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This paper describes the oscillator based on high- overtone bulk acoustic resonator (HBAR) at 3.7 GHz for chip scale atomic clock (CSAC) application. The HBAR is built on a 400 mum thick sapphire with Al/ZnO/Al sandwich layers having thicknesses 0.1 mum/0.88 mum/0.1 mum, which ensures the best acoustic coupling and highest Q (above 19,000) at around 3.7 GHz. The oscillators have been designed, fabricated and measured to have very low phase noise and low power consumption. The oscillator when measured with agilent 4448 spectrum analyzer is measured to oscillate at 3.677 GHz with output signal power of -15 dBm and phase noise of -108 dBc/Hz at 10 kHz offset and -125 dBc/Hz @100 kHz offset, respectively, while consuming only 6.21 mW.