Abstract:
A balun comprises at least two asymmetric coplanar striplines, a first of the striplines coupled to a signal input, and a second of the striplines coupled to a signal output, the at least two asymmetric coplanar striplines configured in a Marchand architecture to receive an unbalanced signal and to output a balanced signal.

Description:
TECHNICAL FIELD 
   The present description relates, in general, to baluns and, more specifically, to Marchand baluns utilizing asymmetric coplanar striplines. 
   BACKGROUND 
   Antennas are typically of two types, namely symmetrical or balanced, and asymmetrical or unbalanced.  FIG. 1  depicts a typical unbalanced antenna  100 . The antenna  100  includes monopole  101  and ground  102 . The input or feed structure is also unbalanced, and may be a coaxial cable  104  with ground shielding  103  or a micro-stripline (not shown). The unbalanced antenna has a single element for the total energy of the signal, which alternates + (positive) and − (negative). Note that the ground plane functions as part of the antenna, and thus strongly affects the performance of the antenna. The antenna can be detuned if the size of the ground place is between 0.25 and 2 wavelengths of the antenna resonant frequency. Other elements connected to the ground plane can also detune the antenna. Since antenna resonant frequency and performance depends on the shape of the device, each antenna needs to be customized, leading to higher design and production costs. However, since there are multiple radiating elements, the antenna  100  is useful for multi-band applications, e.g. mobile phones. However, if the RF module that provides the signal to the antenna  100  is balanced, an additional balun type antenna is required, which introduces additional losses and decreases the antenna&#39;s radiation performance. Examples of antenna  100  are monopole, patch, and PIFA (planar inverted-F antenna). 
     FIG. 2  depicts a typical balanced antenna  200 . The antenna  200  includes loop  201  with ground  204 . The input or feed structure is also balanced, and comprises separate + input  202  and − input  203  for each part of the alternating signal. The feed structure may comprise a coplanar microstrip line or two-wire transmission line. Note that with this arrangement, the ground plane is essentially independent from the antenna and has little effect on the performance of the antenna. Thus, the antenna resonant frequency and performance depends on the shape of the device, and a single antenna can work with a variety of ground plane geometries. However, since this arrangement has a symmetric geometry, the size of the antenna is double that of an equivalent unbalanced antenna. This antenna has a single radiating element and can be configured to operate in wide-band single resonance applications, such a magnetic resonance imaging (MRI) device and other inductive coupling applications. 
   In designing electronic circuits, e.g. mixers or amplifiers, balun antennas are used to link a symmetrical or balanced circuit with an asymmetrical or unbalanced circuit. Thus, a balun can be used to change an unbalanced signal to a balanced signal in order to drive a balanced antenna element, or vice versa.  FIG. 3  depicts a typical Marchand type balun antenna  300 . The Marchand balun has an unbalanced input  303  and a balanced output  301 . The input goes to two coupled line sections  304 ,  305 , the lengths of which are λ/4 (a quarter wavelength) of the input signal. The portions of the line sections that are connected to the outputs are shorted to ground. The portions of the line sections that are connected to the input are connected to an open circuit (OC). The Marchand balun operates through the coupling that occurs between the lines. The balun offers good amplitude balance and phase difference with a relatively wide operating bandwidth. 
   Note that in the balun of  FIG. 3 , the operating bandwidth is mainly controlled by the coupling strength of the two coupled-line sections. There are two types of coupled-lines, namely edge coupled and coplanar coupled.  FIGS. 4A and 4B  depict examples of edge coupled lines  401 , and coplanar coupled lines  402 , respectively. In  FIG. 4A , the signal line  403  couples with line  406 . The couple lines are separated from a ground place  404  by dielectric material  405 . This coupling is referred to an edge coupling. With this arrangement, manufacturing capability limits the coupling strength between a pair microstrips. In  FIG. 4B , the signal line  403  couples with line  406 . The couple lines are separated by dielectric material  405 . Ground plane  404  are adjacent to the couple lines. This arrangement is referred to as a coplanar waveguide configuration or broadside configuration, where one coplanar waveguide (e.g.  403 ) is on the top of the dielectric  405  and another coplanar waveguide (e.g.  406 ) is on the bottom of the dielectric. Strong coupling can be achieved by a pair lines in this arrangement. 
   There are two types of coplanar coupling, namely symmetrical and asymmetrical.  FIGS. 5A and 5B  depict examples of symmetrical  501  and asymmetrical  502  coplanar coupled lines, respectively. In  FIG. 5A , the signal line  503  is coplanar with line  504  and separated by dielectric layer  508 . The ground planes  505  and  506  are also coplanar and separated by dielectric layer  508 . In  FIG. 5B , the signal line  503  is coplanar with line  504  and separated by dielectric layer  508 . However, the ground planes  505  and  507  are not arranged in the same manner as the signal lines. This arrangement is referred to as asymmetric coplanar striplines (ACPS), and can be used to reduce the space for grounding, while still achieving strong coupling. The ACPS striplines will also have a wide bandwidth as the symmetrical coplanar striplines of  FIG. 5A . A Marchand balun based on ACPS coupling has a small size and a wide operating bandwidth. 
   Inhomogeneous media can cause a large difference between the even-mode and odd-mode velocities. A large difference degrades the performance of the balun. An arrangement that has a nonuniform ACPS that is covered with a dielectric can be used to overcome this problem.  FIGS. 6A and 6B  depict different views of a nonuniform ACPS coupler  600 . In this arrangement, the ground place is formed into an irregular shape. This arrangement improves performance of the bandwidth, because it reduces the difference in the even and odd mode velocities through the waveguides. 
   BRIEF SUMMARY OF THE INVENTION 
   Various embodiments of the invention are directed to a nonuniform, asymmetric coplanar stripline Marchand balun and methods for use of such a balun. A balun formed according to embodiments of the invention can have an unbalanced input and a balanced output, or vice versa. Such a balun can be used to feed a balanced antenna from an unbalanced signal feed, or vice versa. 
   One embodiment of the invention is to use ACPS to form a Marchand balun with strong coupling, and thus achieving a balun with wideband characteristic and small in size. The wideband balun is easier to fabricate and small in size for ultrawide bandwidth (UWB) applications. The UWB balun may be formed from one or two PCB layers having two layers of conductors. It is preferable to use a single PCB layer. In contrast, a prior art UWB balun tends to be very complicated and require three or more PCB layers, and thus is large in size. 
   Another embodiment of the invention is to form a balun using an open-circuit stub to introduce a rejection at the middle of the operating band to make a dualband balun. This embodiment simplifies the design of dualband wireless frontend systems. 
   Embodiments of the invention can be used to drive balanced antenna elements in a variety of applications. For example, one or more embodiments can be used to drive balanced antennas in an MRI system. Typical MRI systems use loop antennas to generate a large amount of magnetic field, and the loop antennas can be fed by baluns according to embodiments of the present invention. Further, various embodiments can be used in near-field applications, such as radio frequency identification (RFID). Other applications include the use in single layer superconducting elements. 
   The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which: 
       FIG. 1  depicts a typical unbalanced antenna; 
       FIG. 2  depicts a typical balanced antenna; 
       FIG. 3  depicts a typical Marchand type balun antenna; 
       FIGS. 4A and 4B  depict examples of edge coupled lines and coplanar coupled lines, respectively; 
       FIGS. 5A and 5B  depict examples of symmetrical and asymmetrical coplanar coupled lines, respectively; 
       FIGS. 6A and 6B  depict different views of a nonuniform ACPS coupler; 
       FIGS. 7A and 7B  depict an example of a system using embodiments of the invention and a conventional system, respectively; 
       FIGS. 8A ,  8 B, and  8 C depict different views of an ACPS UBW balun, according to embodiments of the invention; 
       FIGS. 9A and 9B  depict performance graphs of the balun of  FIGS. 8A-8C ; 
       FIG. 10  depicts a schematic diagram of a dual band balun, according to embodiments of the invention; 
       FIGS. 11A ,  11 B, and  11 C depict different views of an example of a dual band balun of  FIG. 10 , according to embodiments of the invention; 
       FIG. 12  depicts a performance graph of an example of the balun of  FIGS. 11A-11C ; and 
       FIG. 13  depicts a performance graph of another example of the balun of  FIGS. 11A-11C . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Embodiments of the invention use asymmetric coplanar striplines (ACPS) to form a Marchand balun with strong coupling, and thus achieving a balun with wideband characteristic and small in size. Embodiments use an open-circuit stub to introduce a rejection at the middle of the operating band to make a dualband balun. The dualband balun simplifies the design of dualband wireless frontend systems. Embodiments of the invention can form a wideband balun that is small in size for ultrawide bandwidth (UWB) applications. 
     FIG. 7A  depicts an example of a system using a dual band balun, according to embodiments of the invention. In this arrangement, system  700  transmits and receives at two frequencies. System  700  includes a dual band balun  701  for coupling the balanced inputs  702  to an unbalanced antenna  703 . A diplexer  704  is used to switch merge/split the different signals. A dual band pass filter  705  is used to condition the signals. In contrast, a convention system  750 , shown in  FIG. 7B  needs two baluns  751 ,  752 , one for each frequency, along with two band pass filters  753 ,  754 . 
     FIGS. 8A ,  8 B, and  8 C depicts different views of an ACPS UBW balun, according to embodiments of the invention.  FIG. 8A  depicts a perspective view of the balun  800 .  FIG. 8B  depicts a top-down view of the upper layer  801  of balun  800 , and  FIG. 8C  depicts a top-down view of the bottom layer  802  of balun  800 . Note that the upper and lower layers are by way of example only, as they could be reversed. The balun  800  comprises balanced ports  803 ,  804  and an unbalanced port  805 . The other ends of lines (i.e. connecting to ports  803  and  804 ) are shorted to ground  806  through the lines with the length ∫ approximately λg/4, λg being the wavelength of operating frequency. In addition, the lines with the length of ∫ on the top layer overlap the line connected to the port  805  on the bottom layer. The balun is formed from two nonuniform ACPS couple lines on two sides of a single layer of a printed circuit board (PCB). The upper layer  801  comprises the balanced ports  803 ,  804  with lines connected to the ground plane (conductor)  806 . Area  807  comprises dielectric material. The ground plane  806  includes wedge portion  810 , which forms a nonuniform ACPS and improves the bandwidth. The dimensions of wedge portion  180  may be adjusted to improve the balun performance for particular frequencies. The bottom layer  802  comprises the unbalanced port  805  and ground plane (conductor)  808 . A portion of the line connected to the port  805  overlap the lines (i.e. connecting to ports  803  and  804 ) on the top layer with a resonator length ∫. Area  809  comprises dielectric material. Vias  811  connect ground planes  806  and  808 . 
     FIGS. 9A and 9B  depict performance graphs of the balun of  FIGS. 8A-8C . The balun of  FIGS. 8A-8C , each of the ports  803 ,  804 , and  805  are coupled to 50 Ohm resistors.  FIG. 9A  depicts the loss of the balun over frequency. Curve  902  depicts the reflection or return loss of the port  805 , and curves  903 ,  904  depict the transmission or insertion loss between the port  805  and the ports  803 ,  804 . Note that the amplitude balance is within ±0.5 dB over 32-50 MHz. Also note that the return loss is less than −10 dB in the working band. The dip in curve  902  indicates low reflection in the working band. The relatively flat and constant nature of curves  903  and  904  indicate good transmission throughout the working band.  FIG. 9B  depicts the phase effects of the balun over frequency. Curve  905  depicts the phase at port  805 , and curve  906  depicts the phase at port  804 . The phase at port  803  would be similar that of port  804 . Note that the phase difference is within ±50 over 30 MHz to 50 MGHz. Thus, as indicated by the performance in the 30 MHz to 50 MHz range, balun  800  is suitable for use in MRI systems or other RF circuits which need the conversion between balanced ports and unbalanced ports. 
     FIG. 10  depicts a schematic diagram of a dual-band balun  1000 , according to embodiments of the invention. The dual-band balun  1000  is a Marchand type balun antenna. The balun has an unbalanced port  1003  and a pair of balanced ports  1001 ,  1002 . The balun includes two coupled line sections  1004 ,  1005 , the length of which is around λ/4 (a quarter wavelength) of the operating frequency. The portions of the line sections that are connected to the balanced ports are shorted to ground. The portions of the line sections that are connected to the unbalanced port are connected to an open circuit (OC) through stub portion  1006 . The stub portion  1006  may be around a quarter wavelength long (a quarter wavelength of the operating frequency). The stub portion  1006  may be implemented by a meandering microstrip to reduce the overall size. This balun has a wide operating bandwidth and introduces a strong rejection at the band center and improves return loss at two separate frequencies. 
     FIGS. 11A ,  11 B, and  11 C depicts different views of an example of a dual band balun of  FIG. 10 , according to embodiments of the invention.  FIG. 11A  depicts a perspective view of the balun  1100 .  FIG. 11B  depicts a top-down view of the upper layer of balun  1100 , and  FIG. 11C  depicts a top-down view of the bottom layer of balun  1100 . Note that the upper and lower layers are by way of example only, as they could be reversed. The balun  1100  comprises balanced ports  1101 ,  1102  and an unbalanced port  1103 . The two coupling areas, which is the overlap between unbalanced port  1103  and balanced ports  1101 ,  1102  are shown as  1104  and  1105 . The balun is formed from two nonuniform ACPS couple lines on two sides of a single layer of PCB. The upper layer comprises the unbalanced port  1103  and ground plane (conductor)  1107 . Area  1108  comprises dielectric material. The upper layer also includes stub portion  1106 . Note that in this example, the stub portion in meandered to reduce the footprint of the stub portion. Note that the meandering is by way of example only as other meander patterns may be used or no meandering may be used. The lower layer comprises the balanced ports  1101  and  1102  and ground place (conductor)  1109 . Area  1110  comprises a dielectric material. Vias  1111  connect ground planes  1107  and  1109 . 
     FIG. 12  depicts a performance graph of an example of a balun of  FIGS. 11A-11C . In this example, each of the ports  1101 ,  1102 , and  1103  are coupled to 50 Ohm loads.  FIG. 13  depicts the performance of balun over frequency. Curve  1201  depicts the reflection or return loss of the unbalanced port ( 1103 ), and curve  1202  depicts the transmission or insertion loss between unbalanced port ( 1103 ) and unbalanced ports ( 1101 ,  1102 ). The location off ƒ 0 , ƒ 1  and ƒ 2  are determined by the stub length. The ƒ 1  and ƒ 2  also depend on the coupling strength of the balun. The return loss can also be adjusted by the stub impedance. The frequencies ƒ 1  and ƒ 2  are lower and upper working bands, respectively. The dips of the blue curve show that the balun has two distinct operating bands. The red curve shows good transmission performance in the working bands. The quarter-wavelength stub corresponds to ƒ 0 . 
     FIG. 13  depicts a performance graph of another example of a balun of  FIGS. 11A-11C . In this example, the balanced port (i.e.  1101 ,  1102 ) is coupled to 100 Ohm resistors, and port  1103  is coupled to a 50 resistor. The stub length is around quarter wavelength at 400 MHz. The balun in this system is used for dual bands which are 200 MHz and 500 MHz bands.  FIG. 13  depicts the performance of balun over frequency. Curve  1301  depicts the reflection or return loss of the unbalanced port  1103 . The locations of ƒ 1  and ƒ 2  are determined by the length of the stub and coupling strength of the balun. The return loss can also be adjusted by the stub impedance. The frequencies ƒ 1  and ƒ 2  are lower and upper working bands, respectively. The dips of the blue curve show that the balun has two distinct operating bands. Note that there is more than 15 dB return loss at the 200 MHz band, and more than 15 dB return loss at the 500 MHz band. 
   It should be noted that while the examples of  FIGS. 9A ,  9 B,  12 , and  13  show performance in specific frequency bands, the scope of embodiments is not so limited. In fact, embodiments can be designed to operate at any radio frequency (RF) band through scaling and shaping. Further, the specific shapes and designs shown herein are exemplary, as other embodiments can take different shapes and/or designs. Moreover, some embodiments of the invention include methods for use of baluns designed according to the concepts described herein. 
   Some embodiments can be deployed in MRI systems to feed balanced antenna elements. Additionally, some embodiments can be used in Near Field Coupling (NFC) applications, such as RFID. Other uses are also possible, such as, e.g., in handheld consumer devices. 
   Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.