An illustrative system includes an amplifier operably connected to a phase shifter. The amplifier is configured to amplify a voltage from an oscillator. The phase shifter is operably connected to a driving amplitude control, wherein the phase shifter is configured to phase shift the amplified voltage and is configured to set an amplitude of the phase shifted voltage. The oscillator is operably connected to the driving amplitude control. The phase shifted voltage drives the oscillator. The oscillator is at an internal resonance condition, based at least on the amplitude of the phase shifted voltage, that stabilizes frequency oscillations in the oscillator.
 The United States Government has rights in this invention pursuant to Contract No. DE-AC02-06CH11357 between the United States Department of Energy, Office of Science, Office of Basic Energy Sciences and the UChicago Argonne, LLC, representing Argonne National Laboratory.