This paper presents the analysis and design of a three-stage low-power CMOS-based voltage-controlled ring oscillator (VCRO) fabricated in standard 130 nm BiCMOS technology. Using the fine and coarse frequency tunability features acquired by employing a variable RC network, the VCRO can work in three different industrial, scientific, and medical (ISM) bands. The theoretical model, corroborated by measurements, demonstrates that incorporating both capacitive and resistive frequency tuning features allows the VCRO to adapt its voltage-controlled oscillator gain (KVCO) variation according to demand. This makes the presented ring oscillator suitable for a broad spectrum of applications such as phase-locked loops (PLLs), where the voltage-controlled oscillator (VCO) must meet strict criteria. The VCRO is powered by a 0.75 V supply and consumes only 1.7 μ ~W, 3.5 μ ~W, and 5.7 μ ~W of DC power at 13.56 MHz, 27.12 MHz, and 40.68 MHz ISM frequencies, respectively. The circuit core occupies an area of 0.0012 ~mm2.