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In this paper we present a direct measurement of the spectral density of both the fractional frequency fluctuations and the phase fluctuations for a rubidium (Rb) maser oscillator. A truncated polynomial is fitted to each independent set of data obtained over a Fourier frequency range between 0.1 Hz and 100 Hz. They are Sy(f) = 1.9 Ã10-25f-2 + 3.4 Ã 10-27f2 and SÂ¿ = 1.2 Ã 10-5f-4 + 1.9 Ã 10-7f0, respectively. A random walk of frequency noise is dominent for frequency below 2.8 Hz while a white phase noise is dominent for the higher frequency range. These results are used to predict time-domain measurements which are then compared to experimental values.