Abstract:
Rectangular-shape resonators as guard traces formed in a region between the victim and aggressor lines are disclosed. No shorting-vias or resistors are required. The rectangular resonators are found to have functions of improving far-end crosstalk (FEXT) and timing jitter in both frequency domain and time domain if the parameters are appropriated selected.

Description:
FIELD OF THE INVENTION 
     The present invention pertains to an architecture for suppressing the far-end crosstalk and the signal timing jitter, particularly to the architecture where rectangular resonators a formed in between two signal transmission lines to improve problems of far-end crosstalk (FEXT) and signal timing jitter. 
     DESCRIPTION OF THE PRIOR ART 
     To attract consumers, the trend of electronic products toward light, slim, short, small, with functional diversification, and processing in high speed become a tendency, it means that not only each integrated circuit is with high-integrity, but the traces layout on the print circuit board are dense and the spacing between adjacent lines are getting smaller and smaller. On the other hand, the processor in the electronic products is usually being operated in high frequency as a result, the wavelength is less than the length of the signal transmission lines. 
     In other words, the signal traces themselves and in between thereof will present a large number of parasitic capacitive coupling and the inductive coupling therein, which make mutual interference and generate noises. Consequently, crosstalk interferences are resulted. 
     To solve crosstalk interference, conventional art provides 3-W rule. Some research found that the line spacing widened up to 3 times the microstrip line width, near-end crosstalk (NEXT) can be reduced to 1%, far-end crosstalk (FEXT) interference reduced to 1.4%, and effective isolating the amount of coupling between the two transmission lines by 70%. In addition, if 98% of the amount of coupling isolation is requested, then it will demand a 10-W rule. Whereas 3-W rule will reduce significantly the trace density, not to mention the 10-W rule, which will further lower the trace density. 
     According to another conventional embodiment, as shown in  FIG. 1 , guard traces are added in between the two signal transmission lines  10 ,  20 . The signal transmission line  10  has one end connected to a driver and the other end connected to a receiver. The guard traces  15  are open terminations. The energy of the aggressive line  10  is also coupled to the guard traces  15  generating ringing noise at the near end and far end of the interference (victim) line  20 . The ringing noise generation is mainly because the ratio of the capacitive coupling between the signal lines  10 ,  20  and guard traces  15  are less than the inductive coupling ratio. According to the embodiment, shorting-vias  5 , hereinafter called shorting-vias, are formed to the guard traces  15  forming grounded guard traces so as to suppress the noise, as shown in  FIG. 1 . Hereinafter the capacitive coupling ratio refers to a ratio of mutual capacitance over self-capacitance: C m /C T , and inductive coupling ratio refers to the ratio of the mutual inductance over self-inductance: L m /L s . 
     Still another known guard traces called serpentine guard traces  16 , as shown in  FIG. 2 . The serpentine guard traces  16  are formed in between the aggressor line  10  and the victim line  20 . The components of the guard traces  16  perpendicular to the signal lines will not have the magnetic field coupling, and the parallel component thereof will increase the capacitive coupling and the inductive coupling. This serpentine protective line  16  is found to be effective to reduce the FEXT and signal timing jitter, but it will increase the NEXT. Another disadvantage is that the serpentine guard traces  16  are demanded to install the resistors, please refer to “A Serpentine Guard Trace to Reduce the Far-End Crosstalk Voltage and the Crosstalk Induced Timing Jitter of Parallel” IEEE TRANSACTIONS ON ADVANCED PACKAGING, pp 809-817, VOL. 31, NO. 4, 2008. Accordingly, it is proposed to replace the resistors by shorting-vias. However, excessive shorting-vias will affect the flexibility of the circuit layout on the back plane of the printed circuit board. By contrast, if the number of the shorting-vias is not enough, the spacing between short-vias is found to have half-wavelength resonance problem in the frequency domain and have the ringing noise problem in the time domain. 
     To overcome the problem of insufficient shorting-vias, a conventional embodiment proposes forming shorting-vias at the two terminals of the guard traces, and additional dielectric material is covered on the printed circuit board so as to compensate the capacitive coupling ratio between the signal line and guard traces. This technology is known as “disposed with a cover plate”. It can eliminate FEXT interference due to guard traces. The disadvantages are that this technology will increase the material cost. On the other hand, in the situation of the printed circuit board has dense elements already, it is difficult to add other materials thereover. 
     In view of the above problems found in the conventional techniques, which indeed improve problems on the crosstalk but the disadvantage including either additional costs or limitations or both during the technique implement. The present invention will provide a new technology, and effective in improving the above problems. 
     SUMMARY OF THE PRESENT INVENTION 
     An object of the present invention is to provide architecture to suppress the FEXT during signal transmission and the signal timing jitter. 
     Another object of the present invention is to provide rectangular resonators in between the signal transmission lines to suppress the FEXT without extra cost. 
     The present discloses guard traces for preventing two parallel transmission lines from far-end crosstalk (FEXT) and timing jitter, comprises forming a plurality of rectangular resonators in between the two parallel signal transmission lines and having long sides thereof perpendicular to a transmission direction. 
     The architecture of rectangular resonators does not need any extra resistors and shorting-vias, but only requires choosing appropriate parameters including the length and the width of the rectangular resonator and the interval in between the rectangular resonators and the spacing of the rectangular resonators to the aggressive line and victim line. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1  shows a schematic diagram according to an embodiment of a prior art, which has shorting-vias as guard traces formed in between two signal transmission lines. 
         FIG. 2  shows a schematic diagram according to another embodiment of a prior art, which has meandering guard traces formed in between two signal transmission lines. 
         FIG. 3  shows a schematic diagram according to an embodiment of the present invention, which has a plurality of rectangular resonators as guard traces formed in between two signal transmission lines. 
         FIG. 4  shows mutual capacitance, mutual inductance between the two signal transmission lines, and a self-capacitance and self-inductance in an equivalent circuit while two signal transmission lines has a rectangular resonator formed in between as guard traces according to an embodiment of the present invention. 
         FIG. 5  is a schematic diagram showing changes in the various width of the rectangular resonator with the inductive coupling and the capacitive coupling. 
         FIG. 6  is a schematic diagram showing changes in the various length of the rectangular resonator with the inductive coupling and the capacitive coupling. 
         FIG. 7  is a schematic diagram showing various interval changes among the rectangular resonators with the inductive coupling and the capacitive coupling. 
         FIG. 8  shows the comparison results between the NEXT and FEXT in the frequency-domain by simulation in accordance with preferred design parameters for rectangular resonator design according to the present invention. 
         FIG. 9  shows the comparison results between the NEXT and FEXT in the time-domain by simulation in accordance with preferred design parameters for rectangular resonator design according to the present invention. 
         FIGS. 10  ( a ) and  10  ( b ) show eye diagrams by simulation in conditions of with and without rectangular resonators, respectively. 
         FIGS. 11  ( a ) and  11  ( b ) show eye diagrams by simulation in conditions of with and without rectangular resonates, respectively in accordance with another simulating conditions. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention provides an architecture of rectangular resonators to solve the problem of crosstalk among signal transmission lines. The crosstalk elimination is implemented almost at no extra cost. It is because the two signal transmission lines, the aggressive line  10  and the victim line  20 , and the rectangular resonators  17  in between, as shown in  FIG. 3 , are formed simultaneously during the trace etching process. It does not require any resistor installation or form any shorting-via. 
     To analyze how a far end crosstalk (FEXT) interference and near-end crosstalk (NEXT) are suppressed by rectangular resonators. An equivalent circuit of rectangular resonators and coupling traces is shown in  FIG. 4 , where C T  is a self-capacitance of the aggressor line  10  and the victim lines  20 ; C m  is a mutual capacitance between the aggressors line  10  and the victim line  20 ; L S  is a self-inductance of the aggressor line  10  and the victim lines  20 , L m  is a mutual inductance between the aggressors line  10  and the victim line  20 ; L R  is the length of the rectangular resonator  17 , as shown in  FIG. 3 ; ½ L RRS  an equivalent self-inductance due to a half length of the rectangular resonator  17 ; C RS  is a capacitance due to parallel plates formed by rectangular resonator and the signal line; C RR  is a parallel plate capacitance generated due to parallel plates formed by rectangular resonator  17  and the reference plane; C RTR  is a parallel plate capacitance generated due to parallel plates formed by a rectangular resonator  17  and its adjacent rectangular resonator  17 . 
     Therefore, as is seen from the equivalent circuit shown in  FIG. 4 , the capacitance C RS  will be increased with the width W R  of the rectangular resonator  17 . Similarly, the capacitance C RS  will be increased with the length L R  of the rectangular resonator  17 , as shown in  FIG. 3 . That is the closer rectangular resonator  17  is to the aggressor line  10  and victim line  20 , the larger of the C RS  will be. However, the wider the width W R  of the rectangular resonator  17  also increases the inductive coupling ratio between the rectangular resonator  17  and the two signal transmission lines.  FIG. 5  shows such a result. 
     To judge whether the rectangular-resonators  17  are capable of improving the problem of crosstalk and signal timing jitter, the following analyses are done. In  FIGS. 5 to 7 , the analyses are base on the following parameters: a ratio of the space S ( FIG. 3 ) between two signal transmission lines over the width w ( FIG. 3 ) of the signal transmission line is by 3:1. For example, two signal transmission lines are spaced by 9 mm and each has a width 3 mm; the dielectric material of the substrate is FR4, has a relative dielectric constant 4.4, and the dielectric substrate has a thickness of 1.6 mm, and the foil thickness is 0.035 mm. In practice, the rectangular resonators  17  can also be used in the situation of S=2W in accordance with the present invention. 
       FIG. 5  is a schematic diagram showing changes in the various width in mm of the rectangular resonator with the mutual inductive coupling and the mutual capacitive coupling. The curve  110  depicts that inductive coupling ratio (L m /L s ) is increase with the width of rectangular resonator  17 . The curve  120  depicts that capacitive coupling ratio (C m /C T ) is increase with the width of rectangular resonator  17  in a similar tendency as the curve  110 . Whereas, the increase in the inductive coupling ratio is dramatically increased with the width of the rectangular resonator  17  far in excess than that of the capacitive coupling ratio. Therefore, according to the changes of two curves  110 ,  120 , in the  FIG. 5 , the width W R  of the rectangular resonator  17  becoming too large is not appropriate. An example of W R  in mm may be 0.5 mm to 1.5 mm. preferably W R =1 mm. Within these ranges, an approaching degree between the Inductive coupled ratio and the capacitive coupling ratio, though is not as that of W R  at 3 mm, but when the width W R  in mm is above 1.5 mm, the inductive coupling ratio will be increase dramatically. 
     Referring to  FIG. 6 , it is a schematic diagram showing changes in the various length in mm of the rectangular resonator with the mutual inductive coupling and the mutual capacitive coupling. The curve  120  depicts that the capacitive coupling ratio (C m /C T ) increased with the length of the rectangular resonator  17 . It is expectable, because the closer the short side of the rectangular resonator  17  to the signal transmission lines  10 ,  20 , mutual capacitance C m  will increase rapidly. The curve  110  shows that the inductive coupling ratio has the same trend of increasing. But apparently the increasing rate of mutual inductance L m  with the length of rectangular resonator  17  is less than that of the mutual capacitance C m . Therefore, viewing from the trend of capacitive coupling and inductive coupling, the length of the rectangular resonator  17  increasing is in favor of the proximity of the inductive coupling ratio with the capacitive coupling ratio. Accordingly, choosing 8 mm as a length of the rectangular resonator  17  is appropriate in accordance with a preferred embodiment. 
     Referring to  FIG. 7 , it is a schematic diagram showing interval changes between the rectangular resonators versus the mutual inductive coupling and the mutual capacitive coupling. Surely, the reduction of the interval will increase the number of rectangular resonators. The curve  120  shows that the capacitive coupling ratio (C m /C T ) is increasing with the reducing of the interval. The curve  110  also shows that the inductive coupling ratio is increasing with the reducing of the interval. Particularly, as the interval goes down to about 2 mm or less, the inductive coupling ratio substantial increases. This is very obvious, excessive number of rectangular resonator  17 , the current path will increase. View from  FIG. 7 , it seems that the capacitive coupling ratio is in the closest proximity to the inductive coupling ratio as the interval is of 2 mm. But in summary the results of  FIG. 5  to  FIG. 7 , an optimum of the interval is in between about 3.5 to 4.5 mm.  FIG. 6  shows that the capacitive coupling ratio is of about 0.0098, while the inductive coupling ratio of about 0.0132 as the length rectangular resonator  17  is of 8 mm. Therefore, the benefit obtained from the interval of 2 mm are not enough to compensate for the disadvantage found in  FIG. 6 . The interval which becomes too small produces inferior results Therefore, the result of a compromise, the interval should be chosen at a value between about 3.5 to 4.5 mm and 4 mm is the most preferred. That is, a preferred interval is of about ⅓ to ½ of the long side of the rectangular resonator. 
     Based on the aforementioned analysis, the design parameters of the preferred rectangular resonators  17  are: 1×8 mm 2  each and the number of rectangular resonators is of 16, assuming the aggressors line and the victim line have a length of 70 mm. 
     Please refer to  FIG. 8 .  FIG. 8  shows the comparison results between the NEXT and the FEXT interference in dB in the frequency-domain in GHz by simulation in accordance with preferred design parameters of the rectangular resonator design of the present invention. The curve  130  illustrates the relationship between the FEXT interference in dB and frequency in GHz without any guard traces. The curve  140  illustrates the relationship between the FEXT interference in dB and frequency in GHz with rectangular resonators. The rectangular resonators  17  of the present invention have a significant effect in the ultra-high frequency range of 1 GHz to 6 GHz. In a range of higher frequencies, for example, the range of 7 GHz to 8 GHz, especially, in the range of 7 GHz to 7.8 GHz, the effect is significant. From the above results, the rectangular resonators  17  of the present invention as guard traces indeed have a significantly improving performance on suppressing the FEXT interference. 
     Please still refer to  FIG. 8 . The curve  160  illustrates the relationship of the NEXT interference in dB against frequency in GHz without any guard traces. The curve  150  illustrates the relationship of the NEXT interference in dB against frequency in GHz with rectangular resonators. The curves  150 ,  160  illustrating the suppression resonance on the NEXT interference in dB is varied with the frequencies in GHz no matter whether the guard traces are present. Since the suppression resonance on the NEXT interference, thus in some ranges of frequencies with the rectangular resonators have better performance but at another range, without any guard has a better performance. But as whole, the signal transmission lines without the rectangular resonators in between have a better performance in average, since the curve  160  is at a lower position. 
     Next, please refer to  FIG. 9 . The curve  180  illustrates the relationship of the FEXT interference in mV against the time domain in ns without any guard traces. The curve  170  illustrates the relationship of the FEXT interference in mV against the time domain with rectangular resonators. In  FIG. 9 , the curves  170 ,  180  illustrate the suppression on the FEXT interference in mV in the time domain 0.3 ns to 0.9 ns. The performance of the rectangular resonators is significantly better than without any guard traces. Out of the foregoing range in the time domain, the effect is not significant with or without the rectangular resonators. But the performance on the suppression the NEXT, without rectangular resonator showing in curve  185  is better than that of with rectangular resonator showing in curve  175 . 
     Consequently, no matter whether viewing from the time domain or from the comparison of the frequency domain, the rectangular resonator  17  architecture can effectively reduce the influence of interference FEXT although the NEXT interference is slightly increased, but in the parallel terminal interface, the FEXT suppression is more important than that of NEXT. Particularly in the UHF range, most of the signals are digitalized circuit. The FEXT noise occurs at the receiving end, if the FEXT not effectively inhibit more likely to cause the signal misjudgment. For example, for DDR (double data rate) or graphic adapter, the influence of the FEXT interference is more important than that of the NEXT, please refer to “FEXT-eliminated stub-alternated microstrip line for multi-gigabit/second parallel links” ELECTRONICS LETTERS 14 Feb. 2008 Vol. 44 No. 4, by S.-K. Lee, et al.” 
     In another aspect, the present invention utilizes the eye diagram by ADS simulation software provided by Agilent Technology to simulate the circuit. To assess the improvement of the rectangular resonator  17  signal timing jitter. Please refer to  FIGS. 10  ( a ) and  10  ( b ). They show the eye diagrams, respectively for without guard trace and with rectangular resonators  17  as guard trace. The simulation using the parameters as above and the input signal to 4V, the rise time of 100 ps, and the data rate of 3 Gbps. 
     Viewing from the eye diagrams in  FIGS. 10  ( a ) and  10  ( b ), the improvement on the amplitude in voltage of eye opening is found; the signal timing jitter part without guard architecture is of 26.04 ps, and with the rectangular resonator  17  architecture is of 20.38 ps. The rectangular resonator architecture improves the signal timing jitter by about 6 ps. 
     On another aspect, the input signals are 2 31 −1 pseudo Random Binary Sequence (PRBS), generated by a signal generator accordingly to the present invention for eye diagram measurement. The input voltage is of 0.5 V and the data rate is of 10 Gbps. The results are shown in  FIGS. 11  ( a ), and  11  ( b ) and table 1. 
     
       
         
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                 Signal timing 
               
               
                   
                 EyeW (ps) 
                 EyeH (mV) 
                 jitter (ps) 
               
               
                   
               
             
             
               
                 Without guard traces 
                 88.46539 
                 411.7608 
                 15.2 
               
               
                 With rectangular 
                 88.58991 
                 401.2587 
                 13.6 
               
               
                 resonators 
               
               
                   
               
             
          
         
       
     
     The rectangular resonator architecture substantial suppression of FEXT noise interference without the problem of the resonance; furthermore, it can suppress a lot of FEXT noise amplitude (411.7608 mV vs. 401.2587 mV. Finally, through the observation of the eye diagram, the rectangular resonator architecture can improve the signal timing jitter phenomenon to 13.6 ps from 15.2 ps due to parity modal rate does not match. The improvement up to 10.52%. 
     The benefits of the present invention are: 
     (1) Forming the rectangular resonators needs only photolithography and etching techniques, and thus they can be formed simultaneously with the traces of the printed circuit board. Therefore, almost no additional costs on. 
     (2) The FEXT interference and timing jitter problem can be effectively improved by the rectangular resonators. 
     (3) Compared to the prior art which needs the guard elements such as the resistor or shorting-via, the present invention provides only simple traces on the board. The resistors may impede the circuit elastic layout and the shorting-vias may need an extra cost. The disadvantages of both are getting improvement in according to the present invention. As is understood by a person skilled in the art, the foregoing preferred embodiments of the present invention are illustrated of the present invention rather than limiting of the present invention. It is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims, the scope of which should be accorded the broadest interpretation so as to encompass all such modifications and similar structures.