An airborne frequency generating unit for the H.F. communication band

The equipment is capable of producing an output frequency having a channel spacing of 1 kc/s in the band 3.500 to 26.499 Mc/s and a stability equal to that of a high-quality reference standard. The final output is provided by a servo-controlled variable oscillator in five subbands, the initial tuning being carried out by a motor-driven variable capacitor after which the oscillator is phase-locked to the desired frequency by comparison with four selectable reference frequencies generated from the 5 Mc/s standard. The control loop contains the necessary mixers, filters, phase discriminator and reactor for locking the oscillator and also the means of controlling the motor. The paper also describes the 5 Mc/s reference oscillator which employs a high quality BT cut crystal mounted in vacuo, operating in a fundamental series mode circuit. The use of an oven whose temperature is controlled to ~0.01 deg C permitsa day-to day stability of better than 1 part in l07 to be obtained.