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Report this Document. Flag for inappropriate content. Related titles. Reflex Klystron used as a low-power Microwave oscillator 2.
Multi cavity klystron used as lowpower microwave amplifier. Drawbacks of klystron amplifiers 1. As the oscillator frequency changes then resonator frequency also changes and the feedback path phase shift must be readjusted for a positive feedback.
The multicavity klystron amplifiers suffer from the noise caused because bunching is never complete and electrons arrive at random at catcher cavity. Hence it is not used in receivers. Cavities used in a two-cavity klystron have high Q values with narrow bandwidths, and thus, individual tuning is awkward. Therefore, two- cavity klystrons are generally used for fixed-frequency applications. The analysis of a reflex klystron is similar to that of a two-cavity klystron to some extent and is subjected to the following approximations: 1.
Cavity grids and repeller are plane and parallel, and also very large in extent. RF field is absent in the repeller space. Electrons are not intercepted by the cavity anode grid. No debunching of electrons takes place in the repeller space. RF gap voltage is small compared to the beam voltage.
The average transit time through the cavity gap d and the transit angle. Thus, we can write that. Power Output and Efficiency To transfer maximum energy to the oscillator, the returning electron must cross the cavity gap when the gap field is maximum retarding.
Therefore, the round trip transit angle of the centre of the bunch is given as follows:. The fundamental component of the current induced in the cavity by the modulated electron beam is. The factor XJ1 X reaches a maximum value of 1. Thus, the maximum efficiency of a reflex klystron is calculated as follows:. Electronic Admittance If an electron returns to the cavity a little before the time T n- , the current lags behind the fields and an inductive reactance is presented to the circuit.
On the other hand, if the electron returns to the cavity a little after the time T n- , the current leads the fields and a capacitive is presented to the circuit. The electronic admittance can be written as follows:. This equation reveals that the phasor admittance is a function of DC beam admittance, DC transit angle, and the second transit of the electron beam through the cavity gap, and is non-linear. Any value of 0 for which the spiral lies in the area at th of the line GjB will yield an oscillation, that is,.
L and C are the energy storage elements of the cavity, Gc is the copper losses in the cavity, Gb is the beam loading conductance, and Gl is the load conductance.
In parametric amplifier as pump source. Signal source in MW generator. Wideband devices. The interaction of electron beam and RF field in the TWT is continuous over the entire length of the circuit.
Slow-Wave Structures As the operating frequency is increased , both the inductance and capacitance in the resonating circuit must be decreased in order to maintain the resonance at the operating frequency. Because the gain-bandwidth product is limited by the resonating circuit, the ordinary resonator cannot generate the large output. Non resonating or slow-wave structures are designed for producing larger gain over a wide bandwidth. Slow-wave structures are special circuits that are used in microwave tubes to reduce the wave velocity in a certain direction so that the electron beam and the signal wave can interact.
In travelling wave tube slow wave structures are used. Amplification Process Travelling wave is propagating in the z-direction then the z component of the electric field where E1 is the magnitude of the z-component of electric field, and p is the axial phase constant and is given by the following relation: the force on the electrons exerted by the axial electric field can be expressed as follows:.
The equation reveals that the magnitude of velocity fluctuation of the electron beam is directly proportional to the magnitude of the axial electric field. This analysis neglects the space charge effect. Conventional Current Determine the relation between the current and the electron beam If the space charge effect is considered, then the electron velocity, charge density, current density, and axial electric field can be written as follows:.
If Then This is known as an electronic equation. Convection current in the electron beam, given in Eq. As a result, the circuit power increases with distance. The electric filed. Wave Mode The wave mode of a TWT is determined by solving the electronics and circuit eqn simultaneously for the propagation constant. We get. These equations reveal the following facts: 1.
This wave propagates at a phase velocity slightly lower than the electron beam velocity, and energy flows from the electron beam to the wave. This wave propagates at the same phase velocity as that of the growing wave and energy flows from the wave to the electron beam.
This wave propagates at a phase velocity slightly higher than the electron beam velocity, and no energy transfer takes place between the electron beam and the wave. Applications Low-noise TWTs are widely used as RF amplifiers in broadband microwave receivers and repeater amplifiers CW high-power tubes are used in tropospheric scatter links due to their high power and large bandwidth. They can also be used in radar for jamming purpose.
Pulsed TWTs are used in airborne and ship-borne radars, as well as in high-power ground-based radars. Due to their long tube life, TWTs are also used as power output tubes in communication satellites.
In a cylindrical magnetron, several reentrant cavities are connected to the gaps. The magnetic field is usually provided by a strong, permanent magnet mounted around the magnetron so that the magnetic field is parallel with the axis of the cathode. The cathode is mounted in the center of the interaction space. The de voltage Vo is applied between the cathode and the anode. The magnetic flux density Bo is in the positive z direction.
When the de voltage and the magnetic flux are adjusted properly, the electrons will follow cycloidal paths in the cathode anode space under the combined force of both electric and magnetic fields as shown. Operation Magnetron theory of operation is based on the motion of electrons under the influence of combined electric and magnetic fields. The law of motion of an electron in a Electric field E field The force exerted by an electric field on an electron is proportional to the strength of the field.
Electrons tend to move from a point of negative potential toward a positive potential. The law of motion of an electron in a magnetic field H field The force exerted on an electron in a magnetic field is at right angles to both the field and the path of the electron.
The direction of the force is such that the electron trajectories are clockwise when viewed in the direction of the magnetic field. When the de voltage and the magnetic flux are adjusted properly, the electrons will follow cycloidal paths in the cathode. At zero magnetic field, the electron take the straight path a, by the influence of electric field only.
For a given Vo if the magnetic field is increased, the electrons take curved path b to reach the anode. At a critical value of magnetic field Bc, The electrons just graze the anode surface at radius b and take the path c to return to the cathode for a given voltage Vo.
This value Bc is called the cut off magnetic flux density. If the magnetic field is greater than Bc all the electrons return to the cathode as shown by a typical path d without reaching the anode.
Operation of the Magnetron mode oscillation N resonant cavities in the anode exists N resonant mode or frequency. The phase difference between the adjacent cavities. Number cavities are eight. Hartree voltage In mode, the phase difference between the adjacent cavities is o. For strong interaction between electron and wave, the phase velocity of the wave is nearly equal to the drift and the oscillations for mode at beam voltage.
RF field id zero and the magnetic field filed exceeds cutoff filed the electrons follow the path b electron return to cathode RF and magnetic field present, the electron path modifies favored electron. Basically, a two-cavity klystron can be converted into an oscillator, but some disadvantages are associated with it.
As we know to design an oscillator, positive feedback must be provided to the input in a way to have a magnitude of loop gain as unity. So, if we design a klystron oscillator using a two-cavity klystron then to have a change in oscillating frequency, the resonant frequency of the two-cavities is also required to be changed. Thereby leading to cause difficulty in generating oscillations.
Thus to overcome the disadvantage, a reflex klystron having a single cavity was invented to have sustained oscillations at microwave frequency. Like a two-cavity klystron, a reflex klystron utilizes the phenomenon of velocity and current modulation to produce oscillations.
However, there exist variation in constructional structure and the respective applications of the two. A reflex klystron consists of a single cavity that performs the action of both buncher and catcher cavity.
As to have oscillations, feedback is needed to be applied at the input which is provided by the oscillator. While moving electrons undergoes velocity modulation and the repeller applies repulsive forces on them. This leads to the formation of a bunch of electrons. Further, this bunching will lead to cause, current modulation. We will discuss the working of a reflex klystron in detail but before that let us see how it is constructed.
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