1. Field of the Invention
This invention relates to methods and apparatus for filtering signals in a subsystem including a power amplifier utilizing a bank of vibrating micromechanical apparatus.
2. Background Art
The need for passive off-chip components has long been a key barrier against communication transceiver miniaturization. In particular, the majority of the high-Q bandpass filters commonly used in the RF and IF stages of heterodyning transceivers are realized using off-chip, mechanically-resonant components, such as crystal and ceramic filters and SAW devices, as illustrated in FIG. 1. Due to higher quality factor Q, such technologies greatly outperform comparable filters implemented using transistor technologies, in insertion loss, percent bandwidth, and achievable rejection. High Q is further required to implement local oscillators or synchronizing clocks in transceivers, both of which must satisfy strict phase noise specifications. Again, as illustrated in FIG. 1, off-chip elements (e.g., quartz crystals) are utilized for this purpose.
Being off-chip components, the above mechanical devices must interface with integrated electronics at the board level, and this constitutes an important bottleneck against the miniaturization of super-heterodyne transceivers. For this reason, recent attempts to achieve single-chip transceivers for paging and cellular communications have utilized alternative architectures that attempt to eliminate the need for off-chip high-Q components via higher levels of transistor integration. Unfortunately, without adequate front-end selectivity, such approaches have suffered somewhat in overall performance, to the point where they so far are usable only in less demanding applications.
Given this, and recognizing that future communication needs will most likely require higher levels of performance, single-chip transceiver solutions that retain high-Q components and that preserve super-heterodyne-like architectures are desirable.
Recent demonstrations of vibrating beam micromechanical (xe2x80x9cxcexcmechanicalxe2x80x9d) resonator devices with frequencies in the VHF range and Q""s in the tens of thousands have sparked a resurgence of research interest in communication architectures using high-Q passive devices as disclosed in the above-noted patent application entitled xe2x80x9cDevice Including A Micromechanical Resonator Having An Operating Frequency and Method of Extending Same.xe2x80x9d Much of the interest in these devices derives from their use of IC-compatible microelectromechanical systems (MEMS) fabrication technologies to greatly facilitate the on-chip integration of ultra-high-Q passive tanks together with active transistor electronics, allowing substantial size reduction.
FIG. 2 illustrates a comparison of MEMS and SAW technologies wherein MEMS offers the same or better high-Q frequency selectivity with orders of magnitude smaller size. Indeed, reductions in size and board-level packaging complexity, as well as the desire for the high performance attainable by super-heterodyne architectures, are principal drivers for this technology.
Although size reduction is certainly an advantage of this technology (commonly dubbed xe2x80x9cRF MEMSxe2x80x9d), it merely touches upon a much greater potential to influence general methods for signal processing. In particular, since they can now be integrated (perhaps on a massive scale) using MEMS technology, vibrating xcexcmechanical resonators (or xcexcmechanical links) can now be thought of as tiny circuit elements, much like resistors or transistors, in a new mechanical circuit technology. Like a single transistor, a single mechanical link does not possess adequate processing power for most applications. However, again like transistors, when combined into larger (potentially, VLSI) circuits, the true power of xcexcmechanical links can be unleashed, and signal processing functions with attributes previously inaccessible to transistor circuits may become feasible.
The Need for High Q in Oscillators
For any communications application, the stability of the oscillator signals used for frequency translation, synchronization, or sampling, is of utmost importance. Oscillator frequencies must be stable against variations in temperature against aging, and against any phenomena, such as noise or microphonics, that cause instantaneous fluctuations in phase and frequency. The single most important parameter that dictates oscillator stability is the Q of the frequency-setting tank (or of the effective tank for the case of ring oscillators). For a given application, and assuming a finite power budget, adequate long- and short-term stability of the oscillation frequency is insured only when the tank Q exceeds a certain threshold value.
Given the need for low power in portable units, and given that the synthesizer (containing the reference and VCO oscillators) is often a dominant contributor to total transceiver power consumption, modern transceivers could benefit greatly from technologies that yield high-Q tank components.
The Need for High Q in Filters
Tank Q also greatly influences the ability to implement extremely selective IF and RF filters with small percent bandwidth, small shape factor, and low insertion loss. As tank Q decreases, insertion loss increases very quickly, too much even for IF filters, and quite unacceptable for RF filters. As with oscillators, high-Q tanks are required for RF and IF filters alike, although more so for the latter, since channel selection is done predominantly at the IF in super-heterodyne receivers. In general, the more selective the filter, the higher the resonator Q required to achieve a given level of insertion loss.
Micromechanical Circuits
Although mechanical circuits, such as quartz crystal resonators and SAW filters, provide essential functions in the majority of transceiver designs, their numbers are generally suppressed due to their large size and finite cost. Unfortunately, when minimizing the use of high-Q components, designers often trade power for selectivity (i.e., Q), and hence, sacrifice transceiver performance. As a simple illustration, if the high-Q IF filter in the receive path of a communication subsystem is removed, the dynamic range requirement on the subsequent IF amplifier, IQ mixer, and A/D converter circuits, increases dramatically, forcing a corresponding increase in power consumption. Similar trade-offs exist at RF, where the larger the number or greater the complexity of high-Q components used, the smaller the power consumption in surrounding transistor circuits.
The Micromechanical Beam Element
To date, the majority of xcexcmechanical circuits most useful for communication applications in the VHF range have been realized using xcexcmechanical flexural-mode beam elements, such as shown in FIG. 2 with clampedxe2x80x94clamped boundary conditions. Although several micromachining technologies are available to realize such an element in a variety of different materials, surface micromachining has been the preferred method for xcexcmechanical communication circuits, mainly due to its flexibility in providing a variety of beam end conditions and electrode locations, and its ability to realize very complex geometries with multiple levels of suspension.
U.S. Pat. No. 6,049,702 to Tham et al. discloses an integrated passive transceiver section wherein microelectromechanical (MEM) device fabrication techniques are used to provide low loss, high performance switches. Utilizing the MEM devices also makes possible the fabrication and use of several circuits comprising passive components, thereby enhancing the performance characteristics of the transceiver.
U.S. Pat. No. 5,872,489 to Chang et al. discloses an integrated tunable inductance network and method. The network utilizes a plurality of MEM switches which selectively interconnect inductance devices thereby providing a selective inductance for a particular circuit.
U.S. Pat. No. 5,963,857 to Greywall discloses an article comprising a micromachined filter. In use, the micromachined filters are assembled as part of a radio to miniaturize the size of the radio.
U.S. Pat. Nos. 5,976,994 and 6,169,321 to Nguyen et al. disclose a batch-compatible, post-fabrication annealing method and system to trim the resonance frequency and enhance the quality factor of micromechanical structures.
U.S. Pat. Nos. 5,455,547; 5,589,082 and 5,537,083 to Lin et al. disclose microelectromechanical signal processors. The signal processors include many individual microelectromechanical resonators which enable the processor to function as a multi-channel signal processor or a spectrum analyzer.
U.S. Pat. No. 5,640,133 to MacDonald et al. discloses a capacitance-based, tunable, micromechanical resonator. The resonators may be selectively tuned and used in mechanical oscillators, accelerometers, electromechanical filters and other electronic devices.
U.S. Pat. No. 5,578,976 to Yao, U.S. Pat. No. 5,619,061 to Goldsmith et al. and U.S. Pat. No. 6,016,092 to Qiu et al. disclose various micromechanical and microelectromechanical switches used in communication apparatus.
U.S. Pat. No. 5,839,062 to Nguyen et al. disclose a MEMS-based receiver including parallel banks of microelectromechanical filters.
U.S. Pat. Nos. 5,491,604 and 5,955,932 to Nguyen et al. disclose Q-controlled microresonators and tunable filters using the resonators.
U.S. Pat. No. 5,783,973 to Weinberg et al. discloses a micromechanical, thermally insensitive silicon resonator and oscillator.
The following articles are of general interest: Nguyen et al., xe2x80x9cDesign and Performance of CMOS Micromechanical Resonator Oscillatorsxe2x80x9d, 1994 IEEE INTERNATIONAL FREQUENCY CONTROL SYMPOSIUM, pp. 127-134; Wang et al, xe2x80x9cQ-Enhancement of Microelectromechanical Filters Via Low-Velocity Spring Couplingxe2x80x9d, 1997 IEEE ULTRASONICS SYMPOSIUM, pp. 323-327; Bannon, III et al., xe2x80x9cHigh Frequency Microelectromechanical IF Filtersxe2x80x9d, 1996 IEEE ELECTRON DEVICES MEETING, San Francisco, Calif., Dec. 8-11, 1996, pp. 773-776; and Clark et al., xe2x80x9cParallel-Resonator HF Micromechanical Bandpass Filtersxe2x80x9d 1997 INTERNATIONAL CONFERENCE ON SOLID-STATE SENSORS AND ACTUATORSxe2x80x9d, pp. 1161-1164.
An object of the present invention is to provide a method and apparatus for filtering signals in a subsystem including a power amplifier utilizing a bank of vibrating micromechanical apparatus to allow the power amplifier to operate with less power consumption.
In carrying out the above object and other objects of the present invention, a method is provided for filtering signals to obtain at least one desired passband of frequencies in a subsystem including a power amplifier to allow the power amplifier to operate with less power consumption. The method includes providing a bank of micromechanical apparatus. Each of the apparatus has at least one passband of frequencies. The method also includes the step of controllably switching the bank of micromechanical apparatus to select the at least one desired passband and to substantially attenuate power outside the at least one desired passband. The method includes the step of vibrating the micromechanical apparatus corresponding to the at least one desired passband.
The subsystem may be an RF transmitter subsystem and the at least one desired passband may be at least one desired channel.
Each of the apparatus may be switchable and tunable.
Each of the devices may have a Q greater than 10 and may be greater than 5000.
The number of devices may number more than 10 and may even number more than 1000.
The apparatus may be devices, filters, circuits or signal processors.
The power amplifier may be a non-linear or low power amplifier.
Further in carrying out the above objects and other objects of the present invention, in a subsystem including a power amplifier, a signal filtering apparatus is provided which filters signals to obtain at least one desired passband of frequencies and also to allow the power amplifier to operate with less power comsumption. The apparatus includes a parallel bank of micromechanical filters. Each of the filters has a passband and is capable of handling amplified signals. The apparatus also includes a controller for selecting at least one desired filter of the bank of filters to thereby provide the at least one desired passband of frequencies.
Each of the filters may have an input connected to a common input and each of the filters has an output connected to a common output.
The controller may include a decoder for controlling application of an appropriate bias voltage to the at least one desired filter.
The apparatus may be a high-Q, high-power filter.
Each of the filters may be switchable and tunable.
The present invention uses MEMS resonators/filters to condition the output/operation of a power amplifier so as to allow it to function with much higher efficiency (and thus, much lower power). For example, MEMS tanks can be used to remove high-order harmonics caused by nonlinearity around a power amplifier, thus allowing it to operate in a nonlinear fashion.
The above object and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.