Learning Lesson

“There are only two forces in the world, the sword and the spirit. In the long run the sword will always be conquered by the spirit.”  -Napoleon Bonaparte 

Velocity ModulationVelocity modulation is defined as that variation in the

velocity of a beam of electrons caused by the alternate speeding up and slowing down of the electrons in the beam.

The electron beam passes through a pair of closely spaced grids, called BUNCHER GRIDS.

Buncher gridsPolarity of AC voltage

Electron beam via Buncher Grids

Buncher and Catcher Cavities

The energy gained by the accelerated electrons is balanced by the energy lost by the decelerated electrons.

A new and useful beam distribution will be formed if the velocity modulated electrons are allowed to drift into an area that has no electrostatic field.

Drift Space

Drift Space

Two cavity klystron amplifierA klystron is a microwave vacuum tube using cavity

resonators to produce velocity modulation of the electron beam and to produce amplification.

Input cavity (buncher cavity) RF energy is coupled in, and the electron beam is velocity modulated .

Output cavity (catcher cavity) the RF energy is coupled through the electron beam by placing the second cavity into the proper position at an optimum distance.

The RF interacting with the electron beam causes a kinetic energy loss from the beam that result in gain.

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(Accelerator grid)(AC voltage)

Two cavity klystron

Two-cavity klystron oscillator The two-cavity amplifier klystron is readily

turned into an oscillator klystron by providing a feedback loop between the input and output cavities.

Multicavity Klystron

In all modern klystrons, the number of cavities exceeds two.

A larger number of cavities may be used to increase the gain of the klystron, or to increase the bandwidth.

Three-cavity klystron

Reflex Klystron OscillatorA reflex klystron consists of an electron gun, a cavity with a pair of

grids and a repeller plate. In this klystron, a single pair of grids does the functions of both the

buncher and the catcher grids.The feedback required to maintain oscillations within the cavity is

obtained by reversing the beam and sending it back through the cavity. Due to this, they deliver energy to the cavity, the result is the oscillation at the cavity producing RF frequency.

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Grid voltage variation - Electron bunching

The variation in grid voltage causes the electrons to enter the space between the grid and the repeller at various velocities.

Traveling Wave Tube (TWT)Traveling-wave tube (TWT) is a high-gain, low-noise, wide-

bandwidth microwave amplifier.The unique feature of the TWT is a helix or coil that surrounds the

length of the tube and the electron beam passes through the centre or axis of the helix.

Surrounding the tube are either permanent magnets or electromagnets that keep the electrons tightly focused into a narrow beam.

Attenuators placed along the RF circuit prevent the reflected wave from traveling back to the cathode.

VELOCITY MODULATION

The electromagnetic field produced by the helix causes the electrons to be speeded up and slowed down, this produces velocity modulation of the beam which produces density modulation.

WANT TO SEE REAL PICTURE?

http://www2.l- 3com.com/edd/products_traveling_wave_tube.htm

The Backward Wave Oscillator (BWO)The electron beam (from an electron gun) passes through a wire

helix and generates an electric field that induces voltage with the helix wire. The resonating electric fields (in and out) produce microwaves in the direction opposite to the electron beam.

The frequency of the radiation is varied by controlling the beam velocity and the helix potential.

Applications of Microwave tubesA klystron can be used either as a generator or as an amplifier of

power, at microwave frequencies.Klystron as power output tubes

1. Satellite communication ground station

2. Radar transmitters

The reflex klystrons are used in1. Radar receivers

2. Local oscillator in microwave receivers

BACKWARD WAVE OSCILLATOR (BWO)

1. Shorter & Thicker TWT

2. Microwave CW oscillator

3. 1-1000 GHz

Applications : Travelling Wave Tubes(TWT)1. Low noise RF amplifier in broad band

microwave receivers.2. Due to long tube life (50,000 hours

against ¼th for other types), TWT is power output tube in communication satellite.

3. Continuous wave high power TWT’s are used in troposcatter links (due to larger power and larger bandwidths).

4. Used in Air borne and ship borne pulsed high power radars.

References

www.navymars.org/national/training/nmo_courses/.../14183_ch2.pdf

www.srmuniv.ac.in/openware_d_loads/u2L6.ppthttp://en.wikipedia.org/wiki/KlystronMicrowave Vacuum Tubes-RFMW Lesson 1B

Part2- YouTubehttp://www2.l-

3com.com/edd/products_traveling_wave_tube.htmhttp://www.allaboutcircuits.com/vol_3/chpt_13/11.html