Abstract:
One embodiment of the invention sets forth a technique for using a resonator as a common-mode filter for attenuating unwanted common-mode frequencies in a differential signal. Filtering these unwanted frequencies may reduce electromagnetic interference from the differential signal pair in nearby electronic devices. Since common-mode filtering is employed, the resonator reduces the unwanted common-mode noise at specific frequencies without distorting the information-carrying differential signals. Further, since the filter is implemented as a PWB resonator, the incremental cost of including this filter on the PWB is relatively small.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Embodiments of the present invention relate generally to the field of electronic devices and more specifically to a technique for reducing common mode noise in differential signal interconnect. 
     2. Description of the Related Art 
     High performance electronic devices employ differential signals to transfer data at very high speeds through printed wiring board (“PWB”) interconnect between integrated circuit devices. However, one side effect of these high speed transfers is that electro-magnetic interference (“EMI”) may be created as a result of distortions of the high speed signals. This EMI may interfere with other nearby electronic devices. In order to reduce such interference, the government strictly regulates the amount of EMI that may be legally emitted from electronic devices, forcing manufacturers to limit the amount of EMI emitted from their products. 
     Signals in an ideal differential pair are designed to be of the same magnitude and out of the phase. Thus, the electromagnetic field generated by each interconnect within the ideal differential pair is canceled, and EMI is minimized. However, in a real word, common-mode signals are inevitably present because of imbalances in a differential pair and because of external noises injected into a differential pair, etc. EMI may be increased substantially from differential signal PWB interconnects when common-mode signals are present. Since these common-mode signals do not contribute to data signals transmitted between integrated circuit devices, filtering the common-mode signals without distorting the differential signals for the data is a desirable technique for reducing EMI. 
     Existing electronic devices transferring data through single-ended signal PWB interconnects employ filters to reduce noise from the data signals. One form of such filtering employs a resonator constructed from one or more of the PWB interconnects. By placing the resonator adjacent to the PWB interconnect, the resonator is able to filter the unwanted frequencies being transmitted within the PWB interconnect. Since techniques for designing a PWB resonator to filter specific frequencies is well known to those skilled in the art, designing PWB resonators to filter these additional frequencies from a PWB interconnect is not discussed herein. 
     One drawback of this approach is that filtering additional frequencies from both signals in a differential signal pair by using resonators located on opposite sides of the PWB interconnects that carry the differential signals may distort one signal relative to the other signal. This distortion arises from small differences in the filtering characteristics of each resonator and may lead to substantial voltage differences between the signals. Since differential signaling communicates data through small voltage differences between the signals, any resonator-induced distortion of one signal relative to the other may distort the data carried by those signals. Thus, such filtering may increase the error rate in that differential signal pair. 
     As the foregoing illustrates, what is needed in the art is a way to filter unwanted common-mode signals in a differential signal pair that avoids one or more of the disadvantages set forth above. 
     SUMMARY OF THE INVENTION 
     One embodiment of the present invention sets forth a system for removing unwanted common-mode noise from a differential signal. The system includes a transmitting device, a receiving device, a first interconnect coupling the positive signal from the transmitting device to the receiving device, a second interconnect coupling the negative signal from the transmitting device to the receiving device, where the first interconnect and the second interconnect are designed in a substantially same manner to maintain the balance between the first interconnected and the second interconnect, and a resonator disposed in between the first interconnect and the second interconnect such that a distance between the first interconnect and the resonator is substantially the same as a distance between the resonator and the second interconnect. Importantly, the resonator is configured to reduce certain common-mode noise at specific frequencies from both a first signal carried by the first interconnect and a second signal carried by the second interconnect without distorting the first signal relative to the second signal or the second signal relative to the first signal. 
     One advantage of the disclosed system is that common-mode filtering is employed, so the resonator reduces common-mode noise at specific frequencies without distorting the information-carrying voltage difference between the differential signals. Further, since the system is implemented as a PWB resonator, the incremental cost of including this type of filter on the PWB is relatively small. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
         FIG. 1  illustrates a resonator configured to reduce unwanted common-mode noise at specific frequencies from a differential signal, according to one embodiment of the invention; 
         FIG. 2  illustrates the resonator of  FIG. 1  that includes a discrete capacitor, for adjusting the resonant frequency of the resonator, according to one embodiment of the invention; 
         FIG. 3  illustrates a system with two resonators configured to reduce a band of unwanted common-mode noise at specific frequencies in a differential signal, according to one embodiment of the invention; 
         FIG. 4A  illustrates a transfer characteristic of the resonator of  FIG. 1 ; 
         FIG. 4B  illustrates a transfer characteristic of the two resonators of  FIG. 3 ; and 
         FIG. 5  illustrates an electronic device in which one or more aspects of the invention may be implemented. 
     
    
    
     DETAILED DESCRIPTION 
     Filtering both signals in a differential signal pair with a common filter characteristic reduces unwanted common-mode noise at specific frequencies from the differential signals without distorting one signal relative to the other signal. In a differential signal pair constructed from a pair of PWB interconnects, configuring both PWB interconnects to include a resonator that is located parallel to and spaced equally between the PWB interconnects filters the common-mode noise on both PWB interconnects with a common filter characteristic. As is well known, the filter characteristics of a resonator are determined by the physical characteristics (e.g., length and dielectric constant) of the resonator, and the spacing between the resonator and the PWB interconnect. Equally spacing a single resonator between two PWB interconnects of the same length and width filters both interconnects with the common filter characteristic of that resonator. Therefore, including a resonator that is equally spaced between the PWB interconnects of a differential signal pair reduces the unwanted common-mode noise at specific frequencies from the signals in both PWB interconnects without distorting a signal in one PWB interconnect relative to a signal in the other PWB interconnect. 
       FIG. 1  illustrates an electronic device  100  that includes a resonator  110  configured to reduce unwanted common-mode noise at specific frequencies from a differential signal carried by PWB interconnects  106  and  108 , according to one embodiment of the invention. As shown, the electronic device  100  includes a transmitter  102 , a receiver  104 , a PWB interconnects  106  and  108  for carrying differential signals, and a resonator  110 . Importantly, the physical characteristics of the PWB interconnects  106 ,  108 , such as their length and width, substantially match. Additionally, the spacing between the resonator  110  and the PWB interconnect  106  substantially matches the spacing between the resonator  110  and the PWB interconnect  108 . Therefore, as previously described, the filter characteristics of the resonator  110  are common to the PWB interconnects  106 ,  108 . Further, as previously described herein, the resonator  110  is configured to reduce the unwanted common-mode noise at specific frequencies from signals in both PWB interconnects  106 ,  108  without distorting the differential signal. 
     In another embodiment of the invention, a discrete capacitor may be designed in the resonator so that the resonant frequency of the resonator can be adjusted after the PWB and the relevant interconnects have been manufactured. As is well-known, adding a discrete capacitor to a resonator changes the effective length of the resonator, thereby changing the resonant frequency of the resonator. Adjusting the resonant frequency of a resonator is desirable because the resonant frequency can be matched to the frequencies that are determined, through laboratory testing, for example, to generate EMI. In this fashion, the EMI emissions generated by an electronic device can be reduced. 
       FIG. 2  illustrates an electronic device  200  that includes the resonator  110  of  FIG. 1  connected to a discrete capacitor  212 , according to one embodiment of the invention. As shown, the electronic device  200  includes the transmitter  102 , the receiver  104 , the PWB interconnects  106  and  108  for carrying a differential signal, the resonator  110 , and the discrete capacitor  212 . In one embodiment of the invention, the capacitor may be a discrete capacitor that is soldered onto the PWB. In other embodiments of the invention, any technically feasible means of adding capacitance to the resonator  110  may be used to adjust the resonant frequency of the resonator without departing from the scope of the invention. Importantly, and as previously described, the added capacitance enables the resonant frequency of the resonator  110  to be adjusted, thereby the common-mode noise at specific frequencies that the resonator  110  reduces from the differential signal carried by PWB interconnects  106  and  108 . 
     In yet-another embodiment of the invention, as described below in  FIG. 3 , a plurality of resonators with different resonant frequencies may equally spaced between the PWB interconnects of a differential signal pair. In this embodiment, the resonant frequencies of the resonators are configured to cooperatively reduce common-mode noise at a much wider band of frequencies from the differential signal than the single resonator of  FIG. 1  is able to reduce. Again, since the spacing between each resonator and a first PWB interconnect matches the spacing between each resonator and a second PWB interconnect, the resonators reduce the unwanted common-mode noise at specific frequencies from signals in both PWB interconnects without distorting the differential signal in the PWB interconnects  106 ,  108 . 
       FIG. 3  illustrates an electronic device  300  configured to include two resonators  310 ,  312  configured to reduce unwanted common-mode noise at a band of specific frequencies from a differential signal carried by PWB interconnects  106 ,  108 , according to one embodiment of the invention. As shown, the electronic device  300  includes the transmitter  102 , the receiver  104 , the PWB interconnects  106  and  108  for carrying a differential signal, the first resonator  310  and the second resonator  312 . 
       FIG. 4A  illustrates a transfer characteristic  400  of the resonator  110  of FIG.  1  that includes a frequency axis  402 , a common-mode gain axis  404 , a frequency response  406 , and a resonant frequency  408 . Importantly, the common-mode gain of the filter is at a minimum at the resonant frequency  408 , indicating the common-mode noise at the specific frequencies that the resonator  110  is configured to reduce from a differential signal. 
       FIG. 4B  illustrates a transfer characteristic  410  of the resonators  310 ,  312  of  FIG. 3  that includes a frequency axis  412 , a common-mode gain axis  414 , a first frequency response  416  corresponding to the first resonator  310 , a second frequency response  418  corresponding to the second resonator  312 , and a combined frequency response  420 . Importantly, the resonant frequencies of the resonators  310 ,  312  cooperatively reduce common-mode noise at a band of frequencies, based on the partially-overlapping frequency responses  416 ,  418 . Again, the frequencies for which the combined frequency response is minimized indicate the common-mode noise at specific frequencies that resonators  310  and  312  are configured to reduce from a differential signal. 
       FIG. 5  illustrates an electronic device  500  in which one or more aspects of the invention may be implemented. As shown, the electronic device  500  includes a graphics adapter  502 , a main memory  506 , a bus  508 , and a microprocessor  504 , which includes a memory controller  516 . The bus  508  is coupled to the graphics adapter  502  and the microprocessor  504  through interfaces  510  and  512 , respectively, and the main memory  506  is coupled to the microprocessor  504  through a memory interface  514 . Importantly, the bus  508 , the interfaces  510  and  512  and/or other interfaces in the electronic device  500  may include one or more PWB interconnects that transmit differential signals and incorporate one or more resonators to filter unwanted common-mode frequencies, as previously described. 
     One advantage of the disclosed resonator designs is that unwanted common-mode noise at specific frequencies may be reduced from differential signals carried in PWB interconnects by equally spacing one or more resonators between the PWB interconnects without distorting the differential signal in the PWB interconnects. Further, since each resonator is implemented as a PWB resonator, the incremental cost of removing the unwanted common-mode frequencies is very small. 
     While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof. For example, the resonator systems disclosed herein may be implemented in any type of printed wiring board structure, whether the interconnects are substantially aligned in a horizontal fashion on the same printed wiring board layer or in a vertical fashion through a plurality of printed wiring board layers. The scope of the present invention is therefore determined by the claims that follow.