Abstract:
A microwave attenuator circuit is disclosed, including a combination of a plurality of quarter wave transformers and a plurality of resistive elements.

Description:
BACKGROUND  
       [0001]     Coaxial attenuators are too bulky and expensive to be implemented on many microwave systems. Distributed ferrite load material on transmission lines have difficulty in realizing repeatable and precise attenuation values because of inconsistencies in the manufacturing the bulk material. Couplers on airstripline are not practical to realize small and precise attenuation values because of difficulties in match due to the unequal even and odd modes association with that type of transmission line.  
         [0002]     Typical lumped element attenuator configurations utilize at minimum three resistors. Each resistor value should be held to very tight tolerances, e.g. on the order of 1% or better. Often active laser trimming is employed to achieve these precise resistor values. Laser trimming is typically preformed on printed resistor-on-ceramic substrates. This operation is prohibited for many large microwave printed circuit boards using non-ceramic material (Teflon® for example) because of the risk of damaging the board by the laser.  
       SUMMARY OF THE DISCLOSURE  
       [0003]     A broadband microwave attenuator circuit is disclosed, including a combination of a plurality of quarter wave transformers and a plurality of lumped element resistors.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0004]     Features and advantages of the disclosure will readily be appreciated by persons skilled in the art from the following detailed description when read in conjunction with the drawing wherein:  
         [0005]      FIG. 1A  is a schematic diagram of an exemplary embodiment of an attenuator device.  
         [0006]      FIG. 1B  is a schematic diagram of an alternate attenuator embodiment.  
         [0007]      FIG. 2  is a side view of an exemplary implementation of an attenuator device according to the schematic diagram of  FIG. 1B , with an upper metal housing removed to illustrate the circuit and resistor pattern formed on a surface of the dielectric substrate.  
         [0008]      FIGS. 3 and 4  are respective left and right cross-sectional side view illustrations of the attenuator circuit of  FIG. 2 .  
         [0009]      FIG. 5  illustrates in cross-section an exemplary embodiment of an attenuator fabricated in a channelized microstrip structure.  
         [0010]      FIG. 6  illustrates in cross-section an exemplary embodiment of an attenuator device fabricated in a channelized inverted microstrip structure.  
         [0011]      FIG. 7  illustrates in cross-section an exemplary embodiment of an attenuator device fabricated in a channelized double-sided air stripline structure.  
         [0012]      FIG. 8  illustrates in simplified schematic form another embodiment of an attenuator, wherein a back to back configuration allows a wider range of attenuation by a factor of two. 
     
    
     DETAILED DESCRIPTION  
       [0013]     In the following detailed description and in the several figures of the drawing, like elements are identified with like reference numerals.  
         [0014]     An exemplary embodiment of this invention is a broadband microwave attenuator using a combination of quarter wave transformers and lumped element resistors.  FIG. 1A  is a schematic diagram of an exemplary embodiment of an attenuator device. RF power P 1  enter into port  1 , and propagates to node a, where it is split between the two quarter wave transformers characterized by impedance Z 1  and Z 2 . A quarter wave transformer is a length of transmission line, of length equivalent to one-quarter wavelength at an operating frequency, functioning to transform a first impedance at a first end of the transformer into a second impedance at the second end of the transformer. The characteristic impedance of the transmission line of the transformer is equal to the square root of the product of the first impedance and the second impedance. Quarter wave transformers are described, for example, in “Foundation for Microwave Engineering,” R. E. Collin, McGraw-Hill, 1966, Chapter five.  
         [0015]     The impedance values of Z 1  and Z 2  determine the amount of power P 2  that travels along the Z 1  transformer, reaches node b and propagates through a quarter wave transformer of characteristic impedance Z 3  into port  2 . Quarter wave transformer Z 3  transforms the impedance at node b to the impedance at port  3 . In an exemplary embodiment, for power entering port  1 , the voltage at nodes b and c will be equal, so that no current flows through resistor R 1 . The proper selection of impedance values Z 1 , Z 2 , R 2  and Z 3 , e.g. using even-odd mode analysis, also realizes a good match at node A to the load impedance at port  2 . Even-odd mode analyses are known in the art, e.g., J. Read and G. J. Wheeler, “A Method of Analysis of Symmetrical Four Port Network”, IRE Trans. MTT, Vol. MTT-4, pages 246-252, October 1956; L. I. Parad and R. L. Moyniham, “Split-Tee Power Divider”, IEEE Trans. MTT, Vol. MTT-13, pages 91-95, January 1965.  
         [0016]     The power P 3  that travels along the Z 2  transformer reaches node c, and is dissipated in the resistor R 2 . The attenuation value of the attenuator circuit of  FIG. 1  is determined by the ratio P 2 /P 1 . Choosing the proper resistor value R 1  allows realization of the same attenuation value when power enter port  2  and exits port  1 .  
         [0017]     By proper selection of impedance values of R 1 , Z 1 , Z 2 , R 2  and Z 3 , a good match may also be realized at port  2  using even-odd mode analysis. The RF match using the configuration in  FIG. 1  may be good across a 20% frequency bandwidth at microwave frequencies in one exemplary embodiment, at an exemplary center frequency of 12.5 GHz. Both R 1  and R 2  are used as termination load resistors and do not impact the attenuation values as Z 1  and Z 2  do. It has been found that, for an exemplary embodiment, R 1  and R 2  may vary as much as 20% without impacting the attenuation. Exemplary values for Z 1 , Z 2 , Z 3 , R 1 , R 2  for a circuit embodiment providing 4.7 dB attenuation are Z 1 =102.8 ohms, Z 2 =2.6 ohms, Z 3 =59.4 ohms, R 1 =106 ohms, and R 2 =36 ohms. In an exemplary embodiment, the impedances presented at ports  1  and  2  may be 50 ohms.  
         [0018]      FIG. 1B  is a schematic diagram of an alternate attenuator embodiment. By adding an addition quarter wave transformer ZT between node A and port  1  and adjusting the other impedance and resistance values, the bandwidth may be broadened, e.g., to up to 40% at microwave frequencies in an exemplary embodiment. An exemplary attenuator as depicted in  FIG. 1B , and with ZT, Z 1 , Z 2 , Z 3 , R 1  and R 2  designed to be 41 ohms, 83 ohms, 43 ohms, 59 ohms, 100 ohms and 35 ohms respectively, has a nominal predicted 4.7 dB attenuation. Across a 4.5 Ghz bandwidth at an X/Ku band from 10.5 GHz to 14.5 GHz, the attenuation is predicted to vary by only 0.2 dB while the predicted match is better than 18 dB.  
         [0019]     An exemplary embodiment of an microwave attenuator  20  illustrated in  FIGS. 2-4  employs an etched strip transmission line pattern for each quarter wave transformer to determine an amount of attenuation through the device. Using the etched transmission line pattern can produce very precision impedance values which then result in very precise control of the attenuation values. In this exemplary embodiment, only two resistors R 1  and R 2  are used to achieve a good match across the operating band, X band, for the device. These resistors can be printed onto the circuit board using resistive ink, mounted as discrete chips using, for example, a conventional solder or conductive epoxy attach method, or using a resistor product such as Ohmegaply™ marketed by Ohmega Corporation.  
         [0020]      FIGS. 3 and 4  are left and right cross-sectional side view illustrations of the attenuator  20 , showing the lower and upper metal housing structures  32  and  34 . These structures may be fabricated of aluminum or other suitable metal. Alternatively, the structures may be fabricated of a plastic material coated with an outer layer of conductive material such as a metal. Each of the housing structures is generally U-shaped in cross-section, so that when the housing structures are joined together as shown in  FIGS. 3 and 4 , an air cavity  36  is defined. The housing structure  42  has a recess  42 A formed therein to receive a dielectric substrate  40 .  
         [0021]      FIG. 2  is a side view of the device  20  taken with the upper metal housing  34  removed to illustrate the circuit and resistor pattern  60  formed on surface  40 A of the dielectric substrate  40 . The substrate can be fabricated from various dielectric materials, e.g., CuClad 250™, ceramic, or 6010 Duroid™. The circuit pattern can be fabricated using photolithographic techniques, by way of example, wherein the surface  40 A is first formed with a conductive layer, e.g. copper, covering the surface. The copper layer can be patterned using photolithographic techniques, selectively removing the copper layer to define a circuit pattern. The circuit pattern includes parallel, separated groundplane regions  80 ,  82  which contact surfaces of the metal housing structure  34 . Matching groundplane regions may also be formed on the opposed surface of the substrate, opposite regions  80 ,  82 .  
         [0022]     The circuit pattern includes a conductor strip  62  having a width selected to provide a characteristic transmission line impedance ZT. At the substrate edge, the strip forms a first I/O port  70 . The circuit pattern also includes conductor strips  64  and  66 , each having an effective electrical length of one quarter wavelength at a frequency within the operating band, e.g. at the center frequency of the operating band. The width of strip  64  is selected to provide a characteristic transmission line impedance Z 1 . The width of strip  66  is selected to provide a characteristic transmission line impedance Z 2 . The strips  62 ,  64  and  66  thus provide respective quarter-wave transformer sections. In an exemplary embodiment, the conductor strip  66  has a tapered configuration at node B to reduce parasitic shunt capacitance and improve the match.  
         [0023]     Ends of the strips  62 ,  64  and  66  are connected at node A. A resistor R 1  is connected at the opposite end of the strip  64  at node B. Resistor R 1  is electrically connected at node B between the strip  64  and the strip  66 . A resistor R 2  is electrically connected between the end of strip  66  and the groundplane  80 . These resistors R 1 , R 2  may be printed onto the circuit board  40  or mounted as discrete chips using, for example, a conventional solder or conductive epoxy attach method.  
         [0024]     The circuit pattern  60  further includes a conductor strip  68  having a width selected to provide a characteristic transmission line impedance Z 3 . In an exemplary embodiment, the conductor strip  68  has a tapered configuration at node B to reduce parasitic capacitance and improve the match. Strip  68  has a first end electrically connected at node B to the adjacent end of strip  64 . A second end of strip  68  serves as the second I/O port  72  of the attenuator device. The resistors R 1  and R 2  and impedances ZT, Z 1 , Z 2  and Z 3  correspond to the similarly named resistors and impedances of the schematic diagram of  FIG. 1B . To implement the attenuator of  FIG. 1A , the conductor strip  62  may be eliminated.  
         [0025]     The exemplary embodiment of an attenuator shown in  FIGS. 2-4  is configured as a channelized single sided air stripline or suspended substrate stripline. The attenuator can be implemented in other transmission line structures. For example, the attenuator can be implemented in channelized microstrip, channelized inverted microstrip, channelized double sided air stripline or high “Q” air stripline, as illustrated in simplified form in  FIGS. 5-7 , respectively.  
         [0026]      FIG. 5  illustrates in cross-section an exemplary embodiment of an attenuator  150  fabricated in a channelized microstrip structure. The attenuator  150  includes a bottom metal housing structure  152  and an upper metal housing structure  154 . The bottom housing structure  152  includes a recessed region to receive the circuit board  40 , which includes a circuit and resistor pattern  60  and groundplane regions formed on upper substrate surface  40 A as in the embodiment of  FIGS. 2-4 . The top housing structure  154  has an open channel formed therein to define an air cavity  158 . The lower surface of the substrate is in contact with the lower housing structure  152 .  
         [0027]      FIG. 6  illustrates in cross-section an exemplary embodiment of an attenuator device  170  fabricated in a channelized inverted microstrip structure. The attenuator  170  includes a housing structure  172  having a generally U shaped channel formed therein to define an air cavity  176 . The circuit board  40  is inverted, so that the circuit and resistor pattern  60  is formed on surface  40 A facing inwardly into the air cavity. The groundplane regions  80 ,  82  contact surfaces of a recessed region  172 A of the housing structure  172 .  
         [0028]      FIG. 7  illustrates in cross-section an exemplary embodiment of an attenuator device  180  fabricated in a channelized double-sided air stripline structure. The attenuator includes lower conductive housing structure  182  and upper conductive housing structure  184 . The housing structures each form a general U-shaped configuration to define an air cavity  186  when the housing structures are assembled together as shown in  FIG. 7 . A dielectric circuit board  40  is captured between the housing structures, and has groundplanes  80 ,  82  which contact mating surfaces of the upper housing structure  184 . The board  40  has respective circuit and resistor patterns  60 A and  60 B formed on opposite sides of the board. In an exemplary embodiment, the patterns  60 A and  60 B are identical to each other and to the circuit pattern  60  shown in  FIG. 2 .  
         [0029]     The range of attenuation for an exemplary attenuator device illustrated in  FIGS. 2-4  may be limited by the achievable etched trace width of the quarter wave transformers for a given transmission line dimensional cross section.  FIG. 8  illustrates in simplified schematic form another embodiment of an attenuator  200 , wherein a back to back configuration allows a wider range of attenuation, e.g., by a factor of two in an exemplary embodiment. As with the embodiment of  FIG. 1B , the attenuator includes quarter-wavelength transformers ZT, Z 1  and Z 2  with nodes a, b and c. The attenuator further includes a second set of quarter-wavelength transformers Z 3 , Z 4  and resistances R 3 , R 4 . Resistance R 3  is connected between nodes b and d, at first ends of transformers Z 4  and Z 3 . Resistance R 4  is connected between node d and a groundplane. The opposite, second ends of the transformers Z 3  and Z 4  are connected at node e, which is connected by another quarter wave transformer Z 5  to port  2  of the device  200 .  
         [0030]     In an exemplary implementation of the attenuator  200  of  FIG. 8 , the parameters are designed to have the following values: ZT=41 ohms, Z 1 =88 ohms, Z 2 =41 ohms, R 1 =R 3 =100 ohms, R 2 =R 4 =34 ohms, Z 3 =41 ohms, Z 4 =88 ohms, and Z 5 =41 ohms, to provide a nominal attenuation of 8.6 dB. Across a 4.5 GHz bandwidth at X/Ku band, centered at 12.5 GHz, the attenuation is predicted to vary an exemplary embodiment by only 0.1 dB while the match is predicted to be better than 20 dB.  
         [0031]     Although the foregoing has been a description and illustration of specific embodiments of the invention, various modifications and changes thereto can be made by persons skilled in the art without departing from the scope and spirit of the invention as defined by the following claims.