Two cavity Klystron Amplifier

 Klystron Amplifier

The klystron amplifier can be used as an microwave oscillator or amplifier at low and high frequencies.
There are two kinds of klystron amplifier

1. Multicavity Klystron: used as a low power microwave amplifier
2. Reflex klystron:used as a low power microwave oscillator 

Thus basically klystron amplifiers possess less power than their counterparts magnetron and travelling wave tube.


So lets see  2-cavity klystron amplifier first............


Two Cavity Klystron Amplifier
 2-cavity klystron amplifier works on the following principles

• Velocity modulation
• Current modulation

 So lets learn what exactly is velocity modulation ???? 

 As we know in any tube amplifier we need to have electron beam produced in cathode to anode region.So electrons in electron beam are produced in the cathode to anode region and accelerated by the means of an anode voltage V0 .These electrons are allowed to pass through a pair of buncher grid across which an RF(radio frequency) voltage V1sinωt and these electrons are accelerated or de-accelerated depending on the part of cycle during which they enter gap.The accelerated electrons emerge with a velocity higher than the entering velocity v0 and de-accelerated electrons emerge with a velocity lower than v0.While some electrons pass through zero RF field and hence there is no change in their velocity. This phenomenon of the variation of electrons in electron beam is known as Velocity Modulation.

The analysis for velocity modulation is carried out with following assumptions
1.Electrons leave the cathode with velocity=0 and the beam has uniform density in cross section of beam
2.Space charge effets are not considered
3.Magnitude(V1) of input signal<<< De-accelerating voltage(V0)  

Current modulation will be seen in the working of 2-cavity klystron amplifier

Operation :

Two cavity Klystron Amplifier

  

The electron beam is created with the help of the cathode.This electron beam attains a high velocity due to the accelerating anode.This beam passes through the buncher cavity.This beam then passes through the drift space having length 'L' and finally through the catcher cavity(the name is referred as catcher since the output is obtained at this cavity).Finally the beam is collected by the collector(Collector electrode).
              The RF input(microwave) is given at the buncher cavity which we want to be amplified.The anode voltage V0 and buncher cavity gap having length 'd' are adjusted such that time taken by beam to pass through d is less than quarter time period of input RF signal.The beam is focussed to travel axially so that it doesn't spread by the means of applied external magnetic field.
                                                                                                                               Now when the electron beam passes through the buncher cavity during positive half cycle of RF input signal,velocity of beam increses  whereas during the negative cycle half cycle of input RF signal velcity of beam decreases . This is the concept of velocity modulation which we have see earlier. In the moving frame of the electron beam, the velocity modulation is equivalent to a plasma oscillations. Plasma oscillations are rapid oscillations of the electron density in conducting media such as plasmas or metals(The frequency only depends weakly on the wavelength). So in a quarter of one period of the plasma frequency, the velocity modulation is converted to density modulation, i.e. bunches of electrons.Now lets see this procedure with the help of Applegate diagram..........

Applegate Diagram

Thus the electron beam is velocity modulated to form bunches or undergoes density(Current modulation)  with input RF signal.This current modulation of beam produces amplification of RF signal input at the catcher cavity.Thus what we obtain finally is the amplification of RF input signal.
One important observation is that the phase of output signal is opposite to that of input signal.Also many harmonics are generated during amplification.One way to remove this harmonics is to tune the catcher cavity to the fundamental frequency or any other harmonic desired.


Analysis:
Due to potential difference V0 between anode and cathode ,the electrons form a high current  density beam with velocity u0

                                                uo=sqrt(2eV0/m)
                                                  e= charge of an electron
                                                     m= mass of electron
 The rime taken by the electron beam to cross cavity gap 'd' = Transit time

                                             Transit time= t2-t1=tg=(d/u0)
                                               Transit angle = Θg=ωtg

RF input signal is given to the buncher cavity

 Thus the average RF input in the gap of buncher cavity is 


                                                          V(av)= ∫V1sinωt.dt                 (t1<t<t2)
 The output comes out to be
                                                   V(av)=V1/ωtg[sin(ωt1+Θ(g/2))sin(ωtg/2)]
                                       where  ß=[sinΘ(g/2)/Θ(g/2)]=buncher cavity beam coupling coefficient

                                                   V(av)=V1ßsin(ωt1+Θ(g/2)

                                                                                                                                                                                                                                            
When the electron passes through the buncher cavity their velocity either increases or decreases depending on the state of RF input cycle

 Let u(av)=velocity of electron at mid of gap

                                                    u(av)/u0 =sqrt[V0+V(av)/V0]
                                         Therefore    u(av)/u0 =sqrt[1+{V1ßsin(ωt1+Θ(g/2)/V0}]

                                               where m= V1ß/V0= depth of modulation
            

                                            u(av)=u0=sqrt[1+msin(ωt1+Θ(g/2)]
Thus we see that electron in beam are velocity modulated by input RF signal with a modulation depth of [V1ß/V0].



Excitation of cavity:

It is desired that that grid spacing be as small as possible to ensure maximum coupling.To achieve this transit angle (Θg) should be kept small......which means that transit time(tg) should be kept small. Also the the Q factor of the cavity must be high.

For excitation two techniques are used 

1. Cylindrical cavity excitation by axial current
2. Cylindrical cavity excitation by velocity modulated beam 

Excitation by axial current 

• Axial current flows through cavity to excite it
• For coupling efficiency we have to modify the dimensions 
• Axial symmetry guarantees TM(0m0) modes  

 Excitation by velocity modulated beam 

• Velocity modulated beam excites cavity
• For coupling efficiency DC potentail and dimensions of the cavity should be modified
• Axial symmetry guarantees TM(0mn) modes  

Characteristics:

• Frequency - 250 MHz to 100 GHz
• Gain- 16 to 70 dB
• Bandwidth-10-50 Mhz
• Power- 10kW- 500kW(continuous mode) and 30MW (pulsed mode)
• Noise Figure- 15 to 20 dB
• Efficiency-60%

Applications:

• UHF TV Transmitter
• Troposphere scatter transmitter
• Communication Satellites
• Radar Transmitter
• Also as a power oscillator 

High-power klystron used for spacecraft communication at the Canberra Deep Space Communications Complex.

                        Above is an actual picture of Two cavity Klystron Amplifier used for space communication purposes....................Thus klystrons are very helpful  at microwave frequencies as amplifiers and oscillators.  In the next post we will see the variant of two cavity klystron called Reflex klystron................. 

Microwave tubes......classification and working

Lets see about different types of microwave tube amplifiers.............................
But before that letz see what are microwave tube amplifiers???


Microwave tubes may seem "old school" for college whippersnappers, but one tube can do the work of many, many solid state power amplifiers. The tube industry is alive and well, but there is not a great number of engineers in this field. If you are thinking about making a career out of tubes, this could work out, because the average age of a real tube engineer is somewhere around 90!!!
                                                                                                                                                  Microwave tubes have special additional components compared to amplifier tubes such as resonant cavities that usually can't be built from glass.The reason for this is that to create a resonant cavity we require vacuum inside the resonant cavity...and since glass can be complex shaped it cannot hold vacuum .

Why do we use microwave tubes and not the conventional low frequency tubes?


Conventional low frequency tubes like triodes fail to operate at microwave frequencies(MF) because the electron transit time from cathode to grid becomes do large that it cannot produce microwave oscillations.In order for an amplifier to work efficiently at the desired frequency the propagation times must be insignificant. And we see that conventional tubes have a significant propagation times and hence cannot be used at microwave frequencies.
                                                                                   The device parameters for this tubes starts taking a  dominating part in circuit and hence successful oscillations aren't met.There are also other limitations attched to them:

• Interelectrode capacitance
• Dielectric losses
• Lead inductance effect
• Effects due to radiation losses and radio frequency(RF) losses
• Skin effect(which is is the tendency of an alternating current (AC) to become distributed within a conductor such that the current density is largest near the surface of the conductor, and decreases with greater depths in the conductor)
• Gain-bandwidth limitations     

                                  CLASSIFICATION



                          Microwave tubes can be broadly classifies into two categories

                  1.O-TYPE Linear Tubes (Travelling tube amplifiers,Klystrons)

In O-Type tube , a magnetic field whose axis coincides with the electron beam is used to hold the beam togetheras it travels the length of the tube

                  2.M-TYPE Tubes (Magnetrons and cross field devices) 

 

This is just a rough classification of the microwave tubes...................
Basically there are only main two types of microwave tubes

1. Tubes with electromagnetic cavity(klystrons and magnetrons)
2. Tubes with slow wave circuits(traveling wave tubes)

We will see the tubes in detail in the next post...............

Comparison between C -Band and K-band

Let's see the comparison between C- Band and Ku-Band.........which are the most important frequency bands used for satellite communication.............



C-Band

Although the equipment required for C-band is in inexpensive than Ku-band,less number of satellites in the C-band can be  accommodated in the geo-stationary orbit....which is the most useful orbit available.The reason for less number of satellites is that the spacing that can be obtained for C-band satellites in geo-stationary orbit is more(2,2.5 and 3.5 degrees) since the beams are wider.

                                                                                                                                               One more disadvantage attached to C-band is that since the frequencies involved in C -band are less than K-band it involves the use of large and expensive satellite antenna (typically about 15 meters in diameter) .Also the C-band is used by terrestrial microwave systems ,hence earth stations(ES) must be located from them.

                                                                                                                                          The most important advantage of C-band is that they are immune to atmospheric attenuation.Rainfall and cosmic noise don't appreciably attenuate C-band signals. Also ionospheric scintillations are small compared to Ku-band.Hence they are mostly used for television broadcasting purposes.

C-Band Radar Dish antenna

                          

Ku-Band

The advantages involve with Ku-band is far greater than C-band.Since the frequencies involved are high smaller antennas are required (typically 2 metres) and hence are cheap to install in buildings and towers.Due to high frequency the signals tend to be narrower and hence donot spread out much.The energy in the beam is concentrated and directed and hence receiving signals is comparitvely easier than C-band.
                      Multiple beams could use same frequency on different points on earth in the form of space division multiplexing (SDM) which is a channel access method based on creating parallel spatial pipes next to higher capacity pipes through spatial multiplexing , by which it is able to offer superior performance in radio multiple access communication systems.SDM can be implemented using phased array techniques. Hence beams can crisscross each other without interference.Because the beams are narrower large number of satellites in Ku-band can be accommodated in the geo-stationary orbit.
                                 The uplink frequencies(14-14.5 GHz) are not used by the terrestrial systems but the downlink frequencies (11.7-12.2GHz) are used by them.Hence uplink transmissions don't observe interference but downlink transmission do.The most important disadvantage is the it suffers more atmospheric attenuation than C-band. It suffers absorption by clouds,rain and atmospheric depolarisation(change in the polarisation of the desired signal) and signal scintillations.So we require high gain both in transmitting and receiving equipments.
                                                                                          One practical application of Ku-band is the Direct to Home (DTH) satellite broadcasting.This is solely because we require smaller antennas appropriate for home users. Also by increasing FM deviation of  video carrier in satellite uplink we can obtain improved FM carrier to noise ratio (CNR) .Greater FM deviation means wider FM bandwidth signals which can be converted into equivalent AM signals.Thus the installalations cost involved in Ku-band are greatly reduced.

Ku-Band DTH dish antenna

                     



Now the question arises which one is better ?


Both the C-band and Ku-band deliver high quality transmissions .So basically it depends on the choice of the interested frequency band and the location where the connection is required.For example in the Middle East Ku-band is efficient to use.


Upcoming Ka-Band?

Ka-band(27-40 GHz) is the newest band in satellite communication which offers high bandwidth communication.It is currently used for vehicle speed detection by law enforcement. It is also used by the Kepler which is a is a space observatory launched by NASA to discover earth like planets orbiting other stars.It basically downlinks the scientific data collected at the telescope. The main disadvantage of Ka-band is that it more signal attenuation in rainy conditions.However it is going to be used by upcoming Iridium Next satellite series.


                                    

Satellite Communication -----General Principle

Satellite Communication:

Now what is basically satellite communication?
But before that you  must know what is communication satellite?

Communication satellite is nothing but a satellite placed in an orbit around the earth the earth that carries aboard some communication equipments ,thus enabling communication to be established between different points on earth

Basic Satellite system

The basic satellite system consists of satellites and earth stations(ES). The user is  connected with the earth station through a terrestrial or dedicated link. The base band signals from the user are processed at the earth station. The earth station modulates signal on the carrier frequency (in GHz range) and this modulated signal is amplified in a High Power Amlpifier (HPA) and radiated into space using narrow beamwidth antennas (like paraboloid reflector or Cassegrain ).
                                                                                                                The satellite receives the modulated signals through a high gain antenna.The received signal is amplified using a Low Noise Amplifier(LNA) and retransmitted to the destination earth station using the same antenna.
The path from ES to satellite is called uplink. And the path from satellite to ES is called downlink.

Now one might wonder that if the satellite is receiving and transmitting through the same antenna ,then is the satellite using same frequency for uplink and downlink?


The answer is NO. The reason why uplink and downlink frequency are not same is related to satellite trans-receivers.

• Trying to receive and retransmit an amplified version of the same uplink waveform at same satellite will cause unwanted feedback or ring around from the downlink waveform back into receiver.
• The satellite generates a signal that would jam its own receive.

Morever the uplink frequency is kept higher than downlink frequency.
There are basic two reasons for this:

• At high frequency attenuation is more and so power required will also be more .And this calls for the use of HPA with excellent ratings and good heat sinks.This will definitely increase the weight of equipment. For earth station an increase in weight will not make any significant difference but for an ES it will surely do!!
• ES must direct the beam towards satellite with high directionality and little beam spillover(losses) . Since beamwidth is inversely proportional to frequency.Hence if uplink frequency is kept high we would get high directive antennas

FREQUENCY BANDS used in Satellite Communication:

1. C-Band : Uplink (5.925-6.425 GHz), Downlink (3.7-4.2 GHz), Bandwidth (0.5 GHz).                It is basically used for television broadcast.       
2. Ku- Band: Uplink (14-14.5 GHz), Downlink (11.7-12.2 GHz), Bandwidth (0.5 GHz). It is used for television broadcast or fixed point services. Also used for non military applications.
3. Ka-Band: Uplink (27-30 GHz) or (30-31 GHz), Downlink (17-20 GHz) or (20 or 21GHz), Bandwidth (3 or 1 GHz). It is mostly uded for commercial broadcasting or military use.