Special Purpose Ammonia Frequency Standard a Feasibility Study

We have investigated the feasibility of a special purpose frequency standard based on microwave absorption in ammonia gas (N15H3). Such a device would potentially fill a need in certain communications and navigation applications for an oscillator which has medium stability, greater accuracy (~10-9) than that provided by crystal oscillators, but a cost significantly smaller than that of more sophisticated atomic frequency standards. A device was constructed using a stripline oscillator near 0.5 GHz whose multiplied output was frequency-locked to the absorption of the 3-3 line in N15H3 (~22.8 GHz). Output between 5 and 10 MHz was provided by direct division from the 0.5-GHz oscillator. Observed stability was 2 x 10-10 from 10 to 6000 s, and reproducibility (accuracy) is estimated to be ±2 x 10-9. The unique features of this device, which include 1) high-performance stripline oscillator, 2) digital servo techniques, 3) unique oscillator-cavity servo, 4) pressure shift compensation scheme, and 5) acceleration insensitivity, are discussed. Areas for further study are noted.