Citation
Abstract
A new fast autotuner for the hydrogen maser has been implemented. By modulating the cavity QO, 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 autotuning methods and significant advantages over both of them. In comparison to line Q modulation, where the frequency shift induced by a change in the atomic line O 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 Q modulator is a valuable analytical tool in maser alignment. Its advantage over signal injection schemes, which give somewhat lower statistical deviation, is a lack of systematic perturbations, including independence from connecting cable lengths. We have developed and tested a PIN-diode cavity Q modulator which gives no incidental frequency shift over a very wide range of operation. Modulated at 200 Hz, it allows 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. Calculations show that long-term stability of 3 X 10-!3/\/7 should be achievable with typical masers.
Details
- Volume
- 42-91
- Published
- November 15, 1987
- Pages
- 295–303
- File Size
- 446.4 KB