Dynamic noise-feedback and mode-coupling mechanism silences the VCXOs phase noise

Reference signal source requires highly quality factor resonating element, such as micro-machined or piezoelectric crystal that acts as an electro-mechanical resonating module in VCXO circuits. Apart from the apparent advantages of these electromechanical resonators they suffer from multi-mode resonance and cross talks. The selection of the desired resonance mode is challenging when the spacing between resonance modes is relatively small. This limits the fabrications of crystal oscillator as a reference sources in modern IC processes. The novel approach (mode-selection and mode-feedback techniques) reported in this paper optimizes the performance of VCXO, even those with relative low Q resonators, for low phase noise and good frequency stability. The typical measured phase noise for 155.6 MHz VCXO is typically - 137 dBc/Hz at 100 Hz offset from the carrier, and to author knowledge, this is the best phase noise performance for this class of VCXO so far reported.