Minimization of rms phase error in a phase locked loop by dithering of a frequency reference

Abstract

A novel and useful apparatus for and method of minimizing the phase distortions experienced at the output of a phase locked loop (PLL) by dithering of its input frequency reference to overcome additive interference that is parasitically suffered on it. The frequency reference signal is dithered in a controlled manner using either indirect or direct coupling. The dither signal may be a single clock or is generated by switching between two or more of the existing clock signals generated, or may be produced by a dedicated pseudo-random noise generator having specific spectral properties. In indirect coupling, the dither signal is coupled through a bond wire sufficiently close in proximity to the frequency reference circuit input. This dominates the jitter inflicted onto the frequency reference signal and upconverts its spectral content to higher frequency, thus eliminating the more damaging low-frequency jitter caused by the interfering RF signal. In direct coupling, the dither signal is coupled to the reference frequency input using a network of components directly connected thereto.

1 . A method of minimizing the impact of interference to a frequency reference input from a radio frequency (RF) signal, said method comprising the steps of:
 generating a dithering signal; and   coupling said dithering signal to said frequency reference input to generate a modulated frequency reference signal modulated with said dithering signal.   
     
     
         2 . The method according to  claim 1 , wherein said step of generating comprises generating a phase dithering signal. 
     
     
         3 . The method according to  claim 1 , wherein said step of generating said dithering signal comprises switching between a plurality of clock signals of different frequencies. 
     
     
         4 . The method according to  claim 1 , wherein said step of coupling comprises applying said dithering signal to a first bond wire located in close physical proximity to a second bond wire corresponding to said frequency reference input. 
     
     
         5 . The method according to  claim 1 , wherein said step of coupling results in intermodulations that effectively shift a significant portion of interference energy outside a particular band of interest. 
     
     
         6 . The method according to  claim 1 , wherein said step of coupling comprises coupling said dithering signal via an external coupling circuit to a first bond wire located in close physical proximity to a second bond wire corresponding to said frequency reference input. 
     
     
         7 . The method according to  claim 1 , wherein said step of coupling comprises directly coupling said dithering signal to said frequency reference input via an external coupling circuit. 
     
     
         8 . The method according to  claim 7 , further comprising a resistive divider network in series with said external coupling circuit, said resistive divider network operative to control the magnitude of said dithering signal. 
     
     
         9 . The method according to  claim 1 , wherein said step of coupling said dithering signal effectively moves the zero crossings of said frequency reference signal such that the frequency at which said transmit RF signal is an integer multiple of said reference frequency is randomized. 
     
     
         10 . The method according to  claim 1 , wherein the frequency of said RF signal is an integer-N multiple of said frequency reference input. 
     
     
         11 - 45 . (canceled) 
     
     
         46 . The method according to  claim 1 , wherein said dithering signal is periodic. 
     
     
         47 . The method according to  claim 4 , wherein said dithering signal is periodic. 
     
     
         48 . The method according to  claim 5 , wherein the dithering signal is periodic. 
     
     
         49 . The method according to  claim 6 , wherein said dithering signal is periodic.