Patent Application
20100123532
1. Field of the Invention
The invention relates generally to microwave systems and, more particularly, to high-speed front-end shutter components.
2. Description of the Related Art
Microwave systems have become increasingly important to electronic systems in many different fields, including defense applications. Modern military platforms are highly dependent on microwave systems for their on-board communications, radar and electronic warfare systems. The ability to protect these systems from high energy threats, such as high power microwave (HPM) weapons, directed energy weapons, or electromagnetic pulses (EMPs) that arise from nuclear blasts, is paramount to the effectiveness of the military.
Microwave receiver front-ends typically include a high-sensitivity low-noise amplifier (LNA) which is particularly vulnerable to high energy exposure. Receiver front-ends are, by functional necessity, well-coupled to electromagnetic energy from the environment via an antenna. As a result, the receiver front-end components (i.e. the entire RF to IF chain) are vulnerable to semiconductor junction breakdown, arcing, thermal damage and electromigration-induced damage that may accompany a high energy electromagnetic attack. Therefore, receiver front-end systems require power limiters to isolate the vulnerable components during a high power electromagnetic attack.
The current state of the art falls roughly into two categories; solid state diode limiters or plasma discharge limiters. Solid state emitter devices provide fast response (˜1 ps); however they can only handle a maximum peak power of approximately 100 kW and typically handle only 10 W to 100 W over the duration of a 1 ms HPM attack. Plasma discharge tubes provide protection against significantly larger power levels but suffer from slower switching times. Present state of the art power limiters for microwave receiver front-ends do not sufficiently protect against the extraordinarily high electric fields generated by EMPs, HPM, or directed energy weapons. Hence, there is a need for a capable power limiter solution.
One embodiment of an electronic shutter device according to the present invention comprises the following elements. An input terminal is connected to receive an input signal. A thermally-activated electrical transition element is connected to accept said input signal and transmit an output signal. The transition element operates in an insulating state and transmits a substantial portion of the input signal when an operating temperature is below a critical temperature. The transition element functions in a reflective state and blocks a substantial portion of the input signal when the operating temperature of the transition element is at or above the critical temperature. An output terminal is connected to pass an output signal from the transition element.
One embodiment of a transmission line system according to the present invention comprises the following elements. A transmission line having an input terminal is connected to receive an input signal, and an output terminal is connected to pass an output signal. A thermally-activated shutter is disposed between the input and output terminals. The shutter operates in an insulating state and transmits a substantial portion of the input signal when an operating temperature is below a critical temperature. The shutter operates in a reflective state and reflects a substantial portion of said input signal when the operating temperature of the shutter is at or above the critical temperature.
One embodiment of a receiver system according to the present invention comprises the following elements. An antenna is disposed to receive an input signal. A receiver circuit processes the input signal and produces an output signal. The antenna is adapted to connect to the receiver circuit through a transmission line. A thermally-activated shutter is disposed in the transmission line between the antenna and the receiver circuit. The shutter operates in an insulating state and transmits a substantial portion of the input signal when an operating temperature is below a critical temperature. The shutter operates in a reflective state and reflects a substantial portion of the input signal when the operating temperature of the shutter is at or above the critical temperature. An output device is connected to manage information related to the output signal.
a and
Embodiments of the present invention as disclosed in the claims provide an electronic shutter device designed to protect receiver front-ends and other sensitive circuits from HPM pulse events such as HPM weapons, directed energy weapons, or EMPs. The electronic shutter device incorporates thin-film vanadium oxide (VOX) materials that exhibit a change in resistivity of over four orders of magnitude as their temperature is varied over a narrow range near a known critical temperature. A high-energy pulse causes ohmic heating in the shutter device, resulting in a state change in the VOX material when the critical temperature is exceeded. During the state change the VOX material transitions from an insulating state (high resistance) to a reflective state (low resistance). In the insulating state, the shutter device transmits the majority of the signal. When the shutter device is operating in the reflective state, most of the signal is reflected and prevented from passing into the electronics on the output side of the shutter device.
Embodiments of the invention are described herein with reference to schematic illustrations of idealized embodiments of the invention. As such, variations from the shapes of the illustrations as a result, for example, of manufacturing and/or mounting techniques are expected. Embodiments of the invention should not be construed as limited to the particular shapes of the elements illustrated herein but are to include deviations in shapes that result, for example, from manufacturing. Thus, the elements illustrated in the figures are schematic in nature; their shapes are not intended to illustrate the precise shape of the element and are not intended to limit the scope of the invention. The elements are not drawn to scale relative to each other but, rather, are shown generally to convey spatial and functional relationships.
The annular embodiment of the transition element 210 is made of alternating concentric rings of a conductive material 212 and a transition material 214. The conductive material 212 can comprise any highly conductive material including metals such as gold, silver, platinum, or metal alloys. One group of materials that are known to have acceptable transition properties are oxides of vanadium (VOX), such as vanadium dioxide (VO2) and vanadium sesquioxide (V2O3). Thin films of VOX may be photolithographically patterned on a substrate such as single-crystal sapphire, for example.
In one embodiment, the annular transition element 210 comprises alternating rings of gold (Au) as the conductive material 212 and thin film VOX as the transition material 214. A thin film (˜500 nm) of VOX at temperatures below a critical temperature (TC=67° C. for VO2) exhibits insulating behavior. Electromagnetic energy incident on such a film suffers minimal attenuation. At temperatures above the critical temperature, the film behaves like a metal and the reflection coefficient approaches unity. Quality VO2 films deposited on sapphire exhibit DC resistivity changes in excess of a factor of 104 with values ranging from approximately 1 Ω·cm in the insulating state to 10−4 Ω·cm in the metallic state. One advantage provided by this material is found in using the lower conductivity of the cold insulating state to provide ohmic “self” heating of the film during an incident HPM pulse. With proper design, the ohmic heating can rapidly drive the film into its hot reflective state.
The temporal response of the shutter device 200 is described as follows. At the start of the HPM event, the normally insulating VOX transition element 210 is absorbing energy from the HPM via ohmic heating. Within approximately 10 ns, the VOX film undergoes an insulator to metal phase transition that activates the reflective state of the shutter 200, reflecting more than 99.9% of the incoming destructive pulse energy. The shutter 200 stays in this reflective state to provide isolation for the remaining duration (up to 1 ms) of the HPM attack. The provided isolation may exceed 60 dB. After the attack, the VOX film rapidly cools and transitions back to its normal insulating state, returning the shutter to its low-loss transmit mode. The thin film VOX can provide activation and recovery times of less than 10 ns and 100 μs, respectively.
a and 3b each show a graph modeling the electrical properties of a shutter device according to an embodiment of the present invention over a range of temperatures.
a shows the resistivity of the shutter device as a function of temperature. The horizontal axis represents a normalized inverse of temperature (1000/T, where T is in kelvin) such that temperature decreases in the positive direction (i.e., to the right of the origin). The vertical axis is the log of resistivity (log Ω·cm).
b is a graph of attenuation versus temperature of one embodiment of a shutter device according to the present invention. This graph models shutter having a VO2 thin film with a thickness of 580 nm operating at a frequency of 38.5 GHz. The attenuation (dB) remains steady until the critical temperature is reached at around 67° C. At this temperature, the shutter 200 transitions from the insulating state to the reflective state, indicated by a sharp increase in signal attenuation (i.e., attenuation becomes more negative). Thus, the shutter 200 passes a very small portion of the signal at the input terminal 202 when the shutter 200 is operating in the reflective state. In the reverse direction, as the system cools to a temperature slightly lower than the critical temperature the shutter 200 transitions back from the reflective state to the insulating state and the majority of the signal is passed to the output terminal 204. An acceptable insertion loss for the shutter 200 is less than 3 dB while preferably providing a reflective state isolation of approximately 60 dB or better.
a illustrates a cross section of a transition element 500 according to an embodiment of the present invention. The transition element 500 has an annular shape with alternating rings of transition material 502 and conductive material 504. A cross section of a conductor 506 running through the center of the transition element 500 is also shown. This particular embodiment comprises thin film VOX as the transition material 502 and a perfect electrical conductor (PEC) as the conductive material 504. The PEC material can comprise any highly conductive material including metals such as gold, silver, platinum, or metal alloys. The conductivity of the VOX is approximately 33 S/m in the cold insulating state and approximately 330,000 S/m in the hot reflective state.
b shows a wedge-shaped section 510 of the transition element 500 with some exemplary dimensions shown. In this particular embodiment, the annular transition element 500 has an outer radius of approximately 4.33 mm and an inner radius of approximately 1.87 mm. It is understood that other dimensions can readily be used to accommodate a particular shutter design.
In some embodiments, the receiver system 700 can comprise a trigger element 710. The trigger element 710 is used to manually trigger a state transition in the shutter device 704. Several different types of trigger elements can be used. For example, the trigger element 710 can comprise a laser. In such an embodiment, the laser may be turned on to quickly heat the shutter device 704 to the critical temperature to cause a state transition. The trigger element 710 can also comprise a circuit that sends a trigger signal to the shutter device 704 that causes the state transition. The trigger signal can be electrical, thermal, optical, or any other type of signal that can initiate a state change. Thus, the system 700 can operate in a passive mode where the state change is triggered only by the input signal, or the system 700 can operate in an active mode where the state change is initiated with a trigger signal. The active mode triggering scheme may be helpful if an HPM event is detected prior to reaching the antenna 702 or if such an event can be anticipated.
Many known subtractive processes may be used to modify the substrate, including etching, grinding, and ablation. Other processes may also be used. The substrate 806 may be modified after the materials 802, 804 are deposited or prior to the deposition process.
Although the present invention has been described in detail with reference to certain preferred configurations thereof, other versions are possible. For example, the shutter device may be adapted for use in many different types of transmission systems. Examples of embodiments that work for coaxial and waveguide transmission lines have been provided; nonetheless, it is understood that the technology may be incorporated into almost any transmission line. Therefore, the spirit and scope of the invention should not be limited to the versions described above.
