Phase cancellation in a phase-locked loop

Abstract

A phase-locked loop (PLL) including a multiplexer with multiple inputs, each input coupled to receive a different reference clock. A time-to-digital converter (TDC) generates a TDC output value based on a phase difference between a reference clock from the multiplexer and a feedback clock. An averager circuit coupled to an output of the TDC. An adder circuit is coupled to outputs of the TDC and the averager circuit. A loop filter is coupled to an output of the adder circuit.

What is claimed is: 
     
         1 . A phase-locked loop (PLL), comprising:
 a multiplexer with multiple inputs, each input coupled to receive a different reference clock;   a time-to-digital converter (TDC) to generate a TDC output value based on a phase difference between a reference clock from the multiplexer and a feedback clock;   an averager circuit coupled to an output of the TDC;   an adder circuit coupled to outputs of the TDC and the averager circuit;   a loop filter coupled to an output of the adder circuit.   
     
     
         2 . The PLL of  claim 1 , wherein, upon reconfiguring the multiplexer to select a different reference clock to provide to the TDC, the averager circuit is to determine an average of the TDC output value. 
     
     
         3 . The PLL of  claim 2 , wherein the adder is to subtract the average determined by the averager circuit from the current TDC output value. 
     
     
         4 . A phase-locked loop (PLL), comprising:
 a multiplexer with multiple inputs, each input coupled to receive a different reference clock;   a time-to-digital converter (TDC) to generate a TDC output value based on a phase difference between a reference clock from the multiplexer and a feedback clock;   an averager circuit coupled to an output of the TDC, the averager circuit to determine an average of multiple TDC output values to produce an average value;   an adder circuit coupled to outputs of the TDC and the averager circuit, the adder circuit to subtract the average value from current TDC output values to produce adder circuit output values; and   a digital loop filter coupled to an output of the adder circuit to filter the adder circuit output values;   wherein, upon configuring the multiplexer to provide a different reference clock to the TDC, the averager circuit is to determine the average value and the digital loop filter's output is to be maintained at a fixed output value.   
     
     
         5 . A method, comprising:
 causing a frequency control word output from a digital loop filter to remain fixed;   changing an input to a time-to-digital converter (TDC) from a first reference clock to a second reference clock;   determining an average of output values from the TDC to generate an average value;   generating a second value based on the average value and the current TDC output values; and   causing the digital loop filter to dynamically generate new frequency control words based on the second value.   
     
     
         6 . The method of  claim 5 , wherein generating the second value comprises subtracting the average value from the current TDC output values.