We report the first observation of above-threshold maser oscillation in a whispering-gallery(WG)mode resonator, whose quasi-transverse-magnetic, 17 th -azimuthal-order WG mode, at a frequency of approx. 12.038 GHz, with a loaded Q of several hundred million, is supported on a cylinder of mono-crystalline sapphire. An electron spin resonance (ESR) associated with Fe 3+ ions, that are substitutively included within the sapphire at a concentration of a few parts per billion, coincides in frequency with that of the (considerably narrower) WG mode. By applying a c.w. 'pump' to the resonator at a frequency of approx. 31.34 GHz, with no applied d.c. magnetic field, the WG ('signal') mode is energized through a three-level maser scheme. Preliminary measurements demonstrate a frequency stability (Allan deviation) of a few times 10 −14 for sampling intervals up to 100 s.

No abstract

In the characterization of the phase noise of a component, it is common practice to measure the cross-spectrum density at the output of two phase detectors that simultaneously compare the component output signal to a common reference. This technique, which is based on correlation and averaging, allows the rejection of the phase detector noise. On the other hand, it is known that the interferometer exhibits lower noise floor and higher conversion gain than other phase detectors suitable to radio-frequency and microwave bands. Thus, we experimented on an improved instrument in which the phase noise of a component is measured by correlating and averaging the output of two interferometers. The measurement sensitivity, given in terms of noise floor, turns out to be limited by the temperature uniformity of the instrument, instead of the absolute temperature T. This feature makes the instrument suitable to investigate the spectrum Sφ(f) of phase fluctuations below kBT/Po, i.e., the thermal energy kBT referred to the carrier power Po. The described method is suitable to the implementation of instruments in a wide frequency range, from some 100 kHz to 40 GHz and beyond. In principle, this method can also be exploited for the measurement of amplitude noise. Theory and experimental proof are given.

This article reports the design, the breadboarding, and the validation of an ultrastable cryogenic sapphire oscillator operated in an autonomous cryocooler. The objective of this project was to demonstrate the feasibility of a frequency stability of 3x10(-15) between 1 and 1000 s for the European Space Agency deep space stations. This represents the lowest fractional frequency instability ever achieved with cryocoolers. The preliminary results presented in this paper validate the design we adopted for the sapphire resonator, the cold source, and the oscillator loop.

No abstract

A frequency counter measures the input frequency ν averaged over a suitable time τ , versus the reference clock. High resolution is achieved by interpolating the clock signal. Further increased resolution is obtained by averaging multiple frequency measurements highly overlapped. In the presence of additive white noise or white phase noise, the square uncertainty improves from σ 2 ν ∝ 1/τ 2 to σ 2 ν ∝ 1/τ 3 . Surprisingly, when a file of contiguous data is fed into the formula of the two-sample (Allan) variance σ 2 y (τ ) = E{ 1 2 (y k+1 − y k ) 2 } of the fractional frequency fluctuation y, the result is the modified Allan variance mod σ 2 y (τ ). But if a sufficient number of contiguous measures are averaged in order to get a longer τ and the data are fed into the same formula, the results is the (non-modified) Allan variance. Of course interpretation mistakes are around the corner if the counter internal process is not well understood.

No abstract

No abstract