Patent Publication Number: US-7215558-B2

Title: EMI shield for transceiver

Description:
The present application is a continuation of U.S. patent application Ser. No. 10/033,688 filed Dec. 27, 2001 now U.S. Pat. No. 6,980, 439, entitled “EMI Shield For Transceiver”. The U.S. patent application Ser. No. 10/033,688 is hereby entirely incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The described invention relates to the field of integrated circuits. In particular, the invention relates to reducing electromagnetic interference (EMI) from a transmitter. 
     2. Description of Related Art 
     A transmitter operating at high frequencies, such as 10 Ghz, emits electromagnetic (EM) radiation from its integrated circuit package. The EM radiation may interfere with other electronic equipment. Thus, reducing EMI from a high-frequency transmitter is important. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram showing one embodiment of a transceiver. 
         FIG. 2  is a 3-D diagram of a transceiver showing a metal shield  210  positioned to cover the oscillator  120 . 
         FIG. 3  is a 3-D diagram that shows one example of a metal shield. 
         FIG. 4  is a 3-dimensional diagram showing a cutaway view of a transceiver. 
         FIG. 5  is a cross-sectional side-view of the transceiver of  FIG. 4 . 
         FIG. 6  is a cross-section side-view that shows an alternate embodiment of a transceiver. 
     
    
    
     DETAILED DESCRIPTION 
     A metal shield is used for reducing EMI from a transmitter or transceiver. The integrated circuit package for the transmitter/transceiver comprises a printed circuit board, a non-metal connector, and a metal casing. The printed circuit board includes a ground ring around the non-metal connector. The metal casing substantially encloses the printed circuit board and has an opening that allows access to the non-metal connector. The metal casing has a metal lip that makes physical and electrical contact with the ground ring of the printed circuit board. In one embodiment, a second metal shield may be employed to reduce clock jitter. 
       FIG. 1  is a schematic diagram showing one embodiment of a transceiver  10 . The transceiver  10  comprises a transmitter  20  and a receiver  30 . The receiver  30  receives optical data  32 , converts it to electrical data via an optical-to-electrical converter  34 , and deserializes the electrical data with a deserializer  36  to provide an electrical signal  38 . The transmitter  20  receives electrical data  40 , converts it to optical signals and sends the optical data  172  out via an optical interconnect, such as an optical fiber. 
     The transmitter  20  comprises a phase lock loop (PLL)  110 , an oscillator  120 , a serializer  130 , and an electrical-to-optical converter  170 . In one embodiment, the oscillator  120  is a voltage-controlled oscillator (VCO). The PLL  110  receives a reference input  42  and provides a voltage  112  to the VCO  120 . The VCO  120  provides a frequency signal f VCO    114  to the PLL  110 , and the PLL  110  provides a clock signal  116  to the serializer  130 . 
     In one embodiment, the serializer  130  comprises a clock multiplier unit (CMU)  140 , a multiplexer  150  and an amplifier  160 . The CMU  140  multiplies the clock signal provided to it by the PLL  110 , and provides the multiplied clock signal to the multiplexer (MUX)  150  which serializes input data  40 . In one embodiment, the MUX  150  is a 16:1 multiplexer, and the CMU  140  multiplies the clock signal  116  by 16 to yield a 10 GHz clock signal  142 . The output of the MUX  150  is amplified by amplifier  160  and provided to the electrical-to-optical converter  170 , which then sends out the optical data  172 . 
     Clock jitter generated in the transceiver  10  is a composite of inherent clock jitter based on the quality of the reference signal and the oscillator  120  and PLL  110 , as well as noise such as pattern-dependent noise from, e.g., the switching of the MUX  150 . Clock jitter may cause data reliability problems if the jitter is too high. In one embodiment, a metal shield  210  is placed around the oscillator  120  to reduce the clock jitter. The metal shield combined with a ground plane, as will be described with respect to  FIGS. 2 and 3 , form a Faraday cage around the oscillator  120 . This reduces the electromagnetic interference (EMI) of other components from interfering with the oscillator  120 , which results in reduced clock jitter for the transmitter. 
       FIG. 2  is a 3-D diagram of a transceiver showing a metal shield  210  positioned to cover the oscillator  120 . A ground ring  212  on the printed circuit board (PCB)  250  surrounds the oscillator and is coupled to one or more ground planes of the PCB  250 . In one embodiment, the ground ring  212  is coupled to the ground planes through vias in the PCB  250 . In one embodiment, two or more holes  230  are used to help align the metal shield  210  as will be discussed below. 
       FIG. 3  is a 3-D diagram that shows one example of a metal shield  210 . In one embodiment, the metal shield  210  has a plurality of protrusions. Attachment protrusions  310  allow the metal shield to be coupled to the ground ring and the printed circuit board. In one embodiment, the metal shield is soldered to the ground ring via the attachment protrusions  310 . In another embodiment, the attachment protrusions  310  may feature a hole that allows a screw to hold the metal shield  210  to the printed circuit board  250 . 
     Positioning protrusions  320  allow the metal shield to be aligned properly on the printed circuit board. In one embodiment, the positioning protrusions  320  are inserted into holes  230  ( FIG. 2 ) in the printed circuit board. 
       FIG. 4  is a 3-dimensional diagram showing a cutaway view of a transceiver  400 . A non-metal connector  410  is attached to a printed circuit board (PCB)  420 . A ground ring  430  surrounds the non-metal connector  410 . In one embodiment, the “non-metal connector” comprises a plastic or ceramic, but also comprises metal interconnects for providing data, control, and status I/O to and from the transceiver. 
     A metal casing  440  substantially encloses the PCB  420 . In one embodiment, the metal casing  440  comprises a top portion  440   a  that substantially covers a top portion of the PCB  420 , and a bottom portion  440   b  that substantially covers a bottom portion of the PCB  420 . The metal casing  440  makes physical and electrical contact with the ground ring  430 . The top portion  440   a  and/or bottom portion  440   b  may comprise a heat sink  450  having multiple fins. 
     In one embodiment, a top perimeter ground ring  442  surrounds a perimeter of the top surface of the PCB  420 , and the top portion of the metal casing  440   a  makes electrical contact with the perimeter ground ring  442 . 
       FIG. 5  is a cross-sectional side-view of the transceiver of  FIG. 4 . A bottom perimeter ground ring  444  may surround a bottom perimeter of the PCB  420 . Vias  460  couple the top perimeter ground ring  442  with the bottom perimeter ground ring  444 . In one embodiment, the vias  460  are spaced intermittently around the top and bottom perimeter ground rings  442 ,  444 . 
     The ground ring  430  surrounds the non-metal connector  410 . In one embodiment, the ground ring  430 , top perimeter ground ring  442 , and bottom perimeter ground ring  444  are all electrically coupled together by vias  460 ,  462  and an internal ground plane  470 . A conductive gasket  464  may be inserted between the top conductive portion of the metal casing  440   a  and the ground ring  430  to provide a good electrical coupling. 
     In one embodiment, the top portion of the metal casing  440   a  at least partially overlaps a perimeter of the bottom portion of the metal casing  440   b  through their respective edges  480  and  482 . Alternatively, the bottom portion  440   b  could be manufactured to overlap a perimeter of the top portion  440   a.    
       FIG. 6  is a cross-section side-view that shows an alternate embodiment of a transceiver  500 . The transceiver circuitry resides on a PCB that is enclosed by a metal casing  540 . In one embodiment the metal casing comprising a top half  540   a  and a bottom half  540   b  that make physical and electrical contact with one another. In one embodiment, a conductive gasket (not shown) may be inserted between the two metal casings  540   a ,  540   b  to provide a better contact. In one embodiment, a conductive gasket (not shown) may be inserted between the two metal casings  540   a ,  540   b  to provide a better contact. 
     A non-metal connector  510  is coupled to the PCB  520  and extends up through an opening in the metal casing  540  so as to be accessible from outside of the integrated circuit package. A ground ring  530  on the PCB  520  surrounds the non-metal connector  510 . 
     In one embodiment, the metal casing  540  includes a metal lip  570  that makes physical and electrical contact with the ground ring  530  of the PCB  520 . A conductive gasket  572  may be inserted between the metal lip  570  and the ground ring  530  to provide good electrical coupling. 
     Thus, a metal shield for reducing EMI from a transceiver is disclosed. However, the specific embodiments and methods described herein are merely illustrative. For example, a conductive gasket may serve at an interface along with any of the described ground rings. Numerous modifications in form and detail may be made without departing from the scope of the invention as claimed below. The invention is limited only by the scope of the appended claims.