Loop filter for current-controlled-oscillator-based phase locked loop

Abstract

A loop filter of a phase-locked loop (PLL) that uses a current-controlled oscillator (CCO) includes a capacitor, a voltage-to-current (V-to-I) converter, and a charge pump. The input node of the loop filter receives a first current from an external charge pump. The combination of the capacitor and the V-to-I converter generates a first component of the output current of the loop filter based on the first current. The charge pump of the loop filter generates a second component of the output current. The loop filter is implemented without the need for a zero-frequency-determining resistor, the resistor instead being realized by the product of the first current, the second component of the output current and the transconductance of the V-to-I converter. Phase noise reduction in the PLL, as well as implementation of the loop filter with a smaller area, are thus made possible.