I. Introduction
High quality factor (Q) on-chip resonators are highly sought for ultra low phase noise oscillators in high performance wireless transceivers. With the advent of 5G technologies, demand of low phase noise, low power, phase locked loop (PLL) free radio frequency (RF) synthesizers have risen to support the huge number of channels around mmWave frequencies. For example, at 30 GHz carrier frequency, continuous channel bandwidth of about 800 MHz to 1 GHz are possible in 5G technologies and there could exist multiple such channels. Traditionally, for RF carrier synthesis, piezoelectric quartz crystals, with reasonable frequency stability having Q > 105 are used in oscillators. However, due to its bulky size, integration of quartz crystals in standard IC technology is very difficult. Moreover, these quartz crystals are also limited to MHz frequencies (<500MHz), which makes PLL essential for >10 GHz frequency generation, which in turn increases the power consumption while significantly increasing system complexity. The other choice for mmWave oscillators is using on-chip LC tanks. However, problem with LC oscillators is that it occupy huge area. Moreover, its frequency stability is limited due to the limited Q (<30) of the on-chip LC tank. Injection locking technique or a PLL can be used to improve the performance of an LC oscillator but at the cost of increased power and area. For low power and area efficient RF carrier generation, PLL-free direct resonator based systems seem more viable, which require more versatile devices with monolithic IC integration capabilities. Moreover, transceiver arrays in multiple-input-multiple-output fashion for 5G applications also need individual RF carrier generators, where a fixed frequency carrier is required from each transceiver and tuning range of oscillator need not to be very wide.