Patent ID: 7656236

Claim:
A frequency synthesizer, comprising: an input terminal and an output terminal; a loop filter having a differential input; a digital phase detector comprising a first input coupled to the input terminal, a second input coupled to the output terminal, and an output coupled to the differential input of the loop filter, wherein the digital phase detector is configured to operate at a first phase comparison frequency, and wherein the digital phase detector is configured to produce a first signal proportional to the phase difference between signals applied to the first and second inputs of the digital phase detector; and an analog phase detector comprising a first input coupled to the input terminal, a second input coupled to the output terminal, and a differential output alternating current (AC) coupled to the differential input of the loop filter, wherein the analog phase detector is configured to operate at a second phase comparison frequency, and wherein the analog phase detector is configured to produce a first differential signal corresponding to residual noise of the digital phase detector; wherein the first phase comparison frequency is different from the second phase comparison frequency.