Abstract:
A circuit having a clock signal synchronizing device with capability to filter clock-jitters is disclosed. One embodiment provides a delayed locked loop with capability to filter clock-jitter. Further, the invention relates to a clock signal synchronizing method with capability to filter clock-jitter.

Description:
BACKGROUND 
       [0001]    The invention relates to a circuit having a clock signal synchronizing device, in one embodiment a delayed locked loop (DLL) with capability to filter clock-jitter. Further, the invention relates to a clock signal synchronizing method with capability to filter clock-jitter. 
         [0002]    A conventional DLL may—as is illustrated in FIG.  1 —e.g., includes a phase detector  11 , a variable delay element  12 , and a constant delay element  15 . 
         [0003]    An incoming clock signal is received at an input of the variable delay element  12 , delayed by a variable delay of the latter, relayed to the constant delay element  15 , delayed by a constant delay of the latter, and then forwarded to the phase detector  11 . The phase detector  11  compares the phase of the incoming clock signal received at a first input of the phase detector  11  with the delayed clock signal received from an output of the constant delay element  15  at a second input of the phase detector  11 . If the two received clock signals are not in phase, the phase detector  11  adjusts the variable delay of the variable delay element  12 , i.e., increases the variable delay by a predetermined process. In other cases, the phase detector  11  may decrease the variable delay by a predetermined process. 
         [0004]    Then, a new cycle begins and the incoming clock signal is delayed by the adjusted variable delay of the variable delay element  12  and the constant delay of the constant delay element  15  and is then relayed to the second input of the phase detector  11 . The phase detector  11  compares the received delayed clock signal with the incoming clock signal and, again, adjusts the variable delay of the variable delay element  12 , i.e., increases (or decreases) the variable delay by a predetermined process if the two clock signals are not in phase. Then again, a new cycle begins and the process is iterated until the DLL is “locked”, i.e. the incoming clock signal and the delayed clock signal which is the outgoing clock signal are in phase. 
         [0005]    It is noted that the phase detector  11  either increases the variable delay in each cycle until the two clock signals at its input are in phase or decreases the variable delay in each cycle until the two clock signals are in phase. 
         [0006]    Thus, it is guaranteed that the DLL locks after a certain maximum of cycles at the latest, the maximum being the period of the incoming clock signal divided by the predetermined process for adjusting the variable delay of the variable delay element  12 . 
         [0007]    Therefore, a DLL requires only a rather short period of time to reach a “locked state”, i.e. the incoming and outgoing clock signals are in phase. However, if the incoming clock signal includes clock-jitter the clock-jitter of the incoming clock signal will be directly transferred to the outgoing clock signal. Such clock-jitter can severely degrade the quality of outgoing data since the falling and rising edges of a read-data-eye during which outgoing data is to be transmitted will become blurred. 
         [0008]    In contrast to a conventional DLL, a phase lock loop (PLL) is capable of filtering clock-jitter of an incoming clock signal. However, a PLL requires a substantially longer period of time to reach a “locked state”. 
         [0009]    Therefore, there e.g., exists a need for a clock signal synchronizing device which both filters clock-jitter of an incoming clock signal and requires only a short period of time to reach a “locked state”. 
         [0010]    For these or other reasons, there is a need for the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The accompanying drawings are included to provide a further understanding of the present invention and are incorporated in and constitute a part of this specification. The drawings illustrate the embodiments of the present invention and together with the description serve to explain the principles of the invention. Other embodiments of the present invention and many of the intended advantages of the present invention will be readily appreciated as they become better understood by reference to the following detailed description. The elements of the drawings are not necessarily to scale relative to each other. Like reference numerals designate corresponding similar parts. 
           [0012]      FIG. 1  illustrates a simplified schematic diagram of an exemplary conventional DLL. 
           [0013]      FIG. 2  illustrates a simplified exemplary schematic diagram of a circuit having a DLL according to one embodiment. 
           [0014]      FIG. 3  illustrates a simplified schematic diagram of an exemplary implementation of the phase interpolator of the DLL in  FIG. 2 . 
           [0015]      FIG. 4  depicts a graph showing a Matlab simulation result for the jitter of an incoming and outgoing clock signal filtered by a simulated DLL according to one embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    In the following Detailed Description, reference is made to the accompanying drawings, which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. In this regard, directional terminology, such as “top,” “bottom,” “front,” “back,” “leading,” “trailing,” etc., is used with reference to the orientation of the Figure(s) being described. Because components of embodiments of the present invention can be positioned in a number of different orientations, the directional terminology is used for purposes of illustration and is in no way limiting. It is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope of the present invention. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims. 
         [0017]    As used herein, the term “electrically coupled” or “connected” is not meant to mean that elements must be directly coupled or connected together, and intervening elements may be provided between the “electrically coupled” or “connected” elements. 
         [0018]      FIG. 2  illustrates a simplified exemplary schematic diagram of a circuit having a DLL according to one embodiment. In one embodiment, the circuit is part of an integrated circuit. 
         [0019]    The DLL  20  includes a delay control circuit  21 , a delay circuit  22 , hereinafter referred to as delay line, a phase interpolator  23 , a phase interpolator control circuit  24 , an input  28 , and an output  29 . 
         [0020]    The delay control circuit  21  has a first input connected to the input  28  of the DLL  20  via connection  201  and connection  201   b  and a second input connected to an output of the delay line  22  via connection  203 , connection  203   b , and connection  203   d . A first output of the delay control circuit  21  is connected to a second input of the delay line  22  via connection  204  and a second output is connected to an input of the phase interpolator control circuit  24  via connection  205 . 
         [0021]    The delay line  22  has a first input connected with an output of the phase interpolator  23  via connection  202  and the second input connected with the first output of the delay control circuit  21 . An output of the delay line  22  is connected with the second input of the delay control circuit  21  and is also connected with a second input of the phase interpolator  23  via connection  203 , connection  203   b  and connection  203   c.    
         [0022]    The phase interpolator  23  has a first input connected to the input  28  of the DLL  20  via connection  201  and connection  201   a , the second input connected to the output of the delay line  22 , and a third input connected to an output of the phase interpolator control circuit  24  via connection  206 . The output of the phase interpolator  23  is connected to the input of the delay line  22 . 
         [0023]    The phase interpolator control circuit  24  has its input connected to the second output of the delay control circuit  21  and its output connected to the third input of the phase interpolator  23 . 
         [0024]    The delay line having a variable delay is initialized with a predetermined value, which may be calculated, for example, by using a suitable algorithm and which represents a delay expected for a corresponding circuit. During operation of the DLL  20 , the variable delay of the delay line  22  is controlled by the delay control circuit  21 . 
         [0025]    The phase interpolator  23  receives two clock signals at its inputs, an incoming clock signal at its first input and a delayed clock signal received from the output of the delay circuit  22  at its second input and adds the two clock signals with variable quantifiers. The variable quantifiers are controlled by the phase interpolator control circuit  24  and represent factors with which the two clock signals are multiplied before adding them. The incoming clock signal is multiplied with a factor of (1−p) and the delayed signal is multiplied with a factor of p, p being a real number greater than or equal to 0 and smaller than or equal to 1. 
         [0026]    However, for a correct operation of the DLL  20 , the DLL has first to be in a “locked state”, i.e. the incoming clock signal and an outgoing clock signal which is the delayed clock signal at the output of the delay line have to be phase aligned, before activating the phase interpolator  23 . When the phase interpolator  23  is not activated the factor p is set to 0 by the phase interpolator control circuit  24  which results in forwarding the incoming clock signal without any modification as, in this case, the incoming clock signal is multiplied with 1, whereas the delayed clock signal is multiplied with 0. Therefore, when the DLL  20  is not in a “locked state” the delay line  22  receives the incoming clock signal at its first input and delays the incoming clock signal by the variable delay controlled by the delay control circuit  21 . 
         [0027]    Thus, as long as the DLL  20  is not in a locked state, the DLL  20  operates in a way analog to a conventional DLL: 
         [0028]    The delay line  22  receives the incoming clock signal at its first input and delays the incoming clock signal by the variable delay controlled by the delay control circuit  21 . The delay control circuit  21  compares the phases of the incoming clock signal and the delayed clock signal and, if the two clock signals are not in phase, adjusts the variable delay of the delay line  22 , i.e., increases the variable delay by a predetermined process. In other embodiments, the delay control circuit  21  may decrease the variable delay by a predetermined process. 
         [0029]    The delayed/outgoing clock signal is then relayed from the output of the delay line  22  to the second input of the delay control circuit  21 . Then, a new cycle begins and the delay control circuit compares the outgoing clock signal with the incoming clock signal and, again, adjusts the variable delay of the delay line  22 , i.e. increases (or e.g., decreases) the variable delay by a predetermined process if the two clock signals are not in phase. The resulting outgoing clock signal is then relayed from the output of the delay line  22  to the second input of the delay control circuit  21 . Then again, a new cycle begins and the process is iterated until the DLL is “locked” i.e. the incoming clock signal and the outgoing clock signal are in phase. 
         [0030]    It is noted that the delay control circuit  21  either increases the variable delay in each cycle until the two clock signals at its input are in phase or decreases the variable delay in each cycle until the two clock signals are in phase. 
         [0031]    Thus, it is guaranteed that the DLL locks after a certain maximum of cycles at the latest, the maximum being the period of the incoming clock signal divided by the predetermined process for adjusting the variable delay of the delay line  22 . 
         [0032]    The delay line  22  may include a counter which counts the number of predetermined processes by which the variable delay of the delay line  22  is increased (decreased). Each time the counter receives a respective signal from the delay control circuit via connection  204 , a count of the counter is increased (decreased) by 1. In this case, the count (together with the predetermined value for the initialization of the delay line  22 ) specifies the value of the variable delay of the delay line  22 . 
         [0033]    The DLL  20  may further include a delay circuit having a constant delay, hereinafter also referred to as constant delay element, which may be placed directly after the delay line  22 . The constant delay of the constant delay element may adjusted suitably to replace the abovementioned initialization of the delay line  22  with the predetermined value so that the variable delay of the delay line starts with 0. 
         [0034]    As illustrated above, the DLL will be in a “locked state” after running through a certain limited number of cycles. 
         [0035]    Up to this point, clock-jitter has not been taken into account. As in a conventional DLL, incoming clock-jitter has been directly transferred. In general, however, this is not critical since, during a start of a system, an associated controller responsible for generating the (incoming) clock signal—and also for introducing clock-jitter—includes a rather low activity whereas low activity of the controller means low clock-jitter of the generated clock-signal. Activity of the controller will not be high until a certain time and at this point, when the activity of the controller increases, the DLL  20  will already be in a “locked state”. 
         [0036]    Only then, when the incoming clock signal and the outgoing clock signal are in phase, the phase interpolator control circuit  24  receives a respective signal from the delay control circuit  21  via connection  205  and activates the phase interpolator. 
         [0037]    As mentioned before, in the phase interpolator  23 , the factor p is set to 0 by the phase interpolator control circuit  24  when the phase interpolator  23  is not activated. For controlling the phase interpolator  23 , the phase interpolator control circuit  24  may send a control signal having a respective value for the factor p: To deactivate the phase interpolator  23  and cause the phase interpolator  23  to remain deactivated, respectively, the phase interpolator control circuit may send a “0” to the phase interpolator. To activate the phase interpolator  23  and cause the phase interpolator  23  to remain activated, respectively, the phase interpolator control circuit  24  may send a signal indicating a real number greater than 0 and smaller than or equal to 1 to the phase interpolator. 
         [0038]    The phase interpolator  23  statically phase-mixes the incoming clock signal and the outgoing clock signal (signal at the output of the delay line) to a contribution of p for the outgoing clock signal and to a contribution of (1−p) for the incoming signal. 
         [0039]    In general, the clock-jitter is introduced by the clock-jitter of the incoming signal and is transferred to the outgoing signal. In a conventional DLL, the clock-jitter of the incoming clock signal is directly transferred to the outgoing signal. In a DLL according to one embodiment, however, clock-jitter of the incoming clock signal is filtered by the phase interpolator  23  by phase-mixing the incoming and outgoing clock signals for several cycles: 
         [0040]    At its first input, the phase interpolator receives the incoming clock signal and weights (i.e. multiplies) it with a factor of (1−p). At its second input, the phase interpolator receives the outgoing clock signal from the output of the delay line  22  and weights (i.e. multiplies) it with a factor of p. Then, the two weighted clock signals are added. The resulting compounded signal is then relayed to the delay line  22  and delayed. The delayed compounded signal which represents the outgoing signal is then fed back to the second input of the phase interpolator  23  and a new cycle starts. To filter the clock-jitter of the incoming signal effectively, multiple cycles are carried out in the way described above to average out uncorrelated clock-jitter of the incoming clock signal. 
         [0041]    As the DLL  20  is in a “locked state” when the phase interpolator  23  is activated, the incoming and outgoing clock signals are identical except for a phase difference of a integer multiple n of the clock signal period 2π, i.e. n*2π, and except for clock-jitter. However, the clock-jitter of the outgoing signal is also “delayed” or rather phase shifted by p*2π. 
         [0042]    Thus, the signal generated by the phase interpolator is an overlap signal of two “in-phase” signals and is therefore, of course, also in phase with the incoming signal. Under the, in general, justified assumption that the clock-jitter of the incoming clock signal is uncorrelated, in particular non periodical, the “original” clock-jitter of the incoming clock signal and the “delayed” clock-jitter of the outgoing clock signal add in the same way as clock-jitters of different sources. Therefore, after several feedback loop cycles, multiple waves (clock signals) in the resonantly operated delay line  22  are overlapped such that the uncorrelated clock-jitters are averaged out. 
         [0043]    A first-order mathematical description of the clock-jitter filtering of the DLL  20  operated resonantly will be given in the following. 
         [0044]    The voltage level V out (t) of the outgoing clock signal can be written as: 
         [0000]        V   out ( t )=(1 −p )· V   in ( t−T )+ p·V   out ( t−T ) 
         [0045]    For the second cycle, V out  can be written as: 
         [0000]    
       
         
           
             
               
                 V 
                 out 
               
                
               
                 ( 
                 t 
                 ) 
               
             
             = 
             
               
                 
                   ( 
                   
                     1 
                     - 
                     p 
                   
                   ) 
                 
                 · 
                 
                   
                     V 
                     in 
                   
                    
                   
                     ( 
                     
                       t 
                       - 
                       T 
                     
                     ) 
                   
                 
               
               + 
               
                 p 
                  
                 
                   [ 
                   
                     
                       
                         ( 
                         
                           1 
                           - 
                           p 
                         
                         ) 
                       
                       · 
                       
                         
                           V 
                           in 
                         
                          
                         
                           ( 
                           
                             t 
                             - 
                             
                               2 
                                
                               T 
                             
                           
                           ) 
                         
                       
                     
                     + 
                     
                       p 
                       · 
                       
                         
                           V 
                           out 
                         
                          
                         
                           ( 
                           
                             t 
                             - 
                             
                               2 
                                
                               T 
                             
                           
                           ) 
                         
                       
                     
                   
                   ] 
                 
               
             
           
         
       
     
         [0046]    For the N th  cycle, V out (t) can be written as: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       V 
                       out 
                     
                      
                     
                       ( 
                       t 
                       ) 
                     
                   
                   = 
                   
                     
                       ( 
                       
                         1 
                         - 
                         p 
                       
                       ) 
                     
                     · 
                     
                       
                         
                           ∑ 
                           
                             n 
                             = 
                             1 
                           
                         
                         N 
                       
                        
                       
                         
                           p 
                           
                             n 
                             - 
                             1 
                           
                         
                         · 
                         
                           
                             V 
                             in 
                           
                            
                           
                             ( 
                             
                               t 
                               - 
                               nT 
                             
                             ) 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0047]    wherein: 
         [0048]    V in (t) is the voltage level of the incoming clock signal; 
         [0049]    T is the delay of the delay line. 
         [0050]    As the DLL  20  is operated in resonance, T is an integer multiple m of the incoming clock period t ck : T=m*t ck    
         [0051]    First, an incoming clock signal not having any clock-jitter is to be considered. In this case, the following equation is true for V in (t): V in (t−n*T)=V in (t−n*m*t ck )=V in (t) 
         [0052]    Herewith, equation (1) can be written as: 
         [0000]    
       
         
           
             
               
                 V 
                 out 
               
                
               
                 ( 
                 t 
                 ) 
               
             
             = 
             
               
                 
                   V 
                   in 
                 
                  
                 
                   ( 
                   t 
                   ) 
                 
               
               · 
               
                 ( 
                 
                   1 
                   - 
                   p 
                 
                 ) 
               
               · 
               
                 
                   
                     ∑ 
                     
                       n 
                       = 
                       1 
                     
                   
                   N 
                 
                  
                 
                   
                     p 
                     
                       n 
                       - 
                       1 
                     
                   
                    
                   
                       
                   
                    
                   or 
                 
               
             
           
         
       
       
         
           
             
               
                 V 
                 out 
               
                
               
                 ( 
                 t 
                 ) 
               
             
             = 
             
               
                 
                   V 
                   in 
                 
                  
                 
                   ( 
                   t 
                   ) 
                 
               
               · 
               
                 ( 
                 
                   1 
                   - 
                   p 
                 
                 ) 
               
               · 
               
                 
                   ∑ 
                   
                     n 
                     = 
                     0 
                   
                   
                     N 
                     - 
                     1 
                   
                 
                  
                 
                     
                 
                  
                 
                   p 
                   n 
                 
               
             
           
         
       
     
         [0053]    for N→∞, and p&lt;1, the upper equation is an infinite geometric series and can be transformed to: 
         [0000]    
       
         
           
               
             
               
                 
                   
                     
                       
                         V 
                         out 
                       
                        
                       
                         ( 
                         t 
                         ) 
                       
                     
                     = 
                       
                      
                     
                       
                         
                           V 
                           in 
                         
                          
                         
                           ( 
                           t 
                           ) 
                         
                       
                       · 
                       
                         ( 
                         
                           1 
                           - 
                           p 
                         
                         ) 
                       
                       · 
                       
                         
                           ∑ 
                           
                             n 
                             = 
                             0 
                           
                           ∞ 
                         
                          
                         
                             
                         
                          
                         
                           p 
                           n 
                         
                       
                     
                   
                 
               
               
                 
                   
                     = 
                       
                      
                     
                       
                         
                           V 
                           in 
                         
                          
                         
                           ( 
                           t 
                           ) 
                         
                       
                       · 
                       
                         ( 
                         
                           1 
                           - 
                           p 
                         
                         ) 
                       
                       · 
                       
                         1 
                         
                           1 
                           - 
                           p 
                         
                       
                     
                   
                 
               
               
                 
                   
                     = 
                       
                      
                     
                       
                         V 
                         in 
                       
                        
                       
                         ( 
                         t 
                         ) 
                       
                     
                   
                 
               
             
           
         
       
     
         [0054]    Therefore, with the incoming clock signal having no clock-jitter the voltage levels of the incoming and outgoing clock signals are equal (V in (t)=V out (t)) and the DLL functions similar to a conventional DLL. 
         [0055]    Next, an incoming clock signal specified by a simple sine wave having phase noise (representing the clock-jitter) is to be considered: 
         [0000]        V   in ( t )= V   i ·exp( j·ωt+j· φ( t ))  (2) 
         [0056]    equation (2) inserted in equation (1) for N→∞ leads to: 
         [0000]    
       
         
           
             
               
                 V 
                 out 
               
                
               
                 ( 
                 t 
                 ) 
               
             
             = 
             
               
                 ( 
                 
                   1 
                   - 
                   p 
                 
                 ) 
               
                
               
                 
                   ∑ 
                   
                     n 
                     = 
                     1 
                   
                   ∞ 
                 
                  
                 
                     
                 
                  
                 
                   
                     p 
                     
                       n 
                       - 
                       1 
                     
                   
                   · 
                   
                     V 
                     i 
                   
                   · 
                   
                     exp 
                      
                     
                       ( 
                       
                         
                           
                             j 
                             · 
                             ω 
                           
                            
                           
                               
                           
                            
                           t 
                         
                         - 
                         
                           j 
                           · 
                           ω 
                           · 
                           nT 
                         
                         + 
                         
                           j 
                           · 
                           
                             ϕ 
                              
                             
                               ( 
                               
                                 t 
                                 - 
                                 nT 
                               
                               ) 
                             
                           
                         
                       
                       ) 
                     
                   
                 
               
             
           
         
       
     
         [0057]    which can be simplified considering ω*nT=ω*n*m*t ck =n*m*2π: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       V 
                       out 
                     
                      
                     
                       ( 
                       t 
                       ) 
                     
                   
                   = 
                   
                     
                       
                         ( 
                         
                           1 
                           - 
                           p 
                         
                         ) 
                       
                       · 
                       
                         V 
                         i 
                       
                       · 
                       
                         exp 
                          
                         
                           ( 
                           
                             
                               j 
                               · 
                               ω 
                             
                              
                             
                                 
                             
                              
                             t 
                           
                           ) 
                         
                       
                     
                      
                     
                       
                         ∑ 
                         
                           n 
                           = 
                           1 
                         
                         ∞ 
                       
                        
                       
                           
                       
                        
                       
                         
                           p 
                           
                             n 
                             - 
                             1 
                           
                         
                         · 
                         
                           exp 
                            
                           
                             ( 
                             
                               j 
                               · 
                               
                                 ϕ 
                                  
                                 
                                   ( 
                                   
                                     t 
                                     - 
                                     nT 
                                   
                                   ) 
                                 
                               
                             
                             ) 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0058]    Considering equation (2), the term (V i *exp(j*ωt)) represents a jitter-free base signal of the incoming signal V in (t). 
         [0059]    Assuming a small value for the phase noise φ(t) the term exp(jφ(t)) representing the clock jitter is approximated using a first order Taylor series approximation (exp(jφ(t))≈1+jφ(t)): 
         [0000]    
       
         
           
               
             
               
                 
                   
                     
                       
                         ∑ 
                         
                           n 
                           = 
                           1 
                         
                         ∞ 
                       
                        
                       
                           
                       
                        
                       
                         
                           p 
                           
                             n 
                             - 
                             1 
                           
                         
                          
                         
                           exp 
                            
                           
                             ( 
                             
                               jϕ 
                                
                               
                                 ( 
                                 
                                   t 
                                   - 
                                   nT 
                                 
                                 ) 
                               
                             
                             ) 
                           
                         
                       
                     
                     ≈ 
                       
                      
                     
                       
                         ∑ 
                         
                           n 
                           = 
                           1 
                         
                         ∞ 
                       
                        
                       
                           
                       
                        
                       
                         
                           p 
                           
                             n 
                             - 
                             1 
                           
                         
                          
                         
                           ( 
                           
                             1 
                             + 
                             
                               jϕ 
                                
                               
                                 ( 
                                 
                                   t 
                                   - 
                                   nT 
                                 
                                 ) 
                               
                             
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   
                     = 
                       
                      
                     
                       
                         
                           ∑ 
                           
                             n 
                             = 
                             1 
                           
                           ∞ 
                         
                          
                         
                             
                         
                          
                         
                           p 
                           
                             n 
                             - 
                             1 
                           
                         
                       
                       + 
                       
                         
                           ∑ 
                           
                             n 
                             = 
                             1 
                           
                           ∞ 
                         
                          
                         
                             
                         
                          
                         
                           
                             p 
                             
                               n 
                               - 
                               1 
                             
                           
                           · 
                           
                             ϕ 
                              
                             
                               ( 
                               
                                 t 
                                 - 
                                 nT 
                               
                               ) 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   
                     = 
                       
                      
                     
                       
                         1 
                         
                           1 
                           - 
                           p 
                         
                       
                       + 
                       
                         
                           ∑ 
                           
                             n 
                             = 
                             1 
                           
                           ∞ 
                         
                          
                         
                             
                         
                          
                         
                           
                             
                               p 
                               
                                 n 
                                 - 
                                 1 
                               
                             
                             · 
                             
                               ϕ 
                                
                               
                                 ( 
                                 
                                   t 
                                   - 
                                   nT 
                                 
                                 ) 
                               
                             
                           
                            
                           
                               
                           
                            
                           
                             ( 
                             
                               
                                 for 
                                  
                                 
                                     
                                 
                                  
                                 p 
                               
                               &lt; 
                               1 
                             
                             ) 
                           
                         
                       
                     
                   
                 
               
             
           
         
       
     
         [0060]    inserted in equation (3): 
         [0000]    
       
         
           
             
               
                 V 
                 out 
               
                
               
                 ( 
                 t 
                 ) 
               
             
             ≈ 
             
               
                 
                   V 
                   i 
                 
                 · 
                 
                   exp 
                    
                   
                     ( 
                     
                       
                         j 
                         · 
                         ω 
                       
                        
                       
                           
                       
                        
                       t 
                     
                     ) 
                   
                 
               
                
               
                 ( 
                 
                   1 
                   + 
                   
                     
                       j 
                        
                       
                         ( 
                         
                           1 
                           - 
                           p 
                         
                         ) 
                       
                     
                      
                     
                       
                         ∑ 
                         
                           n 
                           = 
                           1 
                         
                         ∞ 
                       
                        
                       
                           
                       
                        
                       
                         
                           p 
                           
                             n 
                             - 
                             1 
                           
                         
                         · 
                         
                           ϕ 
                            
                           
                             ( 
                             
                               t 
                               - 
                               nT 
                             
                             ) 
                           
                         
                       
                     
                   
                 
                 ) 
               
             
           
         
       
     
         [0061]    now assuming p→1 (“100% feedback”): 
         [0000]    
       
         
           
             
               
                 
                   V 
                   out 
                 
                  
                 
                   ( 
                   t 
                   ) 
                 
               
               ≈ 
               
                 
                   V 
                   i 
                 
                  
                 
                   exp 
                    
                   
                     ( 
                     
                       jω 
                        
                       
                           
                       
                        
                       t 
                     
                     ) 
                   
                 
                  
                 
                   ( 
                   
                     1 
                     + 
                     
                       
                         ∑ 
                         
                           n 
                           = 
                           1 
                         
                         ∞ 
                       
                        
                       
                           
                       
                        
                       
                         ϕ 
                          
                         
                           ( 
                           
                             t 
                             - 
                             nT 
                           
                           ) 
                         
                       
                     
                   
                   ) 
                 
               
             
             → 
             
               
                 V 
                 i 
               
                
               
                 exp 
                  
                 
                   ( 
                   
                     jω 
                      
                     
                         
                     
                      
                     t 
                   
                   ) 
                 
               
             
           
         
       
     
         [0000]    (for uncorrelated phase noise) 
         [0000]      therefore: V   out ( t )≈ V   i  exp( jφt ) 
         [0062]    wherein (V i *exp(j*φt)) represents the jitter-free base signal of the incoming signal V in (t). 
         [0063]      FIG. 3  illustrates a simplified schematic diagram of an exemplary implementation of the phase interpolator  23  of the DLL  20  in  FIG. 2 . 
         [0064]    In the phase interpolator  23  illustrated in  FIG. 3 , differential clock signals are used. A differential clock signal consists of two complementary clock signals. The “actual” clock signal can be determined by comparing the two complementary clock signals. If the first clock signal of the two complementary clock signals is higher than the second one the “actual” clock signal is, for example, high (“1”). If the second clock signal of the two complementary clock signals is higher than the first one the “actual” clock signal is, for example, low (“0”). 
         [0065]    The phase interpolator  23  includes a power source, an inverter  30 , two sets of transistors  32 ,  33 , and transistors  34 ,  35 ,  36 , and  37 . 
         [0066]    The two sets of transistors  32  and  33  are respectively, for example, 15 transistors each of which can be independently driven by respective gate voltages. In order to control the gates of these transistors, the factor p is converted (not illustrated in  FIG. 3 ) to a thermometer code, which consist in this example of fifteen bits. The proportion of the number of “1&#39;s” to the total number of bits, in this example 15 bits, may represent the factor p. In the following, some examples will be given: 
         [0067]    a factor of 0 is represented by “000000000000000”, 
         [0068]    a factor of 1 is represented by “111111111111111” 
         [0069]    a factor of ⅓ is represented by “111110000000000”, and 
         [0070]    a factor of ⅘ is represented by “111111111111000” 
         [0071]    in thermometer code. 
         [0072]    The first set of transistors  32  are controlled by a received control signal SLC in thermometer code consisting of 15 bits and representing the factor p. Each bit of the thermometer code controls the gate of a respective transistor of the set of transistors  32 . If the respective bit is a “1” the corresponding transistor will be on, i.e. a current will flow through its drain and source. If the respective bit is a “0” the corresponding transistor will be off, i.e. no current will flow through its drain and source. 
         [0073]    The second set of transistors  33  are also controlled by the control signal SLC in thermometer code. However, the control signal SLC is inverted before being applied to the respective gates of the set of transistors  33 , otherwise the control mechanism is the same as the one applied to the first set of transistors  32 . The inversion of the control signal involves that the number of on-transistors of the second set of transistors  33  corresponds to the number of off-transistors of the first set of transistors  32  and vice versa. 
         [0074]    In the following, the functionality of the phase interpolator  23  will only briefly be described on the basis of two extreme examples as the phase interpolator illustrated in  FIG. 3  is a conventional phase interpolator well-known in the art. 
         [0075]    First, the value of SLC is assumed to be “000000000000000”. In this case, each transistor of the second set of transistors  33  is on and each transistor of the first set of transistors  32  is off. Therefore, voltage will be applied only to the transistors  36  and  37  which are connected to the transistors  33 . The transistors  36  and  37  are controlled by differential clock signals clk_ucp and clk_ucn representing the incoming clock signal in  FIG. 2 . In this case, the differential clock signals, clkmix_cp and clkmix_cn, output by the phase interpolator  23  correspond to the incoming clock signal in  FIG. 2  (represented as differential clock signals). 
         [0076]    Next, the value of SLC is assumed to be “111111111111111”. In this case, each transistor of the second set of transistors  33  is off and each transistor of the first set of transistors  32  is on. Therefore, voltage will be applied only to the transistors  34  and  35  which are connected to the transistors  32 . The transistors  34  and  35  are controlled by differential clock signals clk_dcp and clk_dcn representing the outgoing clock signal in  FIG. 2 . In this case, the differential clock signals, clkmix_cp and clkmix_cn, output by the phase interpolator  23  correspond to the outgoing clock signal in  FIG. 2  (represented as differential clock signals). 
         [0077]    For values of SLC lying in between the above two extreme examples, each of the transistors  34 ,  35 ,  36 , and  37  will make some contribution—according to the value of SLC—to the clock signal output by the phase interpolator  23 . For these cases, the phase interpolator  23  statically phase-mixes the incoming and outgoing clock signals to a contribution of p for the outgoing clock signal and to a contribution of (1−p) for the incoming clock signal. 
         [0078]      FIG. 4  depicts a graph illustrating a Matlab simulation result for clock-jitter of an incoming and an outgoing clock signal filtered by a simulated DLL according to one embodiment. 
         [0079]    The upper graph of  FIG. 4  illustrates the time required to lock-in the DLL by displaying the propagation delay of a delay line having a variable delay time comprised in the DLL. 
         [0080]    The lower graph of  FIG. 4  illustrates the progression of clock-jitter of the incoming and outgoing clock signals over time. To allow the DLL to reach a “locked state”, a phase interpolator, designated as “mixer” in the graph, starts with a contribution of 0% for the outgoing clock. During this period, incoming clock-jitter is transferred without modification to the outgoing clock. Between 500 ns and 700 ns, contribution of the outgoing clock signal is increased from 0% to 90% and clock-jitter suppression by the resonant DLL gets visible. 
         [0081]    Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations may be substituted for the specific embodiments shown and described without departing from the scope of the present invention. This application is intended to cover any adaptations or variations of the specific embodiments discussed herein. Therefore, it is intended that this invention be limited only by the claims and the equivalents thereof.