A novel voltage controlled crystal oscillator (VCXO) circuits

Reference signal source requires high quality (Q) factor resonating element, such as micro-machined or piezoelectric crystal that acts as an electro-mechanical resonating element in VCXO circuits. Apart from the apparent advantages of these electro-mechanical resonators they suffer from multi-mode resonance and cross talks. The selection and controlling of the desired mode is critical and challenging task when the spacing between modes is relatively small. The active mode-selection techniques reported in this paper minimizes the mode-jumping and phase noise for overtone mode VCXO, even those with relative low Q factor crystal. The measured phase noise for 290 MHz 3^rd order overtone VCXO is typically -128 dBc/Hz at 1 kHz offset from the carrier, and to authors knowledge, this is the reasonably good phase noise performance for the inexpensive class of VCXOs so far reported.