Long-Term Frequency Stability for a Reflex Klystron without the Use of External Cavities

The reflex klystron is widely used as a tunable low-power oscillator in frequency-modulated microwave radio relay systems, both as the transmitter and as the receiver local oscillator. Automatic frequency control is generally applied to the transmitting klystron to maintain the output frequency within acceptable limits. The frequency control is necessary 945 2 T H E B E L L SYSTEM T E C H N I C A L J O U R N A L , MAY 19(52 because the klystron frequency is dependent on the operating voltages and the ambient temperature. The use of a high-Q stabilizing cavity in the output is not generally feasible since modulation is applied to the repeller of the transmitter klystron to obtain frequency modulation of the output. With well regulated power supplies, most of the frequency error is due to changes in the ambient temperature. This means that automatic frequency control mainly corrects for frequency errors due to ambient temperature changes. Automatic frequency control requires a discriminator, coupled to the output of the system, which will develop an error signal when the frequency drifts away from the desired value. The discriminator usually consists of a high-Q stable reference cavity with associated networks to develop the proper error signal. For electronic automatic frequency control, the error signal is usually used to change the repeller voltage of the transmitting klystron in the direction and magnitude necessary to correct the frequency error. Changing the level of the repeller voltage affects the operating characteristics of the klystron, particularly the modulation linearity.