Spread spectrum modulation technique for frequency synthesizers

Abstract

A spread spectrum modulation technique uses digital control logic to switch back and forth between two feedback divider ratios so that the PLL spreads output clock frequency between two limits determined by the ratios. The spread spectrum control logic can be integrated into any PLL frequency synthesizer.

The invention claimed is:  
     
         1 . A phase lock loop frequency synthesizer having a reference frequency divideable by a first value and a feedback frequency divideable by a second value, the synthesizer comprising: 
 a first system for providing a plurality of first values;    a second system for providing a plurality of second values; and    a control system for determining when to apply said plurality of said first and second values whereby the synthesizer provides a spread spectrum output frequency.    
     
     
         2 . The synthesizer as claimed in  claim 1  includes a detector for detecting the difference between the divided reference frequency and the divided feedback frequency to provide an indication thereof and wherein: 
 said control system is responsive to said detector output to determine when to apply said plurality of first and second values.  
 
     
     
         3 . The synthesizer as claimed in  claim 2  including: 
 a lock detector for determining when the divided reference frequency is equal to the divided feedback frequency to provide a lock signal; and  
 said control system is responsive to said lock signal to change the application of said plurality of first and second values.  
 
     
     
         4 . The synthesizer as claimed in  claim 1  wherein said first system includes a multiplexer for applying a high or low value to divide said reference frequency.  
     
     
         5 . The synthesizer as claimed in  claim 1  wherein said first system includes a multiplexer for applying a high or low value to divide said feedback frequency.  
     
     
         6 . A synthesizer as claimed in  claim 1  wherein said control system uses said reference frequency and said divided reference frequency to change said plurality of first values.  
     
     
         7 . The synthesizer as claimed in  claim 1  wherein said control system uses said feedback frequency and said divided feedback frequency to change said application of said second values.  
     
     
         8 . The synthesizer as claimed in  claim 1  wherein said control system is responsive to said lock signal and said divided reference frequency to change said application of said first value when said signals are synchronized.  
     
     
         9 . The synthesizer as claimed in  claim 1  wherein said control system is responsive to said lock signal and said divided feedback frequency to change said application of said second value when said signals are synchronized.  
     
     
         10 . The synthesizer as claimed in  claim 1  wherein the output of said synthesizer is a spread spectrum within a predetermined range.  
     
     
         11 . A phase locked loop frequency synthesizer having a reference frequency divideable by a first value and a feedback frequency divideable by a second value, both of which a provided to a detector for providing signals in response to phase differences in the divided reference and feedback frequencies, the synthesizer comprising: 
 a first system for providing a high and a low of the first value;    a second system for providing a high and a low of the second value;    a lock detector for detecting the signals from the detector and providing a lock signal when the phases of the divided reference and feedback frequencies are the same;    control logic responsive to said lock signal to cause said first and second systems to provide change between said high and low of the first and second values whereby the synthesizer provides a spread spectrum output frequency within a predetermined spectrum.    
     
     
         12 . The synthesizer as claimed in  claim 11  wherein the detector for providing signals provides up or down signals in response to the divided reference frequency leading or lagging the divided feedback frequency wherein: 
 said control logic is responsive to said up or down signals to cause said first and second systems to provide both high or both low first and second values.  
 
     
     
         13 . The synthesizer as claimed in  claim 12  wherein: 
 said control logic is responsive to said lock signal to cause said first and second systems to change from both high or both low first and second values to both low or both high first and second values.  
 
     
     
         14 . The synthesizer as claimed in  claim 11  wherein said first system includes a multiplexer for applying a high or low first value to divide said reference frequency.  
     
     
         15 . The synthesizer as claimed in  claim 11  wherein said first system includes a multiplexer for applying a high or low second value to divide said feedback frequency.  
     
     
         16 . The synthesizer as claimed in  claim 11  wherein said control system is responsive to said lock signal and said divided reference frequency to change said application of said first value when said signals are in phase.  
     
     
         17 . The synthesizer as claimed in  claim 11  wherein said control system is responsive to said lock signal and said divided feedback frequency to change said application of said second value when said signals are in phase.  
     
     
         18 . The synthesizer as claimed in  claim 11  wherein the output frequency of the synthesizer is a spread spectrum within a predetermined range.  
     
     
         19 . The synthesizer as claimed in  claim 11  wherein the output frequency of the synthesizer is a spread spectrum with the lower frequency 0.5% lower than the higher frequency.