High-linearity, low-spread variable capacitance array

Details High-linearity, low-spread variable capacitance array High-linearity, low-spread variable capacitance array

1998-05-19

2001-01-30

Kinkead, Arnold (Department: 2817)

Oscillators

With frequency adjusting means

With voltage sensitive capacitor

C331S03600C, C331S179000, C334S055000, C334S078000, C455S197200

Reexamination Certificate

active

06181218

ABSTRACT:

FIELD OF THE INVENTION
The invention is an apparatus and method for providing a high-linearity, low-spread variable capacitance array which can be coupled to a wide variety of electronic circuits. More specifically, the invention provides a variable capacitance array which can be used to modify the operation of an oscillator circuit.
BACKGROUND
The communications industry has continually struggled to provide communication equipment at affordable prices. Such communication equipment can include by way of example, mobile and wireless devices such as radios, cellular telephones, cordless phones and the like. Such devices, however, typically operate within narrow frequency ranges with little tolerance for error.
In order to operate at the desired frequencies, such devices often rely on oscillator circuits. Oscillator circuits vibrate or resonate at desired frequencies. A variety of oscillator circuits exist, some of which use transistor networks, feedback circuits, piezoelectric substances and the like.
Conventional systems typically tune an oscillator by varying the amount of capacitance coupled to the oscillator with what are called variable capacitors. In general, a variable capacitor allows one to select a desired capacitive value from a range of capacitive values. For example, adjusting the capacitive value in a variable capacitor alters the amount of capacitance coupled to the oscillator. As a result, the capacitive value modifies the operating frequency of the oscillator.
In many applications, it is desirable to provide a variable capacitor which can be finely tuned. Fine tuning allows a circuit designer to precisely select the value of the variable capacitor with relatively small incremental adjustments. In many conventional variable capacitors, however, the incremental adjustments may not be linear, that is, an incremental change may not be proportionately similar to the previous or following incremental change.
In addition to fine tuned precision, circuit designers also desire the ability to select from a large range of capacitance values. A variable capacitor which provides a relatively wide range of values is said to have a relatively large dynamic range. The large dynamic range is typically achieved by combining capacitors. The ratio of the largest capacitor to the smallest capacitor is often called the capacitance spread. A high-spread capacitance array combines large capacitance values with small capacitance values. On the other hand, in a low-spread capacitance array, the largest capacitor value is closer in value to the smallest capacitor value.
Other applications often need variable capacitors which are stable. That is, once selected, the capacitance value remains relatively constant. In many conventional variable capacitors, however, the value of the selected capacitance can vary due to changes in temperature, operating parameters and external influences.
One common type of variable capacitor used in the communications industry is a varactor diode. In order to improve the performance of varactor diodes, the communications industry has, in some cases, added circuitry to more accurately control the varactor diodes. Unfortunately, varactor diodes are non-linear, thus fine tuning can be difficult to implement. Furthermore, the dynamic range of a varactor diode is dependent on the power supply voltage. The larger the power supply voltage, the larger the dynamic range. Thus, to provide a large dynamic range requires higher voltages.
In addition, conventional varactor diodes and their control circuits are often more sensitive to power fluctuations, can dissipate more power and can add additional weight, can increase manufacturing costs and can consume additional space. As can be appreciated, designers of electronic circuits and particularly designers of hand-held communications devices, desire to improve operating performance, decrease power dissipation, conserve space and reduce weight.
SUMMARY OF THE INVENTION
The present invention provides a unique apparatus and method which varies the capacitance coupled to a circuit. In one embodiment, the variable capacitance comprises a unique variable capacitance array which can be finely tuned within a wide range of capacitor values.
One embodiment of the invention also provides a variable capacitor with increased stability. Thus, the embodiment reduces the amount the capacitance varies or drifts over time. The increased stability improves performance in so called noisy environments wherein operating parameters often fluctuate. Still further, the increased stability improves performance in low cost systems which may use less reliable components.
One embodiment also reduces the number of components required to provide variable capacitors. For example, some embodiments eliminate components such as varactor diodes. As a result, the invention reduces the cost of manufacturing, reduces weight and reduces power dissipation and provides a large dynamic range when compared to varactor diode systems.
One embodiment implements the variable capacitance array on a semiconductor substrate. Advantageously, the unique variable capacitance array accounts for the parasitic capacitance existing in many semiconductor devices. Parasitic capacitance is extraneous capacitance which occurs in semiconductor devices such as integrated circuits. Unlike most discrete components, an integrated circuit comprises a group of densely packed components on multiple layers of material. The proximity of the components and the configuration of the layers produce extraneous (i.e., parasitic) capacitances.
Parasitic capacitances can affect the accuracy and stability of a variable capacitor. In one embodiment, the variable capacitance array accounts for the parasitic capacitance with a variety of components and design configurations. As a result, when compared to discrete components such as varactor diodes, the integrated circuit embodiment of the variable capacitance array provides high-linearity, low spread, improved response to power supply rejections, while reducing power dissipation, and manufacturing costs.
In one embodiment, a wireless communications device has an operating frequency which can be varied with a variable capacitor. The wireless communications device comprises wireless communications circuitry which is configured to transmit and receive data. The wireless communications device further comprises an oscillator which is in communication with the wireless communications circuitry.
The oscillator is configured to generate a signal with a variable which alters the operation of the wireless communications circuitry. Furthermore, the oscillator is configured to vary the frequency of the signal in response to different capacitance values.
The wireless communications device further comprises a first capacitance module and a second capacitance module which are in communication with the oscillator. The first capacitance module comprises a first capacitance value, a first switch and a first parasitic capacitance value. The second capacitance module is in communication with the first capacitance module and comprises a second capacitance value, a second switch, and a second parasitic capacitance value.
The second capacitance value is different in magnitude than the first capacitance value. Furthermore, the ratio of the second capacitance value to the second parasitic capacitance is similar to the ratio of the first capacitance value to the first parasitic capacitance.
The wireless communications device further comprises a control signal which is coupled to the first and second switches. The control signal is configured to selectively enable the first and second switches in a manner which varies the amount of capacitance applied to the oscillator.
In another embodiment, the wireless communications device further comprises a plurality of first capacitance modules and a plurality of second capacitance modules. In yet another embodiment, the control signal varies the amount of capacitance by approximately two femtofarads. In still another embodiment, the control

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