Abstract:
In general, the invention is directed to integration of passive radio frequency (RF) structures with at least one integrated circuit in a single integrated circuit (IC) package. An IC package in accordance with the invention may include, for example, a radio IC, a digital IC, a passive radio frequency balun as well as additional passive RF structures or ICs. Additionally, passive electronic components may further be incorporated in the IC package. For example, the IC package may include a resistor, capacitor, inductor or the like. The components of the IC package may be distributed throughout layers of a multi-layer IC package, such as a multi-layer ceramic package. The different ICs and the passive RF structures may be electrically coupled via conductive traces, which may be varied in thickness and length in order to match input and output impedances of the different ICs and passive RF structures.

Description:
This application claims priority from U.S. Provisional Application Ser. No. 60/404,443, filed Aug. 19, 2002, the entire content of which is incorporated herein by reference. 

   TECHNICAL FIELD 
   The invention relates to integrated circuit packages and, more particularly to integrated circuit packages for radio frequency communication devices. 
   BACKGROUND 
   A circuit package carries a semiconductor device and provides necessary input/output (I/O) interconnections between the semiconductor device and other circuit components. A typical integrated circuit package is designed to provide structure to support and protect the device, and to distribute circuit-generated heat. Furthermore, the integrated circuit package provides connections for signal lines leading into and out of the device, connections that present varying potentials for power and ground, and a wiring structure for I/O signal interconnections within a system. 
   SUMMARY 
   In general, the invention is directed to integration of passive radio frequency (RF) structures with at least one integrated circuit device in a common integrated circuit (IC) package. As will be described herein, an IC package that incorporates passive RF structures with ICs may achieve a low profile, i.e., thickness, compactness, as well as increased IC performance. 
   An IC package in accordance with the invention may include, for example, a radio IC, a digital IC, a passive radio frequency balun as well as additional passive RF structures or ICs. Other passive RF structures that may be incorporated in the IC package along with the balun, radio IC and digital IC include passive RF filters and the like. Additionally, passive electronic components may further be incorporated in the IC package. For example, the IC package may include a resistor, capacitor, inductor or the like. 
   The IC package may be a multi-layer IC package, such as a multi-layer ceramic package, with the internal components, e.g., passive RF structures and ICs, distributed throughout the different layers. The different ICs and the passive RF structures may be conductively coupled via conductive traces formed on the layers, as well as conductive vias that extend between different package layers. Conductively coupling the passive structures RF structures and the different ICs using conductive traces or vias within the package facilitates input and output impedance matching of the different ICs and passive RF structures. For example, the conductive strips may conductively couple the radio IC and the digital IC and have varying lengths and widths to match the input and output impedances of the radio IC and the digital IC. 
   In one embodiment, the invention provides a circuit package comprising at least one integrated circuit device and a passive balun, the integrated circuit device being coupled to the passive balun. 
   In another embodiment, the invention provides an integrated circuit package comprising a radio integrated circuit that converts radio frequency signals to baseband signals, a digital integrated circuit that processes the inbound and outbound baseband frequency signals, and a passive structure coupled to the radio integrated circuit. 
   The invention may provide one or more advantages. In general, integrating passive RF structures along with ICs into a common IC package facilitates a low profile, i.e., thin, IC package while allowing for a large number of input and output connections. Further, interconnecting the internal components, e.g., the different ICs and the passive RF structures, with conductive traces facilitates matching of the input and output impedances of the internal components. For example, the thickness of the conductive traces may be varied to match impedance between a pair of ICs. 
   In addition, because the variance of the dimension of the conductive traces within the package is typically low, the variance on the overall performance of the IC package is low. The low variance facilitates increased performance of the ICs, maintenance of that performance over a high production volume. The multi-layer structure of the IC package permits integration of power planes and ground planes in close proximity to the ICs. This proximity reduces the amount of distortion in the ICs by reducing the parasitic effects associated with surface mounting high speed or high frequency ICs on a printed circuit structure. 
   The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a block diagram illustrating a mobile communication device. 
       FIG. 2  is a schematic diagram illustrating a wireless card for wireless communication. 
       FIG. 3  is an exploded view of an exemplary integrated circuit (IC) package that incorporates at least one integrated circuit device with a passive radio frequency (RF) structure in accordance with the invention. 
       FIG. 4  is an exploded view illustrating a passive RF balun of  FIG. 3  in further detail. 
       FIG. 5  is a schematic diagram illustrating a cross section view of the IC package of  FIG. 3 . 
       FIG. 6  is a schematic diagram illustrating a passive RF balun coupled to a hairpin filter. 
       FIG. 7  is a block diagram illustrating another exemplary passive RF balun arranged on a single layer. 
   

   DETAILED DESCRIPTION 
     FIG. 1  is a block diagram illustrating a mobile communication device  10 . Mobile communication device  10  may communicate with a wired network as well as one or more other mobile communication devices via a wireless communication network. For example, the wireless communication network may include at least one wireless access point coupled to a wired network. The wireless access point permits wireless communication between the wired network and mobile computing device  10 . The wireless access point and mobile computing device  10  may communicate according to one or more Wireless Local Area Network (WLAN) protocols such as those specified by the IEEE 802.11a, 802.11b, 802.11e or 802.11g standards. 
   Mobile computing device  10  may take a variety of forms including a desktop computer, portable computer, personal digital assistant (PDA), mobile telephone, multimedia device, consumer electronics and the like. Mobile computing device  10  is equipped with hardware to provide attachment to the wireless communication network. For example, mobile communication device  10  may include a peripheral device  12 , such as a wireless network card or board coupled to a host computer via an external or internal interface, including Peripheral Component Interconnect (PCI), Mini PCI, Universal Serial Bus (USB), USB-2, Cardbus, IEEE 1394, Small Computer System Interface (SCSI), or Personal Computer Memory Card International Association (PCMCIA) interfaces. 
   More specifically, mobile computing device  10  includes a host  14  coupled to a peripheral device  12 . Particularly, host  14  is coupled to a media access control (MAC)  16  via a host interface (not shown). MAC  16  is further coupled to a digital integrated circuit (IC)  28  via a physical interface. Mobile computing device further includes a radio integrated circuit (IC)  30  and a radio frequency (RF) antenna  24 . MAC  16 , digital IC  28  and radio IC  30  are all incorporated within peripheral device  12 . 
   RF antenna  24  transmits and receives RF signals between device  10  and the access point within wireless communication network. Although  FIG. 1  depicts the use of a single antenna  24 , device  10  may include more than one RF antenna  24  to make use of receive and transmit diversity. Radio IC  30  and digital IC  28  function together as a wireless transceiver. Particularly, radio IC  30  may include circuitry for upconverting signals from baseband to RF for transmission, and downconverting received RF signals to baseband for processing by digital IC  28 . Digital IC  28  handles baseband processing of packets transmitted and received via radio IC  30  and antenna  24 . Digital IC  28  may, for example, encode and decode information carried by packets transmitted and received via radio IC  30  and antenna  24 . 
   MAC  16  interacts with host  14  to facilitate communication between digital IC  28  and applications running on host  14 . Hence, host  14  may be a CPU within a computer, PDA, mobile telephone or some other device. MAC  16 , digital IC  28 , and radio IC  30  may be on a common integrated circuit chip. The common integrated package that includes MAC  16 , digital IC  28  and radio IC  30  may further integrate passive RF structures, such as a balun, in accordance with the invention. 
   Although the techniques of the invention are described for a mobile communication system operating in the RF frequency range, the techniques may be applied to other types of communication systems that operate in different frequency ranges. 
     FIG. 2  is a schematic diagram illustrating a wireless card  22  for wireless communication. Wireless card  22  is just one example of a peripheral device  12  that may incorporate the techniques of the invention. Wireless card  22  includes antennas  24 A and  24 B (hereinafter  24 ), passive RF baluns  26 A and  26 B (hereinafter  26 ), passive RF filters  27 A and  27 B (hereinafter  27 ), a radio integrated circuit (IC)  28  and a digital integrated circuit (IC)  30 . In accordance with the invention, baluns  26 , radio IC  28  and digital IC  30  may all be incorporated into a single circuit package  32 . 
   As described above, antennas  24  receive and transmit signals to and from wireless card  22 . Antennas  24  may, for example, receive signals over multiple receive paths providing wireless card  22  with receive diversity. In this manner, antenna  24 A provides a first receive path, and antenna  24 B provides a second receive path. 
   Wireless card  22  may select, via radio IC  28 , the receive path with the strongest signal. Alternatively, wireless card  22  and, more particularly, radio IC  28  may combine the signals from the two receive paths. More than two antennas  24  may be provided in some embodiments for enhanced receive diversity. Alternatively, only a single antenna  24  may be provided in which case wireless card  22  does not make use of receive diversity. One or both of antennas  24  may further be used for transmission of signals from wireless card  22 . 
   As described above, radio IC  28  may include transmit and receive circuitry (not shown). For example, radio IC  28  may include circuitry for upconverting transmitted signals to radio frequency (RF), and downconverting RF signals to a baseband frequency for processing by digital IC  30 . In this sense, radio IC  28  may integrate both transmit and receive circuitry within a single transceiver component. In some cases, however, transmit and receive circuitry may be formed by separate transmitter and receiver components, e.g., a receive IC and a transmit IC. 
   Baluns  26  couple antennas  24  with radio IC  28 . Specifically, balun  26 A couples antenna  24 A with radio IC  28  and balun  26 B couples antenna  24 B with radio IC  28 . Baluns  26  may transform unbalanced (or single-ended) RF signals from radio IC  28  to balanced (or differential) RF signals for antennas  24  and vice versa, i.e., balanced RF signals from antennas  24  to unbalanced RF signals for radio IC  28 . In some embodiments, however, radio IC  28  may produce balanced signals and antennas  24  may produce unbalanced signals. Baluns  26  may perform impedance transformations in addition to conversions from balanced signals to unbalanced signals. Further, baluns  26  may provide filtering functionality to inbound and outbound signals. Baluns  26  may electrically couple to antennas  24 , e.g., via a conductive strip. Alternatively, baluns  26  may electromagnetically couple to antennas  24 . 
   In addition, filters  27  may be coupled to baluns  26 . Filters  27  may be used to provide filtering in the cases in which baluns  26  do not provide filtering functionality. Alternatively, in the case in which baluns  26  do provide filtering functionality, filters  27  may sharpen the filtering functionality provided by baluns  26 . As will be described, filters  27  may include hairpin filters, notch filters or any other types of filters. 
   As described in  FIG. 1 , digital IC  30  processes inbound and outbound signals and may include a baseband processor and medium access control (MAC) layer hardware. Digital IC  30  may, for instance, encode information in a baseband signal for upconversion to RF by radio IC  28  or decode information from RF signals received via antennas  24  and downconverted to baseband by radio IC  28 . For example, digital IC  30  may provide Fourier transform processing to demodulate signals received from a wireless communication network. Although in the example illustrated in  FIG. 2  radio IC  28  and digital IC  30  are discrete ICs, wireless card  22  may incorporate a single component that integrates radio IC  28  and digital IC  30  onto a common IC. 
   As described above, baluns  26 , filters  27 , radio IC  28  and digital IC  30  may all be incorporated into IC package  32 . For example, IC package  32  may be a multi-layer ceramic package that incorporates baluns  26  and filters  27 , i.e., passive RF structures, with radio IC  28  and digital IC  30 . The passive RF structures, e.g., baluns  26  and filters  27 , may reside on the same layer of IC package  32  as radio IC  28  and digital IC  30 . Alternatively, all or a portion of the passive RF structures may reside on different layers than layers on which radio IC  28  and digital IC  30  reside. Although in the example of  FIG. 2  the only passive RF structures incorporated into IC package  32  are baluns  26  and filters  27 , other passive RF structures may be integrated into IC package  32 . For example, IC package  32  may include other passive RF structures such as a coupler (e.g., a line coupler or a quadrature coupler), as well as passive electronic components such as resistors, capacitors, and inductors. 
   Wireless card  22  illustrated in  FIG. 2  should be taken as exemplary of the type of device in which the invention may be embodied, however, and not as limiting of the invention as broadly embodied herein. For example, the invention may be practiced in a wide variety of devices, including WLAN cards, cellular phones, personal computers (PCs), personal digital assistants (PDAs), and the like. As a particular example, wireless card  22  may take the form of a wireless local area networking (WLAN) card that conforms to a WLAN standard such as one or more of the IEEE 802.11(a), 802.11(b), 802.11(e) or 802.11(g) standards. 
     FIG. 3  is an exploded view of an exemplary IC package  32  that incorporates integrated circuits  28  and  30  with passive RF structures in accordance with the invention. As illustrated in the example of  FIG. 3 , IC package  32  includes multiple layers  34 A– 34 D (hereinafter  34 ). IC package  32  may, for example, be a multi-layer ceramic package. 
   As discussed above, IC package  32  includes a radio IC  28  and a digital IC  30 . Radio IC  28  and digital IC  30  are adjacent to one another and may reside within respective cavities formed within one of layers  34  and, more specifically, in the example of  FIG. 3 , layer  34 A. The cavities within which radio IC  28  and digital IC  30  reside within may extend all the way through layer  34 A or only a portion of the way through layer  34 A. In the case in which the cavities within which radio IC  28  and digital IC  30  reside do not extend all of the way through layer  34 A, a conductive via or other connection may be used to conductively couple radio IC  28  and digital IC  30  to other external or internal components. Alternatively, radio IC  28  and digital IC  30  may not reside within a cavity, but, instead, include one or more conductive extensions, e.g., pins, that electrically couple to conductive traces, a conductive pad, or other conductive connection residing on top side of layer  34 A. Although in the example of  FIG. 3 , radio IC  28  and digital IC  30  reside on the same layer, in some embodiments, radio IC  28  and digital IC  30  may reside on different layers  34  of IC package  32 . 
   In the example illustrated in  FIG. 3 , IC package  32  includes conductive pads  36 A and  36 B (hereinafter  36 ) on layer  34 B. Pads  36  provide conductive bonding for radio IC  28  and digital IC  30 . Specifically, radio IC  28  and digital IC  30  reside within respective cavities that extend all the way through layer  34 A and electrically couple to a respective one of conductive pads  36 . In this manner, radio IC  28  and digital IC  30  may be considered to reside on layer  34 B and layer  34 A may be “built” around radio IC  28  and digital IC  30 . IC package  32  may include other active components such as a power amplifier, other integrated circuits, or the like. 
   IC package  32  incorporates at least one passive RF structure with radio IC  28  and digital IC  30 . In the example illustrated in  FIG. 3 , IC package  32  incorporates baluns  26 A and  26 B, which as described above are exemplary passive RF structures. Baluns  26 A and  26 B may be multi-layer baluns as illustrated. More specifically, a portion of balun  26 A and  26 B may reside on a first layer and another portion of balun  26 A and  26 B may reside on a different layer. One or more layers may reside between the portions of baluns  26 A and  26 B. Using  FIG. 3  as an example, balun  26 A includes portion  26 A′ that resides on layer  34 B and portion  26 A″ that resides on layer  34 D. 
   Balun  26 B is arranged in a similar manner, i.e., a portion  26 B′ resides on layer  34 B and another portion  26 B″ resides on layer  34 D. Although a ground plane  40  and layer  34 C separate the portions of baluns  26  in the illustrated example, any number of layers may separate the portions of baluns  26 . Layer  34 C electrically isolates portions  26 A″ and  26 B″ from ground plane  40 . Portions  26 A′ and  26 A″ may be electrically coupled by a conductive via that extends between layer  34 B, ground plane  40  and layer  34 C. In some embodiments, however, baluns  26  may be formed on a single layer of the multi-layer circuit package  32 . As described above, baluns  26  may transform unbalanced (or single-ended) signals to balanced (or differential) signals, perform impedance transformations in addition to conversions from balanced signals to unbalanced signals, or provide filtering functionality to inbound and outbound signals. Although the example illustrated in  FIG. 3  IC package  32  includes baluns  26 , IC package  32  may only incorporate a single balun  26  or more than two baluns  26 . 
   IC package  32  may further include other passive RF structures, such as filters  27 A and  27 B (hereinafter  27 ), in addition to baluns  26 . Filters  27 A and  27 B couple to portions  26 A″ and  26 B″ of baluns  26 , respectively. Filters  27  illustrated in  FIG. 3  may be constructed from one or more conductive traces formed on dielectric layer  34 B. Filters  27  may comprise notch filters, hairpin filters, or any other type of filter. IC package  32  may include other passive RF structures such as passive RF structure  39 . Passive RF structure  38  may, for example, be a coupler, such as a line coupler or a quadrature coupler. Passive RF structure  38  may be formed from discrete passive components or conductive traces on a dielectric of layer  34 B. IC package  32  may further include individual passive electronic components, such as resistors, capacitors and inductors. 
   Conductive traces, such as microstrip and stripline transmission lines, formed on each of layers  34  may interconnect baluns  26 , radio IC  28 , digital IC  30 , filters  27  and passive RF structure  38  with one another. The conductive traces may be formed by any of a variety of fabrication techniques including chemical vapor deposition, sputtering, etching, photolithography, masking, and the like. Conductive vias may extend between the layers to electrically couple components of one layer to respective conductive traces that reside on a different layer. 
   Integrating passive RF structures, e.g., baluns  26  and filters  27 , with ICs on a single IC chip facilitates input and output impedance matching of the different ICs and passive RF structures using conductive traces. Further, since the variance on dimensions and tolerance of the conductive traces is low, the variance on the overall performance of IC package  32  is low. This low variance increases the performance of the IC chips and holds that increased performance over a high production volume. Further, integration of power planes (not shown) and ground planes, such as ground plane  40 , in close proximity of ICs  28  and  30  result in reduced distortion due to parasitic effects associated with surface mounting high speed or high frequency ICs on a printed circuit board. 
   IC package  32  further includes a conductive pad  42  to which all connections from IC chips  28 ,  30  and passive RF components, such as baluns  26 , are routed to. Conductive pad  42  may, for example, be mounted on a printed circuit board and provide an interface that couples internal components, e.g., baluns  26 , radio IC  28 , and digital IC  30  to external components, such as an antenna or power source. Conductive pad  42  may, for example, be a ball grid array landing pad. Connection of conductive traces from antennas  24  to a section of conductive pad  42  in order to couple to baluns  26  are one example of internal and external components being coupled via conductive pad  42 . Instead of a conductive pad  42 , IC package  32  may have one or more conductive extensions, e.g., pins, that electrically couple to a printed circuit board in order to interface the internal components with external components. 
     FIG. 4  is an exploded view illustrating balun  26 A of  FIG. 3  in further detail. As illustrated in  FIG. 4 , balun  26 A has components formed on more than one layer of multilayer IC package  32 . More specifically, balun  26 A includes a first portion  26 A′ that resides on a first layer  34 B and a second portion  26 A″ that resides on a second layer  34 D. 
   Balun  26 A comprises unbalanced components  50 A and  50 B (hereinafter  50 ) that may be electrically coupled to form an unbalanced balun structure. Unbalanced components  50  may, for example, be electrically coupled by a conductive via  51  that extends between multiple layers of multi-layer IC package  32 . At least one of unbalanced components  50  is further coupled to an unbalanced port  58 . 
   In the example illustrated in  FIG. 4 , unbalanced component  50 A is coupled to unbalanced port  58 . In some cases, however, unbalanced component  50 B may be coupled to unbalanced port  58 . In some embodiments, unbalanced components  50  may not be electrically coupled to one another. In this case, both unbalanced components  50  are coupled to an unbalanced port  58 . Unbalanced components  50  may be conductive elements, such as conductive strips disposed on a dielectric layer. 
   Balun  26 A further includes a balanced balun structure that includes balanced components  52 A and  52 B (hereinafter  52 ). Each of balanced components  52  is electromagnetically coupled to one of unbalanced components  50 . Each balanced component  52  electromagnetically couples more than one side  56 A– 56 F (hereinafter  56 ) of a corresponding unbalanced component  50 . For example, as illustrated in  FIG. 4 , balanced component  52 A electromagnetically couples sides  56 A– 56 C of unbalanced component  50 A and balanced component  52 B electromagnetically couples sides  56 D– 56 F of unbalanced component  50 B. 
   Balanced components  52  may be constructed of balanced elements, such as balanced elements  54 A– 54 D (hereinafter  54 ). For instance, balanced element  54 A may be disposed on layer  34 B adjacent to side  56 A of unbalanced component  50  and balanced element  54 B may be disposed on layer  34 B adjacent to side  56 B of the unbalanced component  50 . Balanced elements  54  may be electrically coupled at one end to form balanced component  52 . In this manner, balanced component  52  electromagnetically couples more than one side of unbalanced component  50 . Each of balanced components  52  is coupled to a balanced port  60 . More specifically, balanced component  52 A is coupled to balanced port  60 A and balanced component  52 B is coupled to balanced port  60 B. 
   Unbalanced component  50 , which may also be a conductive strip, and balanced elements  54  may be of a length equal to approximately a quarter of a wavelength of an operating frequency of balun  26 A. Further, the length and width of balanced elements  54  may be adjusted to achieve a desired impedance transformation between the balanced and unbalanced inputs. 
   Although balun  26 A is described as being disposed on two layers, in some embodiments balun  26 A may be disposed on more than two layers or only a single layer. Unbalanced component  50 A and balanced component  52 A may be formed by any of a variety of fabrication techniques. For instance, a conductive layer (not shown) may be deposited on layer  34 B and shaped, e.g., by etching, to form unbalanced component  50 A and balanced component  52 A. More specifically, the conductive layer may be deposited on layer  34 B using techniques such as chemical vapor deposition and sputtering. The conductive layer deposited on layer  34 B may be shaped via etching, photolithography, masking, or a similar technique to form unbalanced component  50 A and balanced component  52 A. Alternatively, printing techniques may be used to deposit conductive traces on layer  34 B. The conductive layer may include copper, aluminum, or other conductive material. Layer  34 B may include a dielectric material such as silicon oxide, ceramic or other such material. 
   In the same manner, unbalanced component  50 B and balanced component  52 B may be formed on a top side of layer  34 D. Layer  34 C may be used to isolate unbalanced and balanced components  50 B and  52 B from ground plane  40 . However, unbalanced component  50 B and balanced component  52 B may, instead, be disposed on a bottom side of a layer  34 C in order to isolate unbalanced and balanced components  50 B and  52 B from a ground plane  40 . Layer  34 D would then be used to isolate unbalanced and balanced components  50 B and  52 B from the conductive pad  42  ( FIG. 3 ). Further, unbalanced and balanced components  50 A and  52 A do not have to be disposed on different physical layers  34 . For example, unbalanced and balanced components  50 A and  52 A may be disposed on an opposing side of the same dielectric layer as unbalanced and balanced components  50 B and  52 B. 
   As illustrated in  FIG. 4 , portions  26 A′ and  26 A″ of balun  26 A may be oriented such that unbalanced component  50 A is parallel with unbalanced component  50 B. However, portions  26 A′ and  26 A″ may be oriented in any fashion. For example, portions  26 A′ and  26 A″ may be oriented such that unbalanced component  50 A is perpendicular to unbalanced component  50 B. Further, portions  26 A′ and  26 A″ may be oriented such that unbalanced balun component  50 A substantially vertically aligns with unbalanced component  50 B. 
   A ground plane  40  may be placed between layers  34 B and  34 D. Balanced components  52  of the balanced balun structure may be referenced to ground plane  58 , i.e., carry a potential relative to ground plane  58 . Conductive via  51  extends between unbalanced component  50 A and unbalanced component  50 B, i.e., through layer  34 B, ground plane  40 , and layer  34 C to electrically couple unbalanced components  50 . 
   As described above, balun  26 A couples an unbalanced line or device with a balanced line or device. Balun  26 A and, more particularly, unbalanced components  50  receive an unbalanced signal via unbalanced port  58 . Balun  26 A divides the received signal equally between balanced ports  60 . More specifically, electromagnetic coupling between balanced components  52  and associated unbalanced components  50  induces signals on balanced components  52 . For instance, an electromagnetic field from unbalanced component  50 A radiates in all directions. Balanced component  52 A, which electromagnetically couples more than one side  56  of unbalanced component  50 A, induces a signal due to the electromagnetic coupling and transmits the signal via balanced port  60 A. 
   Electromagnetically coupling more than one side of unbalanced component  50 A allows more energy radiated from unbalanced component  50  to be coupled to balanced component  52 A, resulting in reduction of energy loss and greater energy efficiency. A similar phenomenon occurs for unbalanced component  50 B, balanced component  52 B, and balanced port  60 B. The signals output on each of balanced ports  60  are identical except for an approximate 180-degree phase shift. For example, the signal output from balanced port  60 A may have a first phase and the signal output from balanced port  60 B may have a second phase that is a 180-degrees out of phase relative to the phase of the signal output from balanced port  60 A. The signals output via balanced ports  60  are fed to a balanced device, such as radio IC  28 . 
   Signal flow also occurs in the opposite direction. Balanced components  52  each receive a balanced signal from a balanced device via corresponding balanced ports  60 . Balun  26 A combines the balanced signals to create an unbalanced signal and outputs the unbalanced signal to an unbalanced device, such as antenna  14 , via unbalanced port  58 . More particularly, electromagnetic coupling between balanced components  52  and corresponding unbalanced components  50  induce a signal on each of unbalanced components  50 . The signals induced on each of unbalanced components  50  combine via the electric coupling between unbalanced components  50  and are output via unbalanced port  58 . Balun  26 B may be constructed and operate in a manner similar to balun  26 A described above. 
     FIG. 5  is a schematic diagram illustrating a cross section view of IC package  32  of  FIG. 3  from A to A′. As illustrated in  FIG. 5 , IC package  32  is a multi-layer package. More specifically, IC package  32  includes layers  34 A– 34 D (hereinafter  34 ) as well as a ground plane  40  and a conductive pad  42 . 
   A radio IC  28  and a digital IC  30  electrically couple to layer  34 B and extend through layer  34 A. Particularly, layer  34 A may include cavities within which radio IC  28  and digital IC  30  reside. In this manner, layer  34 A may be thought of as “built” around radio IC  28  and digital IC  30 . As described above, radio IC  28  and digital IC  30  may electrically couple to layer  34 B via one or more conductive pads  36 , one or more conductive extensions, e.g., pins, that electrically couple to conductive traces, or the like. 
   IC package  32  further includes balun  26 , which, as described above, constitutes a passive RF structure. Balun  26  may reside on more than one layer of IC package  32 . Particularly, unbalanced components  50 A and  50 B of balun  26  are electrically coupled by a conductive via  51 . As illustrated in  FIG. 5 , conductive via  51  extends between unbalanced component  50 A and unbalanced component  50 B through a layer  34 B and  34 C in addition to a ground plane  40 . 
   Unbalanced component  50 A and a balanced component  52 A are disposed on a top portion of layer  34 B. Unbalanced component  50 B and balanced component  52 B may be disposed on a bottom portion of dielectric layer  34 C. Alternatively, unbalanced component  50 B and balanced component  52 B may be disposed on a top portion of layer  34 D. As described above, unbalanced components  50  and balanced components  52  may be disposed on respective layers  34  by any of a variety of fabrication techniques. 
   Balanced components  52  may be referenced to a common ground plane  40 , i.e., carry a potential relative to ground plane  40 . Alternatively, each of balanced components  52  may be referenced to separate ground planes. 
   In the example of  FIG. 5 , unbalanced component  50 A and unbalanced component  50 B are oriented such that unbalanced components  50  are parallel with respect to one another. However, unbalanced components  50  may be oriented with respect to one another in any manner. For instance, unbalanced components  50  may be oriented such that unbalanced component  50 A is perpendicular to unbalanced component  50 B. 
   On a bottom face of IC package  32  is a conductive pad  42  to which all connections from IC chips  28 ,  30  and passive RF components, such as baluns  26 , are routed to. Conductive pad  42  may, for example, be mounted on a printed circuit board and provide an interface that couples internal components, e.g., baluns  26 , radio IC  28 , and digital IC  30  to external components. In the example illustrated in  FIG. 5 , conductive pad  42  includes a ball grid array  64 . Ball grid array  64  facilitates easy surface mounting of IC package  32  onto printed circuit structures, such as a printed circuit board. More specifically, ball grid array  64  is constructed such that it self-aligns IC package  32  when surface mounting onto printed circuit structures. 
   Radio IC  28 , digital IC  30 , and balun  26  may be dispersed anywhere throughout IC package  32 . For example, a portion or all of balun  26  may reside on a same layer  34  as radio IC  28  and digital IC  30 . Alternatively, all of balun  26  may reside on a different layer  34  from radio IC  28  and digital IC  30 . In addition, radio IC  28  and digital IC  30  may reside on different layers within IC package  32 . 
     FIG. 6  is a schematic diagram illustrating a portion of layer  34 D of IC package  32  in further detail. The portion of layer  34 D illustrated in  FIG. 6  includes a portion  26 B″ of balun  26  electromagnetically coupled with a hairpin filter  62  via a conductive element  63 . Another conductive element  65  electromagnetically couples hairpin filter  62  to an unbalanced component that may be within IC package  32  or external to IC package  32 , such as an external antenna. Conductive elements  63  and  65  may, for example, comprise conductive traces. As illustrated, hairpin filter  62  is coupled to unbalanced component  50 B of portion  26 ″. Portion  26 B″ of balun  26  is described in detail in  FIG. 4 . 
   Hairpin filter  62  includes a U-shaped portion that comprises conductive elements  61 A and  61 B that electromagnetically couple to conductive elements  63  and  65 . More specifically, conductive element  63  extends adjacent to balanced element  54 D and electrically couples to unbalanced component  50 B. Conductive element  61 A of the U-shaped portion of hairpin filter  62  extends adjacent to conductive element  63  to electromagnetically edge couple conductive element  63 . Hairpin filter  62  is further arranged such that conductive element  61 B of the U-shaped portion of hairpin filter  62  electromagnetically edge couples to conductive element  65 . In this manner, hairpin filter  62  filters signals inbound to and outbound from balun  26 . 
   Hairpin filter  62  and, more specifically, conductive elements  61 A may be designed to obtain particular operating frequencies. Particularly, the length and width of conductive elements  61  of hairpin filter  62  determine the operating frequency of hairpin filter  62 . 
   Although the portion of layer  34 D illustrated in  FIG. 6  shows a hairpin filter, other types of filters may couple to balun  26 . For example, balun  26  may be coupled to a notch filter that is formed from one or more conductive traces. Further, layer  34 D may include other filters in addition to hairpin filter  62 . For example, layer  34 D may include a notch filter in addition to hairpin filter  62 . 
     FIG. 7  is a block diagram illustrating another exemplary balun  66  arranged on a single layer  34 . Balun  66  includes unbalanced components  50 A and  50 B (hereinafter  50 ) that are electrically coupled to form an unbalanced balun structure. Unbalanced components may be electrically coupled via a conductive strip  68  that extends from unbalanced component  50 A to unbalanced component  50 B. 
   Balun  66  further comprises a balanced balun structure that includes balanced components  52 A and  52 B (hereinafter  52 ). Balanced components  52  electromagnetically couple respective unbalanced components  50 . More specifically, balanced component  52 A electromagnetically couples more than one side of unbalanced balun component  50 A and balanced component  52 B electromagnetically couples more than one side of unbalanced balun component  50 B. 
   Balanced components  52  may be constructed of balanced elements, such as balanced elements  54 A– 54 D (hereinafter  54 ). For example, balanced component  52 A may consist of a first balanced element  54 A that electromagnetically couples a first side of unbalanced component  50 A and a second balanced element  54 B that electromagnetically couples a second side of unbalanced component  50 A. Balanced elements  54 A and  54 B are electrically coupled to form balanced component  52 A. Balanced component  54 B may be constructed in a similar fashion using balanced elements  54 C and  54 D. 
   Each of balanced components  52  is coupled to a balanced port  60 . More specifically, balanced component  52 A is coupled to balanced port  60 A and balanced component  52 B is coupled to balanced port  60 B. The unbalanced balun structure is coupled to an unbalanced port  58 . More specifically, one or both of unbalanced components  50  is connected to unbalanced port  58 . 
   Balun  66  operates in the same manner as balun  26 A of  FIG. 4 . Unbalanced components  50  receive an unbalanced signal via unbalanced port  58  and divide the received signal equally between balanced ports  60  via electromagnetic coupling between balanced components  52  and associated unbalanced components  50 . The signals output on each of balanced ports  60  are identical except for an approximate 180-degree phase shift. For example, the signal output from balanced port  60 A may have a first phase and the signal output from balanced port  60 B may have a second phase that is a 180-degrees out of phase relative to the phase of the signal output from balanced port  60 A. The signals output via balanced ports  60  are fed to a balanced device, such as balanced radio IC  28  or a balanced antenna  24 . 
   Various embodiments of the invention have been described. For example, the techniques of the invention may be used to incorporate passive structures that operate in different frequency ranges up to millimeter wave frequencies in an integrated circuit package. These and other embodiments are within the scope of the following claims.