Micro wave

Klystron amplifier and reflex klystron 09-05-2012

Definition

• A klystron is a specialized linear-beam vacuum tube (evacuated electron tube).

• used as amplifiers at (for) microwave and radio frequencies to produce high-power carrier waves for communications , the purpose of both the cavities (electrodes) is to control the flow of electrons in the drift space.

Explaination

• Klystron amplifier consist of following parts: cathode, anode ,electron beam, bunch and catcher cavities, drift space.

• The electrons move from cathode to anode in the linear beam tube. the two cavities bunch and catcher coupling with their device make buncher and catcher. The electrons move with the field produced due to radio frequency. The purpose of both cavities are to provide control flow of electrons in the beam by alternately accelerating and decelerating them. The accelerated electrons overtake the decelerate electrons and make a full bunch at catcher cavity. the resonant frequency of the catcher cavity is the same as the buncher cavity. The collector collect the energy of the electron beam and change it X radiation.

Repeller Klystron

• Reflex klystron is a low-power, low-efficiency microwave oscillator another tube based on velocity modulation, and used to generate microwave energy,

• It contain 1. Cathod: which is an electron gun. Fire electron in the

liner beam electron tube.2. Anode: Cavity also acts a Anode having high voltage

applied on it in addition to RF signal3. Repeller: it is an electrode having high negative

voltage apply to it. Precaution is taken that electrons don’t bombard the it.

• Glass Casing: It encloses all the parts stated above• The electrons overshoot the gap in this cavity

Cont….

• continue on to the next electrode, • never reach the repeller at last. 1. electrons in the beam reach some point in the

repeller space and are then turned back, eventually to be dissipated in the anode cavity.

• the returning electrons give more energy to the gap than they took from it on the outward journey, and continuing oscillations take place.

Operation• Let the reference electron is taken as one that passes the gap on its way to the

repeller at the time when the gap voltage is zero and going negative. • This electron is of course unaffected, overshoots the gap, and is ultimately

returned to it, having penetrated some distance into the repeller space.

• An electron passing the gap slightly earlier would have encountered a slightly positive voltage at the gap.

• The resulting acceleration would have propelled this electron slightly farther into the repeller space, and the electron would thus have taken a slightly longer time than the reference electron to return to the gap.

• Similarly, an electron passing the gap a little after the reference electron will encounter a slightly negative voltage.

• The resulting retardation will shorten its stay in the repeller space.

• It is seen that, around the reference electron, earlier electrons take longer to return to the gap than later electrons, and so the conditions are right for bunching to take place. It is thus seen that velocity modulation is converted to current modulation in the repeller space, and one bunch is formed per cycle of oscillations.

Transit time

Practical Considerations• Performance • Reflex klystrons with integral cavities are

available for frequencies ranging from under 4 to over 200 GHz.

• A typical power output is 100 mW, but overall maximum powers range from 3 W in the X band to 10 mW at 220 GHz.

• Typical efficiencies are under 10 percent, restricting the oscillator to low-power applications.

Practical Considerations

• Repeller protection : it is done by keeping the velocity of electrons, not to be exceeded than the repeller capacity to sent it back. For this purpose we have to keep the negative voltage of repeller high.

• Applications The klystron oscillator has been replaced by various semiconductor oscillators in applications, in new equipment.

• However, it will be found in a lot of existing equipment, as a:• 1. Signal source in microwave generators• 2. Local oscillator in microwave receivers• 3. Frequency-modulated oscillator in portable microwave links• 4. Pump oscillator for parametric amplifiers• Additionally, the reflex klystron is still a very useful millimeter and

submillimeter oscillator, producing more power at the highest frequencies than most semiconductor devices, with very low AM and FM noise.

Travelling wave tube