1. Field of Invention
The present invention pertains to the field of band-pass filters. More particularly, this invention relates to electrically-coupled mechanical band-pass filters for use in multiplexed address systems in memory arrays.
2. Background
Portable consumer devices are becoming increasingly sophisticated and requiring structure to generate and/or utilize digital data in increasingly large quantities and small compact sizes. Digital devices, such as digital cameras, may require at least hundreds of megabytes (MB) of data storage built into or attachable to the camera. To provide for this type of data storage application, future storage memories should be relatively low in cost, extremely compact and have sufficient capacities of around 100 MB to one gigabyte (GB). The storage memory should also be low in power consumption, less than one watt, and have relatively rugged physical characteristics to cope with the portable battery powered operating environment.
For archival storage, data need only be written to the memory once. Preferably the memory should have a short access time, in the order of milliseconds, and a moderate transfer rate, such as 2 MB per second. Preferably, the storage memory should be able to be capable of interfacing with a wide variety of industry standard platforms and modules.
One application for meeting this demand involves the use of write-once cross point memory devices. In cross point memory arrays, a matrix of memory elements are formed, each comprising a fuse or anti-fuse and a diode connected in series. The memory elements are formed by a plurality of semiconductor and insulation layers disposed between orthogonal conductive lines or electrodes.
One application for utilizing write-once cross point memory arrays to provide high density archival storage in portable devices is described in co-pending U.S. patent application No. 09/875,356, filed Jun. 5, 2001, entitled xe2x80x9cWrite-Once Memoryxe2x80x9d, the disclosure of which is incorporated herein by reference. The memory system disclosed therein, referred to as portable inexpensive rugged memory (PIRM), provides high capacity write-once memory at low cost for archival storage. This result is realized in part by avoiding silicon substrates, minimizing process complexity and lowering areal density. The memory system includes a memory module formed of a laminated stack of integrated circuit layers constructed on thin-film plastic substrates. Each layer contains a cross-point diode memory array, and sensing of the data stored in the array is carried out from a separate integrated circuit remotely from the memory module.
Because PIRM memory is relatively inexpensive, users will likely acquire a large number of PIRM modules with a variety of stored content. It is important to be able to fabricate and assemble memory modules in straightforward and relatively inexpensive processes that minimize the need for precision while maximizing information storage density and simplifying addressing, reading and writing functions.
The probable lowest cost method of implementation of the PIRM memory system is one in which the controller and other reusable electronics are either embedded in the consumer product or reside in an adapter that inserts into a memory card slot. The PIRM memory module would connect via a proprietary interface to the controller. A major problem in such a system is that the apparatus to make the connection between the controller and the memory module may involve a large number of connections, on the order of 120 or more, a formidable challenge for compact high-density memory devices. Accordingly address multiplexing is needed to reduce the number of external connections to the memory array, such as time division or frequency division multiplexing.
A system for substantially reducing the interconnects to a memory device using frequency division multiplexing is described in co-pending United States Patent Application Ser. No. 09/894,143, entitled xe2x80x9cMethod For Reducing the Number of Interconnections to PIRM Memory Module,xe2x80x9d the disclosure of which is incorporated herein by reference. In that system the addresses of memory elements are spread over a predetermined frequency spectrum in order to express the multiple memory addresses on a single input/output line. To use such a system the multiple addresses must be accessed by means for separating out the addresses according to their assigned frequencies.
Accordingly, what is needed is a band-pass filter system to enable accessing various addresses of a PIRM memory device in accordance to their unique frequencies. This band-pass filter must be highly sensitive to determine small variations in frequencies in order to maintain a large number of addresses in a relatively small frequency spectrum. Moreover, such a band-pass filter must be very small (micrometer range), easily constructed on a thin-film, roll-to-roll plastic substrate and able to interact with other miniature PIRM-type devices on the substrate.
In the PIRM environment, simple RC band-pass filters are not feasible because of unacceptably high power consumption. Such filters also have an attenuation rate of 20 db per decade, which is too low to provide for the necessary number of signals over a given frequency spectrum needed to address a PIRM memory array. Active filters using transistor elements would likely be too expensive. LCR filters could be designed for PIRM, but the fabrication of inexpensive compact inductors is daunting and does not appear to be practically feasible.
A solution to the above problem might be to utilize a mechanically actuated band-pass filter. An example of an electromechanical band-pass filter is shown in U.S. Pat. No. 3,803,521 (Hetzel), in which electro-magnetic transducers are used to convert electrical energy to mechanical energy or the reverse, so as to filter out the desired frequencies. Another application using electrostatic electromechanical resonators is shown in U.S. Pat. No. 3,686,593 (Zakaria), in which a vibrating reed is driven by electrostatic forces between itself and an output electrode and the output is represented by capacitance changes between the reed and an output electrode. Both the Hetzel and Zarkaria systems were designed in the 1970""s and are clearly outdated by present technologies, such as PIRM. Moreover, the Zakarian circuitry utilizes a variation of the air gap between capacitors to generate the output signal. This approach results in a nonlinear relationship between the force and the displacement, making stable operation difficult.
The present invention provides an electromechanical band-pass filter for use in PIRM memory arrays. The system of the present invention utilizes electrically coupled micro-resonators as filters, implementing micro electro-mechanical systems (MEMS) technology in which large numbers of miniature micro-movers are fabricated on a single substrate. The system of the present invention provides a narrow band-pass filter having a sharp frequency roll-off, typically 40 db per decade, and very high impedance that can be implemented in a simple, low temperature, inexpensive process not involving integrated circuitry. The present invention meets the basic needs of PIRM technology by having very low power consumption, high attenuation outside of the pass-band and utilizing simple fabrication techniques compatible with plastic substrates.
In one embodiment of the preferred invention, an electrically-coupled, mechanical band-pass filter fabricated on a substrate includes first and second masses, each mass being independently movable preferably in at least one direction relative to the substrate and to the other mass. A first spring element has one end attached to the substrate and the other end attached to the first mass.
A second spring element has one end attached to the first mass and the other end attached to the second mass. An input transducer is provided for receiving an input frequency and applying an input force to the first mass, the input force having a force component in the direction of motion of the first mass, the input force being influenced by the input frequency. An output transducer is associated with the first and second masses for providing an output frequency influenced by the relative motion between the first and second masses.
Another embodiment of the present invention involves a method of band-pass filtering the frequencies of an oscillator on a substrate common with first and second masses, each mass being independently movable along in at least one direction, and first and second spring elements. One end of the first spring element is connected to the first mass and the other end of the first spring element is connected to the substrate. One end of the second spring element is connected to the first mass and the other end of the second spring element is connected to the second mass. An input force is applied to the first mass, the input force having a component in the direction of motion of the first mass. The input force is influenced by an input frequency provided by an input transducer. An output frequency is generated that is influenced by the relative motion between the first and second masses by means of an output transducer associated with the first and second masses.
Other aspects and advantages of the present invention will become apparent from the following detailed description, which in conjunction with the accompanying drawings illustrates by way of example the principles of the present invention.