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Microwave Devices Faculty of Engineering EMG2026 OBJECTIVE: EXPERIMENT 2 CHARACTERISTICS OF REFLEX KLYSTRON 1. To identify the principle of operation of Reflex Klystron. 2. To identify the relationship between repeller voltage and Power output of Reflex Klystron. 3. To analyse the relationship between repeller voltage and frequency of the microwave signal generated by Reflex Klystron. 4. To analyse the effect of cavity size on frequency of microwave signal generated by Reflex Klystron. APPARATUS: Klystron tube, Klystron power supply, Cavity wavemeter, Isolator, Slotted line probe, SWR meter, Matched load, variable attenuator, and digital multimeter. INTRODUCTION: A Reflex Klystron is a microwave oscillator with a single cavity (resonator). The construction of a reflex klystron is shown in Figure 1(a). The basic configuration in simplified form as shown in Fig 1(b) is considered here for understanding the operation of Reflex Klystron. The various parts in Figure 1(b) are (1) Electron Gun; (2) Resonator; (3) Repeller; and (4) Output coupling. A highly focused beam of electrons passing through the resonator gap will be accelerated or decelerated by the induced current on the resonator. The alternate action of increasing and decreasing the electron velocity will modulate the electron beam into varying dense and sparse of electrons referred to as electron bunches. This electron bunching mechanism is also called intensity modulation or velocity modulation. The repeller electrode is at a negative potential. It stops the movement of the electrons, turn them around, and send them back through the resonator gap. As the repelled electrons re-enter the resonator, they give up their kinetik energy. The field excited in the resonator add in phase with the initial modulating field such that it reinforce the next wave of electron bunching. As a result, this energy serves as a regenerative feedback to sustain the oscillation at the resonant frequency of the resonator. This is the case only if the repeller voltage is set such that the travel time, t o , for the electrons to complete their travel through the gap, turn around, and back through the gap satisfy the following condition: t o ⎨ ⎧ 3 = n + T ⎩ 4⎭ ⎬⎫ n = 1,2,3,…. (1) where T is the period of the RF waveform. If the voltage between the resonator and the repeller is V r , and the distance is d, the deceleration or retardation experienced by the electron may be expressed as eVr d a = / (2) m Revision Sept 2010
Microwave Devices Faculty of Engineering EMG2026 where e is charge of electron and m mass of electron. If an electron leaves the resonator with velocity v o , the displacement of the electron from the resonator, at any time t, is 1 x o − at 2 v t 2 = (3) This equation can be used to calculate the travel time t o taken by the electron to return to the resonator. t r 2v o = = x= 0 a 2v o eV md r (4) Bunching phenomenon in Reflex Klystron Bunching phenomenon in a reflex Klystron can be visualised by studying the electron trajectories in the region between the resonator and the repeller. The parabolic trajectories given by equation (3) are shown in Figure 2. Let us consider an electron A which passes through the resonator gap with velocity v o when the RF voltage at the resonator is zero (changing from positive to negative). This electron returns to the resonator at a time t r later. Another electron B passes through the gap earlier. Because the RF voltage is positive at this instant, the initial velocity v b for electron B is higher than the initial velocity of electron A (v b > v o ). Thus electron B would travel further towards the repeller and take a longer time to return back to the resonator. It will then bunch with electron A. Similarly, an electron C that passes through the gap after A emerges with a lower initial velocity and therefore takes less time to return to the resonator. Electron C catches up with electron A and bunch with electrons A and B. The electron bunches formed as described above would deliver power to the resonator if they pass through the resonator at an instant when the field in the resonator retards the bunch. Referring to Figure 2, we note that this happens when t o = (3/4) T. In general, oscillation will occur when the condition in equation (1) is satisfied. Thus there are several values of repeller voltage V r that satisfy the oscillation condition. These values correspond to various modes of operation. A particular mode of operation may be selected by a choice of repeller voltage. Figure 3 shows the power output versus repeller voltage for various modes of operation (called the reflex Klystron mode curves or mode characteristics). The mode corresponding to n = l occurs at maximum negative repeller voltage. For n = 0, the gain mechanism is usually not strong enough to overcome system losses. Higher order modes (n > 1) may present at lower repeller voltages. Frequency versus repeller voltage variations is also shown in Figure 3. Revision Sept 2010
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