Abstract:
Modular thin film, distributed, lumped element band-pass filters. The filter circuitry is configured on a number of ceramic substrates. The component defining depositions are arranged to overlap and couple to one another with connecting vias. Alternative band-pass filter and delay line circuits are disclosed that provide desirable delay characteristics. Bordering ground conductors and covering ground planes shield lumped impedance resonator and overlapping capacitor elements and/or provide a hermetic seal between the layers. The assemblies are configured to accommodate a range of frequencies and permit pre-fabrication with subsequent laser trimming, assembly and packaging.

Description:
RELATED APPLICATIONS 
     The present application is a continuation-in-part of application Ser. No. 09/257,451, filed Feb. 25, 1999, now U.S. Pat. No. 6,127,906. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to high frequency filters and, in particular, to a distributed, thin film lumped element band-pass filter theoretically capable of operation at frequencies in the range of 800 MHz to 10.0 GHz. 
     Varieties of thin film devices have been constructed for high frequency circuits. Most have been directed to microwave applications. Some devices, such as discrete delay line assemblies, have been constructed for higher frequency applications. Delay lines are frequently used to adjust timing inconsistencies at complex circuitry mounted to complex printed circuit boards. Examples of some discrete, multi-layer, delay line devices constructed on ceramic substrates are shown at U.S. Pat. Nos. 5,030,931; 5,365,203; and 5,499,442. 
     The subject invention provides a discrete, distributed, lumped element, thin film band pass filter. Alternative multi-layer, filter configurations are constructed on ceramic substrates between shielding layers. Resonator, strip line, grounding and necessary termination layers are connected with solder filled vias that extend between the substrates and interconnect elements at the layers. 
     SUMMARY OF THE INVENTION 
     It is a primary object of the present invention to provide a shielded, band-pass filter capable of operation at frequencies in the range of 800 MHz to 10.0 GHz 
     It is a further object of the invention to provide alternative band-pass filter circuits constructed from lumped, thin film components deposited on ceramic substrates. 
     It is a further object of the invention to provide shielded multi-layer filter circuits, wherein the layers are connected with through vias, and wherein various thin film depositions define lumped impedance circuit components, capacitors at overlapping strip conductors, strip conductors and terminations. 
     Various of the foregoing objects, advantages and distinctions of the invention can be found in alternative thin film band-pass filter circuits constructed on ceramic substrates. Circuit elements are constructed with discrete and overlapping thin film depositions. The surface area and thickness of the elements are tailored to desired operating characteristics of the circuit. Filled through vias electrically couple the layers and components together and to circuit terminations. 
     The layers can be pre-processed to determined configurations that permit a range of device characteristics. Specific circuit performance can be obtained by laser trimming the thin film depositions to specific shapes and dimensions, prior to stacking a number of prepared layers and final processing the layers into a desired filter device. 
     Still other objects, advantages and distinctions of the invention will become more apparent from the following description with respect to the appended drawings. To the extent alternative constructions, improvements or modifications have been considered they are described as appropriate. The description should not be literally construed in limitation of the invention. Rather, the scope of the invention should be broadly interpreted within the scope of the further appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Like reference numerals refer to like structure at the various drawings and which are as follows: 
     FIG. 1 is a circuit diagram to an exemplary narrow band-pass filter transform of the invention. 
     FIG. 2 is a circuit diagram of a distributed lumped impedance device that is equivalent to the circuit of FIG.  1 . 
     FIG. 3 is a perspective view, shown in exploded assembly, of a ground and lower lying strip line cover layer (shown in dashed line) and a resonator and bottom termination layer (shown in dashed line) of a 1.9 GHz band-pass filter. 
     FIG. 4 is a plan view of the top ground cover layer of the filter of FIG.  3 . 
     FIG. 5 is a plan view of the top strip line layer of the filter of FIG.  3 . 
     FIG. 6 is a plan view of the resonator layer of the filter of FIG.  3 . 
     FIG. 7 is a plan view of the bottom termination layer of the filter of FIG.  3 . 
     FIG. 8 is a plot of an exemplary frequency response of the filter of FIG.  3 . 
     FIG. 9 is a plan view of a resonator layer of an 800 MHz filter. 
     FIG. 10 is a plan view of the bottom termination layer of the filter of FIG.  9 . 
     FIG. 11 is a circuit diagram to the circuit of FIG.  12 . 
     FIG. 12 is a perspective view, shown in exploded assembly, to the cover layer, signal layers and ball grid array termination layer of another GHz band-pass filter. 
     FIG. 13 is a composite layout schematic of the printed conductors of a nominal 2.1 GHz band pass filter delay line. 
     FIG. 14 is a perspective view, shown in exploded assembly, to the signal layers of the band pass filter of FIG. 13, exclusive of a cover layer and bottom termination layer. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIGS. 1 and 2, generalized circuit diagrams are shown to the distributed thin film band-pass filter circuit  2  of the invention as constructed from discrete and distributed, lumped impedance components. The discrete coupling capacitors CO, 1  through C 3 , 4  and inductor and capacitor components L 1 -L 3 , C 1 -C 3  of FIG. 1 are shown as lumped resonance determining components Z1(L 1 ,C 1 ), Z2(L 2 ,C 2 ) and Z3(L 3 ,C 3 ) at FIG.  2 . The components of FIG. 2 are constructed from tailored thin film depositions that exhibit desired lumped impedances Z1, Z2 and Z3 at the operating frequency range. 
     The filter circuit  2  can be constructed to operate at various frequencies and operating characteristics, depending upon the dimensions of the various depositions that define the circuit elements. The elements Z1, Z2 and Z3 are defined by single thin film depositions. The discrete coupling capacitors C 1  through C 4  are determined by overlapping depositions at adjacent substrates. Necessary ground planes are defined with large depositions that substantially cover, overlying and underlying substrate surfaces. Dielectric materials are positioned between the substrates and passivation materials cover the stacked assemblies. 
     FIG. 3 depicts a band-pass filter centered at 1.95 GHz. The filter circuit  6  of FIG. 9 is centered at 850 MHz. The nominal values of the circuit components vary with the center and side band frequencies and other operating characteristics of a desired filter. For example, the discrete values of the circuit elements of the filter  6  at FIG. 9 are C 1 =1.1 pf; C 2  and C 3 =0.3 pf; and C 4 =1.25 pf. The dimensions of the lumped impedance depositions Z1, Z2 and Z3 are width=0.1 mm; length=25.57 mm; and thickness=0.535 mm. 
     Returning attention to FIG. 3, the filter circuit  4  is shown in exploded assembly on ceramic substrates  8  and  10 . The substrates are approximately 5.0 mm wide×12.4 mm long. Not shown are the associated cover layers that include the circuit termination pads. The construction of the latter layers depends upon the mounting configuration of the circuit package (e.g. multi-pin, surface mount, gull wing etc.). FIGS. 7 and 10 disclose exemplary terminations for the filters  2  and  6 . Otherwise, the substrate  8  defines the coupling capacitors and one ground plane and the substrate  10  defines a resonator layer and second ground plane layer. 
     The specific configuration of the thin film depositions at the substrates  8  and  10  that determine the circuit elements and operating characteristics of the filter  2  are shown in detail at the plan views of FIGS. 4 through 7. FIG. 4 shows the top ground layer  12  and ground plane  13  and through which a number of holes or vias  14  extend. The ground plane  13  overlies the active resonant components Z1-Z3. The vias  14  are positioned to selectively align with other vias and depositions at the other layers. Upon stacking the substrates  8  and  10  and any other stacked substrates (not shown) and filling the previously plated vias  14  with solder, the vias  14  connect the circuit elements as shown at FIGS. 1 and 2. 
     FIG. 5 depicts a strip line layer  15  and number of strip conductors  16 ,  18  and  20  that are formed on the lower surface of the substrate  8 . The conductors  16 ,  18  and  20  are electrically connected by redundant through vias  14  to the ground plane  12  and to strip conductors  22 ,  24  and  26  at the resonator layer  28  shown at FIG.  6 . An intermediate dielectric material (not shown) separates the layers  15  and  28 . The coupled strip conductors  16 - 26  border and shield the distributed impedance elements Z1, Z2 and Z3 at the resonator layer  28  of the substrate  10 . The conductors  16 - 26  and impedance elements Z1-Z3 are each approximately 400 microns wide. The impedance elements Z1-Z3 are also approximately 9.9 mm long. 
     Nodes 1-3 at the ends of the distributed impedance elements Z1-Z3 define one plate of the coupling capacitors C 1 -C 4 . The opposite plates of the coupling capacitors C 1 -C 4  are defined by nodes 0-4 at the bottom termination layer  30 , reference FIG.  7 . Vias  14  extend between the nodes at the layers  28  and  30  to interconnect the distributed capacitors. A bottom ground plane  32  aligns to the active resonant components Z1-Z3. A number of vias  34  provided at the layers  28  and  30  redundantly connect the impedance elements Z1-Z3 to the ground plane  32 . Isolated vias  36  connect the impedance elements Z1-Z3 to the capacitors C 1 -C 4  and provide test points. 
     Termination pads  40  extend from the ground plane  32  and nodes 0 and 4 and connect with terminals, solder pads etc. at the device packaging. The termination pads  40 , many terminations of which are redundant, accommodate a 16-pin package. The number and location of the termination pads  40  is adjusted in relation to the number of substrates  8  and  10  at each filter circuit. That is and depending upon the filter circuit, multiple groupings of the substrates  8  and  10  can be stacked on top of each other and appropriate vias  14 ,  34  and  36  can be provided to obtain necessary circuit couplings. 
     FIG. 8 shows a plot of the operating frequency to DB response characteristic of the filter  2 . The circuit  2  particularly provides narrow band-pass operation centered at 1.95 GHz. 
     FIGS. 9 and 10 depict a resonator layer  50  and termination layer  52  of the 800 MHz band-pass filter  6 . The layers  50  and  52  are constructed on substrates of comparable dimension to the substrates  8  and  10 . The lumped impedance components Z1-Z3 are bordered by strip conductors  22 ,  24  and  26 . The lumped impedance depositions Z1-Z3 extend as serpentine patterns from the nodes 1-3 to the strip conductor  26 . The windings are 100 microns wide and the spacing between windings is 100 microns. The meandering pathway of the serpentine patterns is defined to provide a compact low-frequency construction. The layer  52  provides a ground plane  54  that is duplicated at the overlying substrate  8  (not shown). The nodes 0-4 are sized to reflect necessary capacitance values for the desired filter operating frequency. 
     Although not shown, the ground cover and strip line layers of the substrate  8  for the filter  6  are essentially identical to the layers  13  and  15 , except for the size of the strip conductors  16 - 20  and ground plane  13 . 
     The filters  2  and  6  demonstrate two substantially identical organizations. Depending upon possible ranges of the center and side band frequencies, the various thin film depositions at the layers of the substrates  8  and  10  can be constructed to a preferred oversize condition reflective of an extreme frequency of a relative range of frequencies. The depositions can later be laser trimmed to a desired final size to accommodate other frequencies and operating characteristics for limited volume production runs. Advantageously, the processing time to fabricate the filters can thereby be reduced and simplified to merely trimming, stacking and final packaging the trimmed substrates. Production inventories can thereby be reduced and response time to meet customer needs for tailored filters can be met. 
     FIG. 11 shows a circuit schematic of the multi-layer assembly shown in perspective view and exploded assembly at FIG.  12 . FIG. 12 particularly shows the cover layer  70 , signal layers  72 ,  74  and  76 , ground plane layers  78  and  80  and the BGA termination layer  82  of another multi-layer ceramic microstrip, band-pass filter  84 . The filter  84  operates at a nominal 1.0 gigahertz frequency. The layer  72  provides a pair of inductors  85  and  86  that are coupled to a butterfly capacitor  88  by through vias  89  and  90 . Input and output capacitors  92  and  94  are provided at the layer  76 . Ground-connect buss bars  96  are displaced to the sides of the capacitors  88 ,  92  and  94 . Ground planes  98  and  100  are positioned between the signal layers  72 ,  74  and  76 . Ground strip conductors  102  circumscribe the borders of each of the layers  72 - 82  and vias  14  interconnect the printed components at the layers. 
     FIGS. 13 and 14 depict another microstrip band pass filter  50  that when operated at 2.1 GHz acts as a nominal 14-nanosecond delay line. FIG. 13 depicts a composite view of the layout schematic of the printed conductors  52 - 60  of the circuit  50 . The spacing and offset arrangement between the conductors  52 - 60  is configured to accommodate an operational coupling for the frequencies of interest at one quarter wavelength. 
     The physical multi-layer ceramic assembly  62  of the band pass filter  50  is shown in the exploded assembly at FIG. 14, exclusive of a cover layer and ball grid array (BGA) bottom termination layer. The latter cover and bottom termination layers are similar to those shown at FIG.  12 . Vias  14  interconnect the segmented conductors  54   a  and  54   b  and  58   a  and  58   b  at the signal layers  49 ,  51  and  53  and the ground planes  55 ,  57  and  59 . Ceramic dielectric layers  45  and  47  separate the signal layers  49 ,  51  and  53 . Strip conductors  64 ,  66  and  68  circumscribe the borders of the signal layers  49 ,  51  and  53 . Strip conductors  41  and  43  also circumscribe the borders of the dielectric layers  45  and  47  and facilitate packaging. That is, the strip conductors bond to the adjacent ground planes and strip conductor of each adjacent layer to provide a hermetic seal when the assembly is finally packaged. Arcuate coupling leads  51  and  61  are shown in FIG. 13 that are used as required depending upon the patterning of the signal layers relative to the termination layer and are not present in the layout arrangement of FIG.  14 . 
     While the invention has been described with respect to a number of presently preferred circuit components and assembly constructions, it is to be appreciated still other constructions may be suggested to those skilled in the art. The scope of the invention should therefore be construed broadly within the spirit and scope of the following claims.