1. MICROWAVE VACUUM TUBE DEVICES

2. Why Ordinary Vacuum Tube Devices can not generate Microwave?
Higher value of inter-electrode capacitance, lead inductance and transit time limits the high frequency operation of Vacuum Tube devices.
Reducing the area of electrodes or by increasing the distance between them the inter-electrode capacitance can be reduced but at the same time transit time of the device increases, which in turn reduces the operating frequency.

3. KLYSTRON
The Klystron tube operates on the principle of velocity modulation in stead of current density modulation employed in conventional tubes. Klystron amplifier was introduced by Varian brothers in Later on Klystron oscillator was developed.

4. TWO CAVITY KLYSTRON AMPLIFIER

5. PRINCIPLE OF OPERATION
Electron beam emerges from cathode, interacts with the applied rf wave (signal) within the buncher cavity. By the process velocity of the beam modulates when it comes out the buncher cavity and forms a bunch over the drift space. Finally the bunch moves through the catcher cavity and induces energy into it. Output is taken through catcher cavity.

6. Let a beam of electrons be accelerated by a dc voltage Vdc to pass through a pair of grids to which a sinusoidal rf voltage V1Sinωt1 is applied and vo be the resultant velocity of electron, then from modern physics eVdc=(1/2)mv² where m be the mass and e be the charge of electron.

7. Or vo=√(2eVdc/m) (1) Now voltage appeared at grid 1 = Vdc and voltage at grid 2 = Vdc + V1Sinωt1 Let v1 be the velocity of the electron after leaving grid 2 and V1 be the amplitude of the rf signal voltage, then e(Vdc+V1Sinωt2)=(1/2)mv² or, v1 = √[(2eVdc /m){1+ (V1/Vdc)Sinωt1] = vo[1 + (V1/2Vdc)Sin ωt1] (2)

8. For a particular value of dc voltage (Vdc), the dc velocity (vo) of an electron is constant. Thus the present velocity of the electron beam depends on the the instantaneous value of the signal voltage. This principle is called velocity modulation, which leads to density modulation and finally to bunching.

9. Applegate diagram below explains clearly the distance –time history of an individual electron.

10. Typical operating characteristics of a two cavity Klystron Amplifier
Frequency : Up to 60 GHz
Power output : 100 KW at 60 GHz (CW)
250 KW at 10 GHz (CW)
30 MW at 3 GHz (Pulsed)
Power Gain: About 30 dB
Efficiency : About 40%

11. REFLEX KLYSTRON
Problem of adjustment of feedback path length with the change of frequency does not allow to construct a two cavity klystron amplifier. Hence reflex klystron is a single cavity system operates on the principle of velocity modulation and generates microwave oscillation.

12. Schematic block diagram of Reflex Klystron Oscillator

13. It consists of an electron gun producing a collimated beam of electron, which when passes through resonator cavity gets accelerated under a dc accelerating voltage Vdc. These accelerated beam will be repelled by the –ve voltage of the repeller and finally turns back towards the resonator grid and thus the beam gets velocity modulated and finally forms the bunch. These bunches induce energy when they comes back to the cavity.

14. Tuning Mechanical Tuning
Frequency depends on cavity dimensions. Thus either by flaring the cavity or by reducing the cavity inserting metal rods cavity dimensions can be varied.
Electronic Tuning
Here tuning can be achieved by varying repeller voltage.

15. Typical characteristics of a Reflex Klystron
Frequency : A few GHz to few hundred GHz
Power Output: 10 Mw to 2.5 MW
Repeller Voltage: - 15 to -300 volts
Efficiency : 10%

16. Advantages: Produces high output power
Advantages: Produces high output power. Operates up to a very high frequency.
Disadvantages:
Very low efficiency.
Repeller protection is very much essential.

17. Applications
(i) As a local oscillator in microwave receiver.
(ii) As a microwave signal source
(iii) Pump osc. For parametric amplifier.
(iv) As an oscillator, in frequency modulation of low power microwave link.

18. MAGNETRON
Invented by Hull in 1921.The basic structure of a magnetron is a number of identical resonators arranged in a cylindrical pattern around a cylindrical cathode. Here an electric field is applied perpendicular to the axis of the cathode and a magnetic field parallel to the axis of the cathode. The space between cathode and anode is the interaction space.

19. CUT AWAY VIEW OF MAGNETRON

20. It is thus a crossed field device
It is thus a crossed field device. Under zero magnetic field the electrons emitted by the cathode moves radially in the outward direction towards anode. But the applied magnetic field causes the electrons to move in a curve path within the interaction space. For a critical magnetic field the electrons will describe arcs and grazes the surface of anode and bend back forward to the cathode.

21. Field distribution of a Magnetron within interaction region is shown below where the phase difference between the adjacent cavities is π-radians, hence it is called π-mode of operation of Magnetron.

22. EXPRESSION FOR CRITICAL MAGNETIC FIELD
The force exerted by the magnetic field is given by the relation
F = -(v×B)e
Under the magnetic field electron will rotate in a circular path and at any point this centrifugal force of electron will be balanced by the force exerted by magnetic field.
mv²/r = Bev

23. v = (Be/m)r …(1) From equation (1) we see that B, e, m are constants, hence v∞r and for a particular velocity B∞1/r, i.e., the radius of curvature of the trajectory decreases with the increase of magnetic field. Thus the eefcetive value of magnetic field should be such that the electron will not reach the surface of the anode but will graze it and will rotate around the interaction space between cathode and anode.

24. Magnetron consists of two concentric cylinders of radius ra and rC, where ra> rC. Anode is kept at positive dc potential Vdc with respect to cathode. In cylindrical co-ordinate system the equation of motion of an electron is given by, d dφ e dr — — (r²—) = — B —— …(2) r dt dt m dt

25. Now from equation (1) ω = v/r = Be/m Thus equation (2) becomes 1 d dφ dr — —(r² — )= ω — r dt dt dt On integrating we get dφ ωr² r² — = — + K …(3) dt 2 where K is the integrating constant.

26. Now at the surface of the cathode (i. e
Now at the surface of the cathode (i.e.at r =rc) angular velocity dφ/dt = 0. Thus equation (3) becomes ωrc² —— + K = or, K = - ωrc² /2 Substituting the value of K in equation (3), we get dφ r² — = — ω(r² - rc²) …(4) dt 2

27. If v be the velocity of electron under electric field eVdc = ½ mv² or, v² = 2eVdc/m …(5) But velocity of electron under electric field has two components – radial velocity (vr) and tangential velocity (vφ). Thus, v² = vr² + vφ² = 2eVdc/m

28. dr. dφ. 2eVdc. (—)² + (r — )² = —— …(6)
dr dφ 2eVdc (—)² + (r — )² = —— …(6) dt dt m When an electron is just grazing an anode then r = ra and dr/dt = 0. Thus equation (6) becomes to dφ 2eVdc ra² (—)² = —— dt m

29. ra (dφ/dt) = √(2eVdc/m). …(7) and rewriting equation (4). dφ. 1
ra (dφ/dt) = √(2eVdc/m) …(7) and rewriting equation (4) dφ ra² — = — ω(ra² - rc²) …(8) dt Comparing equation (7) and (8), ω eVdc — — (ra²- rc²)= √(——) ra m or, Be rc² eVdc — — ra (1- —) = √(——) m ra² m

30. The value of the cut-off magnetic field for the operation of a Magnetron is Bo = (1/ra)√(8Vdcm/e)[1/{1- (rc/ra)²}] This means that the applied magnetic field B should be slightly greater than Bo, then for a given Vdc the electron will not reach the anode but will just graze the surface of the anode i.e., the anode current is zero.

31. If there are N number of cavities the phase shift between the adjacent cavities is Ø = 2πn/N where n is an integer. Thus for π mode of operation number of cavities N = 2n i.e even.
The mechanism by which the electron bunching and synchronization with the rf field takes place is called phase focusing.

32. The phase focusing effect is illustrated below

33. An electron at ‘A’ is in the vicinity of the positive anode and the component of the rf field aids the radial dc field, which increases the velocity of electron. Whereas the electron at ‘B’ experiences the equal amount of field due to its position and thus velocity remains unchanged. The electron at ‘C’ nearer to the negative anode and experiences a lower relative magnitude of dc field and thus its velocity decreases.

34. This causes a bunching action around the electron whose relative position is indicated as B. The selective grouping of electrons results in a spoke shaped space charge in the cloud of electrons spinning around the cathode and induces energy into the cavities. The rf power is coupled out from any one of the cavities by using a co-axial loop.

35. Mode Jumping in Magnetron
If the frequencies of the different modes of operation are far apart, the magnetron has a tendency of mode jumping during the operation. It can be overcome by using strapping where two ring are connected only to alternate anode poles. At π-mode each ring is at uniform potential but of opposite polarity and thus present a capacitive loading to the cavities, which lowers the frequency of this mode. For other modes each ring is not at an uniform potential so that current flows in the rings. This places an inductive loading in the cavities to raise the frequency for other modes.

36. Strapped Magnetron

37. Efficiency is quite high.
MAGNETRON
Advantages:
Efficiency is quite high.
Improvement of performance using modified structures.
Disadvantages:
Mode jumping.
Requires a critical magnetic field to start the operation.

38. APPLICATIONS OF MAGNETRON
Mostly used in a transmitter of a radar system.
Used as heating source of a microwave oven.

39. TRAVELLING WAVE TUBES (TWTs)|
|¯¯¯¯¯¯¯¯¯¯¯¯¯|
Forward Wave Backward Wave
Amplifier (FWA) Oscillator (BWO)
| |
|¯¯¯¯¯¯| |¯¯¯¯¯¯¯|
O-type M-type M-type O-type
|¯¯¯¯¯¯¯¯|
Linear M Circular M
Carcinotron Carcinotron

40. M-TYPE TWTA

41. PRINCIPLE OF OPERATION
Electrons emitted by the cathode will initially moves towards anode but the applied magnetic field deflects electron towards collector. Sole does not allow the beam to move further in the downward direction. In M-type TWT electron gains energy from both magnetic as well as electric field.
RF field when moves through the slow wave structure its velocity reduces and when it reacts with electron beam a prolong interaction takes place and by the process the energetic electron beam transfers energy to the rf field.

42. Characteristics of M-type TWTA
Frequency Range: 1.2 to 1.5 GHz
Gain in dB: 10 to 20
Power Output in Watt: 200 to 400
Efficiency: 30 to 40%

43. O-TYPE TWTA

44. Helical type slow wave structure is used between the input and output port, through which the collimated beam of energetic electron moves toward collector and as a result a prolong interaction takes place between the rf field of the slow wave structure and the electron beam.

45. Bunching of Electrons of O-type TWT

46. There are many rf cycles present in the length of the tube
There are many rf cycles present in the length of the tube. Where the rf voltages are positive, nearby electrons will be accelerated and those where rf voltages are negative, nearby electrons were slowed down and electrons velocity remains unchanged for the zero values of the rf field. Thus the velocity of electron beams will be modulated. This velocity modulation results density modulation, which finally leads to bunching of the electron beam.

47. During their motion towards the collector the bunches of electron induces a voltage in the helix by their charges and this induced voltage adds up with the existing voltage of the e.m. wave – which results in an amplification of e.m. wave. Since the interaction takes place along the whole length of the tube, thus the gain of TWTA is proportional to its length.

48. One major problem of TWTA is that due to mismatch of output port reflection results, so a part of the forward wave will come back towards the input and results unwanted oscillation. A graphite is placed to attenuate the reflected signal and thus prevents oscillation but at the same time reduces the gain of the tube.

49. Characteristics of O-type TWTA
Frequency Range: 0.1 to 10 GHz
Power Gain : 20 to 40 dB.
Efficiency: 10 to 40%
Power Output: 1000W (CW)
100 KW (Pulsed)

50. APPLICATION OF TWTs In CW radar and for radar jamming.
At the final stage of a satellite transponder.
As a repeater amplifier in wide band communication link.
As a sweep generator.

51. SLOW WAVE STRUCTURES
Slow wave structures are special type of circuits, used in the microwave tubes to reduce the wave velocity in a certain direction so that a prolong interaction between the electron beam and the signal may take place.

52. DIFF. SLOW WAVE STRUCTURES

53. TRAVELLING WAVE TUBES Advantages: High gain amplifier. Disadvantages:
Long length for higher gain.
Operates at very lower microwave frequency range.

54. BACKWARD WAVE OSDILLATOR (BWO)

55. Here the output port is terminated by rf termination and thus the reflected wave is superimposed on the incident in phase results positive feedback and thus produces oscillation. The generated rf oscillation is then taken out through the input end of the tube i.e., the output terminal is near the gun end of the tube.

56. In linear M-carcinotron the structure is kept straight, i. e
In linear M-carcinotron the structure is kept straight, i.e. slow wave structure along with the sole are kept straight between cathode and collector. To increase the effective length of the tube for higher gain the structure can bend to form a circle and then it is called circular M-carcinotron. Again M-type uses magnetic field for electron deflection but O-type do not require any magnetic field.

57. APPLICATION OF BWO
As a sweep generator, since the frequency of oscillation varies widely.
Can be used for amplitude modulation.
Can be used as a voltage tunable band pass amplifier.

58. COMPARISON BETWEEN DIFFERENT MICROWAVE VACUUM TUBE SOURCES
Parameters Klystron Magnetron TWTs
Frequency Few GHz 1 – 25 GHz 1 to10 GHz
to hundred
GHz
Output MW Several KW Order of KW
Power
Efficiency % 30 – 60 % %
Uses Oscillator Oscillator Oscillator
& &
Amplifier Amplifier

59. Thank You sandip ruidas bengal engineering & science university etc