This article presents a frequency-locked loop (FLL) oscillator using an impedance-sensing frequency detector for use in fully-integrated, on-chip frequency references. By detecting the real and imaginary components of a complex RC impedance, the FLL can be locked into the pole frequency where both components are balanced. A phase-mixing technique is also introduced to reconfigure the output frequency without altering the amplifier operating voltage. A prototype chip demonstrating this IQ-balanced approach was designed and fabricated in a general-purpose 0.18- $\mu \text{m}$ CMOS process. From measured results across multiple dice, the prototype integrated circuit (IC) chips achieve a temperature stability of 22 ppm/°C from −40°C to 80 °C and 10-ppm Allan deviation at an average output frequency of 647 kHz with 1.27 pJ/cycle energy efficiency, and 34.4 ppm/°C from −40°C to 80 °C and 15-ppm Allan deviation at an average output frequency of 1.39 MHz with 0.89 pJ/cycle energy efficiency.