Considerations on High-Performance Frequency Synthesizers and High-Q Oscillators

Frequency generation is an important application in today’s radio frequency (RF) circuit design, especially for RF transceivers. The main challenges are to provide high spectral purity, while keeping the power consumption and area of the system low. In addition, with emerging mobile communication standards e.g. long term evolution (LTE) or 5G, various frequency bands and wide tuning ranges must be provided. This paper gives a brief overview of noise mechanisms, which have to be tackled, and a review of selected state-of-the-art frequency synthesizers (phase-locked-loops (PLLs)), as well as their limitations with respect to spectral purity. A novel interpretation of the figure-of-merit (FoM) for PLLs is shown emphasizing the impact of the resonators quality factor. In addition an outlook on possibilities to overcome the mentioned phase-noise limitations by using high quality factor (high-Q) RF-frequency generation circuit topologies utilizing e.g. surface-acoustic-wave- (SAW-), bulk-acoustic-wave- (BAW-) or micro-electro-mechanical-systems- (MEMS-) resonators.