Abstract:
A multi-channel, dual-band, radio frequency (RF) transmit/receive (T/R) module, for an active electronically scanned array, is provided. The module includes a compact, RF manifold connector and at least four T/R channels. Each of the T/R channels includes a notch radiator, a diplexer coupled to the notch radiator, a power amplifier, including at least one dual-band gain stage, coupled to the notch radiator, a low noise amplifier, including at least one lower-band gain stage and at least one upper-band gain stage, coupled to the diplexer, and a T/R cell, including a phase shifter, a signal attenuator and at least one dual-band gain stage, coupled to the power amplifier, the low noise amplifier and the manifold connector.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to radar systems. More particularly, the present invention relates to transmit/receive modules in compact packages. 
     2. Description of the Background Art 
     A variety of technical problems face one looking to equip an airplane with Ku and Ka band radars (for simplicity, K band radars are referred to with lower case letters, not the official subscripts). Modern radars systems are often implemented as active electronically scanned arrays with hundreds of transmit/receive modules aligned in an array. One advantage of an active electronically scanned array is that it can perform radar scans without physically turning the radar array. This is accomplished by altering the phase of the transmitted radars. By synchronizing the phases of each of the transmit/receive modules, the beam transmitted points in a different direction. However, in order to change the direction of the radar beam (i.e., the main lobe) the transmitted radars must be packed close enough together to work in unison. 
     Ka band radar, short for “K above,” is transmitted at approximately 18-40 GHz. Because such high frequencies are being used, the transmit/receive modules must be packed very tightly. In an active electronically scanned array, the lattice spacing must be approximately half of the wavelength of the highest frequency used. Ka band radar requires five elements per inch. Systems operating in the X band, e.g. 10 GHz, had ten times as much area in which to place transmit/receive modules. The demanding space requirements were too small for the current size of transmit/receive modules. 
     In addition to the size of the modules, a designer must also contend with the size of the connections to and from modules. Prior art designs require bulky connectors connecting a module to a radiating element. Prior art designs also require a connector from the module to a manifold interconnect. The inventors discovered that current connectors did not meet the height requirements of a Ka band radar grid. 
     These issues are compounded where a plane needed both Ku and Ka band radars. The module must be small enough to be able to create an effective array of Ka band radar, but still make room for both Ka band radar technology and Ku band radar technology. Because these two bands are at different frequencies, they must be transmitted and received separately. At the same time, the circuitry for both must be compact enough that it can fit into the Ka space requirements. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention provide a multi-channel, dual-band, radio frequency (RF) transmit/receive (T/R) module for an active electronically scanned array. The module includes a compact, RF manifold connector and at least four T/R channels. Each of the T/R channels includes a notch radiator, a diplexer coupled to the notch radiator, a power amplifier, including at least one dual-band gain stage, coupled to the notch radiator, a low noise amplifier, including at least one lower-band gain stage and at least one upper-band gain stage, coupled to the diplexer, and a T/R cell, including a phase shifter, a signal attenuator and at least one dual-band gain stage, coupled to the power amplifier, the low noise amplifier and the manifold connector. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The same part of an invention appearing in more than one view of the drawing is always designated by the same reference character. Lowercase letters designate different instances of a given part. 
         FIG. 1  is a high three quarters view of a transmit/receive module according to an embodiment of the present invention. 
         FIG. 2  is a schematic block diagram of a single transmit/receive channel according to an embodiment of the present invention. 
         FIG. 3  is a schematic module level block diagram of a four channel transmit/receive module according to an embodiment of the present invention. 
         FIG. 4  is an overhead cutaway view of a four channel transmit/receive module according to an embodiment of the present invention. 
         FIG. 5A  is a phantom view of a Ka/Ku band diplexer according to an embodiment of the present invention. 
         FIG. 5B  is a reproduction of simulated diplexer results according to an embodiment of the present invention. 
         FIG. 6  is an isometric view of a compact connector according to an embodiment of the present invention. 
         FIG. 7  is an isometric view of a DC routing technique according to an embodiment of the present invention. 
         FIG. 8  is a close up view of a notched radiator according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  depicts one embodiment of a transmit/receive module  100  with four integrated radiators  105 . The package for the transmit/receive module  100  illustrated was designed to match the dimensions of the integrated radiators  105 . The integrated radiators  105  depicted are notch radiators. The dimensions of the integrated radiators  105  are in turn governed by the spacing requirements of the Ka band radar grid, because the Ka band is the highest frequency received, and thus integrated radiators  105 , a type of receiver, must be closer to each other to receive the shorter wavelength signals. Each of the four integrated radiators  105  is on a separate channel. The transmit/receive module  100  is depicted without a bulky connector because the radiating elements, i.e. integrated radiators  105 , and the MMICs are all built into the package. In one embodiment, transmit/receive modules  100  similar to the one in  FIG. 1  are mounted into an oval shaped array. The oval shape allows the array to be mounted into the nose of an airplane. For certain missions, it may be desirable to mount the radar array on the underside of an airplane, in which case a rectangular array may be implemented. 
       FIG. 2  illustrates a block diagram of one embodiment of a single channel transmit and receive channel  200 . The receive path begins where the T/R switch  205  connects to the integrated radiator  105  with the radiator connection  280 . The T/R switch  205  is a high power switch which connects the integrated radiator  105  to either the transmit path or to the receive path. Even though the drawings depict a unidirectional arrow, signals may flow in either direction, as is required to transmit and receive. The receive path continues through the diplexer  210  to the LNAs  215 . The diplexer  210  separates Ku and Ka band signals, and is described in more depth at  FIG. 5A . An LNA  215  is used to amplify signals received by the integrated radiators  105 , a type of antenna, because these signals are often too weak to be directly fed into other circuit components. An LNA  215  is a type of amplifier that is optimized to produce as little noise as possible while still meeting amplification requirements for the signal. The LNAs  215  illustrated have two paths of gain stages, one for Ka band signals  220  and one for Ku band signals  225 . As shown, both K band receive paths have multiple gain stages within the LNAs  215 . The Ka band path has an extra gain stage  220 - 3  because Ka is at a higher frequency than Ku, and thus the extra gain provided by a third gain stage  220 - 3  is justified. 
     The LNA  215  output flows across the LNA switch  230  to the T/R cell  235 . The T/R cell  235  provides a series of gain stages  240 . After the first gain stage  240 - 1 , the signal is phase shifted by a variable shifter  245 . After the second gain stage  240 - 2 , the signal is attenuated by a variable resistance  250 , sometimes implemented as a digital attenuator. The T/R cell  235  implements 5 bits of phase shift  245  and 6 bits of attenuation  250 . This allows the T/R cell  235  to transmit or receive one of the four channels  200 . The attenuation allows the beam steering circuitry to control the size of the transmitted signals from each transmit/receive channel  20  relative to each other channel  200 . If an array is malfunctioning such that the right side lobe is too pronounced, the variable resistance  250  can be used to ensure that a smaller side lobe is produced. In other situations, fine grained attenuation may be employed to make small adjustments to the shape of a signal transmitted. The T/R cell  235  has three switches, the manifold interconnect switch  255 , the transmit path switch  260  and the receive path switch  265 . These three switches control the flow of signals through the T/R cell&#39;s three gain stages. The output of the third gain stage  240 - 3  travels across the manifold interconnect switch  255  and the transmit path switch  260  to the manifold interconnect  240 . 
     The transmit path begins at the T/R cell  235 . The T/R cell  235  performs the same function on transmitted signals as it does on received signals. When the manifold interconnect switch  255  is set to the transmit path, the signal will flow across from the manifold connection  285  to the receive path switch  265  to the three gain stages  240 . After being shifted and attenuated, the signal exits the T/R cell  235  via the transmit path switch  260  and continues to the power amplifier  270 . Conversely, the receive path flows as described above. The signal travels from the LNAs  215  to the receive path switch  265 , across the three gain stages  240 , to the transmit path switch  260  and then to the manifold interconnect switch  255 . 
     The T/R cell  235  outputs to the power amplifier  270 . The power amplifier  270  has three gain stages  275  to ensure that the transmitted signal has the requisite signal strength. The power amplifier  270  outputs to the T/R switch  205 , where it is routed to the radiator  105 . In a preferred embodiment, the power amplifier  270 , like the T/R switch  205 , is designed to work at both the Ka and Ku bands. When the T/R switch  205  is integrated with the power amplifier  270 , it may be referred to as a power amplifier switch  205 . 
       FIG. 3  depicts a module level block diagram of one embodiment of the T/R module  100 . There are four radiators  105 , each corresponding to a channel  1 - 4   205 . The receive path begins at a given radiator  105  and continues to a power amplification MMIC  305 . The power amplification MMIC  305  has an integrated T/R switch  205  and power amplifier  270 . In a preferred embodiment, all of the power amplifier MMICs  305  in a transmit/receive module share a single gate regulator ASIC  405  (depicted in  FIG. 4 ). The power amplification MMIC  305  routes the receive path to the diplexer  210 . The diplexer  210  feeds the Ka band components to the Ka band gain stages  240  in the LNAs  215  and feeds the Ku band components to the Ku band gain stages  245  in the LNAs  215 . The Ka band gain stages  240 , the Ku band gain stages  245  and the LNA switch  230  are all housed in a LNA MMIC  310 . The LNA MMIC  310  connects to a T/R cell  235 . 
     The path used to transmit a signal has a number of components in common with the receive path. A signal to be transmitted is provided by the manifold interconnect  315 , and is routed to the T/R cell  235 . The T/R cell  235  directs the signal to the T/R switch  205 , which routes the signal to the radiator  105 . The transmit path does not use the LNAs  215  or the diplexer  210 . By avoiding these band specific devices, the transmit path is identical for both the Ka and Ku bands. Therefore, it may be possible to transmit in both bands at one time. 
     The receive and transmit paths converge at the T/R cell  235 , preferably embodied as a SAD MMIC. The T/R cell  235  interfaces with the manifold interconnect  315  and receives control signals for its channel  200 . The control signals allow the T/R cell  235  to either route signals from the manifold  315  to the transmit path or from the receive path to the manifold interconnect  315 . Like the power amplifier  270 , the T/R cell  235  is a dual band device. 
     All of the MMICs in a transmit/receive module  100 , such as the SAD MMIC  235 , the LNA MMIC  310  and the power amplifier MMIC  305 , share a drain regulator ASIC  410  (depicted in  FIG. 4 ). 
     The control signals are provided to the T/R cell  235  by the control module  320  for each channel  200 . The control module  320  may be implemented as an ASIC. The control module  320  receives six bidirectional DC signals which are used to generate control signals for the T/R cell  235 , the LNA switches  230  and the T/R switch  205 . An ASIC control module is a type of control chip. 
     The control signals allow the T/R cell  235  to interface with beam steering circuitry (not shown). Beam steering refers to changing the main lobe of radar signal. This allows a stationary radar array to point in different directions, often in a sweeping pattern. In certain instances, beam steering circuitry may be employed to enlarge or reduce side lobes of a transmitted signal. Beam steering and lobe adjustment may be accomplished by altering three variables: which transmit/receive modules  100  are addressed; the phase of signals transmitted; and the attenuation of the signals transmitted. Digital signal processors (not shown) are often employed to calculate the particular control signals needed to direct various lobes. A beam steering controller (not shown) includes a memory module, a controller CPU module, an interface timing module, a beam computation module and array interface module. 
     In a preferred embodiment, a manifold interconnect  315  is connected to the T/R cells  235  with an RF network which delivers signals from the manifold interconnect  315  to the T/R cells  235  and transports received signals back to the manifold interconnect  315 . The RF network, part of the manifold interconnect  315 , is an example of a manifold connection. 
       FIG. 3  illustrates a layout of one embodiment of a transmit/receive module  100 . This embodiment is referred to as a “quadpack,” because it provides four channels in a single package. Other embodiments may have eight channels, or another multiple of four channels. Exemplary MMICs have been manufactured by Triquint Semiconductor using pHEMT technology on a state of the art processes. pHEMT stands for pseudomorphic High Electron Mobility Transistor. An HEMT is a transistor where, instead of an n-doped region, there is a junction between two materials with different band gaps. This junction creates a thin layer where the Fermi energy is greater than the energy of the conduction band. This provides for high electron mobility. Pseudomorphism refers, in this case, to stretching a thin layer of a first material over the second. By covering one of the two materials, the junction interfaces with two identical lattice constants. The covered material, however, is not required to have an identical lattice structure, and this allows for a bigger band gap than two materials that have identical lattice constants. The larger band gap provides for improved performance. 
     MMICs are generally manufactured from Gallium Arsenide, Indium Phosphate or Silicon Germanium, so that the devices can operate at the required frequencies. One element of a compact design may be manufacturing a three metal interconnect MMIC from Gallium Arsenide. 
     The placement of the power amplifier  270  is important for transmission, and the switch  205  is integrated with the power amplifier  270  to save space. In a preferred embodiment, a high power T/R switch  205  is used instead of a circulator because traditional circulators may be too large to fit inside of the power amplifier cavity. Power amplifiers  270  have lower linear response requirements than the LNAs  215 . The T/R switch  205  is placed on the front end of the power amplifier MMIC  305  closest to the integrated radiators  105 , and is built into a power amplifier  270  and located in the power amplifier cavity. Each power amplifier  270  and T/R switch  205  is placed directly behind its respective integrated radiator  105 , so that the power amplifier  215  is as close to the integrated radiator  105  as possible. One advantage of placing the power amplifier  270  directly before the integrated radiator  105  is that any potential interference or attenuation is minimized. This helps to ensure that the transmitted signal is not changed before being transmitted. 
     The diplexers  210  are placed in a cavity between the power amplifier cavities and the LNA cavities. Unlike some of the other devices, the diplexers  210  are not placed in line with their respective transmit and receive channels  200 . The MMICs are each separate integrated circuits, whereas the diplexers  210  are, in large part, stripline RF traces embedded in ceramic, a type of ceramic insulation. 
     The LNAs  215  are placed directly after the diplexers  210  to be as close together as possible. LNAs  215  are most effective if used close to the integrated radiators  105  because the less there is between the integrated radatior  105  and the LNAs  215 , the less possibility there is for noise to be introduced. Noise that is introduced before the LNA  215  may be indistinguishable from the signal, particularly if it is at the same frequency. That is, if the noise is within the band that the LNA  215  is designed to amplify, then the noise will be amplified as though it were the signal. Conversely, if this same noise is added to the signal after the LNA  215 , it will be attenuated relative to the signal and thus have a reduced effect on system input. By placing the LNA  215  physically close to the diplexer  210 , feedline losses are reduced. 
     After the LNAs  215 , there are four pairs of T/R cells  235  and control module  320  ASICs, and each pair is placed in a corner. This placement allows space for the gate regulator ASICs  405  and drain regulator ASICs  410  and for the manifold interconnect  315  to be symmetrically routed to each T/R cell  235 . 
     Because the Ka grid may force tight spacing requirements, a number of techniques may be employed to route signals within one embodiment of the module. In order to obtain the benefits of a four-channel architecture, one embodiment of the transmit/receive module  100  utilizes minimum spacing tolerances between all RF and DC lines in most areas of the package layout. The use of thin dielectric tape layers allows for stripline  530 , discussed in more depth in  FIG. 5A , with minimum ground spacing. For example, LTCC tape is sold in thicknesses of 10 mils, but may be cut to 5 mils or less. Smaller ground spacing leads to smaller conductor widths for 50 ohm traces. The thin layers of stripline  530  also allow for multiple layers of high current carrying voltage to be successfully routed in the tight height restrictions. 
     Double rows of grounding vias  535  may be used on both sides of the stripline  545  to keep Ka signals from leaking through to other transmit and receive channels  200 . This dense placement of grounding vias  535  improves the problem of Ka leakage. New techniques in LTCC fabrication such as placing fewer transmit/receive modules  100  on each LTCC panel to reduce shrinkage of the LTCC have been developed to counter the effects of increased via  510  count. 
     Received signals enter the module  100  through one of four integrated notch radiators  105 . A transmit/receive module  100  may have one integrated radiator  105  for each of the four channels  200 , where the term integrated radiator  105  commonly refers to a radiator  105  without a bulky connector attaching the transmit/receive module  100  to a separate radiator or antenna. The desire for both Ka and Ku band radar may prompt some designers to implement integrated radiators  105  that are wideband. 
     In one embodiment, an integrated radiator  105 , such as a wideband notch radiator, couples a stripline  530 , often 50 ohms, with the air, usually 376 ohms, such that a signal may be fed into the stripline  530  and may pass through to the radiating medium with minimal interference. The notch is an aperture cut to form an integrated radiator  105  with a load that matches the ambient radiating medium. The aperture is cut from a dielectric substrate, which also houses the stripline  530 . The substrate sandwiches the stripline  530  and provides insulation. The stripline  530  is connected to the notch with a feed end, and both connecting ends are generally a quarter wavelength long, or a multiple thereof. The integrated radiator  105  is designed for wideband operation using low-temperature co-fired ceramic (LTCC), such as Dupont 943 LTCC. In one embodiment, the stripline feed  505  connects to the power amplifier cavity. 
     In one embodiment, a manifold interconnect  315  may be comprised of a plurality of contiguous RF stripline microwave conductor board members, an example of stripline  530 , which are mutually insulated from one another and include RF coupler sections which abut a pair of relatively shorter tubular coupler members, and which are also adapted to couple transmit RF and receive RF to and from a transmit/receive module  100 . The single connection may provide four channels  200  which are received by a ceramic locus splitter. 
       FIG. 5A  depicts a layout view of a diplexer  210  according to a preferred embodiment. In this embodiment, each diplexer  210  is approximately 0.28×0.16×0.03 (L×W×H, in inches). A diplexer  210  is a single element which can receive input signals at multiple discrete frequency ranges. The diplexer  210  is connected to three ports. Port  1   505  provides a Ku and Ka band signal from the integrated radiator  105 . This signal is divided into Ku band frequencies, which are delivered to Port  2   510 , and Ka band frequencies, which are delivered to Port  3   515 . The Ka band signals are filtered with a rectangular waveguide  520 . This rectangular waveguide  520  provides a cutoff frequency of 28 GHz, and is preferably dielectric filled. The Ku band signals are filtered by a low pass filter  525 . The stripline  530  forms passive elements to create a low pass filter  525  with a cutoff frequency of 20 GHz. Stripline  530  is also known as RF trace. 
     Grounding walls  535  are placed on both sides of the Ku band signal path to provide isolation from other signals. The Ku band signal path is more sensitive to unwanted signals than the Ka band path because the Ku path contains passive components, such as a low pass filter  525 . The rectangular waveguide  520  of the Ka path is shielded. The grounding walls  535  are between two ground planes, one above the diplexer and one below. The grounding walls  535  are formed with a series of grounding vias  540  between the ground planes. Unwanted signals from outside the diplexer  210  encounter the ground planes or the grounding vias  540  and are absorbed into the ground plane rather than interfering with the signals passing through the diplexer  210 . 
     In an embodiment where the diplexer  210  performs a transmit function in addition to receiving, added isolation between Port  2   510  and Port  3   515  may be needed, because these transmitted signals represent noise to the other band. This is less of a concern when receiving because the received signals are amplified after the diplexer  210 , whereas the transmitted signals are amplified and then sent to the diplexer  210 . 
     Ceramics may be used to insulate against unwanted signals as well as grounding techniques. Interface issues between Ku energy operating in a Ka band environment can be solved, in part, by embedding the diplexer  210  in ceramics. 
       FIG. 5B  depicts simulated results of the diplexer shown in  FIG. 5A . The simulation was performed using HFSS™ from Ansoft, a 3D electromagnetic field simulation tool, and depicts S-parameter simulation results. S 11   545  is the signal measured at Port  1   505  based on an input at Port  1   505 . This represents a frequency sweep received by the integrated radiator  105  and transmitted to the diplexer  210 . S 21   550  is the signal measured at Port  2   510  based on the input frequency sweep. It illustrates that frequencies up to 20 GHz are filtered with less than approximately 10 dB of attenuation. As frequencies rise past 20 GHz, the low pass filter  525  provides ever greater attenuation. Many Ka band frequencies will be attenuated by more than 60 dB. S 31   555  is the signal measured at Port  3   515 , the Ka band portion of the signal received by the integrated radiator  105 . As frequencies approach 28 GHz, the attenuation of the Ka band rectangular waveguide  520  drops off. 
       FIG. 6  illustrates a one embodiment of the invention comprising a compact connector  605  and an outer ring  610 . This connector  605  provides a single output from the transmit/receive module  100  to a radar system. The connection between the two should not be higher than the height of the transmit/receive module  100 . In one embodiment, a blindmate microwave connector, such as those supplied by the Gore corporation, may be modified to provide a compact connector. For example, a Gore 60 g connector (part of Gore&#39;s 100 Series of connectors) is 0.095″ across but the ceramic height is less than 0.078″. The 60g connector may be modified to reduce the height of its mating surface and increase the width of the mating surface, to ensure that a minimum of 0.004 square inches of solder area is provided. This modification may be performed by cutting, filing or shaving the connector. The modified Gore brand connector  605  can then be attached to the mating area with enough solder to physically support the connection. References to the Gore brand are for clarity, connectors from other suppliers may be substituted. 
       FIG. 7  depicts routing of signals through “swiss-cheese” openings  705  on a printed wiring board, according to one embodiment. In certain radar applications all DC traces  710  are routed out through a single large opening. In one embodiment of the present invention, DC is routed out through a series of smaller, swiss-cheese openings  705  to reduce cavity resonance. Cavity resonance is, in part, a function of the length of the cavity dimensions. By employing a series of smaller cavities, such as swiss-cheese openings  705 , the peak resonance is reduced, because it is effectively spread between a variety of different cavities. The resonant frequencies produced are, in part, a function of the shape of the cavity. In a preferred embodiment, the swiss-cheese openings  705  may be circular. 
     The DC traces  710  are paired, for balance, and then routed out through the floor of the cavity and then through the wall of the cavity one pair at a time. As depicted, the DC traces  710  are routed through the cavity with vias  715 . The vias  715  are about 5 mils in diameter. On the far side of the cavity, grounding vias  540  are placed that connect to the ground plane, as shown. The use of a pair of grounding vias  540  helps insulate cavity resonance. 
     The DC traces  710  originate from a data bus. Each of the channels  200  has a separate data bus. Each data bus has a control module  320 , such as a module channel controller, often implemented as an ASIC. In one embodiment, each control chip is separately addressable across the manifold interconnect  315 . 
       FIG. 8  depicts a close up view of an notched radiator  105 . The transmitted signal travels between the walls  905 . 
     While this invention has been described in conjunction with specific embodiments thereof, many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the preferred embodiments of the invention as set forth herein, are intended to be illustrative, not limiting. Various changes may be made without departing from the true spirit and full scope of the invention as set forth herein.