A HIGH FREQUENCY BALUN PROVIDED IN A MULTILAYER SUBSTRATE

Abstract: Baluns according to the present invention use both distributed and discrete elements connected together in a multilayer dielectric structure. As distributed elements, coupled striplines are provided in the multilayer dielectric structure. The discrete components are placed on the surface of the multilayer structure and connected with the distributed elements through via-holes. The operating frequency of the balun can be changed by changing values of the discrete components without changing the multilayer structure itself.

Description: A HIGH FREQUENCY BALUN PROVIDED IN A MULTILAYER SUBSTRATE BACKGROUND The present invention is generally directed to baluns and, more particularly, to baluns which are implemented as part of a multilayer structure. A balun (which term comes from the phrase BALanced to UNbalanced) is a passive three port electronic circuit used for conversion between symmetrical and nonsym etrical transmission lines. The signal, for example incoming to a nonsymmetrical port, is divided between two symmetrical ports providing signals which have the same amplitude but with phases which are 180 degrees offset relative to one another on their outputs. Baluns are used, for example, in the construction of balanced amplifiers, mixers and antenna systems. The balun construction depends on the intended operating frequency range. In the microwave frequency range, where the size of the structure is comparable to the wavelength of the signal, distributed element circuit technology is commonly used. In lower frequency ranges, e.g. , up to 2500 MHz, transformer solutions are common in which wires are wound spirally around a highly permeable magnetic core. These conventional balun configurations suffer from a number of problems. The transformer solutions, using the phenomenon of magnetic coupling between transformator wires, are theoretically wide band circuits. In practice, however, compensation for eigen capacitances is needed, especially in the frequency range 400 to 2500 MHz. This means that the physical construction of the transformer-type baluns has to be specifically optimized for operation within its operating frequency bandwidth. Additionally, it is difficult to accurately set the length of the wires to be wound about the core so that baluns which are designed to be the same, actually have substantially the same electrical characteristics. Most existing baluns operating in the high (e g , greater than 2500 MHz) frequency range, give good balun performance only it both symmetrical ports are well matched. In many applications, power matching of the symmetrical ports is not desirable for other reasons. For example, power matching on the symmetrical inputs of mixers or amplifiers worsens their noise parameters Thus, a compromise between power matching and noise matching is needed Moreover, ongoing miniaturization of electronic structures is. in turn, causing the miniaturization of baluns. For example, baluns used in the frequency range 400 to 2000 MHz are not usually bigger than about 20 mπr and are designed for automatic surface mounting onto the end product However, during the production of the baluns themselves, manual mounting is still used because the wires require manual winding around the core and the ends of the wires need to be inserted into electrical connectors on the end product Manual mounting is expensive, time consuming and causes spread in the parameters of the end product, e.g. , a radio receiver, of which the balun is just one of many components. Thus, it would be desirable to provide a balun having better symmetry when working with unmatched loading, which do not require different physical constructions to handle different operating frequency ranges, and which are less expensive to manufacture by allowing automatic integration ot the balun with other circuit components as opposed to manual mounting SUMMARY These and other drawbacks and limitations ot conventional baluns are overcome according to exemplary embodiments of the present invention Baluns according to the present invention use both distributed and discrete elements connected together in a multilayer dielectric structure As distributed elements coupled striplines are provided in the multilayer dielectric structure The discrete components are placed on the surface of the multilayer structure and connected with the distributed elements through via-holes. This provides a number of advantageous balun characteristics. For example, within a range bounded by the characteristics of the dielectric material used in the multilayer structure, the operating frequency range of the balun can be adjusted simply by changing values of the discrete components used to fabricate the balun. In this way, the operating frequency of the balun can be easily changed without necessitating a completely new balun construction. This is a great advantage as compared to, for example, transformer-type baluns for which a completely new construction design was required for different operating frequency ranges. By providing the balun as part of a multilayer structure using known printed circuit and discrete component integration techniques, production gilt is increased and manual mounting of baluns is eliminated. This serves to reduce expense of the end product in which the balun is incorporated. By fabricating the balun as a multilayer structure, the distributed elements can be provided in a layer which is embedded below that on which the discrete components are mounted. This allows the top layer surface area required for the balun to be reduced which further promotes miniaturization of the products in which the balun is incorporated. BRIEF DESCRIPTION OF THE DRAWINGS The foregoing, and other, objects, features and advantages of the present invention will be more readily understood upon reading the following detailed description in conjunction with the drawings in which: Figure 1 is a top view of an exemplary multilayer structure in which baluns according to the present invention can be implemented; Figure 2 is a sectional view of an exemplary multilayer structure in which baluns according to the present invention can be implemented; and Figure 3 illustrates an exemplary circuit topology of baluns according to the present invention. DETAILED DESCRIPTION An example of a multilayer structure in which baluns according to the present invention can be implemented is shown (in a top view) as Figure 1 . Therein, a coupled pair of striplines S I and S2 are illustrated as hidden (i.e.. by way of the dotted lines) since the coupled striplines are embedded in a lower layer of the multilayer structure 10. A stripline is a well known transmission line which can be formed as a conductive metal trace placed in a dielectric media with two parallel ground planes on both sides of the dielectric surface. A coupled stripline is a structure using two striplines having a constant distance between them. In a multilayer structure, coupled striplines can be made as two parallel traces on the same layer with ground planes on layers above and below the layer with traces, or as parallel traces placed on two adjacent layers. The remaining components illustrated in Figure 1 are on the surface or top layer of the multilayer structure 10. For example, the multilayer structure 10 could include surface mounted devices 3 and 5. Surface mounted devices 3 and 5 can be electrically connected to the coupled striplines S2 and S I , respectively, using vias (thru holes) 7 and 9, respectively. As is well known in the art. vias are apertures formed in multilayer structures which are plated with conductive material to establish electrical connections at desired points between different layers in the multilayer structure. Additionally, the exemplary multilayer structure 10 shown in Figure 1 includes microstrips 1 1 and 13 on the surface or top layer. As is well known in the art, microstrips are controlled impedance microwave frequency transmission lines which are formed as conductive metal traces on one side of a dielectric surface with a ground plane on the other side of the dielectric surface. Microstrips 1 1 and 13 are connected to one of the coupled striplines S2 by via-holes 15 and 17, respectively. Also illustrated in Figure 1 is a via 19 which connects the coupled striplines S I and S2 with a ground plane. Figure 2 illustrates a side view of an exemplary multilayer structure in which baluns according to the present invention can be implemented. Although the side view portrays a slightly different component configuration than the multilayer structure of Figure 1 , similar reference numerals are used to refer to similar elements For example, the top layer (denoted Layer 1 in Figure 2) includes a surface mounted device 3 This surface mounted device 3 is connected to one of the coupled striplines S2 which have been fabricated in Layer N-l. In this example, the multilayer structure has four layers, although any number of layers which are equal to or greater than four can be used The via 7 which interconnects surface mounted device 3 with coupled striplines S I and S2 is isolated from the ground planes as seen, tor example, at point 20 on Layer N which illustrates a separation between the via 7 and the metallized ground plane portions 22 Another conductive via 24 provides a connection between the ground plane Layers N and N-2 Each of the four conductive layers illustrated in Figure 2 are separated from adjacent layers by a layer of dielectric material Moreover, the discrete electπcal components provided on Layer 1 of the multilayer structure are electncally isolated from the electπcal components provided on Layer N- l , e g coupled striplines S I and S2, by a ground plane provided as Layer N-2. The ground plane can, for example, be a copper layer of about 17 5 mm in thickness This helps to ensure that the operation of the components provided on Layer 1 is not affected by the provision of electrical impulses to the components on Layer N-l , e.g , by capacitive or inductive coupling effects The operating frequency of the balun is bounded by the electrical parameters of the dielectric layers provided in the multilayer structure (e g , dielectric constant, dielectric losses (loss tangent) and dielectric thickness) For example, if a typical glass-fiber resin material (e g , having a dielectric constant of 4.25, a loss tangent of 0 02 and a layer thickness ot 0 5 mm) is used for the dielectric layers of Figure 2, an operating frequency of baluns according to the present invention can be set to be between 100 MHz and 2 5 GHz The lower value is, in practice, limited by the lengths of the stripline structure The higher frequency value is limited by losses in dielectric layers and higher wavelength transmission modes which are associated with increasing frequency (given a constant dielectric layer thickness). A significant feature of the present invention is that the operating frequency of the balun may be changed within the bounded range by changing the values of the discrete components, without changing the multilayer structure itself. These and other benefits of baluns according to the present invention will become more apparent after reviewing the detailed description of an exemplary balun circuit configuration provided below An exemplary balun according to the present invention is shown in Figure 3. In Figure 3, vias are depicted using shaded circles. As seen by the examples in this figure, some of the vias are connected to a ground plane (e.g. , Layer N-2 and Layer N) while others are connections between the top surface of the multilayer structure and an embedded layer. Port 30 is the nonsymmetπcal port of the balun, while ports 32 and 34 are the symmetrical outputs. Thus, ports 32 and 34 each provide an output having the same amplitude, but whose phases differ by 180 degrees (if the balun is perfectly symmetrical) Port 36 is optionally provided (as is transmission line S7) if an external bias is to be connected for biasing ports 32 and 34. Port 36 can be used, for example, to connect active devices (e.g. , active amplifiers or active mixers) to bias the symmetrical output ports 32 and 34. Transmission line S7 provides electπcal isolation between common node 38 and port 36. When connected to passive devices, port 36 (and transmission line S7) can be omitted. The distributed element part of the balun includes two sections S I and S2 of coupled transmission striplines. Section S I includes striplines S l l and S 12, while section S2 includes striplines S21 and S22. Both sections S I and S2 can have identical characteristic impedances for even and odd modes and can have identical electrical lengths and be coupled together at common node 38 (represented by a dotted line in Figure 3). This can be accomplished by making each stripline S l l , S I 2, S21 and S22 of the same length (e.g. , 6.4 mm), same width (e.g. , 0.2 mm), same thickness and providing a uniform spacing between sections S I and S2 (e.g. , 0. 15 mm). Node 40 of stripline S 12 is connected to the ground planes and node 42 of stripline S I 1 is connected to capacitors C l and C2. The capacitors C l and C2 have values that are chosen based upon the desired operating frequency of the balun to provide proper matching and impedance transformation for the nonsymmetπcal output of the balun. Nodes 44 and 46 of striplines S21 and S22 are connected to striplines S3 and S5 and, together with capacitors C3 and C4, give proper impedance transformation at nodes 48 and 50, respectively. Symmetrical output ports 32 and 34 are connected to nodes 48 and 50 with lines S4 and S6 The proper choice of impedance values and electrical lengths for transmission line elements S3 and S4 and capacitance value of C3 on one side and substantially the same impedance values and electπcal lengths for transmission line elements S5 and S6 and capacitance value for C4 on the other side, determines an output impedance of the terminating balun circuit. Those skilled in the art will appreciate that the output impedance can be varied so that the symmetrical ports provide maximal gain (power matching), minimal noise (noise matching) or a compromise between the two competing objectives depending upon the application The value selected for capacitor C5 gives a proper symmetry of the balun so that the symmetrical outputs have substantially the same amplitude and are as close to 180 degrees offset in phase as possible. This capacitance value depends on the characteristic impedances of the coupled striplines for even and odd modes as well as the electrical lengths of sections S I and S2 at the operating frequency of the balun If used, the characteristic impedance and electrical length of transmission line S7 and the impedance in port 36 have to be taken into account to determine the appropriate capacitance value for C5 to maintain the balun symmetry As mentioned above, coupled stripline sections S I and S2 are embedded within a multilayer structure, e.g. , structure 10, while the discrete components (e.g., capacitors C 1-C5) are provided on Layer 1 or the top surface of the structure. Depending on the design constraints of the balun, striplines (in an embedded layer) or microstrips (on a surface layer) can be used tor transmission line elements S3, S4, S5, S6 and S7. In the example shown in Figure 3, S3 and S5 are fabricated as striplines in an embedded layer (e g . Layer N- l ) along with coupled striplines S I and S2 Transmission line elements S4 and S6 are fabπcated as microstrips on the surface layer along with capacitors C 1 -C5 According to exemplary embodiments of the present invention, baluns are constructed in a multilayer structure using distributed and discrete components such that the operating frequency of the balun can be easily adjusted For example, by adjusting values of capacitors C 1 -C5 , the operating frequency ot the balun can be changed within the range dictated by the dielectric media used (e.g , at least within an octave ot an originally designed operating frequency between 100 MHz and 2 5 GHz) without changing the multilayer structure or adjusting the coupled striplines which are embedded therein For example, using the values in Table 1 below, a balun according to the exemplary embodiment of Figure 3 can operate within the range ot 935-960 MHz By changing the values of capacitors C 1 -C5 to those shown in Table 2, the same balun structure can instead operate at between 425-430 MHz Those skilled in the art will appreciate that other capacitance values can be used to achieve other operating frequency ranges. Table 1 Table 2 C l 1 8pF C l 8 2pF C2 0 47pF C2 12pF C3 4 7pF C3 27pF C4 4 7pF C4 27pF C5 3 3pF C5 33pF The above-described exemplary embodiments are intended to be illustrative in all respects, rather than restrictive, ot the present invention Thus the present invention is capable of many variations in detailed implementation that can be derived from the description contained herein by a person skilled in the art. For example, the nonsymmetπcal port can be used as either an input or an output port, while the symmetrical ports can be used as output or input ports, respectively. All such variations and modifications are considered to be within the scope and spirit of the present invention as defined by the following claims.

Claims: CLAIMS 1 A balun comprising a nonsymmetπcal signal port coupled to a first capacitor said first capacitor being provided on a top layer ot a multilayer structure, a first ground plane layer provided beneath said top layer, a coupled pair of stripline elements provided on a third layer beneath said ground plane layer, said coupled pair ot stripline transmission elements having first, second and third connection nodes, wherein said first capacitor is connected to said first node, a first symmetπcal signal port connected to said second node of said coupled pair of stripline transmission elements, wherein a combination of a first transmission line element, a second capacitor and a second transmission line element are provided between said second node and said first symmetπcal signal port; and a second symmetrical signal port connected to said third node of said coupled pair ot stripline transmission elements, wherein a combination ot a third transmission line element, a third capacitor and a fourth transmission line element are provided between said third node and said second symmetrical signal port, wherein said second and third capacitors are also provided on said top layer of said multilayer structure. 2 The balun ot claim 1 , further comprising a fourth capacitor, provided on said top layer ot said multilayer structure, connected to said first node 3 The balun of claim 2, wherein capacitance values of said first and fourth capacitors are selected to provide impedance matching and transformation to said nonsymmetπcal port 4 The balun ot claim 1 , further comprising a fifth capacitor connected to a common node which interconnects said coupled pair of stripline elements, said fifth capacitor having a capacitance value selected to maintain outputs of said first and second symmetrical ports at substantially the same amplitude and 180 degrees out of phase with respect to one another 5 The balun ot claim 1 , wherein impedance values ot said combination of said first transmission line element, said second capacitor and said second transmission line element are selected to provide a desired tradeoff between power matching and noise matching to outputs of said first and second symmetncal ports 6 The balun of claim 5, wherein said desired tradeoff is optimal noise matching 7 The balun of claim 5, wherein said desired tradeoff is optimal power matching 8 The balun of claim 1 , further comprising a ground plane beneath said third layer 9 The balun ot claim 1 further comprising a biasing port for allowing an external device to bias the first and second symmetrical ports 10. The balun of claim 1 , wherein said first and third transmission line elements are provided on said third layer as striplines and said second and fourth transmission line elements are provided on said top layer as microstrips. 1 1 . A balun comprising: a multilayer layer structure including at least two signal carrying conductive layers each separated from one another by a ground plane layer, said multi-layer structure including: a nonsymmetrical port for receiving or outputting an unbalanced input signal provided on a surface one of said at least two signal carrying conductive layers of said multilayer structure; two symmetrical ports for outputting or receiving, respectively, signals of substantially the same amplitude and which are substantially 180 degrees offset in phase, each provided on said surface one of said at least two signal carrying conductive layers of said multilayer structure; wherein distributed elements of said balun are incorporated in a lower one of said at least two signal carrying conductive layers of said multilayer structure and discrete elements of said balun are provided on said surface one of said at least two signal carrying conductive layers of said multilayer structure. 12. The balun of claim 1 1 , in which said distributed elements include a coupled pair of striplines having substantially the same characteristic impedances and electrcial lengths for even and odd modes. 13. The balun of claim 1 1 , in which said discrete elements include a plurality of capacitors mounted on said surface one of said at least two signal carrying conductive layers of said multilayer structure 14. The balun of claim 1 1 , further comprising' a biasing port for allowing an external device to bias the two symmetrical ports. 15. A balun which can be operated at one of a first operating frequency and a second operating frequency, said balun comprising. a multilayer layer structure including at least two signal carrying conductive layers each separated from one another by a groundplane layer; a nonsymmetrical port for receiving or outputting an unbalanced input signal, said nonsymmetrical port being provided on a surface one of said at least two signal carrying conductive layers of said multilayer structure; two symmetrical ports for outputting or receiving, respectively, signals of substantially the same amplitude and which are substantially 180 degrees offset in phase, each provided on said surface one of said at least two signal carrying conductive layers of said multilayer structure; and a first plurality of components disposed on said surface one of said at least two signal carrying conductive layers of said multilayer structure and at least one component of said balun is disposed on another one of said at least two signal carrying conductive layers of said multilayer structure, whereby said operating frequency of said balun can be changed from said first operating frequency to said second operating frequency by changing said values of said first plurality of components. 16. The balun of claim 15, wherein said first plurality of components include a plurality of capacitors. 17. The balun of claim 15 , wherein said at least one component includes a coupled pair of striplines 18. The balun of claim 1 , wherein said coupled pair ot stripline elements have a fourth node which is connected to said ground plane layer 19. The balun of claim 1 , wherein said first, second and third capacitors are also connected to said ground plane layer.

Inventors: DABROWSKI JERZY, DABROWSKI, JERZY
Applicants: ERICSSON TELEFON AB L M, TELEFONAKTIEBOLAGET LM ERICSSON (PUBL)
Application year: 1997
Main classification: H01P5
IPC codes: H01P5