Patent Application
20100104236
This invention relates generally to a phase shifter and more particularly to a wide-band microwave and mm-wavelength phase shifter.
Astrophysical study of the formation of galaxies and stars and observation and mapping of other astrophysical phenomena is performed in large part by detection of radiation at wavelengths in the microwave and millimeter-wave spectrum. An important tool in the detection of such radiation involves feed horn coupled bolometric detector arrays mounted on satellites or space observatories. Input side components of such detectors, include component blocks such as amplifiers, waveguides, and phase shifters. The input side components are typically maintained at cryogenic temperatures to reduce system noise.
Bolometric detector arrays present particular challenges in fabrication due to the need for operation over large temperature ranges. Existing stripline technologies and solid state switches are not suitable for use over the wide temperature ranges needed for proper bolometric detector operation. For example, suspended stripline phase shifters can only be used with a coherent microwave amplifier preceding the device. Such stripline technologies, which exhibit high loss and cause injection of additional noise, are of limited use in astrophysical applications. Also, as discussed above, sensitive bolometric detectors are incompatible with microwave amplifiers. One example of a solid state switch was provided by Jarosik et al (Design, Implementation, and Testing of the Microwave Anisotropy Probe Radiometers, Astrophysical Journal, Supplement, Series, 145:413-436, April 2003). The devices Jarosik described cannot function below ˜140 K. At 140 K, if Jarosik's devices were to be used with a bolometric detector, they could produce a spurious signal nearly 20 times brighter than the desired science signal.
There is a need for a device that can operate over a temperature range from over 300 K down to below 3 K and that can be used with a bolometer or a coherent microwave amplifier.
In one aspect, the invention relates to a phase shifting device for switching the polarization state of an electromagnetic wave that includes a waveguide having two waveguide sections. Each waveguide section has an exterior rectangular opening defined in an end surface thereof and has an interior opening of predefined cross-sectional shape defined within a body thereof. The exterior rectangular opening and the interior opening of predefined cross-sectional shape are situated along a longitudinal axis of the waveguide. The waveguide sections are separated by a dielectric break. The dielectric break is defined therein and situated substantially collinearly with the longitudinal axis of the waveguide in substantially a center of the waveguide. The phase shifting device also includes a central structure situated along the longitudinal axis of the waveguide. The central structure includes a cylinder having a permeability greater than that of vacuum. The cylinder has two substantially circular end faces situated in perpendicular orientation to a longitudinal axis of the cylinder, and two dielectric cones. Each of the dielectric cones has a base mechanically coupled to an end face of the cylinder and a cone axis situated substantially collinearly with the longitudinal axis of the cylinder. The central structure is supported substantially in the center of the interior opening of predefined cross-sectional shape of the waveguide. The cylinder is substantially situated within the dielectric break of the waveguide. The phase shifting device also includes a magnetic field source. The magnetic field source is configured to generate a controllable magnetic field in the cylinder, wherein the magnetic field switches a polarization of the electromagnetic wave causing a phase shift of the electromagnetic wave of substantially zero degrees when the controllable magnetic field is off and a pre-determined phase shift when the controllable magnetic field is on.
In one embodiment, the interior opening of predefined cross-sectional shape is a circular opening.
In another embodiment, the pre-determined phase shift is substantially 180 degrees.
In yet another embodiment, the pre-determined phase shift is constant to within 1 degree over a 30% or greater fractional bandwidth.
In yet another embodiment, the waveguide includes gold plated copper.
In yet another embodiment, the waveguide includes a superconductor material.
In yet another embodiment, the central structure is supported by one or more dielectric supports.
In yet another embodiment, the one or more dielectric supports include one or more silica washers.
In yet another embodiment, the ceramic cones include an alumina ceramic.
In yet another embodiment, the each of the ceramic cones further include a sheet of microwave absorbing material and each of the sheets of microwave absorbing material are oriented substantially at 90 degrees with respect to the other along a longitudinal axis of the cylinder.
In yet another embodiment, the device further comprises a microwave absorber.
In yet another embodiment, the dielectric break is coated with a microwave absorber.
In yet another embodiment, the solenoid has solenoid windings includes a selected one of metallic windings and superconducting windings.
In yet another embodiment, the dielectric cylinder includes a ceramic or a semiconductor.
In yet another embodiment, the ceramic includes a ferrite ceramic.
In yet another embodiment, the semiconductor includes germanium or garnet.
In another aspect, an interferometer apparatus for strongly enhancing signal reception of an incident electromagnetic wave from a particular direction includes two or more receiving structures to guide the incident electromagnetic wave into the interferometer apparatus. The interferometer apparatus also includes two or more phase shifting devices as described above, each phase shifting device coupled to one each of the receiving structures. The interferometer apparatus also includes two or more detectors coupled to a respective one of the output structures of the two or more phase shifting devices, each detector having a detector electrical output terminal. The interferometer apparatus also includes a processor configured to receive an output signal from each of the detector electrical output terminals, wherein the output signals can be combined and processed to strongly enhance the incident electromagnetic wave from a particular direction.
In one embodiment, at least one of the two or more detectors includes a bolometer.
In another embodiment, at least one of the two or more detectors includes a microwave amplifier.
In yet another embodiment, at least one of the two or more detectors includes a SIS mixer.
In yet another embodiment, the detector is cooled to a temperature below 100 K.
In yet another embodiment, the magnetic field source is an electrical solenoid.
In yet another embodiment, the electrical solenoid includes superconducting windings.
In yet another embodiment, the at least one of the two or more receiving structures and the output structure includes a microwave feedhorn.
In another aspect, a phased array apparatus for transmitting an electromagnetic wave in a particular direction includes two or more input structures, each of the input structures configured to accept an electromagnetic wave to be transmitted by the phased array apparatus in a particular direction. The phased array apparatus also includes two or more phase shifting devices as described above, each phase shifting device coupled to one of the input structures to receive an input signal therefrom and configured to provide as output a respective phase shifted output signal. The phased array apparatus also includes two or more transmitting structures, each of the transmitting structures operatively connected to a respective one of the phase shifting devices and configured to receive as input a respective phase shifted output signal from a respective one of the phase shifting devices, and configured to guide the phase shifted output signal from the phase shifting device into a transmission medium.
In one embodiment, the two or more transmitting structures include planar antennae.
In yet another aspect, a phase shifting device for switching the polarization state of an electromagnetic wave includes a waveguide having two waveguide sections. Each waveguide section has an exterior rectangular opening defined in an end surface thereof and has an interior opening of predefined cross-sectional shape defined within a body thereof. The exterior rectangular opening and the interior opening of predefined cross-sectional shape is situated along a longitudinal axis of the waveguide. The waveguide sections are separated by a dielectric break. The dielectric break is defined therein and situated substantially collinearly with the longitudinal axis of the waveguide in substantially a center of the waveguide. The phase shifting device for switching the polarization state of an electromagnetic wave also includes a central structure situated along the longitudinal axis of the waveguide. The central structure includes a structure having a cross section including a polygon having N sides (where N is an integer) having a permeability greater than that of vacuum. The structure has a cross section including a polygon having N sides having two end faces situated in perpendicular orientation to a longitudinal axis of the structure having a cross section including a polygon having N sides, and two dielectric pyramidal structures having N sides. Each of the dielectric pyramidal structures has N sides that have a base mechanically coupled to an end face of the structure having a cross section including a polygon having N sides and having an axis situated substantially collinearly with the longitudinal axis of the structure having a cross section including a polygon having N sides. The central structure is supported substantially in the center of the interior opening of predefined cross-sectional shape of the waveguide. The structure has a cross section including a polygon having N sides substantially situated within the dielectric break of the waveguide. The phase shifting device for switching the polarization state of an electromagnetic wave also includes a magnetic field source. The magnetic field source is configured to generate a controllable magnetic field in the structure having a cross section including a polygon having N sides, wherein the magnetic field switches a polarization of the electromagnetic wave causing a phase shift of the electromagnetic wave of substantially zero degrees when the controllable magnetic field is off and a pre-determined phase shift when the controllable magnetic field is on.
In one embodiment, N equals 4.
The foregoing and other objects, aspects, features, and advantages of the invention will become more apparent from the following description and from the claims.
The objects and features of the invention can be better understood with reference to the drawings described below, and the claims. The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the drawings, like numerals are used to indicate like parts throughout the various views.
The inventive phase shifter, an all solid state switching device, can operate from 300 K to 3K. The phase shifter is capable of switching the polarization state of linearly polarized microwave or millimeter-wave radiation in a waveguide at high switch rates. It can be used to switch the phase of the electric field inside a waveguide by 180° by inverting the polarization state of the field. The device can produce a constant phase shift for frequencies over a very wide fractional band (30%) within which its performance specifications remain substantially uniform.
The exemplary phase shifter of
A central structure 120 (interchangeably referred to herein as a “toothpick”) includes cylinder 121 and two ceramic tapered cones 122 mechanically affixed to cylinder 121 for impedance matching to cylinder 121. Cylinder 121 can comprise any dielectric material exhibiting a suitable Faraday rotation at wavelengths of interest, such as mm-microwave wavelengths. Cylinder 121 typically has a permeability (magnetic permeability) greater than the permeability of a vacuum. Ceramic tapered cones 122 can comprise an alumina ceramic. While the exemplary polarization switches discussed herein were built and tested using ferrite cylinders, other ceramic and non-ceramic dielectrics are thought to be suitable for use in such devices as well. For example, the Faraday Effect has been shown to exist in n type doped germanium. (G. Srivastava and P. Kothari, “Microwave Faraday effect in n type germanium”, J. Phys. D: Appl. Phys., Vol. 5, 1972, GB). It is contemplated that cylinder 121 can be made from various types of semiconductor materials, including a number of doped garnet semiconductors as manufactured by the Trans-Tech, Inc. of Adamstown, Md.
Note that a central structure can include shapes other than the cylinder and two tapered cones of the embodiment described above. For example, a similar type of central structure can include a structure having a cross section including a polygon having N sides having a permeability greater than that of vacuum. Such a structure can have a cross section including a polygon having N sides (where N is an integer) having two end faces situated in a perpendicular orientation to a longitudinal axis of the structure having a cross section including a polygon having N sides, and two dielectric pyramidal structures having N sides. Each of the dielectric pyramidal structures can have N sides having a base mechanically coupled to an end face of the structure having a cross section including a polygon having N sides and having an axis situated substantially collinearly with the longitudinal axis of the structure having a cross section including a polygon having N sides. Such a central structure can be supported substantially in the center of the interior opening of a predefined cross-sectional shape of the waveguide. The structure can have a cross section including a polygon having N sides substantially situated within the dielectric break of the waveguide. In some embodiments, N equals 4 or an integer multiple of 4.
Toothpick 120 can be held substantially center aligned along the longitudinal axis of the openings of rectangular waveguide sections 101a and 101b and supported by one or more insulating members, such as insulating members 130.
A yoke 160 and pole pieces 153 can create a magnetic shield. In embodiments including solenoid 150, it is advisable to shield solenoid 150. One reason is to prevent the solenoid 150 AC magnetic field from inducing electrical currents into nearby conductors and to avoid eddy currents in nearby conductive structures. Also, as discussed later, polarization switches can be used independently in array applications involving a plurality of switches 100 where it can be important to prevent switches 100 from interacting with each other. Also, certain types of detectors and amplifiers, such as Transition Edge Superconducting Bolometers and SQUID amplifiers which can be used with polarization switches in some applications can be magnetically sensitive and need to be shielded. Such devices need to be shielded from the earth's field, let alone from solenoids 150 in nearby polarization switches 100, which can be a thousand or more times larger than the Earth's magnetic field. Moreover, coupling between devices can be exacerbated by the AC field from the polarization switch solenoid 150 (as compared to the Earth's DC magnetic field). Because the electromagnetic waves to be phase-shifted need to propagate into and out of phase shifter 100, the shielding cannot fully enclose polarization switch 100, i.e., there cannot be a complete Faraday cage. However, the combination of yoke 160 (typically a cylindrical shell) and pole pieces 153 (essentially washers which come as close to the waveguide as possible) can provide substantial shielding while still allowing entry and exit of signals of interest through the input and output signal ports (
In operation, an incoming electromagnetic wave is propagated into a first section of rectangular waveguide section, such as rectangular waveguide section 101a via port 102a, and coupled into dielectric cylinder 121 via a first ceramic cone 122. The polarization of the incoming electromagnetic wave can be switched by a polarization switching angle as it passes through dielectric cylinder 121 by Faraday polarization rotation according to a magnetic field as caused by the one or more windings 151 of solenoid 150. The electromagnetic wave then continues to propagate out of cylinder 121 via a second ceramic cone 122 and a second section of rectangular waveguide section 101b. The output polarized electromagnetic wave can have a final polarization ranging from no polarization change relative to the incoming signal polarization (for example at zero solenoid 150 current) or to a polarization rotation as the result of polarization switching (non-zero solenoid 150 current). The output signal can be coupled via port 102b out of polarization switch 100 though air typically into another waveguide. Note that for linear polarization, integer multiples of π rotation are equivalent to no rotation.
In some embodiments of a FRS, a toothpick 120 can include “sandwiched” sections of a microwave absorbing material such as a metal or metallization layer within cones 122. For example,
Any suitable magnetic field can be used to cause a polarization switching angle. The controlling field does not-need to be provided by a solenoid such as solenoid 150 fixed in a cutout along the outside surface of rectangular waveguide sections 101a and 101b, as shown in
Phase shifting devices and techniques of the prior art typically insert a constant path length to obtain a fixed amount of phase shift when a switch in the “on” state. Such correspondence between extra path length and degree of phase shift is inherently bandwidth dependent due to the one-to-one relationship between extra path length and degree of phase shift. Thus, a given path length produces a 180° phase shift for only one frequency. Therefore, manufacturers of such devices typically produce a specific path length required for the center of a specified frequency band. Unfortunately, such prior art devices also exhibit an undesirable maximal phase deviation from 180° at the band edges. By contrast, the inventive FRS device, for a 30% fractional bandwidth, can exhibit a relatively small deviation of typically 3° at each of the band edges. Because of its frequency-independent phase shift mechanism (Faraday Effect), a FRS phase shifter can also produce a uniform phase shift with deviations less than 0.1° across the band.
Note that Faraday rotation has been previously used in static microwave devices. However, in such static applications, only one phase or polarization state is used. Microwave isolators, for example, use only a DC magnetic field, typically supplied by a permanent magnet. Inherently non-switched DC field devices, such as isolators, are not suitable for switched phase applications.
The benefits of phase sensitive detection can be further utilized where the phase of the electric field is AC modulated. In such AC switching (between 0° and 180°), the electric field changes sign at the switch rate that can be useful for synthesizing beams. Also, because constant sources of noise are subtracted out, AC switching can be used to encode a signal with a very high common mode rejection ratio.
A prototype phase shifter has been constructed that can rotate the electric field vectors by ±80° (160° total) at 4 K. Even with 160° rotation instead of 180°, the electric filed has a large negative component and the device functions as a ± phase shift, albeit with slightly increase loss compared with the 180° that can ultimately be produced. Failure of the current device to achieve 180° was attributed to the inadequacy of the test stand cryogenic set-up, not as a fundamental limitation of the device. The initial performance measurements successfully demonstrated the FRS principle, a switch ratio (on transmission divided by off transmission is equal to 10 over a 20 GHz band pass), with very low reflection (1%) for the 100 GHz prototype at 4 K. With an upgraded cryostat, it is expected that this ratio will be close to 100.
The FRS phase shifter as described herein can be used as a component in an interferometer. An interferometer includes a group of two or more antennae in which the relative phases of the respective signals feeding the antennae are varied in such a way as to produce a radiation pattern that is strongly reinforced in a desired direction and suppressed in undesired directions. Such radiation patterns allow a beam to be synthesized and scanned rapidly to/from any desired direction. Also, since the inventive phase shifter operates in waveguide modes instead of relying suspended stripline technology, it can typically be inserted into a receiver before any other components, thus greatly reducing the amount of excess noise and/or loss typically associated with prior art techniques.
An interferometer also typically includes a processor configured to receive an output signal from each of a plurality of detectors. Each detector generally has one or more detector electrical output terminals. The output signals can be combined and processed to strongly enhance an incident electromagnetic wave from a particular direction. While much of the front end processing today is typically accomplished by analog processing, as digital electronic components become faster, it is contemplated that such processing could include analog and/or digital processing.
Since inventive phase shifter includes an all solid state design with no moving parts, no maintenance is required. Therefore, the inventive phase shifter offers a significant advantage for use in inaccessible environments such as in satellites, polarimetric remote sensing, and focal planes of large diameter communications transceivers, e.g., NASA's Deep Space Network. With slight modifications, the power handling capability of the device, for radar systems, can be several watts and the power required to achieve polarization switching is extremely low (a few milli Watts) when cooled below 10 K and a few Watts at room temperature. The inventive FRS phase shifter technology is therefore also naturally suited to extreme environments (low temperature and high vacuum), and possesses ideal characteristics for remote sensing components.
Beyond receiver applications of the phase shifter of the present invention, additional remote sensing applications include phased-array planar antennae (which operate similar to interferometers, but “in reverse”, i.e., they broadcast rather than receive). This allows satellites and ground stations to achieve high directivity with small antennae.
