Abstract:
A switch matrix assembly provides for excellent frequency performance while maintaining good isolation, insertion loss, and crosstalk performance. The switch matrix assembly can include an internal bus connector card that helps eliminate or reduce stubs on a matrix star card that is part of the switch matrix. A stack bus connector card can be added in order to allow for identical busbars on other matrix star cards to be connected “stubless.” The switch matrix can also include bus stub isolator card(s) in order for busbar stubs to be broken-off reasonably close to final termination points. The stackable design of the switch matrix assembly allows for a fairly dense design which helps save space, improves serviceability, and improves overall switching performance.

Description:
FIELD OF THE INVENTION 
   The invention relates in general to the field of electronics and more particularly to a switch matrix for use in switching electrical signals. 
   BACKGROUND 
   Designing a switch matrix that provides for low insertion loss while maintaining excellent isolation and crosstalk performance for use in critical situations as when testing amplifiers, receivers or other active devices is very difficult to accomplish. Switching high frequency signals becomes even harder to accomplish without degrading the signals significantly. As the dimensional size of a switch matrix increases, signal path lengths increase causing a degradation of the electrical signals traveling through the matrix, primarily due to the effects of un-terminated stubbing. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features of the present invention, which are believed to be novel, are set forth with particularity in the appended claims. The invention may best be understood by reference to the following description, taken in conjunction with the accompanying drawings, in the several figures of which like reference numerals identify like elements, and in which: 
       FIG. 1  shows an assembled switch matrix viewed from the edge in accordance with an embodiment of the invention. 
       FIG. 2  shows a layout view of a stack bus connector card viewed from the component side in accordance with an embodiment of the invention. 
       FIG. 3  shows a schematic view of a portion of the stack bus connector card in accordance with an embodiment of the invention. 
       FIG. 4  shows a layout view of an internal bus connector card viewed from the component side in accordance with an embodiment of the invention. 
       FIG. 5  shows a schematic view of the internal bus connector card. 
       FIG. 6  shows a matrix star card viewed from the component side in accordance with an embodiment of the invention. 
       FIG. 7  shows a schematic view of the matrix star card in accordance with an embodiment of the invention. 
       FIG. 8  shows a bus stub isolator card viewed from the component side in accordance with an embodiment of the invention. 
       FIG. 9  shows a schematic view of the bus stub isolator card in accordance with an embodiment of the invention. 
       FIG. 10  shows a typical application for this invention which is illustrated in this example as an electronic test system that routes multiple electronic signals from a unit under test to one or more test instruments for measurement and analysis in accordance with an embodiment of the invention. 
   

   DETAILED DESCRIPTION 
   While the specification concludes with claims defining the features of the invention that are regarded as novel, it is believed that the invention will be better understood from a consideration of the following description in conjunction with the drawing figures. 
   The switch matrix of the present invention provides for a direct current to radio frequency switch matrix that provides electrical interconnection from any point to any and all points in the matrix. High frequency performance is achieved for single point-to-point connections by reducing stubs and packing the matrix in a very dense three dimensional stackable package. The use of multiple stackable cards allows for different switch designs to be implemented, while providing for easy replacement of damaged cards within the matrix. 
   Referring now to  FIG. 1 , there is shown a side view of an assembled switch matrix  100  in accordance with an embodiment of the invention. Switch matrix  100  in the illustrative embodiment comprises a non-blocking, full fan-out switch matrix. Switch matrix  100  includes a pair of stack bus connector cards  102  one at each end of the switch matrix  100 . Coupled to the stack bus connector cards  102  are internal bus connector cards  104 . The switch matrix  100  also includes a plurality of matrix star cards  106  and bus stub isolator cards  108  stacked together and electrically interconnected as shown. 
   The switch matrix  100  shown in  FIG. 1  takes up an area of approximately 4.6″×8.25″×9.25″ providing for a compact design, which is very helpful when performing high frequency switching. The dimensional size of the switch matrix can vary based on design requirements and other factors. It should be noted that although the matrix  100  is shown with a certain number of matrix star cards  106 , bus stub isolator cards  108 , stack bus connector cards  102  and internal bus connector cards  104 , different designs may use different number of cards. The term card is an industry phrase that is used to describe an electrical assembly such as a printed circuit board assembly. 
   As shown in  FIG. 1 , matrix  100  includes three columns of internal bus connector cards  104 , matrix star cards  106  and bus stub isolator cards  108  located between the two stack bus connector cards  102 . The different cards are electrically interconnected with each other using electrical connectors such as radio frequency (RF) connectors that allow for press in connections. The connectors used in one embodiment of the invention are MMCX series RF connectors that provide for snap-on mating and can operate at high frequencies. 
   The bus stub isolator cards  108  are preferably placed at a minimum of every four matrix star cards  106  or less so that busbar stubs located on the matrix star cards  106  are electrically “broken-off” reasonably close to the final termination points. In the matrix  100  two stack bus connector cards  102  and two internal bus connector cards  104  are located at opposite ends of the assembly in order to reduce the worst case electrical path distance by approximately half. In one embodiment, the switch matrix  100  comprises a single-wire design in order to achieve a 40 MHz bandwidth specification. The single-wire design also helps improve both crosstalk and isolation. 
   For sake of clarity, the control lines that control the relays found in switch matrix  100  as well as the input and output signal lines have not been shown. In one embodiment all of the cards  102 - 108  are held together using one or more rods that are slid through apertures found in each of the cards, the end of the rods can accommodate a nut or other fastening device to help retain the rods in place. If one of the cards  102 - 108  becomes defective, it becomes very easy to replace it by simply unfastening the rods, snapping off one or more of the cards in the stack to reach the defective card and replacing the card. In another embodiment, the stack cards  102 - 108  can be designed to be cards which can be inserted into a backplane bus. In this embodiment, some of the cards would have edge traces that would allow signal/power lines such as relay control, power and I/O signal lines to interconnect with the cards  102 - 108 . 
   Each of the different stackable cards that make up the switch matrix will now be discussed. Referring now to  FIG. 2 , there is shown a layout view of the stack bus connector bus card  102 . The stack bus connector bus card  102  includes three stack sections  202 ,  204 ,  206  for interconnecting with the three columns of cards  110 - 114  found in the stack matrix  100 . Each stack section  202 ,  204  and  206  as shown includes 25 busbar connectors (B 1 -B 25 ) which are used to electrically interconnect the different cards. Although three stack sections  202 - 206  are used in the embodiment being discussed, a different number of stack sections can be included depending on a particular design requirement. 
   The busbar connectors comprise MMCX series connectors that provide for snap-on mating between connectors from one card to the next in the stack and provide for excellent radio frequency (RF) signal transfer performance at a nominal 50 Ohm impedance. The MMCX connectors are typically soldered onto the printed circuit board and have a very small height, such as less than 0.2 inch. Also found in each of the stack sections  202 ,  204  and  206  are a plurality of relays  208 , section  202  includes 10 relays, section  204  includes 20 relays and section  206  includes 10 relays. In one embodiment the relays are Double-Pole, Double-Throw (DPDT) where the two throws can be placed in series for better isolation or in parallel for better carry current and are rated for one ampere of current, although depending on the design requirements as to what type of signals will be switched; different types of relays can be used. In another embodiment, both throws of the relay could be utilized to switch differential signals by utilizing busbar connectors other than MMCX providing dual coaxial signal paths. 
   In  FIG. 3 , there is shown a schematic view of a portion of the stack bus connector card  102 . Busbars B 6  through B 15  of stacks  202 ,  204  and  206  can be electrically interconnected to each other as shown. For example, busbars B 6  in stack # 1   202 , busbar B 6  in stack # 2   204  and busbar B 6  in stack # 3   206  can be interconnected with each other as shown. The remaining busbars B 1 -B 5  and B 16 -B 25  can also be switched through the stack bus connector card  102 , by joining busbars in the internal bus connector card  104 . The dashed lines show a selectable interconnection using some of the relays found in the stack bus connector card  102 . The selectable interconnects can be selected using an external controller (not shown) coupled to the switch matrix  100 . The control lines that control the individual relays are not shown. The selectable interconnections allow for example busbar B 6  in stack # 1   202  to be electrically interconnected to B 6  in stack # 2   204  or stack # 3   206  or all three to be electrically interconnected to each other for low-frequency or direct current signal routing. Two relays are used to switch between the three corresponding busbars (B 6 -B 15 ). The schematic view also shows breakable stub interconnections  302  that are available for use for different switching requirements. The stack bus connector card  102  allows for electrical signals to go from one stack  202 - 206  to another stack, as such allowing for interconnection of any point in the matrix to any other point. 
   The next card in the switch matrix  100  is the internal bus connector card  104 . A layout view of the internal bus connector card  104  is shown in  FIG. 4 . Each of the internal bus connector cards  104  include 25 busbars B 1 -B 25  that interconnect to the corresponding busbars in the adjoining cards. The internal bus connector card  104  includes 30 relays. The amount of relays and busbars used on the bus connector cards  104  or any of the other cards can of course be adjusted based on particular design requirements. Busbars are shared in the internal bus card  104  of this particular embodiment by using several mathematical relationships. For busbars “n” where n=1 to 5 the sharing relationship is Bn, B(n+10), and B(n+20). For busbars “n” where n=6 to 10 the sharing relationship is Bn and B(n+10). The internal bus card  104  helps improve frequency performance in the switch matrix by minimizing any busbar stubs in a signal path that is selected within the switch matrix  100 . 
   In  FIG. 5  there is shown a schematic view of the internal bus connector card  104 . Two relays are used to switch between busbar sets (e.g., B 1 , B 11  and B 21 , B 5 , B 15  and B 25 , etc.). Breakable stubs  502  are also provided for further design flexibility. 
   Referring now to  FIG. 6 , there is shown a top layout view of a matrix star card  106 . The matrix star card  106  shown includes four 1×10 matrixes per card. Signal cables  602 - 608  are each coupled to one of the four 1×10 matrixes found per card. The electrical schematic for the matrix star card is shown in  FIG. 7 . The “n” in the I/O pin designators (SAn, SBn, SCn, SDn) refers to the matrix star card reference designator number. In the SAn  602  matrix for example, one of the B 1 -B 10  busbar signal lines is selected to be sent out from the matrix. The B[1:25] electrical nodes are the busbars that run vertically between different matrix star cards  106 . When a single matrix star card  106  includes at least one internal bus connector card  104 , stubless connections are possible between busbars on a particular matrix star card  106  as previously discussed. This makes it possible to interconnect any I/O pin to any and all busses, and any I/O pin to any all I/O pins located on the same matrix star card stack  110 ,  112  or  114 . 
   Using at least one stack bus connector card  102  in a switch matrix assembly allows busbars B[6:15] to be connected “stubless” to the identical busbars on other matrix star card stacks  110 ,  112 ,  114 . In conjunction with the internal bus connector card  104  in each stack, it is possible to electrically interconnect any and all I/O pins to any and all busses and to all other I/O pins in the switch matrix  100 . 
   In  FIG. 8 , there is shown a top view of a bus stub isolator card  108 . The bus stub isolator card  108  helps keep the worst case stub length to as small a length as possible. In one embodiment, the bus stub isolator card  108  helps keep stub lengths to a maximum of 0.5 inch or less. The bus stub isolator cards  108  in the switch matrix cause selectable electrical open circuits to occur between matrix star cards  106  at periodic intervals throughout the switch matrix  100 . This is accomplished as shown in the schematic view shown in  FIG. 9  by the relays found in the bus stub isolator card  108  which either provide for an electrical connection between the top and bottom portions of B 1 -B 25  or electrically isolate the top and bottom portions of B 1 -B 25  as required. Each of the relays is controlled by a switch matrix controller which is programmed to switch each of the relays in the switch matrix to a state that minimizes path routing distance, minimizes insertion loss, and maximizes frequency performance depending on the path(s) selected. 
   Referring now to  FIG. 10 , there is shown an electronic test and measurement system in accordance with an embodiment of the invention. The system includes test equipment  1002 - 1006  such as automatic test equipment which is under the control of a controller such as a computer. The test equipment  1002 - 1006  utilized will depend on the type of testing that is required for the equipment under test  1010  (also referred to as a unit under test or UUT). For example, the test equipment  1002 - 1006  may comprise a programmable power supply, frequency synthesizer and other assorted test equipment. A switch matrix  1008  similar to switch matrix  100  which is under the control of a test equipment controller  1012  provides all the necessary signal switching between the test equipment  1002 - 1006  and the equipment under test  1010 . The equipment under test  1010  can be any electronic device or assembly that requires testing. 
   The controller  1012  can comprise a personal computer or specialized controller hardware and/or software, or other control equipment. In one embodiment, the controller  1012  programs the test equipment  1002 - 1006  and selects the switch configuration for switch matrix  1008  during testing. The controller  1012  can also include a display, keyboard, printer and memory for providing an interface to a user and for storage of the test results. The controller  1012  can also be coupled to a computer network so that the test results can be forwarded to a remote site if so required. 
   Switch matrix  100  provides for a very compact and extremely dense switching matrix. By using bus stub isolator cards  108 , the worst case stub is kept to a very short length, thereby providing for improved signal performance. Switch matrix  100  provides for the high frequency performance advantage of a conventional tree switch design and the flexibility of a cross point switch without the problems signal degradation problems caused by stubs. 
   While the preferred embodiments of the invention have been illustrated and described, it will be clear that the invention is not so limited. For example, although star matrix  100  has been shown with a set number of different cards, different designs can not include all cards, include more cards of a particular type, etc. Numerous modifications, changes, variations, substitutions and equivalents will occur to those skilled in the art without departing from the spirit and scope of the present invention as defined by the appended claims.