A very high and stable equivalent resistance can be realized if switched capacitors (SC) are used. The operating characteristics of a three‐phase SC oscillator are measured in which SCs are used in place of resistors in a low‐frequency CR oscillator. The amplitude of the phase shifter outputs is designed to be equal. From the squared sum of the instantaneous values of the outputs, a dc voltage proportional to the square of the oscillation amplitude is obtained to be used as the control voltage of the amplitude stabilizing circuit. The clock frequency of the SC and the oscillation frequency have a linear relationship. The experimental values agree well with the theoretical values. For constant circuit parameters, the oscillation frequency has been varied over more than five digits by changing only the clock frequency. A stable oscillation is observed at an extremely low frequency (10 μHz). A possibility of oscillation at frequencies lower than this value is demonstrated. The condition for steady‐state oscillation condition is given by the two sets of capacitance ratios. For the capacitors made with identical materials, the temperature coefficients are expected to be canceled. The problem of a long rise time of the oscillation at an extremely low frequency is resolved by reduction of the clock frequency after the amplitude has been stabilized within a short time by a high clock frequency. The effect of the equivalent parallel resistance on the characteristics of the ultralow‐frequency SC oscillator is discussed.

The beacon signals from a geostationary satellite were observed using three-element very long baseline interferometry (VLBI), and the phase fluctuations along the baselines between three sites were obtained. The atmospheric phase fluctuations at each observation site were derived from the baseline phase fluctuation data. The fluctuations were classified into three time-interval regions based on the dependence of the Allan standard deviation of the fluctuations on the time interval. In the region where the interval was less than a few seconds and in the one where it was greater than one hundred seconds, the curve of the Allan standard deviation was steep and showed the property of white phase noise. In the region between these two regions, the dependence of the Allan standard deviation on the time interval was weak. The magnitude of the Allan standard deviations for the three observation sites showed time variations in the region where the time interval was longer than a few seconds. Comparison with the weather conditions suggested that the time variations of the Allan standard deviation reflected atmospheric instability above the sites.Index Terms-Allan standard deviation, atmospheric stability model, beacon wave, geostationary satellite, observation site atmospheric phase fluctuation, very long baseline interferometer (VLBI).

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