Patent ID: 7692498

Claim:
A phase locked loop, comprising: a digital controlled oscillator configured to generate an oscillator signal, and having an oscillator input and an oscillator output; a comparator, having a comparator first input coupled to a reference input, a comparator second input coupled to the oscillator output, and a comparator output; wherein the comparator is configured to generate an unwanted periodic pattern as a function of the oscillator signal, where the unwanted periodic pattern includes at least a first interference frequency; and a filter block having a filter block input coupled to the comparator output and having a filter block output coupled to the oscillator input, the filter block comprising a loop filter and a rejection filter, wherein the rejection filter comprises a notch filter comprising an at least second-order recursive filter having one or more filter coefficients for rejecting at least the first interference frequency, wherein the notch filter comprises a plurality of at least second-order recursive filters connected in series which have one or more filter coefficients for rejecting the first interference frequency and further interference frequencies.