Patent Grant
8183952
1. Field of the Invention
The present invention relates generally to RF components, and particularly to RF components such as circulators and isolators.
2. Technical Background
Circulators and isolators are passive multi-port microwave device that are typically used in RF transmission line applications. A typical ferrite circulator includes three ports, and is generally referred to as a Y-junction circulator. In operation, when an RF signal is directed into a first port, the RF signal will be accessible via the second port in sequence, i.e., the port immediately adjacent the input port. The RF signal will be substantially attenuated and will not be available at the third port in the sequence, i.e., the port immediately adjacent to the second port on the other side of the first input port. On the other hand, if an RF signal is directed into the second port, it will be available as an RF output signal at the third port, but will not be available at the first port. Finally, if an RF signal is introduced at the third port, it will be available as an RF output at the first port, but not at the second port. A circulator, therefore, propagates RF power from one adjacent port to the next in a sequential, circular fashion. The RF signal circulation may be right-handed (RH) or left-handed (LH).
The circulation action in circulators/isolators is achieved by utilizing the “gyromagnetic effect” that is characteristic of ferrite materials. Ferrite materials have, in particular, specific magnetic properties which are mainly caused by spinning electrons. The spinning electrons have a magnetic moment and a mechanical moment. With the exposure of the ferrite material to an external magnetic field the magnetic moments can be aligned in parallel to the applied field. If all magnetic moments are aligned, the material is saturated. If another disturbing force, like an RF electromagnetic field, is applied to bring the electron spin out of alignment, a torque will act on the electron spin. The electron will then precess around the axis of the applied field with an angular frequency proportional to the applied field. The behavior of the material can be described mathematically using the Polder permeability tensor. The elements of the tensor are controlled by the RF frequency, the saturation magnetization of the material and the strength of the applied DC magnetic field. If the RF frequency is the same as the precession frequency, the ferrite material is operated at ferromagnetic resonance which also causes dissipation. Circulators and Isolators are generally operated with the magnetic biasing field adjusted to operate above or below ferromagnetic resonance.
When an RF signal is directed into the input port of the circulator, circulating phase shifted versions of the RF signal are induced within the ferrite discs. The degree of phase shift between counter circulating fields is a function of the strength of the DC magnetic field and diameter of the ferrite material. The circulator operates in accordance with the principles of superposition and constructive/destructive interference of counter-rotating RF waves. Using the example from above, when an RF signal is directed into the first port, the counter circulating RF signals are substantially in phase with each other at the second port, and therefore, they constructively interfere and reinforce each other. The amount of signal available at the second port is measured by what is commonly referred to as the insertion loss. In a properly functioning device the insertion loss is typically in the range of a few tenths of a decibel (dB). At the third port, the RF signals are out of phase with each other and substantially cancel each other. The term “substantially” refers to the fact that, in practice, the cancellation is not perfect and a residual signal may be detected. The amount of residual signal available at the third port, appropriately referred to as the “isolation,” is measured by the ratio of the residual signal and the incident signal. The isolation is typically between −25 dB and −30 dB.
A circulator may be configured as an isolator by terminating one of the ports with a “matched load” such that the complex impedance of the load is the complex conjugate of the output port impedance. As noted above, an isolator permits RF signal propagation between the two remaining ports in one direction only. RF power flow in the opposite direction is substantially inhibited. Now that the general operating principles have been briefly touched upon, a similarly brief description of the structure of a junction circulator is provided.
A junction circulator may be configured to include both electrical and magnetic circuit components and may be implemented using a stripline, microstrip or waveguide transmission configuration. The circulator includes a circuit portion having a flat center conductor that has three branches extending symmetrically outward from the central conductive portion. The three branches function as the ports of the circulator and are positioned 120° apart from each other. The center conductor is sandwiched between a pair of ferrite discs. The outer surface of both the top ferrite disc and bottom ferrite disc are in contact with ground planes to thereby form a stripline configuration. A permanent magnet is disposed over each ground plane. The permanent magnets apply a predetermined magnetic field to bias the ferrite discs in a predictable manner. A steel pole member may be inserted between each ground plane/magnet pair. The function of the steel pole member is to ensure that the biasing magnetic field applied to the ferrites is substantially uniform.
Those of ordinary skill in the art will understand that the operating frequency of circulators and isolators is determined by a number of factors like disc diameter, permittivity of the ferrite disc, biasing field level and circuit shape. The operating frequency for a biased-above-resonance (A/R) circulator is generally limited to approximately 4 GHz, while the operation frequency for biased below resonance (B/R) circulators extends up to 30 GHz. Below resonance circulators typically operate over broader frequency range than above resonance circulators.
Some of the drawbacks associated with the various circulators/isolators described above relate to manufacturability and functionality issues. Circulators and Isolators are typically implemented as drop-in or connectorized units. Below resonance (B/R) microstrip circulators are typically comprised of ferrite substrates. The electric circuit is implemented by sputtering the circuit material onto the ferrite substrate. A microstrip circulator operating above 10 GHz must be biased by a strong magnet that is typically comprised of a metallic alloy. The magnet should be aligned and placed precisely over the central conductive portion to prevent the RF field from being disturbed by the presence of a metal object disposed over the microstrip lines. Subsequently, the magnet is bonded to the ferrite surface using relatively precise bonding techniques. Accordingly, this approach is characterized by drawbacks that create significant challenges in a production environment. The other challenge associated with this design relates to the connection of the microstrip circulator with external circuitry. The aforementioned connection is usually accomplished using wire bonding techniques. One issue related to wire bonding techniques relates to discontinuities in the 50 Ohm transmission line impedance that are prone to the excitation of unwanted reflections in the transmission path. Moreover, below resonance micro-strip devices require manual labor both to mount them into an assembly and to provide the necessary RF connections.
What is needed is a surface mountable design that is amenable to automated pick and place manufacturing processes that substantially eliminate or reduce labor-intensive assembly.
The present invention addresses the needs described above by providing a surface mountable design that can take advantage of automated pick and place manufacturing processes that substantially eliminates or reduces labor-intensive assembly.
One aspect of the present invention is directed to a circulator/isolator device that includes an electric circuit having predetermined electrical characteristics. The electric circuit includes at least one conductor disposed on at least one side of a first dielectric substrate. The at least one conductor includes at least one first transmission line, at least one second transmission line and at least one third transmission line that form a transmission line junction at a substantially central region on the at least one side of the first dielectric substrate. The at least one first transmission line includes a first electric circuit port disposed at a first edge of the first dielectric substrate. The at least one second transmission line includes a second electric circuit port disposed at a second edge of the first dielectric substrate. The at least one third transmission line includes a third electric circuit port disposed at a third edge of the first dielectric substrate. At least one second dielectric substrate is disposed adjacent the first dielectric substrate. The at least one second dielectric substrate includes an opening formed in a substantially central region of the at least one second dielectric substrate. The opening is aligned in a predetermined position relative to the transmission line junction formed at the substantially central region of the first dielectric substrate. A ferrite element is disposed in the opening of the at least one second dielectric substrate. The ferrite element is biased below ferromagnetic resonance in the presence of a biasing magnetic field and abuts a predetermined portion of the electric circuit. At least one third dielectric substrate is disposed adjacent the at least one second dielectric substrate. The at least one third dielectric layer includes an interior ground plane formed on an interior major surface of the at least one third dielectric substrate and disposed adjacent the at least one second dielectric substrate and an exterior ground plane formed on an exterior major surface of the at least one third dielectric substrate. The at least one third dielectric substrate includes a first-third dielectric port disposed at a first edge of the third dielectric substrate and coupled to the first electric circuit port, a second-third dielectric port disposed at a second edge of the third dielectric substrate and coupled to the second electric circuit port, and a third-third dielectric port disposed at a third edge of the third dielectric substrate and coupled to the third electric circuit port. The electric circuit, the at least one second dielectric substrate having the ferrite element, and the at least one third dielectric substrate are laminated together to form a surface mountable multi-layer assembly. The surface mountable multi-layer assembly is further characterized by a non-metallic magnetic circuit return path.
In another aspect, the present invention is directed to a method for making at least one surface mountable circulator/isolator device. The method includes disposing at least one conductor on at least one side of a first dielectric substrate to form an electric circuit having predetermined electrical characteristics. The at least one conductor includes at least one first transmission line, at least one second transmission line and at least one third transmission line that form a transmission line junction at a substantially central region on the at least one side of the first dielectric substrate. The at least one first transmission line including a first electric circuit port disposed at a first edge of the first dielectric substrate, the at least one second transmission line including a second electric circuit port disposed at a second edge of the first dielectric substrate, the at least one third transmission line including a third electric circuit port disposed at a third edge of the first dielectric substrate. The method also includes providing at least one second dielectric substrate. The method further includes forming an opening in a substantially central region of the at least one second dielectric substrate. The opening is formed such that it is aligned in a predetermined position relative to the transmission line junction formed at the substantially central region of the first dielectric substrate. The method further includes disposing the at least one second dielectric substrate adjacent the first dielectric substrate. The method further includes disposing a ferrite element in the opening of the at least one second dielectric substrate. The ferrite element is biased below ferromagnetic resonance in the presence of a biasing magnetic field. The ferrite element abuts a predetermined portion of the electric circuit. The method further includes disposing at least one third dielectric substrate adjacent the at least one second dielectric substrate. The at least one third dielectric layer includes an interior ground plane formed on an interior major surface of the at least one third dielectric substrate and disposed adjacent the at least one second dielectric substrate and an exterior ground plane formed on an exterior major surface of the at least one third dielectric substrate. The at least one third dielectric substrate includes a first-third dielectric port disposed at a first edge of the third dielectric substrate and coupled to the first electric circuit port, a second-third dielectric port disposed at a second edge of the third dielectric substrate and coupled to the second electric circuit port, and a third-third dielectric port disposed at a third edge of the third dielectric substrate and coupled to the third electric circuit port. The method further includes laminating the first dielectric substrate, the at least one second dielectric substrate having the ferrite element disposed in the opening, and the at least one third dielectric substrate to form at least one surface mountable multi-layer assembly. The at least one surface mountable multi-layer assembly is further characterized by a non-metallic magnetic circuit return path.
In yet another aspect, the present invention is directed to a method for making an RF assembly. The method includes the steps of providing a circulator/isolator device that includes an electric circuit having predetermined electrical characteristics. The electric circuit includes at least one conductor disposed on at least one side of a first dielectric substrate. The at least one conductor includes at least one first transmission line, at least one second transmission line and at least one third transmission line that form a transmission line junction at a substantially central region on the at least one side of the first dielectric substrate. The at least one first transmission line includes a first electric circuit port disposed at a first edge of the first dielectric substrate. The at least one second transmission line includes a second electric circuit port disposed at a second edge of the first dielectric substrate. The at least one third transmission line includes a third electric circuit port disposed at a third edge of the first dielectric substrate. At least one second dielectric substrate is disposed adjacent the first dielectric substrate. The at least one second dielectric substrate includes an opening formed in a substantially central region of the at least one second dielectric substrate. The opening is aligned in a predetermined position relative to the transmission line junction formed at the substantially central region of the first dielectric substrate. A ferrite element is disposed in the opening of the at least one second dielectric substrate. The ferrite element is biased below ferromagnetic resonance in the presence of a biasing magnetic field and abuts a predetermined portion of the electric circuit. At least one third dielectric substrate is disposed adjacent the at least one second dielectric substrate. The at least one third dielectric layer includes an interior ground plane formed on an interior major surface of the at least one third dielectric substrate and disposed adjacent the at least one second dielectric substrate and an exterior ground plane formed on an exterior major surface of the at least one third dielectric substrate. The at least one third dielectric substrate includes a first-third dielectric port disposed at a first edge of the third dielectric substrate and coupled to the first electric circuit port, a second-third dielectric port disposed at a second edge of the third dielectric substrate and coupled to the second electric circuit port, and a third-third dielectric port disposed at a third edge of the third dielectric substrate and coupled to the third electric circuit port. The electric circuit, the at least one second dielectric substrate having the ferrite element, and the at least one third dielectric substrate are laminated together to form a surface mountable multi-layer assembly. The surface mountable multi-layer assembly is further characterized by a non-metallic magnetic circuit return path. The method for making the RF assembly further includes providing an RF assembly substrate, the RF assembly substrate including at least one printed circuit formed thereon. The method for making the RF assembly further includes disposing a solder paste on the RF assembly substrate in accordance with the at least one printed circuit. The method for making the RF assembly further includes positioning the surface mountable multi-layer assembly at a predetermined position on the RF assembly substrate using an automated process. The solder paste is reflowed such that portions of the surface mountable multi-layer assembly are electrically and mechanically bonded to predetermined portions of the at least one printed circuit.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.
Reference will now be made in detail to the present exemplary embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. An exemplary embodiment of the circulator/isolator of the present invention is shown in
As embodied herein and depicted in
The dielectric layer 12 may be fabricated using a suitable non-ferrous dielectric material configured to support the electric circuit trace 120. Accordingly, the dielectric layer 12 may be formed using dielectric materials suitable for printed circuit boards (PCBs). As such, the dielectric layer 12 may be fabricated using suitable materials such as polytetrafluoroethylene (PTFE), for example. The present invention, however, should not be construed as being limited to PTFE. The dielectric layer 12 may also be fabricated using any suitable materials such as combinations of PTFE and woven glass fibers, PTFE and random micro fiber glass, ceramic, etc. The electric circuit 122 may be formed on one side of the dielectric layer 12 using standard PCB manufacturing techniques, such as physical vapor deposition (PVD), chemical vapor deposition (CVD), etc. In one embodiment of the present invention, for example, the dielectric layer 12 is implemented using commercially available ROGERS RO-3003 board.
The electric circuit 120 is implemented using any suitable conductive material, such as gold (Au), silver (Ag), copper (Cu), aluminum (Al), titanium (Ti), etc. Initially, the material is deposited on the surface of dielectric layer 12. A pattern of the electric circuit is transferred to the surface of the plated dielectric using photolithographic techniques. Subsequently, the conductive material that is exposed during the photolithographic imaging procedure is removed from the layer 12 such that the electric circuit 120 remains. Thus, the electric circuit 120 is formed as a circuit trace that includes no discontinuities or irregularities.
Referring back to
A top dielectric layer 18 is disposed over the interior layer 16. The top dielectric layer 18 includes a ground plane 180 (e.g., a copper foil material) formed on most of the exposed surface thereof. The top layer 18 also provides an interior ground-plane 188 that is adjacent to, and in contact with, the ferrite disk 14. The upper ground plane 180 is connected to the lower ground plane 188 by way of metalized vias formed in the dielectric layer 18. A number of the vias connecting the top and the bottom ground plane may be used to suppress resonant modes.
The exposed surface of dielectric layer 18 also accommodates castellated signal through holes. For example, the dielectric layer 18 includes three exterior RF ports (182, 184, 186) formed along exterior edges of layer 18. The exterior RF ports 182, 184, 186 are connected to electric circuit ports 122, 124, and 126, respectively, by way of metalized vias 162, 164, and 166, respectively, formed along the edge of the interior layer 16.
Finally, a permanent magnet 20 may be bonded to the upper surface of the top layer 18 to provide the necessary magnetic biasing field to the ferrite element. In other embodiments, the magnetic bias may be provided by an external magnet, a solenoid, or by way of other such biasing means.
Again, the dielectric layers 12, 14, 16 and 18 may be formed using a suitable material including any suitable composite PTFE board. As implied above, in one embodiment of the present invention, the dielectric layers 12, 14, and 16 may be implemented using composite polytetrafluoroethylene (PTFE) materials. In one embodiment, the dielectric substrates may be implemented using commercially available ROGERS boards (e.g., RO-3003). The dielectric constant of a composite PTFE board such as the RO-3003 is approximately equal to 3.0. Depending on its function within the laminated multi-layer assembly, a PTFE board may include a copper layer (e.g., 0.5 ounce) disposed over the PTFE dielectric layer. Of course, the copper layer may function as a ground plane. The multi-layer PTFE structure is bonded to create a multi-layer laminate structure. In another embodiment of the present invention, the dielectric layers are fabricated using a ceramic material such as LTCC. The electric circuit 120 may also be formed by a screen printing process. The ground planes may also be printed or etched upon the LTCC layers using any suitable circuit trace process.
In general, the circulator depicted in
In an alternate embodiment of the present invention, the permanent magnet 20 is not part of the assembly of either
The ferrite junctions depicted herein are biased below ferromagnetic resonance and primarily utilized in a frequency range having a lower bound of approximately 2 GHz. The device may be employed in applications up to about 30 GHz. The ferrite material is saturated in the presence of the biasing magnetic field. The biasing magnet provides the necessary static magnetic field inside the ferrite element(s). The flux lines are closed through air and no additional housing structures are necessary. In order to improve the magnetic shielding of the device and/or to better utilize the permanent magnet, ferromagnetic return path structures may be employed.
As embodied herein and depicted in
The middle dielectric layer 12 again includes an electric circuit 120 formed thereon. The electric circuit 120 includes three transmission lines (122, 124, 126) that extend outwardly from a central junction. Surface 128 is disposed on the side of dielectric layer 12 opposite the electric circuit layer. In this embodiment a second circuit trace 120′ (not shown) conforming to the electric circuit 120 is disposed on the undersurface 128. The upper and lower circuit traces (120, 120′) are connected together using metalized vias formed in the interior of the dielectric layer 12. The use of the two ferrites provides an increase in performance relative to bandwidth, insertion loss, and isolation.
Like the first embodiment, the dielectric layer 12 may be fabricated using a suitable non-ferrous dielectric material configured to support the electric circuit traces 120, 120′. Accordingly, the dielectric layer 12 may be formed using dielectric materials suitable for printed circuit boards (PCBs). As such, the dielectric layer 12 may be fabricated using suitable materials such as polytetrafluoroethylene (PTFE), for example.
The present invention, however, should not be construed as being limited to PTFE. The dielectric layer 12 may also be fabricated using any suitable materials such as combinations of PTFE and woven glass fibers, PTFE and random micro fiber glass, ceramic, etc. The electric circuit 122 may be formed on one side of the dielectric layer 12 using standard PCB manufacturing techniques, such as physical vapor deposition (PVD), chemical vapor deposition (CVD), etc. In one embodiment, the dielectric substrates may be implemented using commercially available ROGERS boards (e.g., RO-3003). The dielectric constant of a composite PTFE board such as the RO-3003 is approximately equal to 3.0. Those of ordinary skill in the art will understand, however, that other materials may be employed with various dielectric properties. For example, the dielectric constant may be altered based on impedance matching characteristics. Thus, the dielectric constant may be 3.0, 6.0, 10.0 or higher. Depending on its function within the laminated multi-layer assembly, a PTFE board may include a copper layer (e.g., 0.5 ounce) disposed over the PTFE dielectric layer. Of course, the copper layer may function as a ground plane. The multi-layer PTFE structure is bonded to create a multi-layer laminate stripline structure. The electric circuit traces (120, 120′) may be comprised of any suitable conductive material such as gold (Au), silver (Ag), copper (Cu), aluminum (Al), titanium (Ti), etc. and formed using any suitable manufacturing techniques.
In the symmetric stripline embodiment depicted in
An upper dielectric layer 18 is disposed over the interior layer 16 whereas a lower dielectric layer 19 is disposed under the interior layer 17. The top dielectric layer 18 includes a ground plane 180 formed on most of the exposed surface thereof. The top layer 18 also provides an interior ground-plane 188 that is adjacent to, and in contact with, the ferrite disk 14. The upper ground plane 180 is connected to the lower ground plane 188 by way of metalized vias formed in the dielectric layer 18. A number of the vias connecting the top and the bottom ground plane may be used to suppress resonant modes. Similarly, the bottom dielectric layer 19 includes a ground plane 198 formed on a majority of the exposed surface thereof. The lower dielectric layer 18 also provides an interior ground-plane 190 that is adjacent to, and in contact with, the ferrite disk 15. The upper ground plane 190 is connected to the lower ground plane 198 by way of metalized vias formed in the dielectric layer 19.
The dielectric layer 18 includes three exterior RF ports (182, 184, 186) formed along exterior edges of layer 18. The exterior RF ports 182, 184, 186 are connected to metalized vias 162, 164, and 166, respectively, formed along the edge of the interior layer 16. The vias (162, 164, 166) are connected to the electric circuit ports 122, 124, and 126, respectively. The electric circuit ports 122, 124, and 126 are also connected to the metalized vias 172, 174, and 176, respectively, formed along the edge of the interior layer 17. The undersurface of dielectric layer 19 includes three exterior RF ports (192, 194, 196) formed along exterior edges of layer 19. The exterior RF ports 192, 194, 196 are connected to electric circuit ports 122, 124, and 126, respectively, by way of metalized vias 172, 174, and 176, respectively, formed along the edge of the interior layer 17. Thus, there is complete electrical connectivity from top to bottom in the stripline device of
Like the previous embodiment, the dielectric layers 12-18 may be formed using a suitable material including any suitable composite PTFE board. As implied above, in one embodiment of the present invention, the dielectric layers 12, 14, and 16 may be implemented using composite PTFE boards such as commercially available ROGERS RO-3003 boards. In another embodiment of the present invention, the dielectric layers are fabricated using a ceramic material such as LTCC. The electric circuit 120 may be formed by a screen printing process. The ground planes may also be printed or etched upon the LTCC layers using any suitable circuit trace process.
In general, the circulator depicted in
As in the asymmetric stripline embodiment, the ferrite junctions depicted herein are biased below ferromagnetic resonance and primarily utilized in a frequency range having a lower bound of approximately 2 GHz. The device may be employed in applications up to about 30 GHz. The ferrite material is saturated in the presence of the biasing magnetic field. The biasing magnet provides the necessary static magnetic field inside the ferrite element(s). The flux lines are closed through air and no additional housing structures are necessary. In order to improve the magnetic shielding of the device and/or to better utilize the permanent magnet, ferromagnetic return path structures may be employed.
Referring to an example application depicted in
All references, including publications, patent applications, and patents, cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein.
The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. The term “connected” is to be construed as partly or wholly contained within, attached to, or joined together, even if there is something intervening.
The recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein.
All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate embodiments of the invention and does not impose a limitation on the scope of the invention unless otherwise claimed.
No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.
It will be apparent to those skilled in the art that various modifications and variations can be made to the present invention without departing from the spirit and scope of the invention. There is no intention to limit the invention to the specific form or forms disclosed, but on the contrary, the intention is to cover all modifications, alternative constructions, and equivalents falling within the spirit and scope of the invention, as defined in the appended claims. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.
This application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Patent Application Ser. No. 61/168,977 filed on Apr. 14, 2009, the content of which is relied upon and incorporated herein by reference in its entirety.
| Number | Name | Date | Kind |
|---|---|---|---|
| 3573666 | Caffrey et al. | Apr 1971 | A |
| 3854106 | Tresselt | Dec 1974 | A |
| 4276522 | Coerver | Jun 1981 | A |
| 6011449 | Jussaume et al. | Jan 2000 | A |
| 6433649 | Miura et al. | Aug 2002 | B2 |
| 6844789 | Lombardi et al. | Jan 2005 | B2 |
| 6917250 | Lombardi et al. | Jul 2005 | B2 |
| 20050040908 | Mazzochette et al. | Feb 2005 | A1 |
| 20060139118 | Vaughn et al. | Jun 2006 | A1 |
| Number | Date | Country | |
|---|---|---|---|
| 20100259336 A1 | Oct 2010 | US |
| Number | Date | Country | |
|---|---|---|---|
| 61168977 | Apr 2009 | US |
