Transcript UNIT 3 PPT
LINEAR BEAM TUBES BY: P. Vijaya & M. Niraja • A wide range of semiconductor devices have been developed for detection, mixing, amplification, attenuation etc. • Microwave tubes are preferred over vacuum tubes. At microwave frequency range, the conventional tubes become less effective when used as an amplifier and oscillator. • Microwave tubes usually operate on the theory of velocity modulation, a concept that avoids the problems encountered in conventional tubes. The liberation of electrons from the surface of a substance is called electron emission. In metals, the electrons in the outermost orbit are very loosely held by the nucleus. Those loosely attached electrons called free electrons, can be easily detached by some external energy. The liberation of electron is possible only when external energy supplied to a metal is equal to or more than the work function. This external energy may be supplied from variety of sources such as heat energy, kinetic energy supplied by the moving electron or energy stored in the electrical field. Accordingly there are four main methods of obtaining electron emission from the surface of metal Thermionic emission Secondary emission Photo-electric emission High Field emission • A thermionic emission include heating of the metal. • When a metal is heated, some of heat energy is converted into kinetic energy which accelerates the motion of free electrons. • When the temperature is raised sufficiently, these electrons acquire sufficient energy equal to work function of the metal. • Consequently they overcome the opposing forces and leave the metal surface. An electronic device in which electrons flow through vacuum is called vacuum tubes. A vacuum tube consists of a evacuated glass envelope which contains a cathode, an anode and one or more electrodes called grids. Diode, triode, tetrode, pentode • Vacuum tubes are voltage controlled device. • These can operate at very high voltages. • High power can be easily developed by vacuum tubes. According to the number of electrodes, vacuum tubes can be classified as (i) Vacuum diode (ii) Vacuum triode (iii) Vacuum Tetrode (iv) Vacuum Pentode Conventional tubes such as triodes, tetrodes and pentodes are useful only at low microwave frequencies. These tubes cannot operate at high frequencies due to their limitations at those frequencies. The conventional tubes become less effective at microwave frequency range when these are used as an amplifier and oscillator. The limitations of conventional tubes at high frequencies is due to : (a) Inter-electrode capacitance effect (b) Lead Inductance effect (c) Transit Time effect The capacitance exists when two pieces of metal are separated by a dielectric. Vacuum has a dielectric constant of 1. The elements of the triode are made up of metal and are separated by dielectric material . So there must exist capacitance between them. This capacitance is called interelectrode capacitance. • The common lead inductance is the inductance associated with the common connection of vacuum triode. • This effect is more when the frequency of the signal is high. • As the frequency increases, the inductive reactance increases and due to high inductive reactance there is an input matching problem. The lead inductance affects the performance of vacuum triode with most of input voltage lost across inductance and only small fraction of input reach to terminal for amplification. These inductances form unwanted tuned circuit with the capacitance and parasitic oscillations are produced. As frequency increases, the reactance increases. • The time taken by an electron to travel from cathode to anode is called transit time. • At low frequencies, the transit time is very small i.e. the electrons reach instantaneously the anode plate from cathode. • At high frequencies, the transit time becomes large because the source driving the grid becomes loaded and the gain of the vacuum tube becomes less than unity. This loading is due to the dissipation of the power at the grid. The effect of loading is such that the noise in the circuit increases. To minimize this effect, the distance between the electrodes is to be reduced and high voltage must be applied. This will increase the interelectrode capacitance. Klystron tube is a vacuum tube that can be operated either as an oscillator or as an amplifier at microwave frequencies. Two basic configurations of klystron tubes are : 1. Multi-cavity klystron which is used as a low power microwave amplifier. 2. Reflex klystron which is used as a low power microwave oscillator. As power output tubes 1. in UHF TV transmitters 2. in troposphere scatter transmitters 3. satellite communication ground station 4. radar transmitters As power oscillator (5 – 50 GHz), if used as a klystron oscillator The reflex klystrons are used in 1. Radar receivers 2. Local oscillator in microwave receivers 3. Signal source in microwave generator of variable frequency 4. Portable microwave links 5. Pump oscillator in parametric amplifier • TWT is an amplifier that makes use of distributed interaction between electron beam and a travelling wave. • It is mainly used for amplification of high frequencies. i.e. 3000 MHz or above. • Its principle feature is based on a slow wave structure. the RF wave propagate at the speed of light, while electron beam propagate at much slow velocity. Therefore the mechanism that reduces RF wave phase velocity in a TWT is a slow wave structure. 1. Low noise RF amplifier in broad band microwave receivers. 2. Repeater amplifier in wide band communication links and long distance telephony. 3. Due to long tube life (50,000 hours against ¼th for other types), TWT is power output tube in communication satellite. 4. Continuous wave high power TWT’s are used in troposcatter links (due to larger power and larger bandwidths). 5. Used in Air borne and ship borne pulsed high power radars. Used for high power/high frequency combination Tubes generate and amplify high levels of microwave power more cheaply than solid state devices Conventional tubes can be modified for low capacitance but specialized microwave tubes are also used High-power oscillator Common in radar and microwave ovens Cathode in center, anode around outside Strong dc magnetic field around tube causes electrons from cathode to spiral as they move toward anode Current of electrons generates microwaves in cavities around outside Magnetron has cavities all around the outside Wave circulates from one cavity to the next around the outside Each cavity represents one-half period Wave moves around tube at a velocity much less than that of light Wave velocity approximately equals electron velocity Important for pulsed tubes like radar transmitters Peak power can be much greater than average power Ton D TT Pavg PP D Used in high-power amplifiers Electron beam moves down tube past several cavities. Input cavity is the buncher, output cavity is the catcher. Buncher modulates the velocity of the electron beam Electric field from microwaves at buncher alternately speeds and slows electron beam This causes electrons to bunch up Electron bunches at catcher induce microwaves with more energy The cavities form a slow-wave structure Uses a helix as a slow-wave structure Microwaves input at cathode end of helix, output at anode end Energy is transferred from electron beam to microwaves