverse electric or simply TE mode. TE mode is also referred as 'H' mode. The second mode is t transverse magnetic or simply TM mode. It is also referred as 'E' mode. The different modes of each class are represented by double subscripts such as TE10.

    8. When travelling along a waveguide have a phase velocitY and will be attenuated.

    9. Waves a wave reaches the end of a waveguide it is reflected unless the load impedance is adjusted carefully to absorb the wave.

    10. An irregularity in a waveguide produces reflections as is the case with the transmission line.

    11. A particular mode will propagate down a waveguide with low attenuation only if the wavelength of the wave is less than the critical cutoff value, determined by the dimensions and geometry of the guide.

    12. Waveguide behaves as a high pass filter allowing only those modes which have higher frequencies, than the critical cutoff frequency. So, one can design a waveguide to allow only that mode of interest to propagate.


    Gunn Diode and Tunnel Diode

  • Gunn Diode

    Gunn diodes are most commonly used microwave oscillators. It was discovered in 1963 by Gunn; he discovered the bulk transferred effect in Ga As and InS.

    Gunn Effect in Microwave Engineering: 

    If a voltage of about 50V is applied across a thin slice of Ga As, then negative resistance will be encountered under certain conditions. Basically, these consist merely in applying a voltage gradient across the slice in excess of about 3700 V/cm. Oscillations occur across the slice if connected to a suitably tuned circuit. Thus we see that the voltage gradient across the slice of Ga As is very high, giving rise to high electron velocity and so oscillations that occur are at microwave frequencies, with a cavity normally being used as a tuned circuit. This effect is called 'Gunn Effect'.

    Gunn Effect is a bulk property of semiconductors and so it does not depend, like other semiconductor effects on either junction or contact properties. It is independent of total voltage or current, and is not affected by magnetic fields or different types of contacts. This effect occurs only in N-type materials as it is associated with electrons and not with holes. The electric field in V/cm is the factor resulting in the presence or absence of oscillations. A threshold value of 3.7kV/cm is the minimum to excite oscillations. The frequency of oscillations produced is related closely to the time that electrons take to traverse such a slice of N-type material due to the voltage applied. This proposes that a group of electrons is formed by cycle (called domain) and it reaches at the positive end of the slice to stimulate oscillations in the related tuned circuit.

    It is quite possible to use small slices of Ga As mounted directly in a cavity without encapsulation.

    Now-a-days, commercial Gunn diodes, encapsulated in packages are available. A typical Gunn diode is shown in Figure. The device is grown epitaxially onto a copper or gold plated molybdenum electrode, out of Ga As doped with silicon, tellurium or selenium. The substrate is very highly doped Ga As with the top layer highly doped, whereas, the active layer is less heavily doped.
    Gunn Diode Construction
    A typical commercial diode uses a 10V supply and has a typical power dissipation of 1W and a D.C. current of 10mA. Its power output is 20mW with an efficiency of 2% and frequency of oscillation lying between 8 and 12 GHz. The Gunn diodes oscillate anywhere in the X-band with 20% mechanical tuning arrangement and about 400 MHz of electrical tuning.


    Gunn diodes can replace reflex klystron in all fields such as receiver local oscillators, parametric amplifier pumps, signal generators, frequency modulated power oscillators. It is also used in short range communication links, RADAR for small boats, etc.

    Tunnel Diode

    Tunnel diodes are fabricated by doping the semiconductor materials at a very high level, one in one thousand or one in one hundred. Germanium and gallium arsenide (Ga As) are used to fabricate tunnel diodes. Construction details are shown in Figure. It is also called Esaki diode. Ge and Ga As are usually used to fabricate tunnel diodes because of high electron mobility and reasonable gap energy. It is possible for the carriers to tunnel through the potential barrier if it narrow enough (typically 10-6 cm) and if available energy level exists on the other side.

    Tunnel Diode Construction
    The V-I characteristics of a tunnel diode are shown in Figure.
    Tunnel Diode VI Characteristics
    The right hand rising portion is the normal forward biased diode region of the device. The current variation in the vincinity of the origin is due to quantum mechanical tunneling of electrons through narrow space charge region of the junction. As the applied voltage is increased from zero, tunneling current first increases and then decreases to zero. This decreases in current with increasing voltage results in a negative resistance region. When the forward voltage is further increased, the tunneling effect cease, and current increases as in the case of an ordinary PN diode.
    In reverse bias, tunnel diodes behave like a conductor.

    Applications: Tunnel diodes are useful in high frequency circuits, microwave oscillators, parametric amplifiers, etc. 


    BP Measurement Methods

  • BP Measurement Methods

    Blood pressure is considered a good indicator of the status of the cardio-vascular system. Blood pressure measurement can save many persons from an untimely death by providing warnings of very high blood pressure (hypertension) in time to provide treatment. There are two different methods available today for the measurement of blood pressure namely direct method and indirect method.

    In routine clinical tests, blood pressure is usually measured by means of an indirect method using a sphygmomanometer. On the other hand, direct blood pressure measurement do provide a continuous readout or recording of the blood pressure.

    I) Indirect Method of Blood Pressure Measurement or Non Invasive Measurement

    Usually indirect method uses sphygmomanometer setup for measurements. This method is easy to use and can be automated. But it has a disadvantage that it doesn’t provide a continuous readout or recording of the pressure variations and only systolic and diastolic arterial pressure readings can be obtained with no indication of the details of the pressure waveform.  Indirect method fails when the blood pressure is very low (when the patient is in shock).

    a) Auscultatory Method

    Here stethoscope is used along with sphygmomanometer for measurement. Sphygmomanometer consists of an inflatable pressure cuffs. The cuff consists of a rubber bladder inside an elastic fabric covering that can be wrapped around the upper arm and fastened with either hooks or a Velcro fastener. The cuff is normally inflated (putting pressure) manually with a rubber bulb (green ball) and deflated (removing pressure) through a needle valve.


    To obtain a blood pressure measurement with a sphygmomanometer and a stethoscope, the pressure cuff (rubber strap) on the upper arm is first inflated to a pressure well above systolic pressure. At this point no sounds can be hearers through the stethoscope,  which is placed over the brachial artery, because it have been collapsed by the pressure of the cuff that crushes around the arm.  The pressure on the cuff is then reduced gradually. As soon as the cuff pressure falls below systolic pressure, small amount of blood start to flow passing the cuff through the artery and korotkoff sounds starts to hear through the stethoscope. The pressure of the cuff that is indicated on the manometer when the first korotkoff sound is heard is recorded as the systolic blood pressure. As the pressure in the cuff continues to drop, the korotkoff sounds continue until the cuff pressure is no longer sufficient to suppress he vessel (artery) during any part of the cycle. Below this pressure, the korotkoff sounds disappear making the value of diastole pressure. This familiar method of locating systolic and Diastolic pressure by listening to the korotkoff sound is called ausculatory method.

    Ausculatory Method

    Palpatory Method

    Palpatory Method is similar to ausculatory method, except that the physician identifies the flow of blood in the artery by feeling the pulse of the patient from the cuff instead of listening through stethoscope.

    b) Oscillometric Method

    It is annon invasive or indirect blood pressure technique, measures the amplitude of oscillations that appear in the cuff pressure signal which are created by expansion of the arterial wall each time blood is forced through the artery. The compression cuffs entrained air volume are used to identify blood pressure values. The cuff pressure signal increases within the systolic pressure region, reaching a maximum when the cuff pressure is equal to mean arterial pressure. As the cuff pressure drop below this point, the signal strength decreases proportionally to the cuffs air pressure losing rate.

    There is no clear transition in the cuff pressure oscillations to identify diastolic pressure while blood is forced through the artery. But we uses specific proprietary algorithms to estimate and record diastolic pressure also. When the cuff pressure is raised quickly to pressure higher than the systolic pressures, it is observed that the radial pulse disappears, because that much cuff pressure cause the underlying artery to be completely occluded (closed). Pressure oscillations occur in the cuff pressure due to the artery which is pulsating just under the upper edge of cuff with slow cuff pressure reductions, and when cuff pressure is just below systolic pressure, blood quickly flow through artery and the cuff pressure oscillations becomes larger.
    Oscillometric Method

    c) Ultrasonic non invasive measurement

    This is an indirect method for blood pressure measurement. It employs a sensor which detects the motion of the blood vessel walls in various states of occlusion (blocking or partial blocking).

    Construction and working

    There will be 2 small  transmitting and receiving crystals(8 MHz) on the arm. The transmitted signal is focused on the vessel wall and the blood. the reflected signal which is shifted in frequency is detected by the receiving crystal and then decoded. The difference in frequency ranges from 40 to 500 Hz, which is proportional to the velocity of wall motion and the blood velocity. As the cuff pressure is increased above diastolic pressure but below systolic pressure, the blood vessel opens and closes with each heart beat. This opening and closing of vessel is detected by the ultrasonic system. When the applied pressure is further increased, the time between the opening and closing decreases until they coincide. The reading at this point is the systolic pressure. When the pressure in the cuff is reduced, the time between opening and closing increases until the closing signal from one pulse coincide with the opening signal from the next pulse. The reading at this point is diastolic pressure.

    II) Direct Blood Pressure Measurement

    Direct method provides a continuous readout or recording of the blood pressure waveform, and is considerably more accurate than the indirect method. But in Direct measurements, it is required to puncture (make hole) the blood vessel in order to introduce the sensor. This limits their use to those cases in which the condition of the patient.
    Direct measurement of blood pressure usually obtained by 3 methods:
    a) Percutaneous insertion
    b) Catheterization  (vessel cut down)
    c) Implantation of a transducer in a vessel or in the heart.

    There is another method such as clamping transducer on the intact artery has also been used, but they are not common.

    a) Percutaneous insertion

    For this method, a local anesthetic is injected near the site of insertion, then the vessel is occluded and a narrow hollow needle is inserted at a slight angle towards the vessel. When he needle is in place, a catheter is fed through the hollow needle with some sort of a guide. When the catheter is secured in place inside the vessel, the needle and guide are withdrawn. Catheter is a long tube that introduced into the heart or major vessels.

    b) Catheterization  

    Apart from obtaining blood pressures in the heart chambers and vessels, this technique is also used to obtain blood samples from the heart for oxygen-content analysis and to detect the location of the abnormal blood flow pathways. Measurement of blood pressure with a catheter can be achieved in two ways:

    Introduce a sterile saline solution into the catheter so that the fluid pressure is transmitted to a transducer outside the body.

    Pressure measurement is obtained at the source. Here the transducer is introduced into the catheter and pushed to the point at which the pressure is to be measured or he transducer is mounted at the tip of the catheter.

    c) Implantation Technique

    This technique involves major surgery and thus is normally employed only in research experiments. They have the advantage of keeping the transducer fixed in place in the appropriate vessel for long period of time.


    Travelling Wave Tube in Microwave Engineering

  •  TWT

    The travelling wave tube (TWT) is an amplifier which makes use of a distributed interface between an electron beam and a travelling wave. To extend the interface between an electron beam and an RF field, it is compulsory to make sure that they are equally moving in the similar way with almost the equal velocity. It differs from the multi-cavity klystron in which the electron beam travels, but the RF field stays motionless. The electron beam moves with a velocity preside over by the anode voltage which for a typical value of anode voltage can be as high as 0.1 vc where vc is the velocity of light in vaccum.

    The RF field transmits with a velocity identical to the velocity of light vc. The slow-wave structures to retard RF field, either use a helix or a wave guide arrangement.

    Helix Travelling Wave Tube

    The construction of a usual TWT using a helix is shown in Figure. It has an electron gun to create a slight electron beam, which in turn is gone through the centre of a extensive axial helix. A magnetic focusing is given to stop the beam from scattering and to direct it through the centre of the helix.
    The amplified signal comes out at the output or further end of the helix under suitable operating setting.
    When the RF signal applied spread around the turns of the helix in which it generates an electric field at the middle of the helix. The RF field moves with a velocity of light, the axial electric field go forward with a velocity of light, multiplied by the ratio of helix pitch to helix boundary, So when the velocity of the electron beam moving through the helix equals the rate of progress of the axial field, then communication takes place between them. On an average, the electrons transport energy to the wave on the helix. Thus the signal wave develop and improved output is attained. The TWT can be thought of a limiting case of a multi-cavity klystron. Here all the electrons centred about the axis are velocity modulated.

    To suppress the oscillations from being spontaneously generated in a TWT, it is necessary to prevent internal feedback arising from reflections due to impedance mismatch. This problem is solved by placing an attenuator in some convenient place in the tube. When the attenuator is placed at the input end of the tube, the bunching of tube remains unaffected because the attenuator attenuates both forward and reverse waves.

    There are two types of TWTs. Lowpower, Low noise TWT and High power, Fairly noise TWT. These may be pulsed also. The noise level of low power is from 4 to 8 dB at about 0.5 to 16GHz. The power ranges from 5 to 25 W at frequencies upto 40 GHz. High power TWTs have narrower bandwidth. They operate between 0.5 to 95 GHz. The power output can be 25 kW, CW near 3 GHz or 10 MW pulsed at 3 GHz.

    Applications  of TWT: 

    The low noise TWTs are used as RF amplifiers in broadband microwave receivers and as repeater amplifiers in wide band communication links. They are also used in communication satellites with working lives in excess of 50,000 hours. Pulsed high power TWTs are used in airborne, shipborne RADAR and in high power ground based RADAR.


    Klystron in Microwave Engineering

  • Klystron in Microwave Engineering

    A Klystron is a velocity modulated tube, in which the velocity modulation process produces a density modulated stream of electrons. Figure shows the '2-cavity klystron amplifier'. It is distinguished that a high velocity electron beam is created. It is a high positive potential with respect to cathode. Magnetic focusing is used here.
    Klystron Amplifier
    A 2-cavity klystron amplifier comprises of a cathode focusing, electrodes, two buncher grids divided by a extremely small space appearing a gap A, two catcher network with a small gap B trail by a collector. The input and output are taken from the tube via resonant cavities. The separation between buncher grids and catcher grids is called, 'drift space'. The electron beam passes gap A in the buncher cavity to which RF signal to be amplified is applied and is then allowed to drift freely without any interference from RF fields until it reaches the gap B in the output or catcher cavity.

    The first grid controls the number of electrons in the electron beam and serves to focus the beam. The velocity of electron in the beam is determined by the beam accelerating potential. On leaving the region of first grid the electrons pass through the grids of buncher cavity. The grids of the cavity allow the electrons to pass through, but confine the magnetic fields within the cavity. The space between the grids is called 'inter-action space'. When electrons travel through this space, they are subjected to RF potentials at a frequency determined by the cavity reasonably frequency or the input frequency. The amplitude of this RF potential between the grids is determined by the amplitude of the incoming signal in case of the amplifier, or by the amplitude of the feedback signal from the second cavity if used as in oscillator. Then oscillations will be excited in the second cavity which is of a power much higher than in the buncher cavity, so that a large output can be taken.


    A multicavity klystron can be used as a medium or high power amplifier in the UHF and microwave ranges, for both continuous and pulsed operations. The power ranges vary from 10kW to 25MW. The power gain ranges from 30dB to 60dB.


    ISDN - Principles, Objectives, Services, Architecture, Channels

  • ISDN - Principles:

    Integrated Services Digital Network (ISDN) is a high -speed, fully digital telephone service. It is an ITU-T (CCITT) standard for an end-to-end global digital communication system providing fully integrated digital services. The principles of ISDN from the point of ITU-T International Telecommunication Union - Telecommunications Standardisation Sector). CCITT (Consulative committee for International Telegraphy and Telephony are :

    1. Support of voice and non-voice execution by means of a partial set of standardised services.
    2. Support for switched and non-switched (committed lines) applications.
    3. Reliance for 64 - kbps connections.
    4. Aptitude in the network.
    5. Layered protocol architecture.
    6. Diversity of configurations are likely to execute ISDN.

    ISDN - Objectives :

    Activities currently under way are leading to the development of a worldwide ISDN. The key objectives of ISDN are :


    2. Transparency : It should provide a transparent transmission service, thereby permitting users to develop applications and protocols with the confidence that they will not be affected by the underlying ISDN.

    3. Separation of Competitive Functions

    4. Leased and Switched Services : The ISDN should provide dedicated point-to-point services and switched services, thereby allowing the user to optimise implementation of switching and routing techniques.

    5. Cost-related Tariffs

    6. Smooth Migration: ISDN interfaces should evolve from current interfaces, and provide a migration path for users.

    7. Multiplexed Support : ISDN should provide low-capacity support and multiplexed support.

    ISDN - Services :

    The purpose of the ISDN is to provide fully integrated services to users. These services are divided into three categories :

    1. Bearer Services : They can be provided using circuit -switched, packet - switched, frame - relay or cell - relay networks.

    2. Teleservices : In teleservicing, the network may change or process the contents of the data. These services be in contact to layers 4 to 7 of the OSI model. Teleservices rely on the facilities of the bearer services and are designed to accommodate complex user needs without the user having to be aware of the process details. Teleservices include telephony, teletex, telefax, videotex, telex and teleconferencing.

    3. Supplementary Services : Supplementary services are those that provide additional functionality to the bearer services and teleservices. These services include reverse teleservices. These services include reverse charging, call waiting and message handling.

    ISDN - Architecture :

    Figure shows the block diagram of ISDN. ISDN supports a new physical connector for users, a digital subscriber loop (link from end user to central or end office), and modifications to all central office equipment. The area which most attention has been paid by standards organisations is that of user access. A common physical interface has been defined to provide, in essence, a DTE - DCE connection. The same interface should be usable for the telephone, computer terminal and videotex terminal.

    Protocols are needed for the exchange of control information between user device and the network. Provisions must be made for high-speed interfaces to a digital PBX or a LAN.
    The subscriber loop consists of twisted pair links between the subscriber and the central office carrying 4kHz analog signals.

    The digital central office connects many ISDN subscriber loop signals to the IDN (Integrated Digital Network). It provides access to both the circuit - switched, packet switched networks, dedicated lines and time - shared, transaction oriented computer services.
    Block Diagram of ISDN
    ISDN — Channels

    To allow flexibility, digital pipes between customers and the ISDN office (Subscriber loops) are organised into multiple channels of different sizes. The ISDN standard defines three channel types, each with different transmission rates. They are:

    1. Bearer (B) channel - 64 kbps
    2. Data (D) channel - 16 or 64 kbps
    3. Hybrid (H) channel - 384 (H 0), 1536 (H 11) and 1920 (H 12) kbps.

    B Channels : The B channels is the basic user channel. It can carry any type of digital data in full - duplex mode as long as the required data rate doesn't exceed 64 kbps. It carries transmissions end-to-end. It is not designed to demultiplex a stream midway in order to separate and divert transmissions to more than one recipient.

    D Channels : The D channel serves two purposes - to carry signalling information to control circuit - switched calls on associated B channels at the user interface and for packet - switching or low - speed telemetry at times when no signalling information is waiting.

    H Channels : H channels are provided for user information at higher bit rates. The user may employ such a channel as a high - speed trunk, or the channel may be subdivided according to the user's own TDM scheme. Some of the applications of H channels include fast facsimile, video, high - speed data, high - quality audio, teleconferencing and multiple - information streams at lower data rates.


    BPSK System with Block Diagram

  • Binary Phase Shift Keying (BPSK) is a form of phase modulation using two different carrier phases to signal 1 and 0. BPSK is the simplest type of PSK. In binary phase shift keying (BPSK), the phase of a stable amplitude carrier signal is toggled amid two values with respect to the two likely signals m1 and m2 equivalent to binary 1 and 0, respectively. Normally, the phases are separated by 1800.

    The two signals are therefore given by: 
    S1 (t) = √( 2Eb/Tb) cos (2πfct)
    S2 (t) = √( 2Eb/Tb) cos (2πfct +π ) = -√( 2Eb/Tb) cos (2πfct),

    where fc is the carrier frequency. The relationship among bit information and waveform signal output and initial phase from modulator is illustrated as Figure.
    This system may be represented using a single basis function
    φ1 (t) = √( 2/Tb) cos (2πfct),    0 ≤ t ≤ Tb
    with the two signal vectors amplitude given by

    S1 = Eb
    S2 = -Eb

    Where Eb is the signal energy which is also in his case the energy per bit.

    A coherent binary PSK system is therefore characterized by having a signal space that is one dimensional with a signal constellation consisting of two message points of equal and opposite amplitude. Such signals with equal energy and a cross correlation coefficient of -1 are called antipodal.

    This signal space diagram is shown in Figure. If the symbols are equiprobable then the rule for deciding which symbol was transmitted is to choose the closest message point. The BPSK transmission system is illustrated as Figure.
    BPSK System Block Diagram
    From the Figure, we can describe the BPSK transmission system as 3 parts:
    • Transmitter
    • Receiver
    • AWGN Channel.

    Transmitter Part :

    By using the block diagram in Figure above, if we generate, the binary number randomly we will get the information' sequences. Information generated: {b(i)} = 1, 0, 0,…..1.

    This bit information sequences will modulate the carrier frequency, and the phase of the carrier frequency will shifted as function of binary information. 

    The bit information "1" will not shift the carrier phase, and the bit information "0" will shift the carrier phase by 1800 or radians.

    Receiver Part :

    Basically the receiver part as similar with the transmitter part, but the function is contradictive. After demodulate process by using local signal oscillator and filtering by using LPF, the base band signal will be decided.

    If the amplitude of the base band signal is < 0 it's decides that a binary "0" was send and if the amplitude of the baseband signal is 0, the receiver decides that binary "1" was send by the transmitter.

    This description is made by assumption that all of the, process of modulator, Band Pass Filter, demodulator and filtering is work perfectly and inter symbol inter fervencies (ISI) is not happen.

    The AWGN Channel :

    The signal output from transmitter part is propagated through the channel, which is corrupted by the additive white Gaussian noise, and illustrated in the Figure.

    Thus the received signal at the receiver part will have a form :

    r(t) = s(t) + n(t); 0 t

    where n(t) denotes a sample function of the additive white Gaussian noise (AWGN) process with variance value σ2 = No/2 Watt/Hertz. Here we made an assumption that the mean value of the AWGN is 0.


    Recalling that our BPSK signal is s(t) = Ac m(t) cos(ωct) , we can use the convolution theorem to calculate its spectrum. 

    The spectrum of cos(wct) is [δ(f - fc) + 6(f + fc)]/2, so
    S(f)= (Ac/2)M(f - fc) + (Ac/2) M(f + fc)
    while M ( f ) is called the fourier transform of m(t).

    To calculate the specific sequence of logic states is to be known. If the bits are assumed random, then the power spectrum of m(t) is identical to the power spectrum of a single bit. Hence we have

    |S(f)|2 = Tb sin c2 [Tb(f – fc)] 

    This is illustrated in Figure and compared to the MSK spectrum.
    BPSK Spectrum
    The wider main lobe of BPSK as compared to MSK, and the much larger side lobes show that BPSK is not bandwidth efficient as MSK.

    The bit rate of BPSK is Rb = Tb. The bandwidth is well approximated by B 1/ Ts = 1/ Tb

    The spectral efficiency is  πBPSK = Rp/B = (1/Tb)/(1/Tb) = 1 bps/Hz


    Digital Carrier Modulation Techniques

  • To transmit digital data over copper wires, they can be transmitted directly with two voltage levels +A and -A because digital data is a string 1's and 0's. But if they are to be transmitted through space using antenna, it is necessary to modulate the incoming data on to a sinusoidal carrier wave. The data may be the output of PCM system or digital computer output. Amplitude modulation, frequency modulation and phase modulation are commonly used Digital Carrier Modulation Techniques for an analog signal. But, in digital communication, the modulation process involves switching or keying the amplitude, phase or frequency of the carrier according to the incoming digital data. As the modulating signal consists of only two levels, the digital carrier modulation techniques are known as Amplitude Shift Keying (ASK), Phase Shift Keying (PSK) and Frequency Shift Keying (FSK).
    Various Shift Keying Systems
    Modulation Techniques:

    In amplitude shift keying, the amplitude of the carrier is switched between two levels. ON and OFF. The resultant waveform consists of ON pulse representing binary '1' and OFF pulse representing binary '0'. The ASK waveform is shown in Figure.

    In frequency shift keying, the frequency of the carrier is switched between to frequencies. The resultant wave form consists of high frequency carrier representing binary '1' and low frequency carrier representing binary '0'. The FSK waveform is shown in Figure (a).

    If the carrier phase is switched between two levels, we get phase shift keying. The PSK waveform is shown in Figure (c). In PSK and FSK methods and amplitude of the carrier remains constant.

    All the above systems use a pair of signals to represent binary symbols 1 and 0. So, these systems are known as Binary Shift Keying systems. (BPSK, BFSK, BASK). Generally, the modulator produces one signal of an available set of M = 2N district signals in response to N bits of source data at a time. This type of digital modulation is known as M - ray shift keying system. (M - ray PSK, M - ray FSK, M - ray ASK). Binary phase shift keying is a special case of M - ray shift keying with M = 2 and N = 1. In ASK, PSK and FSK, a single parameter of the carrier i.e., amplitude, frequency and phase of the carrier undergoes modulation. Sometimes, the amplitude and phase of the carrier are varied to produce single modulation techniques, amplitude phase shift keying (APSK). This type of techniques is known as hybrid modulation technique.


    Types of Pulse Modulation

  • Pulse Modulation is divided into two as Analog and Digital.

    Analog Modulation is divided into Pulse Amplitude Modulation, Pulse Time Modulation, Pulse Width Modulation or Pulse Duration Modulation and Pulse Position Modulation

    Digital Modulation is divided into Pulse Code Modulation and Delta Modulation. All the Pulse Modulations are explained below with the help of diagram.

    The types of Pulse Modulation are shown in the Figure below


    In pulse amplitude modulation (PAM), the information, signal is sampled at the sampling rate and the carrier pulse In pulse is discretely amplitude - modulated at the sampling frequency. For faithful transmission, the signal should It-119 The information is transferred to e sampled sufficiently. the pulse at the sampling frequency.

    PAM is not often used because the amplitude of pulses not constant and goes against the basic advantage of NI is system. Therefore, in a practical PAM system, the pulses are made to perform frequency modulation of the carrier rather than amplitude modulation.

    PAM Generation :

    A PAM modulator circuit is shown in Figure. This circuit is a simple emitter follower. In the absence of clock signal, the output follows the input. The modulating signal is applied as the input signal. Another input to the base of the transistor is the clock signal. The frequency of the clock signal is made equal to the desired carrier pulse train frequency. The amplitude of the clock signal is so chosen that the high level is at ground (OV), and the low level is at some negative voltage which is sufficient to bring the transistor in the cut-off region. Thus when the clock signal is high, the circuit behaves as an emitter follower, and the output follows the input modulating signal. When the clock signal is low, the transistor is cut-off and the output is zero.

    Thus the output waveform, shown in Figure is the desired PAM signal.

    PAM Detection:

    A PAM demodulator circuit is just an envelope detector followed by a low pass filter. The diode and R - C combination network act as the envelope detector. This is followed by a second order OP-AMP low pass filter (LPF) to have a good filtering characteristic. Thus, for the received PAM Signal as the input signal, the desired demodulated signal is show in figure is the output.
    PAM Modulator

    In this system, we have fixed amplitude and starting time of each pulse, but the width of each pulse is made proportional to the amplitude of the signal at that instant.
    PWM Generation:

    The clock signal of the desired frequency is applied flow which negative edge triggering pulses are derived with the help of a diode and a R1 – C1 combination which works as differentiator.

    These negative trigger pulses applied to the pin 2 of IC 555, which is working as the monostable multivibrator makes an excellent voltage-to-time converter. These pulses decide the starting time of the PWM pulses. The end of pulses depends upon R2 - C2 combination on the modulating signal applied at pin 5.

    The width of the pulses depends upon the value of the modulating signal and thus the pin 3 output is the desired PWM.

    PDM uses in communication:

    1. It is often used in the high powered audio amplifiers used to modulate AM transmitters.
    2. It has also been used for telemetry systems.
    3. Though still an analog mode, it is more robust than PAM because it is insensitive to amplitude changes due to noise and distortion.
    4. PWM still works if synchronization between transmitter and receiver fails.

    Disadvantages of PDM :

    1. PDM pulses are of varying width and therefore of varying power content.

    PWM Modulator

    Generation and demodulation of PPM

    The Pulse position modulation obtained from PWM as shown in Figure in PWM, it is seen that each pulse has a leading edge and a trailing edge. Their position depends on pulse width, which is determined by the signal amplitude at the instant. In order to it obtain the PPM signal what is required is first a PWM signal and removal of the leading edges of PWM pulses.
    In demodulation of PPM, it is first converted into PWM, this is done with a flip-flop or bistable multivibrator. One input of the multivibrator get trigger pulses from a local generator which is synchronized by trigger pulses gets from the transmitter, in addition to these triggers are utilized to switch off one of the stages of the flip-flop. The PPM pulse are fed to the other base of the flip-flop and switch that stage ON (actually by switching the other one OFF). The resulting PWM pulse train is then demodulated.

    The PWM signal is applied to pin no.2 through the diode R1 - C1 combination. Thus the input to pin no.2 is the negative trigger pulse which corresponds o the trailing edges of PWM wave form. The IC 555 timer is working in a monostable mode and the width of the pulse is constant (governed by R2 - C2 combination).

    The negative trigger pulses decide the starting time of the output pulses and thus the output at Pin no.3 is the desired pulse position modulated (PPM) signal.


    PCM is the mainly used digital modulation system. In PCM the accessible range of signal voltages, separated into levels, and each is given a binary number. Every sample is then symbolized by the binary number representing the level adjoining to its amplitude, and this number is conveyed in serial form. In linear PCM, levels are separated by equal voltage gradations.
    The number of levels available depends on the number bits used to express the sample value. The number of levels is given by

    N = 2m

    Here N  is the number of levels, and
    m is the number of bits per sample

    Problem:  Estimate the number of levels if the number of bits per sample is as follows.

    (a) 8 (as in telephony)
    (b) 16 (as in compact disc audio systems)

    Solution :

    a) The number of levels with 8 bits per sample
    N = 2m = 28 = 256

    b) The number of levels with 16 bits per sample N = 2m = 216 = 65,536.

    This process is called 'quantizing'.

    For example, if the signal amplitude is 3.2 volts at one instant it will be sent as a digit 3 only but as 3.2 volts pulse. As there are 8 levels (23) as taken in this example the bits required to transmit the levels are 3. The digit 3 will be sent as 011. That is, OPP. Where O represents no pulse and represent pulse. However, if we take yet another level 5 volts the binary digit is 101, POP this will be sent as 101 or POP. This makes the demodulation of the signal easier.

    Generation of PCM Signals :

    In short in the pulse code modulation process: the signal is continuously sampled. It is quantized, coded and then transmitted. Quantizing is done to the nearest standard amplitude. The code is binary number when sufficient quantizing levels are used the signal will resemble the same as is from an analog system.

    A group of pulses representing a sample is termed a word. The word consists of a supervisory or signalling bit and the binary bits. So a sample is expressed by n + 1 bits where 2n is the chosen number of standard levels.

    Demodulation of PCM:

    The PCM signal is given to a quantizer. This stage eliminates the noise and requantizes the signal. The output of quantizer is given to a decoder. The decoder is a Digital to Analog converter. It performs the reverse operation of the encoder. The PAM signal is reconstituted. A filter is used to reject the unwanted frequency components. The output of the filter will be the original signal.

    Advantages of PCM:

    1. PCM is immune to noise and interference.
    2. It is easy to generate PCM signals.

    Disadvantages of PCM :

    1. PCM requires very complex circuitry.
    2. PCM requires large band width compared to analog signal.

    Applications of PCM:

    1. It is used for telephony.
    2. It is used in data transmission.
    3. It is used in space communications.