592 



THE GENERATION OF MICROWAVE POWER 



density-modulated electron beam formed in this way shows that it contains 

 a component of RF current at the frequency of the drive signal supplied to 

 the first cavity resonator. Therefore, when a second cavity resonator 

 similar to the first is placed along the electron beam, this RF current 

 flowing through the impedance of the resonant circuit will develop a voltage 

 across the gap or capacity of the cavity resonator. If an external load is 

 coupled to this second cavity resonator, power may be delivered to the 

 load. This power is normally greater than the drive power supplied to the 

 first cavity resonator, and the klystron therefore acts as an amplifier. For a 

 two-cavity amplifier as shown in Fig. 1 1-1 6, a power gain of 10 db is typical. 



Klystron Oscillators. If part of the energy developed in the second 

 cavity is fed back into the input cavity, the two-cavity klystron can be 

 operated as an oscillator. The energy may be fed back in any of several 

 ways. The simplest way is to cut an aperture in a common wall separating 

 the first and second cavities so that the electromagnetic energy will couple 

 through the aperture. Oscillations will only occur when the phase of the 

 signal fed back to the first cavity is such as to cause regeneration rather 

 than degeneration. The total phase delay of the feedback loop depends in 

 part upon the transit time of the electrons between the grids of the first 

 cavity and the grids of the second, and this transit time depends upon 

 the voltage between cathode and anode. Modes of oscillation are found as 

 the voltage is changed; this is illustrated in Fig. ll-18a. Fig. ll-18b shows 



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A 



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VOLTAGE 

 (a) 



FREQUENCY 

 (b) 



Fig. 11-18 Modes of Oscillation of a Two-Cavity Klystron Oscillator. 



how this mode pattern changes with operating frequency. Because the 

 operating voltage and output power change with frequency, and because of 

 the mechanical difficulty of tuning the two cavities properly for oscillation, 

 two-cavity klystron oscillators are almost never used for applications where 

 the frequency must be changed. They have found wide application for 

 low-power continuous-wave radars that are used in aircraft and missiles as 

 ground-speed indicators (see Sec. 6-5). These CW radars require that the 



