tube device

8/10/2019 Tube Device

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GENERATION OF MICROWAVE

SIGNAL Microwave Tubesklystron, reflex

klystron, magnetron and TWT.

Diode semiconductorTunnel, Gunn,

Impatt, Varactor diodes, PIN, LSA,

Schottky barrier diode.


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CROSSED-FIELD AND LINEAR-BEAMTUBES

Klystrons and Traveling-Wave tubes are

examples of linear-beam tubes These have a focused electron beam (as

in a CRT)

Magnetron is one of a number of crossed-

field tubes

Magnetic and electric fields are at right

angles


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Introduction

Microwave tubes are constructed to overcome thelimitations with conventional tubes and UHF tubes.

Principle of Operation:-

The basic principle of operation of microwave tubeinvolves transfer of power from a source of DC

voltage to source of AC voltage by means of a current

density modulated electron beam.

The same can be achieved by accelerating electrons in

a static electric field and retarding them in an AC field.


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Contd.. The density modulation of the electron beam allows

more electrons to be retarded by an AC field than

accelerated by DC field, which therefore makes possiblea net energy be delivered to the ac electric field.

Classification of microwave tubes:-

The classification is based on different factors. Their mechanism of producing density modulation.

The acceleration and retardation of electrons in the ac field.

Important types are:-

Klystron amplifier (two cavity,multy cavity)

Klystron oscillator (reflex klystron)

Magnetron

Traveling Wave Tube(TWT)


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Microwave Tubes

1 MW

1 KW

1W

1mW

0.3 1 3 10 30 100

300 Frequency (GHz)

Averag

epower

Microwave

tubes

Microwave

semiconductor

devices

Lower weight

Smaller size

Longer life time

Higher power

Limited life time

High vacuum

High potential


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Two possible methods of achieving high

output power in microwave system

Low power

semiconductoroscillator

High power tube

amplifier

High power

tube

oscillator


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Important Parameters Peak power Average power

Efficiency Gain

Bandwidth Frequency

Harmonic and spurious power Intermodulation products

Manufacturability at low cost

Relativecomplexity

of operation

Relativeoperating

voltage

Relativespurious

level

Gain

(dB)(%)

Relative

BW (%)Type

1Low26-1520-501-10Gridded tube

2High140-6030-701-5Klystron

3High330-5020-4030-120Helix tube

3High330-5020-405-40Coupled

cavity tube


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10 MW

1 MW

100 KW

10 KW

1 KW

100 W

0.3 1 3 10 30 100

300 Frequency (GHz)

Average power

Klystron

Coupled

cavity

TWT

Helix

TWT

Gridded tube

1000 MW

100 MW

10 MW

1 MW

100 KW

10 K W

0.3 1 3 10 30 100

300Frequency (GHz)

Peak power

Coupledcavity

TWT

Klystron

Gridde

d tube

Helix

TWT


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Conventional Tubes Conventional Device tubes cannot be

used for frequencies above 100MHz 1. Interelectrode capacitance

2. Lead Inductance effect

3. Transit time effect 4. Gain Bandwidth limitation

5. Effect of RF losses (Conductance,

dielectric) 6. Effect due to radiation losses


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Efficient Microwave tubes usually operate

on the theory of electron velocitymodulation concept

The electron transit time is used in the

conversion of dc power to RF power


13/73PH0101 Unit 2 Lecture 6 13

Klystron Tubes

A klystron is a vacuum tube that can beused either as a generator or as anamplifier of power, at microwavefrequencies.
http://en.wikipedia.org/wiki/Image:Klystron.enp.gif


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KLystron

Used in high-power amplifiers

Electron beam moves down tube past

several cavities.

Input cavity is the buncher, outputcavity is the catcher.

Buncher modulates the velocity of the

electron beam

KLYSTRON


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KLYSTRON

There are two basic configurations of klystron

tubes

1. Reflex Klystron used as a low-power

Microwave oscillator

2. Multi cavity klystronused as low-power

microwave amplifier


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The major element are;

An electron gun to form and accelerate a beam of electrons

A focusing magnet to focus the beam of electrons through thecavities

Microwave cavities where the electron beam power isconverted to microwave power

A collector to collect the electron beam after the microwavepower has been generated

A microwave input where the microwave signal to beamplified is introduced into the klystron

A microwave output where the amplified microwave power istaken out


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Principle

Velocity modulated tube

High velocity electron beam is generatedby 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 theelectrons, while the output is taken from

the tube via resonant cavities with the aid

of coupling loops


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Two grids of the buncher cavity are

separated by a small gap A while the twogrids of the catcher cavity are separated by

a small gap B.


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REENTRANT CAVITIES

Cavities with metallic boundariesextended into the interior of the cavity.

Coaxial cavity support infinite number

of resonant frequencies.


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Reentrant cavities

M lti it Kl t


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Multicavity Klystron


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PH0101 Unit 2 Lecture 6 22

Applications

As power output tubes

1. in UHF TV transmitters

2. in troposphere scatter transmitters3. satellite communication ground station

4. radar transmitters

As power oscillator(550 GHz), if usedas a klystron oscillator

T C i Kl O ill


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Two Cavity Klystron Oscillator

Klystron amplifier can be converted into oscillator by

feeding back a part of catcher output to into the buncher inproper phase.

But the feed backing should be satisfy Barkhausen criterion.

A= 1

The schematic is same as of amplifier expect the feedbackneed to be added.

The feedback must be adjusted to give correct polarity and

amplitude which is basically depends on cavity tuning.

The criterion for oscillation is given by

Where + /2 is the phase angle difference between buncher and catcher

cavity.

is the total phase shift between resonator and the feedback cable.


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Contd

If the value of is zero means the oscillations are in

phase.

The maximum power output is obtained at this condition.

Also when a small change in the de accelerating voltage

it cause change in the frequency since transit angle varies.

Tuning of the oscillator is done by adjusting the grid

voltage, accelerating voltage and tuning the cavities.

High frequency oscillations are obtained by controlling

the temperature of the resonators.


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Buncher The cathode controls the number of electrons in the electron

beam and focuses the beam. The voltage between thecathode and the cavity resonators (the buncher and the

catcher, which serve as reservoirs of electromagnetic

oscillations) is the accelerating potential and is commonly

referred to as the beam voltage. This voltage accelerates theDC electron beam to a high.


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KLYSTRON

A klystron is a specialized linear-beam vacuum tube,

invented in 1937 by American electrical

engineers Russel and Sigurd Varian, which is used as

an amplifier for high radio frequencies, from UHF up

into the microwave range or generator of power at mwfrequencies.


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Two Cavity Klystron Amplifier

Two cavity klystron amplifier is basically a

velocity modulated tube.Construction:-

Here a high velocity electron beam is formed , focusedand sent down along a glass tube through an input cavity(buncher ), a field free drift space and an output cavity(catcher ) to a collector electrode.

The anode is kept positive potential with respect tocathode.

The electron beam is passed through the gap A .

The input and output is taken from the tube via resonantcavities with the help of coupling loops.


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Two cavity klystron amplifier


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OPERATION Operation can be best explained by the help of apple

gate diagram


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Contd

Initial stage

At point Bon the input RF cycle, the alternating voltage is

zero and going to be positive.

At this instant, the electric field across gap A is zero and an

electron which passes through the gap A is unaffected by the

RF signal.

Let this electron is called the reference electron eR and travels

with an unaffected velocity v0 = . Where V is the anode to cathode voltage


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ContdSecond stage

At point C of the input RF cycle again we are passingelectron called late electron el as compared to the referenceelectron.

the late electron is subjected to maximum positive RF

voltage and hence travel towards gap B with an increasevelocity than V0 (V>V0 ) , and these electron overtakesreference electron.

Similarly an early electron ee that passes thorough the gap A

slightly before the reference electron is subjected to themaximum negative field.

So these electron travels with the reduced velocity than v0.

Later these electrons eeare catch up by reference electron.


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As a result of these actions the electrons in the bunching limit

gradually bunch together as they travel down in the driftspace from gap A to B.

The drift space converts the velocity modulation into current

modulation.


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Two cavity Klystron Amplifier


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VELOCITY MODULATION

Electric field from microwaves at

buncher alternately speeds and slows

electron beam This causes electrons to bunch up

Electron bunches at catcher induce

microwaves with more energy The cavities form a slow-wave

structure


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The initial velocity of electron is

V0= electron accelerating high DC

voltage Microwave signal is applied at input, the

gap voltage at buncher becomes, V1

tube device

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