Abstract:
A laminated low-profile dual filter module for telecommunications devices and method provides both Groupe Spécial Mobile (GSM) and Digital Cellular System (DCS) transmit filters in a small package. The filter module comprises multiple layers of ceramic substrate with metal circuit patterns sandwiched between. Two separate filters are implemented within the layers, with a first filter comprising a first set of layers and the second filter adjoining within a second set of layers. Resonators for each filter are positioned at the opposite sides of the module, in order to avoid coupling between the resonators and ground layers are interspersed for isolation. Capacitors are implemented by a first plate defined by an area on one metal layer with the adjacent layers providing ground planes that form the second plate.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to radio-frequency (RF) and microwave filters and more specifically for a dual RF filter having a small package size for integration within telecommunications devices. 
     BACKGROUND OF THE INVENTION 
     RF and microwave filters are used in telecommunications devices to reject out-of-band signals, filter undesired harmonics and spurious components from signals prior to transmission and for performing various other functions within the telecommunications devices. 
     In particular, a Groupe Spécial Mobile. (GSM) filter is typically required in the output of the transmitter for GSM transceivers and a Digital Cellular System (DCS) filter is typically required in the output of the transmitter for DCS transceivers. Both are typically required for a dual-mode telecommunications transceiver. Similar filters are also needed in wireless local area network (LAN) and wireless personal area network (WPAN) devices, such as BLUETOOTH and 802.11 devices. The above-mentioned filters occupy a significant amount of circuit board area and package volume within a dual-mode transceiver. 
     Small size and light weight components are critical to reducing the size of telecommunications transceivers and the trend is toward progressively smaller devices. Therefore, it would be desirable to reduce the size of required filters within telecommunications transceivers and further desirable to provide such reduction in a dual-mode telecommunications receiver wherein both a GSM and a DCS filter are required. 
     SUMMARY OF THE INVENTION 
     The above objectives of reducing filter size and providing a GSM and DCS filter that required less package volume and circuit board area are provided in a low-profile dual filter module that integrates two RF filter in one low-profile package. The dual filter module includes a first and second filter section each formed by multiple dielectric layers with metal circuit patterns sandwiched in between. The construction provides isolation between two filters in the same small package. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a pictorial diagram depicting a top projected view of a filter module in accordance with an embodiment of the present invention; 
     FIG. 1B is a pictorial diagram depicting a bottom projected view of the filter module of FIG. 1A; 
     FIG. 2A is a schematic diagram of a first filter section as embodied within the filter module of FIGS. 1A and 1B; 
     FIG. 2B is a schematic diagram of a second filter section as embodied within the filter module of FIGS. 1A and 1B; 
     FIGS. 3A-3M are pictorial diagrams depicting metal patterns within the layers of the filter module of FIGS. 1A and 1B; 
     FIG. 4 is a set of graphs depicting the electrical operation of the filter module sections within the filter module of FIGS. 1A and 1B; 
     FIG. 5A is a pictorial diagram depicting a cross-section of a circuit module assembly incorporating the filter module of FIGS. 1A and 1B; and 
     FIG. 5B is a pictorial diagram depicting a top view of the circuit module assembly of FIG.  5 A. 
     The invention, as well as a preferred mode of use and advantages thereof, will best be understood by reference to the following detailed description of illustrative embodiments when read in conjunction with the accompanying drawings, wherein like reference numerals indicate like parts throughout. 
    
    
     DETAILED DESCRIPTION 
     Referring now to the figures and in particular to FIG. 1A, a filter module  10 , in accordance with an embodiment of the present invention is depicted. Filter module  10  includes a plurality of layers  16 , each comprising a dielectric with metal circuit patterns that may be etched, sputtered or separately formed (stamped or die cut) between the dielectric layers. Layers  16  can be made of varying thickness and dielectric constant in order to achieve a particular capacitance for a given surface area. Capacitors within filter  10  are implemented with two ground plates opposing a central plate in order to achieve a higher capacitance value than with a single plate design. However, an alternative embodiment of the invention, one or both filters are implemented using a series capacitor and shunt inductors, wherein neither plate of the capacitor is connected to ground. 
     The layers in exemplary filter  10  are formed from a ceramic tape that is generally available for low temperature co-fired ceramic (LTCC) fabrication. The metal patterns are formed on each of layers  16 , then layers  16  are compressed together and fused in a low-temperature bonding process. Terminals on an external surface of filter module  10  are plated for subsequent attachment to external circuits. 
     Within the layers, circuits are formed by the metal patterns that achieve inductors, capacitors and inductive coupler stages, as well as shield planes for preventing coupling between the filter stages, between components within the filter stages and between the filters and the outside environment. 
     Filter module  10  is a dual filter module having a first section  12  formed from a first set of layers from among the plurality of layers  16  and a second set of layers  14 . Referring now to FIG. 1B, a set of terminals  18  are disposed on the bottom side of filter  10  for connection to external circuits. 
     In the exemplary-embodiment, filter module comprises a Groupe Spécial Mobile (GSM) filter and a Digital Cellular System (DCS) filter having separate terminals  18  for connection to GSM and DCS circuits within an assembly. The circuit assembly is desirably a very thin circuit module having a thickness on the order of 1 millimeter, and as such, present off-the-shelf filters are either too thick, or require a large amount of circuit module area. 
     The present invention provides a dual filter module  10  that incorporates two filters within a single stack of layers  16 , while effectively isolating the filters from each other and achieving a similar level of performance as two currently available single filter modules. 
     Referring now to FIG. 2A, a schematic diagram of a first (DCS) low-pass filter section is depicted. Inductor L 1  and capacitor C 1  form a first resonant circuit that can be viewed as a resonator or as a lumped-parameter LC circuit. Inductor L 3  and capacitor C 2  form a second resonant circuit. Inductor L 2  provides coupling between these two resonant circuits as well as part of the filtering function. The first resonant circuit (L 1 , C 1 ) and the second resonant circuit (L 3 ,C 2 ) are positioned at opposite corners of their respective layers  16  within filter module  10 , providing isolation that enhances the performance of filter module  10 . 
     Referring now to FIG. 2B, a schematic diagram of a second (GSM) low-pass filter section is depicted. Inductor L 11  and capacitor C 11  form a first resonant circuit that can be viewed as a resonator or as a lumped-parameter LC circuit. Inductor L 14  and capacitor C 13  form a second resonant circuit while capacitor C 12  adds to the filtering function. Inductor L 12  provides coupling between these resonant circuits as well as part of the filtering function. The first resonant circuit (L 11 , C 11 ) and the second resonant circuit (L 14 , C 13 ) are positioned at opposite corners of their respective layers  16  within filter module  10 , providing isolation that enhances the performance of filter module  10 . 
     While the above illustrated filter sections within filter module  10  are both low-pass filters and are for dedicated bands, specific filter implementations should not be viewed as limiting the present invention. Combinations of high-pass, low-pass, band-pass and notch filters can all be achieved within the multiple filters of the present invention, and more than two filters may be stacked using the same techniques and structures. Further, bandpass filters may be formed by a cascade of a high-pass filter and a low-pass filter connected in series within filter module  10 . In the above mentioned cascade configuration, filter module  10  need only include a single input and a single output terminal. 
     Referring now to FIGS. 3A-3M the specific metal circuit patterns implemented within layers  16  of filter  10  are depicted. Bottom layer  16 A (depicted in FIG. 3A) is a metal layer used to isolate the circuits of filter  10  from external circuit traces or components beneath filter  10  in an installation. Gaps are provided to avoid capacitive loading on vias leading to external terminals  18  and the terminals themselves. 
     Layer  16 B (depicted in FIG. 3B) includes a portion of inductor L 1  coupled to input terminal IN 1  and metal areas forming ground plates for capacitors C 1  and C 2 . Layer  16 C (depicted in FIG. 3C) includes the remainder of inductor L 1  and the central plate of capacitor C 1 , positioned to the right side of the layers as shown. Inductor L 3  and the central plate of capacitor C 2  are positioned to the opposite side (corner) of layer  16 C from inductor L 1  and capacitor C 1 , providing isolation between the resonant circuits. Layer  16 D (depicted in FIG.  3 D)includes the other ground plates for capacitors C 1  and C 2  and a shield for inductor L 1 . Layer  16 E (depicted in FIG. 3E) includes a first portion of inductor L 2  connected to output terminal OUT 1 . Layer  16 F (depicted in FIG. 3F) includes the remainder of inductor L 2  connected to output terminal IN 1 . 
     Layer  16 G (depicted in FIG. 3G) includes shield layers for isolation capacitors and other circuit elements between the two filter sections. Notably the central area is empty, as thin metal present between inductor stages will tend to increase coupling rather than reduce coupling, as induced currents in the metal would increase mutual coupling between adjacent inductors. Likewise layer  16 H (depicted in FIG. 3H) has no metal in the central area, while implementing central plates for capacitors C 12 A (which forms a portion of capacitor C 12 ), C 13 A (which forms a portion of capacitor C 12 ), C 11 A (which forms a portion of capacitor C 12 ) and metal shield sections. 
     Layer  16 J (depicted in FIG. 3I includes more metal shield sections (forming ground plates for capacitors C 11 , C 12  and C 13 ). Layers  16 G- 16 J together provide an isolation distance between inductive couplers L 2  and L 12  of the two filter sections. Layer  16 K (depicted in FIG. 3J) includes the central plates of capacitors C 11 , C 12  and C 13 . (Capacitor plate  12 B and capacitor plate  12 A from layer  16 H are connected in parallel to provide capacitor C 12  of FIG. 2B, Capacitor plate  11 B and capacitor plate  11 A from layer  16 H are connected in parallel to provide capacitor C 11  of FIG.  2 B and Capacitor plate  13 B and capacitor plate  13 A from layer  16 H are connected in parallel to provide capacitor C 13  of FIG.  28 ). Layer  16 K also includes a portion of inductor L 12  that is connected to input terminal IN 2 . 
     Layer  16 L (depicted in FIG. 3K includes another section of inductor L 12  and the third ground plates of capacitors C 11 , C 12  and C 13 . Inductor L 12  is connected to inductor L 14  and output terminal OUT 2  on layer  16 M (depicted in FIG. 3L Layer  16 M also includes inductor L 11  connected to input IN 2 . Inductor L 11  and capacitor C 11  on layer  3 K are positioned on the opposite side from the other components within the second filter stage, increasing isolation between the resonant circuits of the second filter stage. 
     Finally, layer  16 N (depicted in FIG. 3M) provides a shield at the top of filter  10 , excluding the central region that would degrade the operation of inductors (and increase coupling between the filter sections). In general, a metal layer, even if thick enough to prevent coupling to the inductors would increase filter insertion loss and reduce the Q of the resonators, degrading the operation of filter  10 . 
     Referring now to FIG. 4, graphs depicting the operation of the filter sections of filter  10  are depicted. The DCS filter transmission loss 40 shows a rejection within the band of interest of greater than 25 dB, with even better rejection around the second harmonics of the in-band frequencies. The return loss 42 of the DCS section is shown as being better than −12 dB within the DCS transmit band. The transmission loss 44 of GSM filter is better than 30 dB out of band, extending past the third harmonic of in-band signals, and the return loss 46 is better than −10 dB for the in-band signals. 
     Referring now to FIGS. 5A and 5B, a circuit module  50 , in accordance with an embodiment of the present invention is depicted. Circuit module  50  is of a type having an overmold thickness  54  of approximately 1 mm. If standard filters were used, the filters would occupy approximately twice the area which filter  10  requires. Integrated circuits  52  and other components  60  are mounted on a substrate  56  and are shown in overmold  54  cut-away  58  that exposes the internal components of circuit module  50 . Overmold  54  may be an injection-molded plastic resin or other suitable encapsulant or overmold  54  may be a thin cover attached by an adhesive. 
     The above description of embodiments of the invention is intended to be illustrative and not limiting. Other embodiments of this invention will be obvious to those skilled in the art in view of the above disclosure and fall within the scope of the present invention.