Learning Reflex Klystron work project make money

Microwave Engineering – 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. 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 Repeller 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. Operation of Reflex Klystron The operation of Reflex Klystron is understood by some assumptions. The electron beam is accelerated towards the anode cavity. Let us assume that a reference electron er crosses the anode cavity but has no extra velocity and it repels back after reaching the Repeller electrode, with the same velocity. Another electron, let”s say ee which has started earlier than this reference electron, reaches the Repeller first, but returns slowly, reaching at the same time as the reference electron. We have another electron, the late electron el, which starts later than both er and ee, however, it moves with greater velocity while returning back, reaching at the same time as er and ee. Now, these three electrons, namely er, ee and el reach the gap at the same time, forming an electron bunch. This travel time is called as transit time, which should have an optimum value. The following figure illustrates this. The anode cavity accelerates the electrons while going and gains their energy by retarding them during the return journey. When the gap voltage is at maximum positive, this lets the maximum negative electrons to retard. The optimum transit time is represented as $$T = n + frac{3}{4} quad where : n : is :an :integer$$ This transit time depends upon the Repeller and anode voltages. Applications of Reflex Klystron Reflex Klystron is used in applications where variable frequency is desirable, such as − Radio receivers Portable microwave links Parametric amplifiers Local oscillators of microwave receivers As a signal source where variable frequency is desirable in microwave generators. Learning working make money

Learning Types of Transmission Lines work project make money

Types of Transmission Lines The conventional open-wire transmission lines are not suitable for microwave transmission, as the radiation losses would be high. At Microwave frequencies, the transmission lines employed can be broadly classified into three types. They are − Multi conductor lines Co-axial lines Strip lines Micro strip lines Slot lines Coplanar lines, etc. Single conductor lines (Waveguides) Rectangular waveguides Circular waveguides Elliptical waveguides Single-ridged waveguides Double-ridged waveguides, etc. Open boundary structures Di-electric rods Open waveguides, etc. Multi-conductor Lines The transmission lines which has more than one conductor are called as Multi-conductor lines. Co-axial Lines This one is mostly used for high frequency applications. A coaxial line consists of an inner conductor with inner diameter d, and then a concentric cylindrical insulating material, around it. This is surrounded by an outer conductor, which is a concentric cylinder with an inner diameter D. This structure is well understood by taking a look at the following figure. The fundamental and dominant mode in co-axial cables is TEM mode. There is no cutoff frequency in the co-axial cable. It passes all frequencies. However, for higher frequencies, some higher order non-TEM mode starts propagating, causing a lot of attenuation. Strip Lines These are the planar transmission lines, used at frequencies from 100MHz to 100GHz. A Strip line consists of a central thin conducting strip of width ω which is greater than its thickness t. It is placed inside the low loss dielectric (εr) substrate of thickness b/2 between two wide ground plates. The width of the ground plates is five times greater than the spacing between the plates. The thickness of metallic central conductor and the thickness of metallic ground planes are the same. The following figure shows the cross-sectional view of the strip line structure. The fundamental and dominant mode in Strip lines is TEM mode. For b<λ/2, there will be no propagation in the transverse direction. The impedance of a strip line is inversely proportional to the ratio of the width ω of the inner conductor to the distance b between the ground planes. Micro Strip Lines The strip line has a disadvantage that it is not accessible for adjustment and tuning. This is avoided in micro strip lines, which allows mounting of active or passive devices, and also allows making minor adjustments after the circuit has been fabricated. A micro strip line is an unsymmetrical parallel plate transmission line, having di-electric substrate which has a metallized ground on the bottom and a thin conducting strip on top with thickness ”t” and width ”ω”. This can be understood by taking a look at the following figure, which shows a micro strip line. The characteristic impedance of a micro strip is a function of the strip line width (ω), thickness (t) and the distance between the line and the ground plane (h). Micro strip lines are of many types such as embedded micro strip, inverted micro strip, suspended micro strip and slotted micro strip transmission lines. In addition to these, some other TEM lines such as parallel strip lines and coplanar strip lines also have been used for microwave integrated circuits. Other Lines A Parallel Strip line is similar to a two conductor transmission line. It can support quasi TEM mode. The following figure explains this. A Coplanar strip line is formed by two conducting strips with one strip grounded, both being placed on the same substrate surface, for convenient connections. The following figure explains this. A Slot line transmission line, consists of a slot or gap in a conducting coating on a dielectric substrate and this fabrication process is identical to the micro strip lines. Following is its diagrammatical representation. A coplanar waveguide consists of a strip of thin metallic film which is deposited on the surface of a dielectric slab. This slab has two electrodes running adjacent and parallel to the strip on to the same surface. The following figure explains this. All of these micro strip lines are used in microwave applications where the use of bulky and expensive to manufacture transmission lines will be a disadvantage. Open Boundary Structures These can also be stated as Open Electromagnetic Waveguides. A waveguide that is not entirely enclosed in a metal shielding, can be considered as an open waveguide. Free space is also considered as a kind of open waveguide. An open waveguide may be defined as any physical device with longitudinal axial symmetry and unbounded cross-section, capable of guiding electromagnetic waves. They possess a spectrum which is no longer discrete. Micro strip lines and optical fibers are also examples of open waveguides. Learning working make money

Learning Components work project make money

Microwave Engineering – Components In this chapter, we shall discuss about the microwave components such as microwave transistors and different types of diodes. Microwave Transistors There is a need to develop special transistors to tolerate the microwave frequencies. Hence for microwave applications, silicon n-p-n transistors that can provide adequate powers at microwave frequencies have been developed. They are with typically 5 watts at a frequency of 3GHz with a gain of 5dB. A cross-sectional view of such a transistor is shown in the following figure. Construction of Microwave Transistors An n type epitaxial layer is grown on n+ substrate that constitutes the collector. On this n region, a SiO2 layer is grown thermally. A p-base and heavily doped n-emitters are diffused into the base. Openings are made in Oxide for Ohmic contacts. Connections are made in parallel. Such transistors have a surface geometry categorized as either interdigitated, overlay, or matrix. These forms are shown in the following figure. Power transistors employ all the three surface geometries. Small signal transistors employ interdigitated surface geometry. Interdigitated structure is suitable for small signal applications in the L, S, and C bands. The matrix geometry is sometimes called mesh or emitter grid. Overlay and Matrix structures are useful as power devices in the UHF and VHF regions. Operation of Microwave Transistors In a microwave transistor, initially the emitter-base and collector-base junctions are reverse biased. On the application of a microwave signal, the emitter-base junction becomes forward biased. If a p-n-p transistor is considered, the application of positive peak of signal, forward biases the emitter-base junction, making the holes to drift to the thin negative base. The holes further accelerate to the negative terminal of the bias voltage between the collector and the base terminals. A load connected at the collector, receives a current pulse. Solid State Devices The classification of solid state Microwave devices can be done − Depending upon their electrical behavior Non-linear resistance type. Example − Varistors (variable resistances) Non-Linear reactance type. Example − Varactors (variable reactors) Negative resistance type. Example − Tunnel diode, Impatt diode, Gunn diode Controllable impedance type. Example − PIN diode Depending upon their construction Point contact diodes Schottky barrier diodes Metal Oxide Semiconductor devices (MOS) Metal insulation devices The types of diodes which we have mentioned here have many uses such as amplification, detection, power generation, phase shifting, down conversion, up conversion, limiting modulation, switching, etc. Varactor Diode A voltage variable capacitance of a reverse biased junction can be termed as a Varactor diode. Varactor diode is a semi-conductor device in which the junction capacitance can be varied as a function of the reverse bias of the diode. The CV characteristics of a typical Varactor diode and its symbols are shown in the following figure. The junction capacitance depends on the applied voltage and junction design. We know that, $$C_j : alpha : V_{r}^{-n}$$ Where $C_j$ = Junction capacitance $V_r$ = Reverse bias voltage $n$ = A parameter that decides the type of junction If the junction is reverse biased, the mobile carriers deplete the junction, resulting in some capacitance, where the diode behaves as a capacitor, with the junction acting as a dielectric. The capacitance decreases with the increase in reverse bias. The encapsulation of diode contains electrical leads which are attached to the semiconductor wafer and a lead attached to the ceramic case. The following figure shows how a microwave Varactor diode looks. These are capable of handling large powers and large reverse breakdown voltages. These have low noise. Although variation in junction capacitance is an important factor in this diode, parasitic resistances, capacitances, and conductances are associated with every practical diode, which should be kept low. Applications of Varactor Diode Varactor diodes are used in the following applications − Up conversion Parametric amplifier Pulse generation Pulse shaping Switching circuits Modulation of microwave signals Schottky Barrier Diode This is a simple diode that exhibits non-linear impedance. These diodes are mostly used for microwave detection and mixing. Construction of Schottky Barrier Diode A semi-conductor pellet is mounted on a metal base. A spring loaded wire is connected with a sharp point to this silicon pellet. This can be easily mounted into coaxial or waveguide lines. The following figure gives a clear picture of the construction. Operation of Schottky Barrier Diode With the contact between the semi-conductor and the metal, a depletion region is formed. The metal region has smaller depletion width, comparatively. When contact is made, electron flow occurs from the semi-conductor to the metal. This depletion builds up a positive space charge in the semi-conductor and the electric field opposes further flow, which leads to the creation of a barrier at the interface. During forward bias, the barrier height is reduced and the electrons get injected into the metal, whereas during reverse bias, the barrier height increases and the electron injection almost stops. Advantages of Schottky Barrier Diode These are the following advantages. Low cost Simplicity Reliable Noise figures 4 to 5dB Applications of Schottky Barrier Diode These are the following applications. Low noise mixer Balanced mixer in continuous wave radar Microwave detector Gunn Effect Devices J B Gunn discovered periodic fluctuations of current passing through the n-type GaAs specimen when the applied voltage exceeded a certain critical value. In these diodes, there are two valleys, L & U valleys in conduction band and the electron transfer occurs between them, depending upon the applied electric field. This effect of population inversion from lower L-valley to upper U-valley is called Transfer Electron Effect and hence these are called as Transfer Electron Devices (TEDs). Applications of Gunn Diodes Gunn diodes are extensively used in the following devices − Radar transmitters Transponders in air traffic control Industrial telemetry systems Power oscillators Logic circuits Broadband linear amplifier Learning working make money

Learning Transmission Lines work project make money

Microwave Engineering – Transmission Lines A transmission line is a connector which transmits energy from one point to another. The study of transmission line theory is helpful in the effective usage of power and equipment. There are basically four types of transmission lines − Two-wire parallel transmission lines Coaxial lines Strip type substrate transmission lines Waveguides While transmitting or while receiving, the energy transfer has to be done effectively, without the wastage of power. To achieve this, there are certain important parameters which has to be considered. Main Parameters of a Transmission Line The important parameters of a transmission line are resistance, inductance, capacitance and conductance. Resistance and inductance together are called as transmission line impedance. Capacitance and conductance together are called as admittance. Resistance The resistance offered by the material out of which the transmission lines are made, will be of considerable amount, especially for shorter lines. As the line current increases, the ohmic loss $left ( I^{2}R : loss right )$ also increases. The resistance $R$ of a conductor of length “$l$” and cross-section “$a$” is represented as $$R = rho frac{l}{a}$$ Where $rho$ = resistivity of the conductor material, which is constant. Temperature and the frequency of the current are the main factors that affect the resistance of a line. The resistance of a conductor varies linearly with the change in temperature. Whereas, if the frequency of the current increases, the current density towards the surface of the conductor also increases. Otherwise, the current density towards the center of the conductor increases. This means, more the current flows towards the surface of the conductor, it flows less towards the center, which is known as the Skin Effect. Inductance In an AC transmission line, the current flows sinusoidally. This current induces a magnetic field perpendicular to the electric field, which also varies sinusoidally. This is well known as Faraday”s law. The fields are depicted in the following figure. This varying magnetic field induces some EMF into the conductor. Now this induced voltage or EMF flows in the opposite direction to the current flowing initially. This EMF flowing in the opposite direction is equivalently shown by a parameter known as Inductance, which is the property to oppose the shift in the current. It is denoted by “L“. The unit of measurement is “Henry(H)“. Conductance There will be a leakage current between the transmission line and the ground, and also between the phase conductors. This small amount of leakage current generally flows through the surface of the insulator. Inverse of this leakage current is termed as Conductance. It is denoted by “G“. The flow of line current is associated with inductance and the voltage difference between the two points is associated with capacitance. Inductance is associated with the magnetic field, while capacitance is associated with the electric field. Capacitance The voltage difference between the Phase conductors gives rise to an electric field between the conductors. The two conductors are just like parallel plates and the air in between them becomes dielectric. This pattern gives rise to the capacitance effect between the conductors. Characteristic Impedance If a uniform lossless transmission line is considered, for a wave travelling in one direction, the ratio of the amplitudes of voltage and current along that line, which has no reflections, is called as Characteristic impedance. It is denoted by $Z_0$ $$Z_0 = sqrt{frac{voltage :: wave :: value}{current :: wave :: value}}$$ $$Z_0 = sqrt{frac{R + jwL}{G + jwC}}$$ For a lossless line, $R_0 = sqrt{frac{L}{C}}$ Where $L$ & $C$ are the inductance and capacitance per unit lengths. Impedance Matching To achieve maximum power transfer to the load, impedance matching has to be done. To achieve this impedance matching, the following conditions are to be met. The resistance of the load should be equal to that of the source. $$R_L = R_S$$ The reactance of the load should be equal to that of the source but opposite in sign. $$X_L = -X_S$$ Which means, if the source is inductive, the load should be capacitive and vice versa. Reflection Co-efficient The parameter that expresses the amount of reflected energy due to impedance mismatch in a transmission line is called as Reflection coefficient. It is indicated by $rho$ (rho). It can be defined as “the ratio of reflected voltage to the incident voltage at the load terminals”. $$rho = frac{reflected:voltage}{incident:voltage} = frac{V_r}{V_i} : at : load : terminals$$ If the impedance between the device and the transmission line don”t match with each other, then the energy gets reflected. The higher the energy gets reflected, the greater will be the value of $rho$ reflection coefficient. Voltage Standing Wave Ratio (VSWR) The standing wave is formed when the incident wave gets reflected. The standing wave which is formed, contains some voltage. The magnitude of standing waves can be measured in terms of standing wave ratios. The ratio of maximum voltage to the minimum voltage in a standing wave can be defined as Voltage Standing Wave Ratio (VSWR). It is denoted by “$S$”. $$S = frac{left |V_{max} right |}{left |V_{min} right |} quad 1:leq S leq infty$$ VSWR describes the voltage standing wave pattern that is present in the transmission line due to phase addition and subtraction of the incident and reflected waves. Hence, it can also be written as $$S = frac{1 + rho }{1 – rho }$$ The larger the impedance mismatch, the higher will be the amplitude of the standing wave. Therefore, if the impedance is matched perfectly, $$V_{max} : V_{min} = 1:1$$ Hence, the value for VSWR is unity, which means the transmission is perfect. Efficiency of Transmission Lines The efficiency of transmission lines is defined as the ratio of the output power to the input power. $% : efficiency : of : transmission : line : eta = frac{Power : delivered : at : reception}{Power : sent : from : the : transmission : end} times 100$ Voltage Regulation Voltage regulation is defined as the change in the magnitude of the voltage between the sending