Microwave Tubes

ADDIS ABABA INSTITUTE OF TECNOLOGY 'Theory and application of Microwave tube'7/19/2013 1

ADDIS ABABA INSTITUTE OF TECNOLOGYDEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING

MICROWAVE DEVICE AND NETWORKSECEG6312

THEORY AND APPLICATION OF MICROWAVE TUBE

Prepared by: Henok MeleseID: GSE/1100/03

What is a “Microwave?” Microwaves are electromagnetic waves whose frequencies range from

about (300 MHz – 300 GHz) or wavelengths in air ranging from 100 cm –1 mm.

The word Microwave means very short wave, which is the shortest wavelength region of the radio spectrum and a part of the electromagnetic spectrum.


What is microwave tube ?

Microwave tubes are vacuum tubes that Microwave tubes are vacuum tubes that perform generation and amplification in the microwave portion of the frequency spectrum

A vacuum tube is an electron device in which an electron beam is interacting with an electromagnetic wave

Tubes that are efficient in microwave range usually operate on the theory of VELOCITY MODULATION


Principle of Vacuum Tubes

• The energy is transferred from the electron-beam to the EM-wave


Velocity Modulation

The interchange of power is accomplished by The interchange of power is accomplished by using the principle of electron VELOCITY MODULATION and low-loss resonant cavities in the microwave tube

Velocity modulation has stages like producing stream of electrons , producing electron beam , producing BUNCHES of electrons and taking useful power from the beam

To clearly understand the microwave tube let’s start by understanding how electrons and electric field interact


How electrons and electric fields interact

An electron has mass and thus exhibits kinetic energy when in motion

The amount of kinetic energy in an electron is directly proportional to its velocity; that is, the higher the velocity, the higher and the energy level

An electron can be accelerated or decelerated by an electrostatic field.

Figure shows an electron moving in an electrostatic field. The direction of travel is against the electrostatic lines of force .

The negatively charged electron will be attracted to the positively charged body and will increase in velocity.

As its velocity increases, the energy level of the electron will also increase

Where does the electron acquire its additional energy? The only logical source is from the electrostatic field.

Moving electron gaining velocity and energy


How electrons and electric fields interact (cont’d)

Figure shows an electron Figure shows an electron moving in an electrostatic field. The direction of travel is same with the electrostatic lines of force

The negatively charged body will repel the electron and cause it to decrease in velocity so an electron will decelerate

When the velocity is reduced, the energy level is also reduced.

The energy lost by the electron is gained by the electrostatic field.

Moving electron losing energy and velocity.


Velocity Modulation‘ producing stream of electrons and producing electron beam ‘

The operation of a velocity-modulated tube depends on a change in the velocity of the electrons passing through its electrostatic field

Velocity 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

This variation is usually caused by a voltage signal applied between the grids through which the beam must pass.

The first requirement in obtaining velocity modulation is to produce a stream of electrons which are all traveling at the same speed.

The electron stream is produced by an electron gun(electron gun is shown in figure)

Electrons emitted from the cathode are attracted toward the positive accelerator grid and all but a few of the electrons pass through the grid and form a beam

Electron gun with buncher grids.


Velocity Modulation ‘producing BUNCHES of electrons’

The electron beam then passes through a pair of closely spaced grids, called BUNCHER GRIDS. Each grid is connected to one side of a tuned circuit

The alternating voltage which exists across the resonant circuit causes the velocity of the electrons leaving the buncher grids to differ from the velocity of the electrons arriving at the bunchergrids

So produces bunches of electrons Till now no useful power has been

produced at this point. However, a new and useful beam

distribution is formed

produces bunches of electrons


Velocity Modulation‘Taking useful power from the beam’

The electron bunches will induce an RF voltage in the grid gap of the second cavity called CATCHER CAVITY causing it to oscillate

the induced grid-gap voltage will cause to decelerate the electron bunches as they arrive at the gap

Since the largest concentration of electrons is in the bunches, slowing the bunches causes a transfer of energy to the output cavity

Placing of the catcher cavity at proper place , bunches are slowed down when they arrive at the cavity

The areas between bunches arrive at the cavity when the voltage is correct polarity to increase the velocity of the electrons and the beam absorbs energy

The areas between the bunches have very few electrons, so the energy removed from the beam is much greater than the energy required to speed up the electrons between the bunches.

Therefore, if the second cavity is properly positioned, useful energy can be removed from a velocity modulated electron beam

Removing energy from a velocity-modulated beam


Types of Microwave Tubes

In a linear beam tube, the electron beam travels in the In a linear beam tube, the electron beam travels in the same direction as the magnetic field .

In crossed-field tubes Magnetic and electric fields are at right angles


Klystron

klystrons are Velocity modulated Microwave tubes used either as an amplifier in the microwave region or as an oscillatorDue to the number of the resonant

cavities klystrons are divided up into Two- or Multi-cavity klystrons, and Reflex or Repeller Klystrons


Two-Cavity KlystronFunctional and schematic diagram of a two-cavity klystron

physical construction and mode of operation of a two-cavity klystron


CATHODE • When the tube is energized, the emits electrons

CONTROL GRID

• Focused into a beam by low positive voltage

ACCELERATOR GRID

• Accelerated by a very high positive dc potential that is applied

BUNCHER GRIDS

• Due to the cavity resonator superimposes an ac potential on the dc voltage The oscillations within the cavity produce an oscillating electrostatic field between the buncher grids that is at the same frequency as the natural frequency of the cavity

• The direction of the field changes with the frequency of the cavity. These changes alternately accelerate and decelerate the electrons of the beam passing through the grids

CATCHER GRIDS

•Absorb energy from the electron beam•May be used either as an oscillator or an amplifier•A signal applied at the buncher grids will be amplified if the feedback path is removed.


Principle

Velocity modulated tubeVelocity modulated tubeHigh velocity electron beam is generated by an

electron gun and sent down along a gas tube through an input cavity (BUNCHER), drift space (FIELD FREE) and an output cavity (CATCHER) to a collector electrode anode.

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


The Multi-cavity Klystron

Adding intermediate cavities Adding more cavities is roughly the same as

adding more stages to a conventional Two-cavity klystronThe effect of the intermediate cavities is to

improve the electron bunching implies improves amplifier gain, power output, and efficiency.


Applications

As power output tubes1. in UHF TV transmitters2. in troposphere scatter transmitters3. satellite communication ground station4. radar transmitters

As power oscillator (5 – 50 GHz), if used as a klystron oscillator


Reflex Klystrons

The reflex klystron has been the most used source of microwave power in laboratory applications.


Construction A reflex klystron consists of an A reflex klystron consists of an

electron gun, a cavity with a pair of grids and a repeller plate as shown in the diagram.

In this klystron, a single pair of grids does the functions of both the buncher and the catcher grids.

The main difference between two cavity reflex klystron amplifier and reflex klystron is that the output cavity is omitted in reflex klystron and the repeller or reflector electrode, placed a very short distance from the single cavity, replaces the collector electrode. Functional diagram of a reflex klystron


Working

The cathode emits electrons which The cathode emits electrons which are accelerated forward by an accelerating grid with a positive voltage on it and focused into a narrow beam.

The electrons pass through the cavity and undergo velocity modulation, which produces electron bunching and the beam is repelled back by a repeller plate kept at a negative potential with respect to the cathode.

On return, the electron beam once again enters the same grids which act as a buncher, there by the same pair of grids acts simultaneously as a buncher for the forward moving electron and as a catcher for the returning beam.


Working

The feedback necessary for electrical oscillations is developed by reflecting the electron beam, the velocity modulated electron beam does not actually reach the repeller plate, but is repelled back by the negative voltage.

The point at which the electron beam is turned back can be varied by adjusting the repeller voltage.

Thus the repeller voltage is so adjusted that complete bunching of the electrons takes place at the catcher grids, the distance between the repeller and the cavity is chosen such that the repeller electron bunches will reach the cavity at proper time to be in synchronization.

Due to this, they deliver energy to the cavity, the result is the oscillation at the cavity producing RF frequency.

Wiring with a repeller klystron


Performance Characteristics

1. Frequency: 4 – 200 GHz2. Power: 1 mW – 2.5 W 3. Theoretical efficiency : 22.78 %4. Practical efficiency : 10 % - 20 %5. Tuning range : 5 GHz at 2 W – 30 GHz at 10

mW


Applications

The reflex klystrons are used in1. Radar receivers2. Local oscillator in microwave receivers3. Signal source in microwave generator of

variable frequency4. Portable microwave links5. Pump oscillator in parametric amplifier


Traveling Wave Tube

Traveling Wave Tube (TWT) is the most versatile microwave RF power amplifiers.

The main virtue of the TWT is its extremely wide band width of operation.


Basic structure of a Traveling Wave Tube (TWT) The basic structure of a TWT consists of a cathode and filament heater

plus an anode that is biased positively to accelerate the electron beam forward and to focus it into a narrow beam.

The electrons are attracted by a positive plate called the collector, which has given a high dc voltage.

The length of the tube is usually many wavelengths at the operating frequency.

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


Features The unique feature of the TWT is a 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.

The microwave signal to be amplified is applied to the end of the helix near the cathode and the output is taken from the end of the helix near the collector.

The purpose of the helix is to provide path for RF signal.

The propagation of the RF signal along the helix is made approximately equal to the velocity of the electron beam from the cathode to the collector


Functioning The passage of the microwave signal down the helix The passage of the microwave signal down the helix

produces electric and magnetic fields that will interact with the electron beam.

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.

Density modulation causes bunches of electrons to group together one wavelength apart and. these bunch of electrons travel down the length of the tube toward the collector.

Simulation performed with the self-consistent particle in cell (PIC) solver of CST PARTICLE STUDIO(fig)


Functioning The electron bunches induce

voltages into the helix which reinforce the voltage already present there. Due to that the strength of the electromagnetic field on the helix increases as the wave travels down the tube towards the collector.

At the end of the helix, the signal is considerably amplified. Coaxial cable or waveguide structures are used to extract the energy from the helix.

Simulated Output signals (fig)ADDIS ABABA INSTITUTE OF TECNOLOGY

'Theory and application of Microwave tube'7/19/2013 28

Advantages

1. TWT has extremely wide bandwidth. Hence, it can be made to amplify signals from UHF to hundreds of gigahertz.

2. Most of the TWT’s have a frequency range of approximately 2:1 in the desired segment of the microwave region to be amplified.

3. The TWT’s can be used in both continuous and pulsed modes of operation with power levels up to several thousands watts.


Performance characteristics1. Frequency of operation : 0.5 GHz – 95 GHz2. Power outputs:

5 mW (10 – 40 GHz – low power TWT)250 kW (CW) at 3 GHz (high power TWT)

10 MW (pulsed) at 3 GHz3. Efficiency : 5 – 20 % ( 30 % with depressed

collector)


Applications of TWT1. Low noise RF amplifier in broad band microwave receivers.2. Repeater amplifier in wide band communication links and

long distance telephony.3. Due to long tube life (50,000 hours against ¼th for other

types), TWT is power output tube in communication satellite.

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

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


Magnetron Is self-contained microwave oscillator that Is self-contained microwave oscillator that

operates differently from the linear-beam tubes.

Fig :-Major elements in the Magnetron oscillatorADDIS ABABA INSTITUTE OF TECNOLOGY


Construction Construction Each cavity in the

anode acts as an inductor and the interaction space acts as a capacitor.

These two form a parallel resonant circuit and its resonant frequency depends on the value of L of the cavity and the C of the slot.

The frequency of the microwaves generated by the magnetron oscillator depends on the frequency of the RF oscillations existing in the resonant cavities

Anode assemblyThis image cannot currently be displayed.


Cavity Magnetrons


Magnetron Cavity Resonator


Forms of Cavity

a) slot- type b) vane- type c) Rising sun- type d) Hole-and-slot- type


Basic Magnetron Operation Operation of Magnetron can be subdivided into four phases

• Production and acceleration of an electron beam in a dc field phase 1

• Velocity-modulation of the electron beam phase 2• Formation of electron bunches by velocity modulation

(here in form of a “Space-Charge Wheel”) phase 3• Dispense energy to the ac field Phase 4


Phase 1: Production and acceleration of an electron beam

Magnetron is a cross field device as the electric field between the anode and the cathode is radial whereas the magnetic field produced by a permanent magnet is axial.

A high DC potential can be applied between the cathode and anode which produces the radial electric field.

Depending on the relative strengths of the electric and magnetic fields, the electrons emitted from the cathode and moving towards the anode will traverse through the interaction space

In the absence of magnetic field ( B = 0), the electron travel straight from the cathode to the anode due to the radial electric field force acting on it (the blue path )

The electron path under the influence of different strength of the magnetic field


Phase1: Production and acceleration of an electron beam (cont’d)

If the magnetic field strength is increased slightly, due to lateral force the electron path bends(green , yellow path).

If the strength of the magnetic field is made sufficiently high then the electrons can be prevented from reaching the anode (red path).

The magnetic field required to return electrons back to the cathode just grazing the surface of the anode is called the critical magnetic field ( Bc ) or the cut off magnetic field.

If the magnetic field is larger than the critical field (B > Bc ), the electron experiences a greater rotational force and may return back to the cathode quite faster

The electron path under the influence of different strength of the magnetic field


Phase 2: Velocity-modulation of the electron beam

The electric field in the magnetron oscillator is a product of ac and dc fields

The dc field extends radially from adjacent anode segments to the cathode

The ac fields, extending between adjacent segments, are shown at an instant of maximum magnitude of one alternation of the rf oscillations occurring in the cavities

The ac field of each individual cavity increases or decreases the dc field like shown in the figure.

the electrons which fly toward the anode segments loaded at the moment more positively are accelerated in addition. These get a higher tangential speed.

On the other hand the electrons which fly toward the segments loaded at the moment more negatively are slow down. These get consequently a smaller tangential speed.

The high-frequency electrical field


Phase 3 : Forming of a “Space-Charge Wheel”

On reason the different speeds of the electron groups a velocity modulation appears

The cumulative action of many electrons returning to the cathode while others are moving toward the anode forms a pattern resembling the moving spokes of a wheel known as a “Space-Charge Wheel”, as indicated in Figure

One of the spokes just is near an anode segment which is loaded a little more negatively. The electrons are slowed down and pass her energy on to the ac field

At any particular instant, one set of alternate poles goes positive and the remaining set of alternate poles goes negative due to the RF oscillations in the cavities.

As the electron approaches the anode, one set of alternate poles accelerates the electrons and turns back the electrons quickly to the cathode and the other set alternate poles retard the electrons, thereby transferring the energy from electrons to the RF signal.

Rotating space-charge wheel in twelve-cavity magnetron


Phase 4: Dispense energy to the ac field

Recall that an electron moving Recall that an electron moving against an E field is accelerated by the field and takes energy from the field.

Also, an electron dispenses energy to a field and slows down if it is moving in the same direction as the field (positive to negative).

The electron spends energy to each cavity as it passes and eventually reaches the anode when its energy is expended

Thus, the electron has helped sustain oscillations because it has taken energy from the dc field and given it to the ac field

Path of a single electron under influence of the electric RF-field


Cross sectional view of the anode assembly


Performance Characteristics

1. Power output: In excess of 250 kW ( Pulsed Mode), 10 mW (UHF band), 2 mW (X band), 8 kW (at 95 GHz)

2. Frequency: 500 MHz – 12 GHz3. Duty cycle: 0.1 %4. Efficiency: 40 % - 70 %


Applications of Magnetron

1. Pulsed radar is the single most important application with large pulse powers.

2. Voltage tunable magnetrons are used in sweep oscillators in telemetry and in missile applications.

3. Fixed frequency, CW (continuous-wave)magnetrons are used for industrial heating and microwave ovens.


Reference

Microwave Engineering, second editionby David Microwave Engineering, second editionby David M Pozar, University of Massachusetts at Amherst.

Microwave Engineering, with wireless applications by S.R.Pennock and P.R.Shepherd(University of Bath)

An introduction to guided waves and microwave circuits by Robert S Elliott UCLA

Foundations for Microwave Engineering, second edition by Robert E Collin, Case Western Reserve University.

And other small papers from internet ADDIS ABABA INSTITUTE OF TECNOLOGY


Date post:	01-Dec-2015
Category:	Documents
Upload:	henok-melese-keraga
View:	22 times
Download:	1 times

Microwave Tubes

Documents