Compact radio frequency harmonic filter using integrated passive device technology

A radio frequency (“RF”) harmonic filter circuit as disclosed herein is fabricated using integrated passive device (“IPD”) technology. The RF harmonic filter circuit is configured to provide second, third, and fourth harmonic rejection while providing good input and output impedance matching. The RF harmonic filter circuit employs only one IPD loop inductance (preferably used for a second harmonic resonance circuit), which results in a significant die/package size reduction. The RF harmonic filter circuit also employs a combined circuit that performs input and/or output impedance matching and third harmonic rejection.

TECHNICAL FIELD

The present invention relates generally to electronic components. More particularly, the present invention relates to radio frequency (“RF”) harmonic filters fabricated using integrated passive device (“IPD”) technology.

BACKGROUND

The prior art is replete with electronic devices and components designed for high frequency data communication applications. A common practical application for such devices and components is cellular telephony systems. In this regard, the need for component integration will increase as module sizes decrease for high performance cellular phones with advanced features. Cellular phone radio transmitters use several passive components for functions such as filtering, impedance matching, and switching. For example, a harmonic filter is used for signal selectivity over radio bands, such as the 824-915 MHz AMPS/GSM band or the 1.71-1.91 GHz DCS/PCS band. Practical harmonic filters for use in these bands are specifically designed to reject the second, third, and fourth harmonic frequencies from an RF input signal.

In conventional IPD implementations, an RF harmonic filter includes at least two loop inductors, which represent the bulk of the physical space of the device, which is typically on the order of approximately 1 mm2. In addition, conventional RF harmonic filter designs employ distinct input impedance matching and output impedance matching circuit elements, which inherently contribute to the overall size of the device. In accordance with the current trend toward miniaturization, a smaller device footprint is desirable, especially if such a smaller footprint can be achieved without a significant increase in manufacturing cost or complexity.

Accordingly, it is desirable to have a compact, low cost, RF harmonic filter that can be fabricated as an IPD. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.

BRIEF SUMMARY

An RF harmonic filter device configured in accordance with an embodiment of the invention is implemented as an IPD using only one IPD loop inductor. The RF harmonic filter employs at least one circuit that functions as a combined harmonic resonance and impedance matching circuit. The elimination of an IPD loop inductor results in a reduction in the footprint of the device, thus reducing the overall size and packaging requirements of the RF harmonic filter device.

The above and other aspects of the invention may be carried out in one form by an RF harmonic filter circuit fabricated using IPD. The RF harmonic filter circuit includes a substrate, only one IPD loop inductance formed on the substrate, and a harmonic resonance circuit formed on the substrate, where the harmonic resonance circuit includes the IPD loop inductance.

DETAILED DESCRIPTION

The following detailed description is merely illustrative in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description.

The invention may be described herein in terms of functional and/or schematic components. It should be appreciated that such components may be realized in any number of practical ways. For example, an embodiment of the invention may employ various elements, e.g., conductive traces, wire bonds, integrated passive devices, semiconductor substrate materials, dielectric materials, or the like, which may have characteristics or properties known to those skilled in the art. In addition, those skilled in the art will appreciate that the present invention may be practiced in conjunction with any number of practical RF circuit topologies and applications and that the harmonic filter circuits described herein are merely example applications for the invention.

For the sake of brevity, conventional techniques related to RF circuit design, RF signal propagation, RF impedance matching, semiconductor process technology, integrated passive device fabrication, and other aspects of the circuits (and the individual operating components of the circuits) may not be described in detail herein. Furthermore, the connecting lines shown in the various figures contained herein are intended to represent example functional relationships and/or physical couplings between the various elements. It should be noted that many alternative or additional functional relationships or physical connections may be present in a practical embodiment.

The following description refers to elements or features being “connected” together. As used herein, unless expressly stated otherwise, “connected” means that one element/feature is directly or indirectly connected to another element/feature, and not necessarily mechanically. For example, although the various schematics depict example arrangements of elements, additional intervening elements, devices, features, or components may be present in actual embodiments (assuming that the functionality of the circuits are not adversely affected).

FIG. 1is a schematic diagram showing the general circuit topology of an RF harmonic filter100,FIG. 2is a perspective view of an example IPD layout for a high band RF harmonic filter200having the circuit topology shown inFIG. 1, andFIG. 3is a perspective view of an example IPD layout for a low band RF harmonic filter300having the circuit topology shown inFIG. 1. RF harmonic filters100/200/300share a number of features and, where applicable, the following description of common features is intended to apply to RF harmonic filters100/200/300individually and collectively.

An RF signal enters harmonic filter100at an input port102(corresponding to a conductive RF input pad), and a filtered RF signal within the desired RF band is provided at an output port104(corresponding to a conductive RF output pad). In the practical layouts shown inFIG. 2andFIG. 3, the RF energy propagates over conductive traces formed on an insulating (semiconducting) substrate. Harmonic content associated with the RF input signal is rejected by three harmonic resonance circuits: a second harmonic resonance circuit106/206/306; a third harmonic resonance circuit108/208/308; and a fourth harmonic resonance circuit110. Second harmonic resonance circuit106/206/306is realized as an LC tank circuit (inductor L1in parallel with capacitor C1), third harmonic resonance circuit108/208/308is realized as an LC tank circuit (inductor L7in parallel with capacitor C5), and fourth harmonic resonance circuit110is realized as an LC series combination (capacitor C3in series with inductor L5). Harmonic filter100also includes an input impedance matching circuit112and an output impedance matching circuit114. Input impedance matching circuit112is realized as an LC series combination (capacitor C2in series with inductor L4), and output impedance matching circuit114is realized as an LC series combination (capacitor C4in series with inductor L6). The specific inductor and capacitor values of the harmonic filters are selected according to the desired filtering characteristics and the desired output frequency band. For example, harmonic filter300may be suitably configured for operation with AMPS/GSM applications (824-915 MHz), while harmonic filter200may be suitably configured for operation with DCS/PCS applications (1710-1910 MHz).

In practice, IPDs can be used to effectively reduce component and module sizes. As used herein, an IPD is a passive electronic device or a passive electronic component that can be fabricated using semiconductor process technology. An IPD can be produced with very high precision, excellent reproducibility, and low cost in high quantities by utilizing semiconductor wafer processing technologies. The layouts of harmonic filter200and harmonic filter300represent IPD realizations, where all of the depicted elements are formed on the same substrate (e.g., a semiconductor or insulating substrate such as GaAs, glass, or ceramic) using the same semiconductor process technology (i.e., the fabrication or manufacturing process by which the IPD is formed). In harmonic filters200/300, inductors L1and L7are realized as conductive RF signal line loops (bridges are employed at the respective “intersections” of each inductor to insulate the inductor loops from the respective C1and C5transmission lines), and the C1, C2, C3, C4, and C5capacitors are formed as IPDs on the substrate in the desired locations. The L1and L7inductors in harmonic filter200are smaller in size, include less loops, and have lower inductances than the L1and L7inductors in harmonic filter300. Notably, inductors L4, L5, and L6(not shown inFIGS. 2 and 3) are realized as wire bonds or other inductance elements connected between respective contact pads (numbered210,212, and214inFIG. 2; numbered310,312, and314inFIG. 3) and ground pads, which may be off-chip. Thus, inductors L4, L5, and L6are not actually part of the IPD itself, and harmonic filters100/200/300may be referred to as “two inductor” IPDs.

FIG. 4is a schematic circuit diagram of a high band RF harmonic filter400configured in accordance with an embodiment of the invention, andFIG. 5is a perspective view of an example IPD layout for RF harmonic filter400. Referring toFIG. 4, harmonic filter400generally includes an RF input node402, an RF output node404, a second harmonic resonance circuit406connected between RF input node402and RF output node404, a combined third harmonic resonance and input matching circuit408connected to RF input node402, and a combined fourth harmonic resonance and output matching circuit410connected to RF output node404. In operation, RF input node402may be connected, via an inductance L2, to a component, device, circuit, or termination412that provides an input impedance (typically 50 ohms) for harmonic filter400, and RF output node404may be connected, via an inductance L3, to a component, device, circuit, or termination414that provides an output impedance (typically 50 ohms) for harmonic filter400.

Second harmonic resonance circuit406includes an inductor L1connected in parallel with a capacitor C1, where inductor L1and capacitor C1are each connected between RF input node402and RF output node404. Second harmonic resonance circuit406is suitably configured to reject the second harmonic component of the RF input signal. Combined third harmonic resonance and input matching circuit408includes a capacitor C2connected in series with an inductor L4. As shown inFIG. 4, combined third harmonic resonance and input matching circuit408is connected between RF input node402and ground. Combined third harmonic and input matching circuit408is suitably configured to reject the third harmonic component of the RF input signal while establishing a desired input impedance for harmonic filter400. Combined fourth harmonic resonance and output matching circuit410includes a capacitor C3connected in series with an inductor L5. As shown inFIG. 4, combined fourth harmonic resonance and output matching circuit410is connected between RF output node404and ground. Combined fourth harmonic and output matching circuit410is suitably configured to reject the fourth harmonic component of the RF input signal while establishing a desired output impedance for harmonic filter400.

In an alternate embodiment, harmonic filter400may employ a combined fourth harmonic resonance and input matching circuit (in lieu of combined third harmonic resonance and input matching circuit408), and a combined third harmonic resonance and output matching circuit (in lieu of combined fourth harmonic resonance and output matching circuit410). Of course, such an alternate embodiment would require appropriate design and optimization to accommodate the combined functions. Furthermore, although the preferred embodiment maintains the position of the second harmonic resonance circuit as shown inFIG. 4, an alternate embodiment may utilize that position for the third or fourth harmonic resonance circuit, with the second harmonic resonance circuit being combined with the input or output matching circuit as described herein.

Referring toFIG. 5, an RF signal enters harmonic filter400at an RF input pad416(which corresponds to RF input node402inFIG. 4), and a filtered RF signal within the desired RF band is provided at an RF output pad418(which corresponds to RF output node404inFIG. 4). In the practical layout shown inFIG. 5, the RF energy propagates over conductive traces formed on an insulating (semiconducting) substrate419. The IPD layout shown inFIG. 5depicts elements formed on the same substrate419(e.g., a semiconductor or insulating substrate such as GaAs, glass, or ceramic) using the same semiconductor process technology (i.e., the fabrication or manufacturing process by which the IPD is formed). In this regard, inductor L1is realized as a conductive RF signal line loop inductor (air or dielectric bridges are employed at the respective “intersections” of inductor L1to insulate the inductor loops from the transmission lines utilized for the C1capacitor).

In practice, the C1, C2, and C3capacitors are each realized as an IPD capacitance formed on substrate419in the desired locations. IPD capacitance C1is connected between RF input pad416and RF output pad418(in the example embodiment, inductance L1forms bridges over the transmission line associated with IPD capacitance C1). In this regard, the input end of capacitor C1corresponds to its RF input node and the output end of capacitor C1corresponds to its RF output node. IPD inductance L1is also connected between RF input pad416and RF output pad418. Thus, IPD capacitance C1is connected in parallel with IPD inductance L1. IPD capacitance C2is connected between RF input pad416and a ground pad420, and IPD capacitance C3is connected between RF output pad418and a ground pad422.

Notably, inductors L4and L5(not shown inFIG. 5) are realized as wire bonds, conductive traces, or other inductance elements connected between respective ground pads420/422and grounding pads or other ground potential locations, which may be off-chip. In other words, these inductance elements establish ground connections to ground pads420/422. Thus, inductors L2, L3, L4, and L5need not actually be part of the IPD substrate itself, and harmonic filter400may be referred to as a “single inductor” IPD. In practical embodiments, harmonic filter400includes only one IPD loop inductor. As mentioned above, IPD capacitance C2and the L4inductance element connected to ground pad420form the combined third harmonic resonance and input matching circuit, which is connected to RF input pad416, while IPD capacitance C3and the L5inductance element connected to ground pad422form the combined fourth harmonic resonance and output matching circuit, which is connected to RF output pad418.

In accordance with one practical embodiment of harmonic filter400, IPD inductance L1, IPD capacitance C1, IPD capacitance C2, IPD capacitance C3, the L4inductance element, and the L5inductance element are suitably configured to provide a filter response that rejects harmonic frequencies corresponding to a pass band of 1.71 GHz to 1.91 GHz. To accomplish this objective, the input and output matching circuit component values are tuned to make the combined circuits resonate at the desired third and fourth harmonic frequencies while still providing good input and output impedance matching. Typical component values for such an example embodiment are contained in Table 1.

TABLE 1Typical Component Values for High Band Harmonic FilterL13.0nHL20.7nHL30.7nHL40.35nHL50.35nHC10.45pFC21.4pFC31.5pFZi50ΩZo50Ω

In accordance with known semiconductor fabrication techniques, harmonic filter400, including IPD inductance L1, IPD capacitances C1, C2, and C3, RF input pad416, RF output pad418, ground pads420/422, and other elements shown inFIG. 5may be formed on a common semiconductor substrate419using a plurality of metal layers and a number of dielectric layers. The metal layers are deposited and the desired conductive traces are etched or otherwise formed from the metal layers. The metal layers are typically referred to as “metal 1,” “metal 2,” “metal 3,” and so on to indicate the order in which they are deposited or formed onto the substrate during the fabrication process. In accordance with one practical embodiment, at least portions of the IPD capacitances are formed from the metal 2 layer and the loops of IPD inductance L1are formed from the metal 3 layer. In accordance with one practical semiconductor process technology, metal 1 elements are approximately 0.6 μm to 2.0 μm thick; metal 2 elements are approximately 2.5 μm thick, and metal 3 elements are approximately 10 μm thick.

Harmonic filter400can be fabricated using IPD process technology in a manner that results in a smaller footprint relative to harmonic filter200shown inFIG. 2. Notably, harmonic filter400eliminates one of the two IPD loop inductors employed by harmonic filter200, eliminates one of the LC tank circuits employed by harmonic filter200, combines an input matching circuit functionality with a harmonic resonance circuit functionality, and combines an output matching circuit functionality with a harmonic resonance circuit functionality. Even though the overall die size is considerably smaller, the operation of harmonic filter400does not suffer. In this regard,FIG. 6is a graph showing simulated insertion loss (S21) characteristics for harmonic filter400.FIG. 6illustrates the frequency rejection at the second, third, and fourth harmonics.

FIG. 7is a schematic circuit diagram of a low band RF harmonic filter500configured in accordance with an embodiment of the invention, andFIG. 8is a perspective view of an example IPD layout for RF harmonic filter500. Referring toFIG. 7, harmonic filter500generally includes an RF input node502, an RF output node504, a second harmonic resonance circuit506connected between RF input node502and RF output node504, and a combined third harmonic resonance, input matching, and output matching circuit508connected between RF input node502and RF output node504(circuit508is also connected to ground at one node). In accordance with a practical embodiment, the combined circuit may also be configured to function as a fourth harmonic resonance circuit. In operation, RF input node502may be connected, via an inductance L2, to a component, device, circuit, or termination510that provides an input impedance (typically 50 ohms) for harmonic filter500, and RF output node504may be connected, via an inductance L3, to a component, device, circuit, or termination512that provides an output impedance (typically 50 ohms) for harmonic filter500.

Second harmonic resonance circuit506includes an inductor L1connected in parallel with a capacitor C1, where inductor L1and capacitor C1are each connected between RF input node502and RF output node504. Second harmonic resonance circuit506is suitably configured to reject the second harmonic component of the RF input signal. Combined third harmonic resonance, input matching, and output matching circuit508includes a capacitor C2connected at one end to RF input node502and connected at the other end to an inductor L6. The other end of inductor L6is connected to a node514. In other words, the C2/L6series combination is connected between RF input node502and node514. Combined third harmonic resonance, input matching, and output matching circuit508also includes a capacitor C3connected between RF output node504and node514, and an inductor L5connected between node514and ground. Combined third harmonic resonance, input matching, and output matching circuit508is suitably configured to reject the third harmonic component of the RF input signal while establishing a desired input impedance and a desired output impedance for harmonic filter500. Optionally (or inherently), combined third harmonic resonance, input matching, and output matching circuit508may also perform rejection of the fourth harmonic component of the RF input signal.

Although the preferred embodiment maintains the position of the second harmonic resonance circuit as shown inFIG. 7, an alternate embodiment may utilize that position for the third or fourth harmonic resonance circuit, with the second harmonic resonance circuit being combined with the input/output matching circuit as described herein. Furthermore, the L6inductance need not be located as shown inFIG. 7. For example, the L6inductance may be located in series with the C3capacitor, the L6inductance may be “divided” into two or more inductors, or the like.

Referring toFIG. 8, an RF signal enters harmonic filter500at an RF input pad516(which corresponds to RF input node502inFIG. 7), and a filtered RF signal within the desired RF band is provided at an RF output pad518(which corresponds to RF output node504inFIG. 7). In the practical layout shown inFIG. 8, the RF energy propagates over conductive traces formed on an insulating (semiconducting) substrate520. The IPD layout shown inFIG. 8depicts elements formed on the same substrate520(e.g., a semiconductor or insulating substrate such as GaAs, glass, or ceramic) using the same semiconductor process technology (i.e., the fabrication or manufacturing process by which the IPD is formed). In this regard, inductor L1is realized as a conductive RF signal line loop inductor (air or dielectric bridges are employed at the respective “intersections” of inductor L1to insulate the inductor loops from the transmission lines utilized for the C1capacitor).

In practice, the C1, C2, and C3capacitors are each realized as an IPD capacitance formed on substrate520in the desired locations. IPD capacitance C1is connected between RF input pad516and RF output pad518(in the example embodiment, inductance L1forms bridges over the transmission line associated with IPD capacitance C1). IPD inductance L1is also connected between RF input pad516and RF output pad518. Thus, IPD capacitance C1is connected in parallel with IPD inductance L1. IPD capacitance C2is connected between RF input pad516and a ground pad522(which may be realized as a portion of a conductive trace524), and IPD capacitance C3is connected between RF output pad518and a ground pad526(which may be realized as a portion of a conductive trace524). In the example embodiment shown inFIG. 8, ground pad522and ground pad526are connected together, and integrated with, conductive trace524. Conductive trace524functions as another IPD inductance (corresponding to inductance L6inFIG. 7) for harmonic filter500. This second IPD inductance is connected between the “ground” ends of IPD capacitance C2and IPD capacitance C3. As shown inFIG. 8, conductive trace524may be configured as a non-looped inductor, in contrast to IPD inductance L1.

Notably, inductor L5(not shown inFIG. 8) is realized as a wire bond, conductive trace, or other inductance element connected between ground pads526(or any suitable area of conductive trace524) and a grounding pad or other ground potential location, which may be off-chip. In other words, this inductance element establishes a ground connection to ground pad526and, in turn, to IPD capacitance C3. Furthermore, the small amount of inductance needed for inductor L6can be realized with conductive trace524connected between IPD capacitances C2and C3, rather than a loop inductor. Thus, inductors L2, L3, and L5need not be part of the IPD substrate itself, and harmonic filter500may be referred to as a “single inductor” IPD. In practical embodiments, harmonic filter500includes only one IPD loop inductor. As mentioned above, IPD capacitance C2, IPD capacitance C3, IPD inductance L6, and the L5inductance element connected to ground pad526form the combined third harmonic resonance, fourth harmonic resonance, input matching, and output matching circuit508, which is connected between RF input pad516and RF output pad518.

In accordance with one practical embodiment of harmonic filter500, IPD inductance L1, IPD capacitance C1, IPD capacitance C2, IPD capacitance C3, IPD inductance L6, and the L5inductance element are suitably configured to provide a filter response that rejects harmonic frequencies corresponding to a pass band of 824 MHz to 915 MHz. To accomplish this objective, the matching circuit component values are tuned to make the combined circuit resonate at the desired third harmonic frequency, while still providing good input and output impedance matching. In practice, tuning for resonance at the third harmonic frequency will also provide some rejection at the fourth harmonic frequency. Typical component values for such an example embodiment are contained in Table 2.

TABLE 2Typical Component Values for Low Band Harmonic FilterL16.0nHL20.7nHL30.7nHL50.1nHL60.6nHC11.4pFC23.0pFC33.3pFZi50ΩZo50Ω

Harmonic filter500can be fabricated using IPD process technology in a manner described above, which results in a smaller footprint relative to harmonic filter300shown inFIG. 3. Notably, harmonic filter500eliminates one of the two IPD loop inductors employed by harmonic filter300, eliminates one of the LC tank circuits employed by harmonic filter300, and combines the functionality of an input matching circuit, a harmonic resonance circuit, and an output matching circuit into a single sub-circuit. Even though the overall die size is considerably smaller, the operation of harmonic filter500does not suffer. In this regard,FIG. 9is a graph showing simulated insertion loss (S21) characteristics for harmonic filter500.FIG. 9illustrates the frequency rejection at the second, third, and fourth harmonics.