Low voltage, high performance, GHz range STW clocks with BAW crystal stability

An alternative method for improving the temperature stability and the close-to-carrier phase noise performance of low-voltage (LV) surface transverse wave (STW) based oscillators in the lower GHz frequency range is presented. The method is verified by a 1 GHz STW based oscillator (STWO), injection locked to the 10-th overtone of a 100 MHz bulk acoustic wave (BAW) crystal controlled comb spectrum oscillator (CCCSO). Within the injection locking bandwidth (ILBW) of the STWO which is selected to be large enough to compensate for temperature induced frequency shifts and fabrication tolerances of the STW devices, the STWO tracks the 10-th overtone of the CCCSO and adopts its close-in phase noise which is typically 10 to 30 dB better compared to the free running STWO. At carrier offsets, larger than 10 kHz, the STWO can no longer track the noise modulation of the CCCSO, therefore, the phase noise at such offsets, down to the noise floor, is dominated by the much lower phase noise of the STWO. On the other hand, the temperature stability of the system is determined by the BAW crystal and is typically by a factor of 10 better compared to the free running STWO. Suppression of the fundamental 100 MHz CCCSO frequency and its undesired overtones from the STWO output spectrum is performed by two STW resonators identical to the one stabilizing the STWO. The laboratory prototype is operated from a 1.25V dc source, consumes 50 mA of dc current and maintains lock over a (-40 to +85)deg C temperature range. The suppression of the 100 MHz CCCSO overtone residues in the output spectrum is -60 dB referenced to the -14 dBm output signal at 1 GHz. The phase noise suppression at 1 kHz carrier offset is estimated as -130 dBc/Hz which represents about 26 dB improvement compared to the free running STWO