Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
2000-03-25
2003-12-23
Flynn, Nathan J. (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S288000
Reexamination Certificate
active
06667506
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to the field of floating gate devices and more specifically to a programmable variable capacitor which incorporates a floating gate device.
BACKGROUND OF THE INVENTION
General
Electronic devices perform several functions, including digital, analog and memory. Analog devices fall into many categories with one major category being that of frequency selective devices. Examples include voltage-controlled oscillators (VCO), narrow band tuned amplifiers and resonant tuning circuits. In general, frequency selectivity is performed by circuits comprising inductors and capacitors which are assembled in well known circuit topologies such as to exhibit frequency selective behavior. Examples include band-pass filters and input matching networks.
A key characteristic of resonant circuits is bandwidth, which is the frequency band over which the circuit passes a signal. Bandwidth is often described in both absolute terms (measured in Hz) and in relative terms (measured as a percentage of the center frequency). Both wide and narrow bandwidth circuits find widespread application in modern electronic communication systems. Communication systems are often categorized as wired or wireless, but the issues related to frequency selectivity are similar for both cases. In all cases, optimum performance is obtained when a circuit is tuned to ensure that its center frequency and bandwidth are matched to the center frequency and bandwidth of the application.
Many wired and most wireless communication systems (e.g. radios, a term used herein which is understood to refer not only to radios specifically, but to communication systems in general) are considered narrow band in that the entire allowed spectrum (e.g., in a cellular phone system) is typically no more than a few percent of the center frequency. In such systems, resonant circuits are typically tuned by mechanical techniques to align them to the broadcast frequency. Tuning is required because typical components and manufacturing techniques are generally not precise enough to achieve the desired alignment with the broadcast frequency accurately and inexpensively.
In many communication systems, it may also be necessary to adjust the frequency of individual devices. In the cellular phone example, multiple handsets operate within a single cell and it is often necessary that each phone operate at an assigned frequency (a so-called channel) that can vary from cell to cell and even from call to call. Such frequency agility is typical of wireless systems including AM/FM radios, television (both broadcast and cable), cell phones, pagers, mobile radios and virtually all other modern communications systems. It may also be desirable for a communication system to operate in multiple bands, which currently requires multiple tuned circuits. If a single circuit could be re-tuned, significant cost, weight and power consumption would be realized. These requirements for all forms of frequency agility place numerous requirements on the design and manufacture of the electronic devices performing communications functions.
Tuned Circuits
Tuned circuits exhibit a response that is dependent on the frequency of an applied signal. The simplest tuned circuit is an L-C circuit, a circuit that is well known in the electronics industry. In the absence of any resistance, a pure L-C circuit would respond to a radian frequency of (1/LC)
1/2
, where L is the inductance and C is the capacitance. Hence doubling the value of the capacitance would reduce the center frequency by about 30%. This pure L-C circuit would also have an infinitely high Q. However, including resistance of the inductor, capacitor and wires of a non-ideal, i.e. real or physical, L-C circuit reduces the Q to values typically between 10 and 100.
Many variations on the tuned circuit theme have been used, including multiple components connected in an almost infinite number of topologies. Each topology has a characteristic response, but in general, their key features are center frequency, bandwidth and transition region. In each design, tradeoffs between efficiency (high Q) and bandwidth (typically wider for lower Q) must be tolerated and acceptable compromises determined. In general, a radio's bandwidth is first determined by the system specification, then the highest Q components that are consistent with the system cost and specification are selected. However, since all components have manufacturing variations, tuned circuits usually require adjustment to get them to operate at their designed frequency.
Certain tuned circuits are designed to operate in a narrow segment (channel) of a system's bandwidth. Such circuits are critically important to a radio's performance since they must be much narrower than the overall system and they must be frequency agile. The most common such circuit is the aforementioned VCO. The element within the VCO that actually causes frequency shifting is a variable capacitor, also referred to as a varactor.
Varactors
Frequency agility is usually provided by a circuit which changes frequency in response to an applied voltage, i.e., a circuit often referred to as a voltage controlled oscillator, or VCO. Typically, a VCO circuit includes a component referred to as a varactor (contraction of variable-capacitor) or varicap or voltacap, i.e., a capacitor which changes value in response to an applied voltage. The term varactor will be used herein to refer to all of these types of devices. Presently, many varactors are made from semiconductor materials such as silicon and utilize devices that typically include a p-n junction (e.g., a diode). These devices use the well-known effect that a diode's depletion capacitance decreases as the D-C voltage applied across the p-n junction increases (when applied in a reverse bias condition). While such devices provide the variable capacitance required to adjust the tuning of a resonant circuit, they have numerous drawbacks, including relatively high resistance (hence a low quality factor, Q), large variations in their value of capacitance and large variations in their voltage sensitivity. Nonetheless, these devices are found in most modern radios.
Quality factor, Q, is a ratio of the capacitive effect to the resistive effect with high Q values being desirable. In diode varactors, it is necessary to use highly resistive material to form the variable capacitance, which in turn creates relatively high resistance. In this type of device, Q factors above 10 at 2 GHz are considered good, and are often listed as high-Q devices. Highly resistive material is also highly sensitive to variations in its processing conditions, which in turn causes large variations in the value of capacitance and the change in capacitance per unit applied voltage. In production, a typical high Q varactor design can exhibit a 30-50% variation in its capacitance values from component to component, even though the same materials and manufacturing processes are used to produce the individual components.
These variations in component value result in errors in the frequency of the VCO. These frequency errors are often greater than the entire bandwidth of the system; hence the radio operates incorrectly (and often in violation of license limits). To correct for this error and the combined errors of other critical components, most modern VCO's are tuned to the correct frequency in a labor-intensive, expensive and often mechanical process. For example, it is often necessary to use mechanical tuning capacitors and laser-trimmed capacitors.
SUMMARY OF THE INVENTION
The present invention addresses the above described shortcomings in varactor design and production. The present invention provides a design and method for producing a superior varactor which can be electronically tuned and shipped with improved accuracy and which can be electronically tuned in the assembled circuit to permit for correction of errors due to other components in the circuit. When compared to currently available varactors, the varactor of the present i
Cable James S.
Reedy Ronald E.
Andújar Leonardo
Flynn Nathan J.
Fuierer Marianne
Hultquist Steven J.
Peregrine Semiconductor Corporation
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