RF Active Circuit Design אקטיבייםRF תכנון מעגלי 1 Prepared by: Prof. Anatoly Patlakh Conventional Tubes Disadvantage Conventional Device tubes cannot be used for frequencies above 100MHz: • • • • • • Inter-electrode capacitance Lead Inductance effect Transit time effect Gain Bandwidth limitation Effect of RF losses (conductance, dielectric) Effect due to radiation losses Solutions • Efficient Microwave tubes usually operate on the theory of electron velocity modulation concept • The electron transit time is used in the conversion of dc power to RF power Microwave tubes classification Microwave tubes can be broadly classifies into two categories 1. O-TYPE Linear Tubes (travelling-wave tube, Klystrons) a) these are linear beam tubes, i.e. the magnetic field is in parallel with the d.c. electric field and is used just to focusing the beam as it travels the length of the tube. b) in these tubes, electrons receive potential energy from the dc voltage before they arrive in the microwave interaction region this energy is converted into their kinetic energy. 2. M-TYPE Tubes (magnetrons and cross field devices) a) these are cross field devices i.e. the dc magnetic field and the dc electric field are perpendicular to each other. b) in these tubes, the electrons emitted by the cathode are accelerated by the electric field and gain velocity but the greater their velocity, the more their path is bent by the magnetic field. Travelling-wave tube Klystron 5 Klystron Klystron- an electron (vacuum) tube for the amplification or generation of microwaves by means of velocity modulation There are two basic configurations of klystron tubes- Floating drift tube and Reflex klystron : • Reflex Klystron used as a low-power Microwave oscillator • Multi cavity klystron used as low-power microwave amplifier Multi cavity Klystron Reflex Klystron Klystron The klystron is a linear beam device, that is, the electron flow is in a straight line focused by an axial magnetic field. The velocities of electrons emitted from the cathode are modulated by MW electromagnetic field to produce a density-modulated electron beam. Klystron Classification Reflex Multi-cavity Two-cavity klystron Grids Principle of operation • High velocity electron beam is generated by an electron gun and sent down along a vacuum tube through an input cavity (BUNCHER), drift space (FIELD FREE) and an output cavity (CATCHER) to a collector electrode. • The input buncher cavity is exited by the RF signal, (the signal to be amplified) which will produce an alternating voltage of signal frequency across the gap of first resonator. • This voltage generated at the gap is responsible to produce bunching of electrons or velocity modulation of the electron beam. • The anode is kept positive to receive the electrons, while the output is taken from the tube via resonant cavities with the aid of coupling loops • Two grids of the both buncher and catcher cavities are separated by a small gap. Applegate Diagram Two-Cavity Klystron Construction Multicavity Klystron Buncher cavity Cathode Filament (heater) Electron beam Intermediate cavity Catcher cavity Collector There is a graphical representation of 4 cavities klystron tube. In the first cavity (the input cavity), the beam is excited by the microwave signal, which is intended be amplified. This generates an alternating signal across the gap of the cavity. The velocity of the electrons passing through the beam will be modulated with the RF input signal. Each of the cavities is successively tuned in such a way as to reproduce a linear amplified input signal. In the output cavity, otherwise known as the ultimate cavity, the RF output signal is coupled to the transmission line and to load. Reflex Klystron • The electron beam emitted from the cathode is accelerated by the grid and passes through the cavity anode to the repeller space between the cavity anode and the repeller electrode . • The feedback required to maintain the oscillations within the cavity is obtained by reversing electron beam emitted from the cathode towards repeller electrode and sending it back through the cavity. • The electrons in the beam are velocity modulated before the beam passes through the cavity the second time and give up the energy to the cavity to maintain oscillations. • This type of a Klystron is called a Reflex Klystron because of the reflex action of the electron beam. Mechanism of Oscillation and Modulation • It is assumed that the oscillations are set up in the tube initially due to noise or switching transients and the oscillations are sustained by device operation. • The electrons passing through the cavity gap interact with RF field and are velocity modulated. • The electrons which encountered the positive half cycle of the RF field in the cavity gap will be accelerated, which encountered zero RF field will pass with unchanged original velocity, and which encountered the negative half cycle will be retarded on entering the repeller space. • All these velocity modulated electrons will be repelled back to the cavity by the repeller due to the negative potential. • The repeller distance L and the voltages can be adjusted to receive all the electrons at a same time on the positive peak of the cavity RF velocity cycle. • Thus the velocity modulated electrons are bunched together and lose their kinetic energy when they encounter the positive cycle of the cavity RF field. • Bunches occur once per cycle centered around the reference electron and these bunches transfer maximum energy to the gap to get sustained oscillations. • For oscillations to be sustained, the time taken by the electrons to travel into the repeller space and back to the gap (transit time) must have an optimum value. https://www.youtube.com/watch?v=Fvud81pYGOg https://www.youtube.com/watch?v=TsBTI3tO5-8#t=48.7891254 Magnetron A magnetron is a type of diode vacuum tube in which the flow of electrons from a central cathode to a cylindrical anode is controlled by crossed magnetic and electric fields The magnetron is a high-powered vacuum tube, that works as selfexcited microwave oscillator. Crossed electron and magnetic fields are used in the magnetron to produce the high-power output. The relatively simple construction has the disadvantage that the magnetron usually can work only on a constructively fixed frequency. http://www.radartutorial.eu/transmitters/Magnetron.en.html.08 Magnetron 22 Magnetron Basic Operation Cycloid Trochoid Electron in crossed magnetic and electrical fields 23 24 B1<B2<B3<B4<B5 B1=0 The electron path under the influence of different strength of the magnetic field 25 λ/4 Forms of the anode block in a magnetron a) b) c) d) 26 slot- type vane- type rising sun- type hole-and-slot- type Each cavity work like a resonant circuit 27 resonant cavity interaction space cathode resonant cavity filament leads Cutaway view of a magnetron π-Mode Construction and working principles- multi-cavity magnetron The high-frequency electrical field 29 The equivalent circuit of the cavity of 4-resonator magnetron 30 π-Mode π-Mode ½π-Mode ¾π-Mode π 1/3 φ0=2πn/N n=N/2 Modes of the magnetron (Anode segments are represented “unwound”) 32 _ A + B _ + 33 Paths followed by electrons Space-Charge Rotating space-charge wheel in an twelve-cavity magnetron 35 36 https://www.youtube.com/watch?v=VkpEQZEGSkE https://www.youtube.com/watch?v=21QvzOeZdBw
