Voltage controlled oscillator capable of linear operation at very low frequencies

A processing system comprising:

• a clocked circuit capable of operating at very low frequency using an external clock signal;
• and
• a phase-locked loop (PLL) coupled to said clocked circuit capable of supplying said external clock signal with high linearity at very low frequency, said PLL comprising:
• a frequency divider circuit for dividing a frequency of said external clock signal by N;
• a phase detector capable of detecting a phase difference between a frequency divided output of said frequency divider circuit and an input reference signal and generating therefrom a phase difference signal;
• a charge pump and loop filter circuit capable of converting said phase difference signal to a +V(IN) control voltage and a −V(IN) control voltage;
• and
• a voltage controlled oscillator (VCO) capable of receiving said +V(IN) control voltage and said −V(IN) control voltage and outputting a VCO output signal having a frequency of oscillation determined by said +V(IN) control voltage and said −V(IN) control voltage, said VCO comprising:
• a storage capacitor capable of being charged linearly by a constant charge current and discharged linearly by a constant discharge current;
• a comparator capable of comparing a voltage on said storage capacitor to an upper threshold voltage and a lower threshold voltage, wherein an output of said comparator drops to a negative saturation voltage (−V(SAT)) when said storage capacitor voltage rises above said upper threshold voltage and said comparator output rises to a positive saturation voltage (+V(SAT)) when said storage capacitor voltage drops below said lower threshold voltage;
• a constant charge current source capable of injecting said constant charge current to said storage capacitor when said comparator output rises to said positive saturation voltage;
• and
• a constant discharge current source capable of draining said constant discharge current from said storage capacitor when said comparator output drops to said negative saturation voltage.