MICROWAVE 1 limitations of vacuum tube in microwave frequency Conventional low frequency tubes like triodes fail to operate at microwave frequencies MF because the electron transit time from cathode to grid becomes do large that it cannot produce microwave oscillations Two cavity Klystron Amplifier Definition Klystrons are a special type of vacuum tubes that find applications as amplifiers and oscillators at microwave frequencies Its principle of operation is velocity modulation Thus the device used for amplifying microwave signals is known as Two cavity Klystron Principle of Two cavity Klystron As we have already discussed in the introduction that Klystron is based on the principle of velocity modulation Thus two cavity klystron amplifier utilizes the kinetic energy of moving electron beam for signal amplification The variation in the velocity of electrons while moving inside the tube is known as velocity modulation This velocity modulation permits bunching of electrons while propagation So the combined energy of bunched electrons is transferred at the output thereby providing an amplified signal Applications The two cavity Klystron finds application in satellite communication UHF TV transmitters as well as radar systems wideband high power communication and troposphere scatter transmitters etc Construction The figure below represents the structure of a two cavity klystron As we can see that the above figure consists of 2 cavities namely the buncher cavity and catcher cavity The RF signal to be amplified is provided at the buncher cavity The electron gun comprises cathode heating element and anode The electron beam is produced by the cathode by making use of a heating element and the high positive potential at the anode provides the required acceleration to the electron beam initially The region between two cavities is known as drift space To allow focussed propagation of electron beam inside the tube an external electromagnetic winding is used that generates a longitudinal magnetic field This is done in order to prevent the spreading of the beam inside the tube The amplified RF signal is achieved at the catcher cavity Also a collector is present near the second cavity that collects the electron bunch Reflex Klystron consists of an electron gun a cathode filament an anode cavity and an electrode at the cathode potential It provides low power and has low efficiency Construction of Reflex Klystron The electron gun emits the electron beam which passes through the gap in the anode cavity These electrons travel towards the Repelled electrode which is at high negative potential Due to the high negative field the electrons repel back to the anode cavity In their return journey the electrons give more energy to the gap and these oscillations are sustained The constructional details of this reflex klystron is as shown in the following figure It is assumed that oscillations already exist in the tube and they are sustained by its operation The electrons while passing through the anode cavity gain some velocity Characteristic principle of Reflex Klystron This microwave generator is a Klystron that works on reflections and oscillations in a single cavity which has a variable frequency Reflex Klystron consists of an electron gun a cathode filament an anode cavity and an electrode at the cathode potential It provides low power and has low efficiency Applications of Reflex Klystron Radio receivers Portable microwave links Parametric amplifiers Local oscillators of microwave receivers As a signal source where variable frequency is desirable in microwave generators Magnetron A magnetron is a device that generates high power electromagnetic wave It is basically considered as a self excited microwave oscillator And is also known as a crossed field device Operating Principle A magnetron is basically a vacuum tube of high power having multiple cavities It is also known as cavity magnetron because of the presence of anode in the resonant cavity of the tube The operating principle of a magnetron is such that when electrons interact with electric and magnetic field in the cavity then high power oscillations get generated Applications of Magnetron A major application of magnetron is present in a pulsed radar system in order to produce a high power microwave signal Magnetrons are also used in heating appliances like microwave ovens so as to produce fixed frequency oscillations Tunable magnetrons find their applications in sweep oscillators Construction of Magnetrons The figure here shows a magnetron with 8 cavities A cylindrical magnetron has a cylindrical cathode of a certain length and radius present at the center around which a cylindrical anode is present The cavities are present at the circumference of the anode at equal spacing Also the area existing between anode and cathode of the tube is known as interaction space region It is to be noted here that there exists a phase difference of 180 between adjacent cavities Therefore cavities will transfer their excitation from one cavity to another with a phase shift of 180 Thus we can say that if one plate is positive then automatically its adjacent plate will be negative Advantages of Magnetron Magnetrons are a highly efficient device used for generation of the high power microwave signal The use of magnetrons in radar can produce radar system of better quality for tracking purpose It is usually small in size thus less bulky Disadvantages of Magnetron It is quite expensive Despite producing a wide range of frequency there exists a drawback in controllability of the generated frequency It offers average power of around 1 to 2 kilowatts Magnetrons are quite noisy A Gunn diode also known as a transferred electron device TED is a form of diode a two terminal semiconductor electronic component with negative resistance used in high frequency electronics Advantages Size is smaller Manufacturing cost is low It reliable and stable at higher frequencies Noise to signal ratio is better Operates at high bandwidth Operating current is high so the power dissipation is also Disadvantages Efficiency is low high High turn ON voltage Temperature stability is low Application of GUNN Diode Used in microwave instruments and receiver circuits Used in automatic door openers traffic signal Used in radio communications Used in military systems Used in tachometers Construction of Gunn Diode As mentioned earlier the GUNN diode is made up of only N type material Lightly doped