I. Field of the Invention
The present invention relates generally to millimeter wave circuits and, more particularly, to a low loss electrical connection between adjacent high frequency circuit boards.
II. Description of Material Art
Electrical connections between adjacent circuit boards in millimeter wave applications present many challenges. Such circuits are used, for example, in automotive radar applications in the 77 gigahertz range. Such applications, furthermore, are typically low powered so that a low loss electrical connection between adjacent circuit boards is important in order to achieve proper operation of the overall circuit.
In such millimeter wave applications, these circuit boards include a ground plane on one side of a substrate and a microstrip on the opposite side of the substrate and thus spaced from the ground plane. The microstrip terminates in a connection pad along one edge of the circuit board. Similarly, an adjacent circuit board also includes a microstrip and ground plane as well as a connection pad along one edge of the second circuit board. The connection pads are aligned with each other and a wire bond or ribbon bond—hereinafter collectively referred to as wire bonds—electrically connects the two connection pads together and thus electrically connects the two microstrips together for transmission of the high frequency (radio frequency-RF) signal.
Due to the high frequency of the RF signal, the wire bonds between adjacent connection pads on the adjacent circuit boards present a small, but significant, inductance. Furthermore, any misalignment between the connection pads and the adjacent circuit boards, or a misalignment of the wire bond or difference in the length of the wire bond will vary the inductance of the wire bond and thus adversely affect the impedance match between the two circuit boards. Such an impedance mismatch creates signal loss in the transmission of the microwave radio frequency signal from one microstrip to the second microstrip on the two circuit boards which, in turn, adversely affects the overall operation of the microwave system.
The present invention provides an approach to connecting microstrips on adjacent circuit boards in millimeter wave radio frequency applications.
In one approach, the size of the connection pads which terminate the microstrips on the adjacent circuit boards is enlarged. Thereafter, a first wire bond connects the connection pads together while a second wire bond extends between the connection pads in parallel with the first wire bond. The provision of two wire bonds in parallel with each other effectively reduces the inductance of the connection between the two connection pads by approximately one half. In some applications, such reduction in the inductance may be sufficient to create an acceptable impedance match between the microstrips on the adjacent circuit boards for the overall operation of the microwave system.
If the provision for the two wire bonds between the adjacent connection pads does not result in an acceptable impedance match between the two connection pads, and thus between the two microstrips on the adjacent circuit boards, one or more capacitors may be added to one or both of the circuit boards. These capacitors may be trimmed in order to obtain a resonant or near resonant circuit at the desired frequency of operation for the system. These capacitors, for example, may comprise MAM or MIM capacitors and may be selectively added to one or both of the circuit boards as required.
Alternatively, one or both of the microstrips may include a stub line extending laterally from the microstrip. One or more capacitors then electrically connect the stub line to the ground plane and these capacitors may be selectively removed from the circuit by interrupting the stub line. Similarly, a trimmable capacitive stub may also be electrically connected to one or both of the microstrips on the circuit boards.
A better understanding of the present invention will be had upon reference to the following detailed description when read in conjunction with the accompanying drawing, wherein like reference characters refer to like parts throughout the several views, and in which:
With reference first to
Each circuit board 20 and 22 includes its own substrate 24 and 26, respectively, constructed of an electrically insulating material. A first microstrip 28 extends along a top of the first circuit board 20 and terminates in a connection pad 30. Similarly, a second microstrip 32 extends along the top of the insulating substrate 26 of the second circuit board 22 and also terminates in a connection pad 34. In order to electrically connect the microstrips 28 and 32 together, the connection pads 30 and 34 are aligned and adjacent each other.
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In order to add capacitance to the connection between the microstrips 28 and 34, one or more wire bonds 60 extend between the ground pads 42 or 46 on either or both of the circuit boards 20 and 22. Consequently, the wire bonds 60 overlie a portion of the microstrip 28 or 32 of their associated circuit board 20 or 22. Since the wire bond itself is made of metal, a small capacitance is created between the wire bonds 60 and the microstrip 28 or 32.
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Furthermore, although the stub line 70 and its associated components are illustrated in
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The spacing between the line sections 82 and the microstrip 28 determines the amount of capacitance added to the overall circuit by the line sections 82. These line sections 82 may be selectively removed by simply removing a portion of the desired line section 82, as shown at 84, thus electrically disconnecting the line section 82 from the circuit.
In practice, the various approaches to add or delete the capacitance in an attempt to obtain a resonant or near resonant circuit may be performed on an individual basis during the manufacture of the overall system containing the two circuit boards 20 and 22. For example, during the manufacture of the microwave system, the impedance match between the two circuit boards 20 and 22 may be tested and the capacitance added or deleted as required until an acceptable impedance match is obtained.
From the foregoing, it can be seen that the present invention provides a simple and yet highly effective mechanism for obtaining an acceptable impedance match between adjacent circuit boards in a microwave system. Having described my invention, however, many modifications thereto will become apparent to those skilled in the art to which it pertains without deviation from the spirit of the invention as defined by the scope of the appended claims.