Abstract:
A miniaturized (compact) millimeter wave hybrid based on a coupling function and associated method for designing millimeter wave electrical elements based on a coupling function is provided. The invention overcomes a drawback of conventional coupled transmission lines structures and equivalent lumped elements in that they are considerably large in size for IC designs making such structures unsuitable for use in RFICs&#39; designs. Hence, such structures are not partial in millimeter wave electrical elements, and more particularly elements that should be integrated in devices for 60 GHz applications.

Description:
TECHNICAL FIELD 
     The present invention relates generally to millimeter wave electronics, and more particularly to millimeter wave electrical elements implementing a coupling function. 
     BACKGROUND OF THE INVENTION 
     The 60 GHz band is an unlicensed band that features a large amount of bandwidth which means that a very high volume of information can be transmitted wirelessly. As a result, multiple applications, that require transmission of a large amount of data, can be developed to allow wireless communication around the 60 GHz band. Examples for such applications include, but are not limited to, wireless high definition TV (HDTV), wireless docking station, wireless Gigabit Ethernet, and many others. The objective of the industry is to integrate 60 GHz band applications with portable devices including, but not limited to, netbook computers, tablet computers, smartphones, laptop computers, and the like. The physical size of such devices is relatively small, thus the area for installing additional circuitry to support 60 GHz applications is limited. 
     Recently developed techniques allow fabricating integrated circuits (ICs) that operate in millimeter wave frequencies. Example for one such technique is the monolithic microwave integrated circuit (MMIC) or radio frequency IC (RFIC). The dimensions of RFICs are relatively small, ranging from around  1  square-millimeter (mm 2 ) to 10 mm 2  and can be mass-produced. 
     However, even when designing a RF circuit as an IC, there still is a need to integrate many electronic elements in the same IC while keeping the size of the fabricated chip as small as possible and while not degrading performance of the IC. In addition, there is a need to fabricate such a design with minimum design constraints. 
     Phase shifters and hybrids are examples for electronic elements that implement a coupling function. Such elements are designed to achieve a specific phase difference and impedance matching between ports. A hybrid is a form of a reciprocal four-port device that provides a phase difference of 90-degrees or 180-degrees between two designated ports. A phase shifter can be implemented based on the hybrid to provide a controllable phase change of a RF signal as part of a phase array antenna. 
     One technique for designing of a hybrid element is based on a coupled transmission lines structure  100  as illustrated in  FIG. 1 . The length of the transmission lines  110  is typically λ/4. However, when fabricating the lines on a multilayer semiconductor substrate the length of each line is shorten due to characteristics of the substrate, but still the length of each metal line  110  is not less than 0.6 mm (600 micron, instead of 1.25 mm). Mutual coupling is achieved when the metal lines  110  are perfectly adjacent to each other. The coupling may be degraded, hence the efficiency of the structure  100  due to increased line spacing, line impedance differences, and/or line lengths less than λ/4 is also degraded. Other conventional techniques for designing a hybrid are the branch-line coupler and the rat-race Hybrid, which typically include 4 transmission lines. 
     The coupled transmission lines structure  110  may be implemented in a form of a lumped element to achieve good coupling. A diagram illustrating such an implementation is shown in  FIG. 2 . In the lumped element  200 , transmission lines are implemented as a LC (pi) network that includes inductors  210  and capacitors  220 . The inductors  210  are wound on the substrate. However, such implementation requires a specific inductors and capacitors, thus resulting in a larger area and more gain losses in comparison to the structure  100 . 
     Thus, one of the drawbacks of the conventional coupled transmission lines structure and an equivalent lumped element is that they are considerably large in size for IC designs. Therefore, such structures are unsuitable for use in RFICs&#39; designs, hence such structures are not partial in millimeter wave electrical elements, and more particularly elements that should be integrated in devices for 60 GHz applications. 
     Therefore, it would be advantageous to provide a solution for designing compact size millimeter wave electrical elements that are based on a coupling function. 
     SUMMARY OF THE INVENTION 
     Certain embodiments of the invention include a method for designing a coupling-function based millimeter wave electrical elements. The method comprises computing a length of a first metal line and a second metal line; computing a first number of turns for the first metal line and a second number of turns for the second metal line, wherein the length and number of turns of each of the first metal line and the second metal line are computed to meet radio-frequency (RF) properties of a millimeter wave electrical element to be designed; determining a width value of each of the first metal line and the second metal line; determining a spacing value between the first metal line and the second metal line; winding the first metal line on a first metal layer according to the first number of turns and winding the second metal line on the first metal layer and, in part, on a second metal layer according to the second number of turn, thereby resulting in a spiraled structure; and setting ports for the spiraled structure to form a complete design of the millimeter wave electrical element. 
     Certain embodiments of the invention further include a miniaturized millimeter wave hybrid. The miniaturized millimeter wave hybrid comprises a first metal line that connects a first port to a third port, wherein the first metal line is wound, a first number of turns, on a first metal layer of a multilayer substrate; a second metal line that connects a second port to a fourth port, wherein the second metal line is wound in a close proximity to the first metal line, a second number of turns, on a first metal layer and, in part, on a second metal layer of a multilayer substrate, thereby resulting in a spiraled structure; wherein the first port receives an input radio-frequency (RF) signal and each of the third port and the fourth port outputs an output RF signal having a phase difference relatively to the input RF signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter that is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other objects, features, and advantages of the invention will be apparent from the following detailed description taken in conjunction with the accompanying drawings. 
         FIG. 1  is a schematic diagram of a coupled transmission lines structure; 
         FIG. 2  is a schematic diagram of a lumped element coupled-lines hybrid; 
         FIG. 3  is a flowchart illustrating a method for designing a coupling function based millimeter wave electrical elements in accordance with an embodiment of the invention; 
         FIG. 4  is a schematic diagram of a 90-degree hybrid in accordance with an embodiment of the invention; 
         FIG. 5  is a schematic diagram of a 90-degree hybrid in accordance with another embodiment of the invention; and 
         FIG. 6  depicts graphs illustrating return loss varying with frequency measured for a 90-degree hybrid designed in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The embodiments disclosed by the invention are only examples of the many possible advantageous uses and implementations of the innovative teachings presented herein. In general, statements made in the specification of the present application do not necessarily limit any of the various claimed inventions. Moreover, some statements may apply to some inventive features but not to others. In general, unless otherwise indicated, singular elements may be in plural and vice versa with no loss of generality. In the drawings, like numerals refer to like parts through several views. 
     Certain exemplary embodiments of the invention provide a method for implementing a coupling function when designing millimeter wave electric elements. Using the method of certain embodiments, efficient hybrids, and phase shifters for millimeter wave frequencies can be designed. In accordance with certain embodiments of the invention, a good coupling function characterized by strong self-inductance and capacitance is achieved by tightly winding two metal lines, such as bifilar lines on the same layer of a multilayer substrate. 
       FIG. 3  shows a non-limiting and exemplary flowchart  300  illustrating a method for designing coupling function based millimeter wave electrical elements in accordance with an embodiment of the invention. Two metal lines are required to design a coupling function based millimeter wave electrical element. At S 310 , the length and a number of turns of each of the metal lines are computed. In accordance with certain embodiments, a “turn” shall have the meaning of a curve on a plane that winds around a fixed center point in the designed element and creates a complete loop (not a closed loop). The effect of the turns is of a wound structure printed on a plane of a multilayer semiconductor substrate. The number of turns and length of the metal lines computed to meet RF properties of the element to be designed. The RF properties include, but are not limited to, operation frequency, phase difference, bandwidth, and impedance matching of the element&#39;s ports. For example, increasing the number of turns improves the lumped effect of the designed structure, thus reducing the losses of the element and the overall size. As another example, the length of the metal lines determine the operating frequency of the designed electrical element (longer lines lead to lower frequency and vice versa). For example, when designing a 90-degree hybrid, the resulting structure resonates between windings of the two metal lines. The resonance frequency is such that the power transferred from an Input port to a Through port of the hybrid is equal to the power delivered from an Input port to a Coupled port of the hybrid. The resonance frequency determines the total line length given the possible number of turns. Higher frequencies require shorter metal lines, and given the size limitation of the hybrid, it leads to less turns. Further, the winding should be performed without closing the “hole” in the spiral structure which is required for magnetic coupling to happen. Therefore, in accordance with an embodiment of the invention, the length and number of turns are determined based on the magnitude of the power transfer from the Input to Through port and from the Input to Coupled port of the design, the crossing point of these power transfers states the resonating frequency. That is, the Input-Through ports transfer declines, while the Input-Coupled power transfer inclines. The crossing point of the power transfers can be changed by adjusting the number of turns to meet a specific frequency. 
     At S 320 , the minimum width of each of the metal line is determined based on the process utilized for fabricating the designed element. Preferably, the width of a metal line should be approximately close to the minimum width allowed by the fabrication process. For example, a deviation of up to 5% from the minimum width for a metal line, as defined by the process, is allowed. In an exemplary embodiment of the invention, the minimum width of a metal line is less than 3 micron. 
     At S 330 , the minimum spacing allowed between two metal lines is determined based on the fabrication process. Preferably, the spacing value is selected to be approximately close to the minimum spacing between two metal lines as allowed by the fabrication process. For example, a deviation of up to 5% from the minimum spacing value is allowed. In an exemplary embodiment, the spacing between the two metal lines is 0.5 micron. 
     To maximize the mutual inductance and capacitance between the metal lines, a fabrication process that can ensure minimum spacing and width values is selected. In accordance with an exemplary embodiment of the invention, a 7-copper metal layers deep sub-micron used for 65 nanometer (nm) CMOS fabrication process is utilized. Such a process provides the capability of fine metal spacing and width, thus strong electromagnetic fields can be achieved for compact structures with tight windings. Further, providing compact structures using a CMOS process causes to relaxation in metal density constraints. 
     At S 340 , the two metal lines each having a length as computed at S 310  and width as determined at S 320  are wound on one layer of the multilayer semiconductor substrate. The metal lines are adjacent to each other at a minimum spacing distance. The number of turns of the two metal lines should be performed as computed at S 310 . Various embodiments for different setting of the length and number of turns for constructing electrical elements are provided below. 
     It should be noted that winding the metal lines does not require following any symmetry requirements and port locus, thus flexibility when designing millimeter wave electrical elements based on a coupling function is allowed. At S 350 , ports of the resulting spiraled structure are defined by specifying the connectivity of each end of each metal line. Various examples for defining the ports are provided below. 
     It should be appreciated that by using minimum line spacing and minimum width, a relatively high number of windings can be achieved. This results in a compact structure characterized by strong mutual inductance and capacitance which is relatively insensitive to port locus. Further, tightly winding the two metal lines results in strong coupling properties as provided by a lumped element without using specific inductors and capacitors in the design. In addition, as will be demonstrated below, millimeter wave electrical elements designed using the disclosed method includes a small number of crosses between the metal lines and low return path losses. 
     The above embodiments can be implemented in any computer aided design (CAD) tools utilized in the design of RFICs. 
       FIG. 4  shows an exemplary diagram of a hybrid design  400  according to an embodiment of the invention. The hybrid  400  is designed to operate in millimeter wave frequencies, e.g., a frequency band of 60 GHz. The hybrid design  400  is constructed using metal lines  410  and  420 . The width of each of the metal lines  410  and  420  and the spacing between them is set to the minimum values allowed by the fabrication process, e.g., a copper metal layers deep sub-micron used in a 65 nanometer CMOS process. 
     The length and number of turns of the metal lines  410  and  420  determine spiraling of the metal lines, thus impact the mutual capacitance and inductance, capacitance to ground, self-inductance of the hybrid  400 . It should be noted that only for the sake of clarity, without limiting the scope of the invention, the metal lines  410  and  420  are illustrated differently. However, this does not imply that the lines  410  and  420  behave differently. 
     In the hybrid design  400 , there are 4 ports  401 ,  402 ,  403  and  404  defined as Input, Isolated, Through, and Coupled ports respectively. The metal line  410  connects port  401  to port  403  and curves for 1.75 turns. The port  402  is connected via 1.5 turns in the metal line  420  to the port  404 . The port  402  is further connected to a termination, e.g., a 50 Ohm resistor. The metal line  420  is fabricated on the same metal layer (e.g., layer  7 ) of a multilayer substrate  440  as of the metal line  410 , except for the section between vias  441  and  442 . In this section, the metal layer  420  underpasses to a lower metal layer (e.g., layer  6 ). In accordance with an exemplary embodiment of the invention, the metal lines&#39;  410  and  420  lengths, widths and spacing are 275 micron, 3 micron, and 0.5 micron respectively. 
     The coupling effects of the hybrid  400  are spread over the design in an uneven manner. Specifically, the bottom part of the design features mutual capacitance and inductance between 4 parallel metal lines (portions of lines  410  and  420 ). On the other hand, the upper left portion of the design provides coupling between two metal lines. Therefore, the hybrid design  400  is effectively equivalent to a lumped element implemented as an LC (pi) network. It should be noted that the inductances and capacitances distribution is a function of the port locations and the overall dimensions of the structure. 
     As the hybrid design  400  is a pi-network equivalent, a connection from port  401  to port  403  functions as a high-pass filter, while the connection from a port  401  to a port  404  behaves as a low-pass filter. Port  402  is terminated. In a non-limiting embodiment, transfer functions of the filters perform a 45-degree phase lead between ports  401  and  404  and a 45-degree phase lag from between ports  401  and  403 . Thus, the total phase difference of between output signals at ports  403  and  404  relatively to port  401  is 90-degree. 
       FIG. 5  shows a schematic diagram of a hybrid design  500  constructed in accordance with another embodiment of the invention. The hybrid design  500  features different port locations in comparison to the hybrid design  400 . Thus, the inductances and capacitances are distributed differently. The hybrid design  500  is a 90-degree hybrid that can be utilized, for example, in a phase shifter antenna. Thus, there is a phase difference between the output signals at ports  503  (Through) and  504  (Coupled) relatively to an input signal at port  501  (Input). The ports  501  and  503  are connected through a metal line  510  and the ports  502  and  504  are connected via a metal line  520 . The port  502  (Isolated) is terminated. 
     In accordance with an embodiment of the invention, illustrated in  FIG. 5 , in order to suppress the effect of return-path flows through a perimeter ground  550 , the hybrid is fabricated over parts  560  of the ground. The ground is typically the first metal layer of the multilayer substrate. Typically, the signal at each port is identified as being measured and driven between the metal lines and the ground metal. Ideally, the ground is a single node which implies that physical locations of all the ports reference metals are actually shorted to each other. Practically, electric voltage is built between these nodes, as the ports are not ideally shorted. Hence, it is important to model the return-path. Further, the return-path should be designed in a way that it does not degrade performance and limits port locus in order to maintain design flexibility. This is typically the case when the return-path flows through the perimeter ground  550 . In accordance with an embodiment of the invention, the hybrid design  500  is structured to limiting the effect of the return-path flowing through a perimeter ground  550  by shorting perimeter ground  550  using the part of metal  560 . In an exemplary embodiment, the length of metal lines  510  and  520  is 275 micron and the number of turns of metal line  510  is 1.75 while the number of turns of the metal line  520  is 1.5. 
     As mentioned above the length and number of turns determine the operation frequency of the electrical element. For example, a 90-degree hybrid operation in a 30 GHz can be designed using the same layout as the hybrid  500 , by lengthening the metal wires. In an exemplary embodiment of the invention, a 30 GHz hybrid is designed with metal lines having length of 560 micron and 490 micron and 1.75 and 1.5 turns respectively. 
       FIG. 6  depicts graphs of return-loss varying with frequency results measured for a 90-degree hybrid operating in a 60 GHz frequency band and constructed in accordance with an embodiment of the invention. The return-loss is a measure of voltage standing wave ratio (VSWR), expressed in decibels (db) and may be caused due to an impedance mismatch. A high value of return-loss denotes better quality of the electrical element under test. The graphs  601 ,  601 ,  603 , and  604  depict the return-loss at ports  401 ,  402 ,  403 , and  404  of the hybrid  400  respectively. During the test, each port is terminated to a 50 Ohm termination. As can be noticed for frequency band from 56 GHz to 70 GHz frequency, each measured return-loss is below −12 db. A person with ordinary skill the art should appreciate that such a result represents a low return-loss value, thus good performance of the hybrid. 
     It is important to note that these embodiments are only examples of the many advantageous uses of the innovative teachings herein. Specifically, the innovative teachings disclosed herein can be adapted in any type of consumer electronic devices where reception and transmission of millimeter wave signals is needed. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the principles of the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. Moreover, all statements herein reciting principles, aspects, and embodiments of the invention, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents as well as equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure.