Fast Autotuning of a Hydrogen Maser by Cavity Q Modulation

G.J. Dick and T.K. Tucker

Abstract: A new fast auto-tuner for the Hydrogen Ma~er has been implemented. By modulating the cavity Q, a phase shift in the maser output signal is induced which is proportional to the cavity tuning error. This phase shift is detected and fed back to a varactor tuner to stabilize the cavity against long term drifts. Cavity Q modulation has similarities to two other auto-tuning methods, and significant advantages over both of them. In comparison to line Q modulation, where the frequency sh~ft induced by a change in the atomic line Q requires a second maser for detection, the high chopping frequency allowed by cavity Q modulation gives rise to a phase shift which requires only the maser's quartz crystal "flywheel oscillator for detection. In comparison to cavity frequency modulation, statistical noise considerations are almost identical. However, a significant advantage is the lack of phase modulation of the maser output, feedback being around the null modulation condition. Furthermore, the 0-modulator is a valuable analytical tool in maser alignment. Its advantage over signal injection schemes, which give somewhat lower statistical deviation, is a lack a+ systematic perturbations, Including independence to connecting cable lengths, We have developed and tested a PIN-diode cavity Q modulator which gives no incidental frequency sh~ft over a very wide range a+ operation. Modulated at 200 Hz, it allowed variations in maser cavity frequency to be compensated with a loop gain greater than 1000. Compensation of incidental amplitude modulation of the output has also been demonstrated.
Published in: Proceedings of the 17th Annual Precise Time and Time Interval Systems and Applications Meeting
December 3 - 5, 1985
DuPont Plaza Hotel
Washington, DC
Pages: 129 - 144
Cite this article: Dick, G.J., Tucker, T.K., "Fast Autotuning of a Hydrogen Maser by Cavity Q Modulation," Proceedings of the 17th Annual Precise Time and Time Interval Systems and Applications Meeting, Washington, DC, December 1985, pp. 129-144.
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