Miscellaneous active electrical nonlinear devices – circuits – and – Specific input to output function – Combining of plural signals
Reexamination Certificate
2001-10-30
2003-11-25
Callahan, Timothy P. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific input to output function
Combining of plural signals
C327S359000, C327S355000, C327S566000, C455S326000
Reexamination Certificate
active
06653885
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to radio frequency (RF) communication devices and more specifically to an RF mixer circuit having signal splitting and phase shifting circuitry (“balun”), wherein the balun is integrated on-chip with the core RF mixer circuit. The invention further relates to methods for making RF mixers integrated on-chip with one or more baluns.
BACKGROUND OF THE INVENTION
Integration of RF Functions
Wireless communication via radio frequency (RF) wave transmission presents numerous technical challenges. Contemporary wireless products typically divide RF functions among several integrated circuits and discrete devices, and separate functions may be manufactured in different technologies. Large scale integration (LSI) is a desirable goal in virtually all electronic manufacturing processes but has proven difficult for RF applications, and consequently modular or hybridized approaches are typically used to manufacture products to accomplish complex functions such as frequency translation or “mixing.”
RF components such as mixer modules have generally been made by combining semiconductor technology, such as gallium arsenide transistors or silicon diodes, with passive component networks, all mounted on a common carrier. Such hybrid assemblies add cost and complexity to the manufacturing process by comparison with fully monolithic or integrated solutions, and modular construction of parts such as mixers can result in poorly performing, bulky modules that must be soldered on to an “integrated” system. Combining on a single chip RF functions, passive components (e.g., resistors, capacitors, inductors), and the control functions, usually fabricated in CMOS circuitry on silicon, for example, could enhance product performance, reliability, and manufacturing flow tolerances, while decreasing size, power consumption, and manufacturing costs. However, attempts at high levels of integration which attempt to combine RF functions on a single chip have proven to be difficult, expensive and generally unsatisfactory.
One earlier approach to partially integrating several different functions on a single substrate is the thin-film hybrid process. The combination of several chips on a thin-film hybrid substrate for RF applications requires precise manufacturing practices and circuit elements. Many processing steps are necessary. For instance, these steps generally include depositing a thin layer of metal on a substrate, coating it, and finally removing the metal layer by etching to form a desired pattern. This process is repeated for the deposition of resistive films to create circuit elements. Ultimately, semiconductor devices are attached to the patterned substrate and these individual chips are interconnected by the patterned transmission lines within the hybrid package.
Such thin-film hybrid fabrication processes can be expensive and time consuming and can have significant yield problems that increase proportionally with the number of integrated circuits on the substrate. Furthermore, modular or hybrid construction processes can be economically inefficient due to the testing and retesting of the various subassemblies required to feed into the end product. The further along in the process before defects are discovered, the greater the waste of resources. Thus, yield losses from the incorporation of multiple chips on a hybrid circuit can be very costly.
Another factor which can increase hybrid cost is variation in stray coupling between closely-spaced RF circuits. This phenomenon is traceable to the placement of lumped element components. The mechanical alignment precision of these components on the hybrid substrate is inherently poor, their sizes and shapes are numerous and varied, and the placement equipment typically has loose registration tolerance for placement accuracy. Stray RF coupling between active components, passive components, and interconnect wiring can be a significant factor in reducing manufacturing yields. These stray coupling variations can make it difficult to achieve repeatable circuit performance, thereby resulting in serious yield problems.
For the above reasons, attempts to integrate systems more completely with monolithic microwave integrated circuits (MMICs) have increased. Many of these efforts have been frustrated, however, by the limited availability of high performance substrates. A number of significant problems arise from using substrates that are not highly insulating. High electrical loss, high inter-element parasitic capacitance, high conductor-to-substrate capacitances, and other deleterious effects can result from using substrates such as gallium arsenide (GaAs) and bulk silicon that are not highly insulating.
Substrates such as gallium arsenide (GaAs) and bulk silicon can have serious disadvantages for the integration of both active and passive RF components in a single chip. For silicon, for example, the performance of passive components can be severely impaired by the conductivity of the substrate. Insertion loss along transmission lines and isolation between non-connected devices are both poor owing to this conductivity. For GaAs, for example, the ability to integrate large numbers of active devices can be limited by a relatively high defect density of the substrate. Both technologies have their individual merits, but they cannot be merged readily into a single system except through modular methods.
Differential Signal Processing
Differential signal processing enhances performance in an RF system. Integrated circuit manufacturing naturally promotes differential design techniques because of the small size, low cost, and superior matching of devices available. For cost reasons, signal routing in the hybrid or module realm tends to be single ended rather than differential. RF signals tend to be routed through expensive cabling and precision machined RF connectors. Naturally, single ended routing cuts costs in half and is very desirable to endpoint manufacturers of RF equipment. Routing single ended signals into and out of differential circuits, on the other hand, introduces problems that must be overcome.
One known type of device for combining differential signals into a single ended output signal is referred to in the art as a “balun” (balanced input/unbalanced output). A balun is often used when it is desired to couple a balanced system or device to an unbalanced system or device, or vice versa. A typical example is the coupling of a two-line (balanced) circuit, such as a cellular telephone transmitting circuit, to a single-line (unbalanced) circuit, such as an antenna. Another example is the use of a balun as a signal splitter/phase shifter used with a balanced mixer, wherein a single ended input signal is split into complementary signals that are 180 degrees out of phase with one another.
Conventional baluns are tightly coupled structures fabricated much like a conventional transformer that uses discrete components, e.g. typically comprising transformer-coupled windings on a ferrite core. When implemented as discrete components in modular design approaches, these baluns require a relatively large amount of board space.
In applications that are sensitive to size and accuracy, e.g., wireless mobile telephones, a balun must meet the criteria of compactness, minimum insertion loss and power wastage, and precise 0-180° phase separation. Although prior art baluns are known which accomplish one or two of these objectives, there are no economical solutions which satisfactorily accomplish all three. Using discrete lumped element components, instead of transformers, to generate the complementary 0-180° signals from a single ended source is an effective method, but it requires that the inductor and capacitor elements used in the networks match one another with high accuracy. This design approach argues strongly in favor of an integrated circuit solution wherein element matching can easily be better than 1%.
Frequency Conversion
A mixer is a critical component of radio-frequency (RF) systems. It is usually the first or second de
Kelly David E.
Mohr Jeff T.
Rodgers Paul L.
Wu John C.
Morrison & Foerster / LLP
Nguyen Hai L.
Peregrine Semiconductor Corporation
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