Experiment No. STUDY OF THE CHARACTERISTICS OF THE KLYSTRON KLYSTRON TUBE OBJECTIVE
To study the characteristics of the reflex Klystron tube and to determine the its electronic tuning range. EQUI!ENTS
Klystron power supply, Klystron with mount, Isolator, Frequency meter, Variable attenuator, detector, B!"to"B! cable and #scilloscope. BLOCK DIA"RA!
THEORY
$eflex Klystron is one of the most commonly used microwa%e &low power' generators. It con%e con%ert rtss (.!. (.!. power power into into micr microw owa%e a%e power power.. The The refl reflex ex Klys Klystr tron on ma)e ma)ess use use of %elo %eloci city ty modulation to transform a continuous electron beam in to microwa%e power. Reflex Klystrons Klystrons Oscillator: Oscillator:
The schematic diagram of a reflex )lystron tube is shown in Fig.*, which uses only a single re" entrant entrantss microw microwa%e a%e ca%ity ca%ity as resonat resonator or.. The electr electron on beam emitte emitted d from from the cathode cathode K is accelerated by the grid + and passes through the ca%ity anode to the repeller space between the ca%ity anode and the repeller electrode.
Mechanism of Oscillation:
(ue to dc %oltage in the ca%ity circuit, $F noise is generated in the ca%ity. This electromagnetic noise field in the ca%ity becomes pronounced at ca%ity resonant frequency. The electrons passing through the ca%ity gap d experience this $F field and are %elocity modulated in the following manner. The electrons as shown in Fig. -. which encountered the positi%e half cycle of the $F field in the gap d will be accelerated, those &reference electrons' b which encountered /ero $F field will pass with unchanged original %elocity, and the electrons c which encountered the negati%e half cycle will be retarded on entering the repeller space.
ll these %elocity modulated electrons will be repelled bac) to the ca%ity by the repeller due to its negati%e potential. The repeller distance 0 and the %oltages can be ad1usted to recei%e all the %elocity modulated electrons at a same time on the positi%e pea) of the ca%ity $F %oltage cycle. Thus the %elocity modulated electrons are bunched together and lose their )inetic energy when they encounter the positi%e cycle of the ca%ity $F field. This loss of energy is thus transferred to the ca%ity to conser%e the total power. If the power deli%ered by the bunched electrons to the ca%ity is greater than the power loss in the ca%ity, the electromagnetic field amplitude at the resonant frequency of the ca%ity will increase to produce microwa%e oscillations. The $F power is coupled to the output load by means of a small loop which forms the center conductor of the coaxial line. 2hen the power deli%ered by the electrons becomes equal to the total power loss in the ca%ity system, a steady microwa%e oscillation is generated at resonant frequency of the ca%ity. Mode of Oscillation: The bunched electrons in a reflex )lystron can deli%er maximum power to the ca% ity at any instant which corresponds to the positi%e pea) o f the $F cycle of the ca%ity oscillation. If T is the time period at the resonant frequency, to is the time ta)en by the reference electron to tra%el in the repeller space between entering the repeller space at b and the returning to the ca%ity at positi%e pea) %oltage on formatting of the bunch, then Thus by ad1usting repeller %oltage for gi%en dimensions of the reflex )lystron, the bunching can be made to
occur at , ,3 ,* 3 * etc. for modes n 4 5, *, , 3, 6, respecti%ely. It is ob%ious that the lowest order N mode 37- occurs for a maximum %alue of repeller %oltage when the transit time to of the electrons in the repeller space is minimum. 8igher modes occur at lower repeller %oltages. 9ince at the highest repeller %oltage the acceleration of the bunched electrons of return is maximum, the power o utput of the lowest mode is maximum. =
Modulation:
By %arying the reflector %oltage about a d.c. %alue, Klystron can be frequency and amplitude modulated simultaneously. For proper square wa%e modulation with *55: modulation index, the reflector %oltage and amplitude of the square wa%e should be set as shown in Fig. -.3. If the square wa%e pea) to pea) amplitude is Vm and Vo is the reflector d.c. %oltage, the total reflector %oltage will switch between &Vo;Vm' and &Vo" Vm'. 2e ha%e to choose Vo and Vm such that &Vo;Vm' is in the mode center and &V o"Vm' is the non" oscillating region for proper square wa%e modulation.
