Passive intermodulation (PIM) is a form of nonlinear distortion that is encountered in a growing number of communications and sensing systems. This distortion is created in passive RF system components such as coaxial connectors, antennas and filters as a result of small nonlinear characteristics of such passive components, and frequently transfers energy from high-power transmit signals to frequencies within the system's receive band, masking low-level receive signals or even saturating sensitive receive circuitry. See, for example, P. L. Lui, “Passive Intermodulation Interference in Communication Systems,” Electronics & Communication Engineering Journal, Vol. 2, June 1990, pp. 109-118. This reference is incorporated herein by reference along with all other references cited herein. Because the distortion products are generated after low-level receive signals are already present in the network, the distortion power cannot be removed by filters and arrives at the receiver along with the desired receive signals.
As a result of the great difference in power between transmitted and receive signals in a communication system, passive intermodulation distortion levels as low as −150 dBc are potentially problematic sources of interference in many systems as the nonlinearity of passive components causes power at transmit frequencies to mix into the system's receive band. Passive intermodulation is most problematic in transmit/receive systems where transmit and receive bands are closely spaced. Communication frequency bands are becoming more densely populated, making passive intermodulation a growing concern in the wireless community, cell phone applications representing one example.
Because PIM distortion cannot usually be mitigated by conventional means such as frequency filtering, many studies have been undertaken to identify the causes of PIM. See, for example, the above-referenced paper by Lui as well as the following papers: M. T. Abuelma'atti, “Prediction of Passive Intermodulation Arising From Corrosion,” IEE Proceedings, Science, Measurement and Technology, Vol. 150, No. 1, 2003, pp. 30-34; F. Arazm, “Nonlinearities in Metal Contacts at Microwave Frequencies,” IEEE Transactions on Electromagnetic Compatibility, Vol. EMC-22, August 1980, pp. 142-149; and J. Wilcox and P. Molmud, “Thermal Heating Contribution to Intermodulation Fields in Coaxial Waveguides,” IEEE Transactions on Communications, Vol. 24, No. 2, February 1976, pp. 238-243. PIM is known to occur at junctions of dissimilar metals and at junctions of metals and oxides. The unsoldered metal-metal junction in a coaxial connector is a major contributor to PIM in many microwave networks. Ferromagnetic conduction metals such as iron or nickel are also well known causes of PIM distortion, especially in coaxial connectors. See, for example, the above-referenced paper by Lui as well as the following references: J. Henrie, A. Christianson, and W. J. Chappell, “Prediction of Passive Intermodulation From Coaxial Connectors in Microwave Networks,” IEEE Transactions on Microwave Theory and Techniques, Vol. 56, No. 1, January 2008, pp. 209-216; and J. C. Pedro and N. B. Carvalho, Intermodulation Distortion In Microwave And Wireless Circuits, Artech House, Boston, Mass., 2003. As a result, efforts have been made to solve the problem through the choice of contact materials, including combinations of base material and plating. For example, low-PIM connectors are available in which the contacts are silver-plated or gold-plated with no nickel undercoat. However, there are tradeoffs with such connectors, most notably a higher cost of materials and manufacturing, and a shorter lifetime in some cases, in exchange for the lower PIM. Low PIM connectors are relatively expensive, bulky, low-bandwidth, and more susceptible to some environmental factors because of their composition. In addition, a need exists for even lower PIM in coaxial connectors and other passive RF components.
The present invention uses a magnetic field to mitigate passive intermodulation distortion (PIM). A preferably strong external static magnetic field is used to significantly lower the intermodulation distortion produced by wireless components which contain ferromagnetic materials such as nickel or steel. The method is useful for mitigating PIM and for locating sources of PIM.
The objects and advantages of the present invention will be more apparent upon reading the following detailed description in conjunction with the accompanying drawings.
For the purpose of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated device and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates.
Although not extensively studied in terms of intermodulation distortion (IM) generation, it is well known that components which contain ferromagnetic materials in their conduction paths generate high levels of IM. See, for example, the above-referenced paper by Lui, as well as the following references: J. Henrie, A. Christianson, and W. J. Chappell, “Prediction of Passive Intermodulation From Coaxial Connectors in Microwave Networks,” IEEE Transactions on Microwave Theory and Techniques, Vol. 56, January 2008, pp. 209-216; and J. C. Pedro and N. B. Carvalho, Intermodulation Distortion In Microwave And Wireless Circuits, Artech House, Boston, Mass., 2003. The cause of this nonlinearity is believed to be the hysteretic relationship between the magnetization of a ferromagnetic material and any imposed magnetic field, such as that caused by current flow through the conductor. The relationship is represented graphically as a nonlinear, sigmoid-shaped B-H curve such as shown in
It has been found, however, that the introduction of an external magnetic field can partially or fully saturate the magnetization of a ferromagnetic metal. This has the effect of decreasing the effective magnetic permeability of the material, lowering the real and imaginary parts of the impedance of the metal. This lowering of impedance due to an external magnetic field is known as magneto-impedance. In linear tests, the change is very small, but it can have a profound impact on the nonlinear behavior of the system.
One embodiment of the present invention applies an external magnetic field to gold-plated SMA (sub-miniature A) connectors, whose nickel underplate layer displays the magneto-impedance phenomenon. In one example, shown in
The external magnetic field also causes a shift of the resonant frequency of the resonator (from 465.8 to 466.1 MHz in this example).
The inventors have found that by appropriately accounting for the interaction between linear and nonlinear components in a system, circuit models of PIM-producing networks can be constructed by modeling passive nonlinear components such as coaxial connectors as simple third-order nonlinear resistors. These models can be very accurate across broad power ranges and network topologies. Further information regarding these models is contained in the following paper by J. Henrie, A. Christianson, and W. J. Chappell: “Linear-Nonlinear Interaction's Effect on the Power Dependence of Nonlinear Distortion Products,” Applied Physics Letters, Vol. 94, March 2009, pp. 114101-1-114101-3. These models can be easily implemented using circuit simulators, allowing one to simulate both the linear and nonlinear properties of PIM-producing networks under the same framework.
Based on such a SPICE model of the microstrip resonator shown in
An Agilent ADS model of the resonator of
The magnetoimpedance phenomenon results in only small changes in the linear behavior of the connector, and for that reason the connector is embedded in the resonator of
Referring to
Because the static magnetic field causes a drop in the resistance of the ferromagnetic component, less power is absorbed by the component and less IM distortion is produced. Another way in which the external magnetic field serves to mitigate the nonlinearity of the ferromagnetic material is by direct linearization of the permeability, as seen in FIG. 1 of a paper by L. V. Panina, K. Mohri, K. Bushida, and M. Noda entitled “Giant Magneto-Impedance and Magneto-Inductive Effects in Amorphous Alloys,” published in the Journal of Applied Physics, Vol. 76, No. 10, November 1994, pp. 6198-6203.
Both of these mechanisms—the lowering of resistance and the linearization of magnetic permeability of the nickel underplate metal—combine to dramatically decrease the PIM output of the connector. As an example,
Because of the DC nature of the magnetic source, this type of PIM reduction is broadband in nature, and thus not subject to the bandwidth limitations of some other proposed techniques.
The prospect of using inexpensive, high-bandwidth SMA and 3.5 mm connectors in applications where previously only large, silver-plated connectors met design specifications is of interest for cost and performance reasons. Also, as shown in
In addition to the various types of coaxial connectors made with ferromagnetic metals, the present invention is contemplated to be useful with antennas and RF filters among other passive RF system components, such as circuit boards with nickel-containing package leads, e.g., as depicted in
Ferromagnetic metals are often included in high-power conduction paths and contribute to unwanted PIM generation. It is also contemplated as part of the present invention that magnetic fixtures may be fashioned to “bias” these ferromagnetic deposits as well, very similar to the gyromagnetic waveguide isolator shown in
Other embodiments of the present invention have the form of ferrite beads that are used on USB cables, shown in
Thus, in various embodiments, the invention provides a magnetic field to reduce the passive intermodulation distortion produced by components containing ferromagnetic material. The introduction of a strong external magnetic field has been shown to reduce the passive intermodulation distortion produced by components containing ferromagnetic material by as much as 40 dB. This effect is useful both for mitigating passive intermodulation and for easily locating dominant sources of it, even in shielded components. A strong external magnetic field diminishes and linearizes the magnetic permeability of ferromagnetic metal, decreasing both the sheet resistance and inductance of the nickel-containing coaxial connectors described herein. Without being bound to a particular theory, the inventors note that inside ferromagnetics like nickel are millions of microscopic magnetic “domains,” each with a random polarization. The inventors herein believe that an electric current passing through this metal dynamically re-orients these domains, causing a highly nonlinear response, and that the addition of a strong DC magnetic field to the material tends to “pin” the domains to a certain orientation, so that the electrical response of the material is linearized to a high degree, resulting in a substantial reduction of nonlinear distortion. The diminution of passive intermodulation power in response to the external magnetic field could be used as a non-invasive method of locating and mitigating the dominant ferromagnetic passive nonlinearities in high-powered systems, without disrupting the geometries of shielded structures such as coaxial transmission lines.
While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only preferred embodiments have been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected.
This application claims the benefit of Provisional Patent Application Nos. 61/184,153, filed Jun. 4, 2009, and 61/244,754, filed Sep. 22, 2009, which applications are hereby incorporated by reference along with all references cited therein.
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/US2010/037465 | 6/4/2010 | WO | 00 | 2/21/2012 |
Publishing Document | Publishing Date | Country | Kind |
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WO2010/141860 | 12/9/2010 | WO | A |
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Number | Date | Country | |
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20120139656 A1 | Jun 2012 | US |
Number | Date | Country | |
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61184153 | Jun 2009 | US | |
61244754 | Sep 2009 | US |