As provided herein, in some embodiments, monolithic oscillators with low phase noise, large swing voltages, wide tuning, and high frequency characteristics are obtained by a monolithic integrated circuit having an oscillator core configured to generate a first output signal, and one or more tuning units operatively coupled to the oscillator core. In some embodiments, the oscillator core is a push-push oscillator core having a bipolar junction transistor, and each of the tuning units uses a FET transistor to present a selectable capacitance. In some embodiments, the tuning units have high-voltage and high-frequency capabilities. In some embodiments, the tuning units use MEMS switches to selectively connect capacitances to the oscillator core. In some embodiments, the oscillator core generates a second signal that has twice the frequency of the first frequency.