Abstract:
A switch matrix comprising a mulitlayer ceramic circuit board having a M×N array of N×N switch modules. Specifically, the ceramic circuit board has a M×N array of mounting sites for accepting individual N×N switch modules. The ceramic circuit board contains all the necessary interconnects to form a M×N RF switch matrix.

Description:
This application claims the benefit of United States Provisional Application No. 60/153,782, filed Sep. 14, 1999, and is hereby incorporated herein by reference. 
    
    
     The invention relates to a RF switch matrix and, more particularly, the invention relates to an integrated RF M×N switch matrix using N×N switch modules. 
     BACKGROUND OF THE DISCLOSURE 
     Switch matrices are critical components in virtually all modern communications systems, especially satellite systems. Such matrices are used principally to route up-link channels to a specific down-link channel. In some instances, switch matrices may have a “broadcast mode” in that an individual incoming signal is simultaneously routed to a number of outgoing channels for transmission. 
     RF switches come in a variety of configurations, the most basic being a single pole, single throw having a single RF input and a single RF output. Typically the RF signal is switched electronically with RF switching diodes or field effect transistors. Such RF switches are combined to form M×N switch matrices and are typically constructed using Monolithic Microwave Integrated Circuitry (MMIC) techniques wherein nonintersecting transmission lines, interconnections, and switch elements are integrated on a multilevel substrate using thin film processing techniques. The MMIC technique is impractical in most instances where there are numerous inputs and outputs to be switched because chip size becomes very large due to circuit complexity. 
     Thus, there exists a need in the art for automated production of efficient, reliable switch matrices that are able to operate in the RF frequency range. 
     SUMMARY OF THE INVENTION 
     The disadvantages associated with the prior art are overcome by forming an M×N RF switch matrix using a multilayer ceramic circuit board that accommodates M×N switch modules, each switch module being an N×N switch matrix. The ceramic structure exhibits low RF loss and the metal base functions both as a heatsink and as a RF ground. The circuit board has an M×N array of mounting sites for accepting the switch modules and contains all the necessary RF routing, bias, and control lines to interconnect the switch modules to from the M×N switch matrix. 
     In one embodiment of the invention, the circuit board is a multilayer ceramic structure on a metal base that is fired using a Low Temperature Ceramic Circuit on Metal (LTCC-M) process. An LTCC-M process provides for zero-shrinkage in the plane of the circuit board allowing for the fabrication of large-area boards with tight tolerances over the entire area of the board. The switch modules can be MMIC devices or can also be fabricated using an LTCC-M process. The switch modules attach to the ceramic circuit board at the mounting sites via wire-bonds or flip-chip bonds. The mounting sites are punch cavities whereby the metal base of each switch module is coupled to the metal base of the motherboard. The switch modules are interconnected by the circuit board to from an M×N switch matrix. In an alternative embodiment of the invention, the switch modules are surface mounted to the circuit board at the mounting sites. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which: 
     FIG. 1 depicts a M×N RF switch matrix; 
     FIG. 2 shows an example of a N×N switch matrix; and 
     FIG. 3 depicts a schematic of an illustrative RF switch for use in a N×N switch matrix. 
    
    
     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. 
     DETAILED DESCRIPTION 
     FIG. 1 depicts one embodiment of an M×N RF switch matrix  100  in accordance with the present invention. The switch matrix  100  comprises a multilayer ceramic circuit board  104  that is adhered to a metal base  102  and a plurality of switch modules  108 . The ceramic circuit board  104  has an M×N array of mounting sites  106  (e.g., 10×10 are shown) for attaching the switch modules  108  to the ceramic circuit board  104 . In the preferred embodiment of the invention, the ceramic circuit board  104  and the metal base  102  are of the type commonly used in an LTCC-M firing process. 
     Multilayer ceramic circuit boards have been in use for several decades and are advantageously used to from integrated RF circuits. The electric properties of ceramics and metals used in an LTCC-M process are suitable for both low and high frequency operation. An LTCC-M manufacturing process is geared to low cost, highly automated production and is well suited to integrated RF sub-system applications. The multi-layer capability of LTCC-M technology allows for the design of compact structures, with short lengths between components, resulting in lower losses and better overall performance. 
     Dimensions of a typical switch matrix are on the order of tens of wavelengths and require tight tolerances in integrated circuit production. LTCC-M exhibits zero shrinkage in the x and y dimensions of the board, allowing for the fabrication of large-area circuit boards and the definition of conductor patterns with tight tolerances over the entire large area of the board. For a detailed description of LTCC-M technology, the reader should refer to commonly assigned U.S. Pat. No. 5,725,808, issued Mar. 10, 1998, and commonly assigned U.S. Pat. No. 5,958,807, issued Sep. 28, 1999. 
     In this illustrative embodiment of the invention, each of the plurality of switch modules  108  comprises a multilayer ceramic circuit board  116  adhered to a metal base  114 , an integrated N×N switch matrix  112 , electronic control circuitry  110 , N input transmission line segments  118 , and N output transmission line segments  120 . As with the main circuit board  104 , the module circuit board  116  is of the type commonly used in an LTCC-M fabrication process. The integrated switch matrix  112  has N inputs and N outputs and is capable of switching any of the N inputs to any of the N outputs. The integrated switch matrix  112  is coupled to the input transmission line segments  118  and the output transmission line segments  120  so as to allow the switch module  108  to function as a N×N switch matrix. In an alternative embodiment of the invention, the switch modules  108  are MMIC devices. 
     Each one of the plurality of switch modules  108  is attached to one of the M×N array of mounting sites  106  via wire-bonds or flip-chip bonds. The mounting sites  106  are punch cavities and the metal base  114  of the module  108  is coupled to the metal base  102  of the main circuit board  104 . The input transmission line segments  118  and the output transmission line segments  120  are coupled to respective transmission segments  122  of the main circuit board  104  by wire-bonds so as to form a M×N switch matrix. In an alternative embodiment of the invention, the modules are surface mounted to the main circuit board  102  at the mounting sites  106 . 
     FIG. 2 shows an example of a N×N switch matrix  112 . The switch matrix  112  comprises N input transmission lines  202 , N output transmission lines  204 , and a plurality of RF switches S ij . The input transmission lines  202  and the output transmission lines  204  are microwave striplines. Each one of the plurality of RF switches S ij  is a PIN diode or similar semiconductor switching element. An i th  one of the input transmission lines  202   i  is coupled to a respective one of the switch cells, S ij . A j th  one of the output transmission lines  204   j  is coupled to a respective one of the switch cells, S ij . In this manner, each one of the input transmission lines  202  is connected to each one of the output transmission lines  204 . Alternative methods of constructing an N×N switch matrix are known to those skilled in the art. 
     FIG. 3 depicts a schematic of an illustrative single pole, single throw RF switch S ij . The switch S ij  comprises a transmission line segment  302  spanning from switch S ij  input to switch S ij  output, a PIN diode  304  having an anode and a cathode, an input coupling capacitor  308 , an output coupling capacitor  310 , and a DC bias source  316  with an inductor  312  (RF choke). The anode of the PIN diode  304  is coupled to the transmission line segment  302  and the cathode of the PIN diode  304  is coupled to ground. he switch S ij  is closed when the diode  304  is reverse biased or unbiased, causing the diode  304  to act as an infinite resistance to RF signals thereby allowing the input signal to pass through the transmission line segment  302  from switch input to switch output. The switch S ij  is open when the transmission line segment  302  is biased at a positive DC voltage sufficient to forward bias the diode  304 , causing the diode  304  to act as a shunt to ground. The input coupling capacitor  308  and the output coupling capacitor  310  block the DC bias voltage from entering the coupled circuitry (not shown). The inductor  312  blocks the RF signal from entering the DC bias  316  circuitry. Alternative switch configurations, for example, MOSFET or other semiconductor type switches, are known to those skilled in the art. 
     Although various embodiments which incorporate the teachings of the present invention have been shown and described in detail herein, those skilled in the art can readily devise many other varied embodiments that still incorporate these teachings.