Oscillators – Solid state active element oscillator – Transistors
Reexamination Certificate
1999-12-13
2001-08-07
Grimm, Siegfried H. (Department: 2817)
Oscillators
Solid state active element oscillator
Transistors
C331S158000, C331S17700V
Reexamination Certificate
active
06271733
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of oscillators, and, more particularly, to an integrated oscillator circuit and associated methods for setting an output frequency.
BACKGROUND OF THE INVENTION
An oscillator circuit generates an output signal at a desired frequency with a specific waveform. Although an oscillator circuit can be used to perform definite functions in different type circuits, such as timing circuits, transmitter/receiver circuits and the like, for example, the desired frequency of the output signal is not typically changed.
There are numerous approaches for an oscillator circuit which uses semiconductor devices, one of which is the Pierce oscillator. The Pierce oscillator includes a crystal, an inverter connected across the crystal, and a respective capacitor connected to a first and second terminal of the external crystal. The inverter provides phase shift and amplification for the oscillating signal. An amplifier is connected either before or after the inverter for providing the output signal which may be used for performing a definite function, such as synchronizing digital logic circuitry. The amplifier has a large gain to accurately detect small changes in the crystal output voltage.
Unfortunately, manufacturing process variations in a metal oxide semiconductor (MOS) integrated oscillator circuit causes frequency dispersion in the output signal. For example, transistor and capacitor process variations cause the desired frequency of the output signal from the integrated oscillator circuit to vary. In addition, frequency dispersion of the integrated oscillator circuit can also be caused by temperature and voltage variations, and aging of the components. Frequency dispersion can be minimized to a certain point in the design of the integrated oscillator circuit. However, some applications require a very small dispersion, such as within 25 part per million (ppm). Consequently, the frequency of the output signal needs to be set or trimmed to the desired frequency.
One approach to setting the frequency of the output signal is to include capacitors having variable capacitance within the integrated oscillator circuit. By including two or more variable capacitors, they can be selectively connected for setting the frequency of the output signal to the desired frequency. For example, U.S. Pat. No. 4,827,226 to Connell discloses a tuning network for an integrated oscillator circuit that includes a variable capacitor, i.e., a varactor, as a shunt element for providing at least one type of adjustment of the oscillating signal. More than one type of adjustment can be provided by including a bank of varactors for each of the shunt elements of the tuning network, in which various individual varactors are selected in binary fashion for setting the output frequency.
Another approach is disclosed in U.S. Pat. No. 5,030,926 to Walden, which is assigned to the current assignee of the present invention. The frequency range over which the output signal of the integrated oscillator circuit is typically adjusted is increased by the addition of a variable capacitor directly to the amplifier within the oscillator circuit. The frequency range can be further increased by selectively connecting an additional pair of variable capacitors to the oscillator circuit.
Unfortunately, selectively connecting variable capacitances within an integrated oscillator circuit has several drawbacks. One drawback is that the control circuit selecting the variable capacitors is relatively complex. Consequently, an increased surface area of the integrated oscillator circuit is necessary to support a large number of components for implementing the complex control circuit. Another drawback is an undesired charge injection condition that results during switching between capacitors if the setting is performed while the integrated oscillator circuit is operating in an electronic system.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an integrated oscillator circuit and associated methods for setting an output frequency without using a complex control circuit.
This and other objects, advantages and features in accordance with the present invention are provided by an integrated circuit comprising an oscillator circuit for generating an output signal having a desired frequency, and comprising a capacitive device having a controllable capacitance value responsive to a control signal. The integrated circuit preferably includes a control circuit for controlling the capacitance value so that the oscillator circuit generates the output signal at the desired frequency.
The control circuit preferably comprises a memory for storing a digital control word, and a control signal generating circuit for converting the digital control word into the control signal for the capacitive device. The control signal generating circuit preferably comprises a pulse width modulation (PWM) circuit having a controllable duty cycle based upon the digital control word. In addition, the control circuit preferably further comprises a filter connected between the PWM circuit and the capacitive device.
Setting the frequency of the output signal to the desired frequency is advantageously performed internal to the integrated circuit via the PWM control signal. The setting may be performed after manufacture to compensate for manufacturing process variations. In addition, the setting may be done on the fly when the integrated circuit is operating in an electronic system to compensate for temperature variations, voltage dispersions and aging of the components within the oscillator circuit.
To set the frequency of the output signal to the desired frequency on the fly, the integrated circuit preferably further comprises a feedback loop between the oscillator circuit and the control circuit. The feedback loop preferably comprises a sensing circuit connected to an output of the oscillator circuit for sensing the frequency of the output signal, and a comparator circuit connected to the sensing circuit for comparing the sensed frequency to the desired frequency. The control signal generating circuit preferably further includes a counter for incrementing or decrementing a value of the digital control word based upon the sensed frequency of the output signal.
Another aspect of the present invention relates to a method for setting a desired frequency of an output signal generated by an oscillator circuit comprising a capacitive device having a controllable capacitance value responsive to a control signal. The method preferably comprises the step of converting a digital control word into the control signal for controlling the capacitance value so that the oscillator circuit generates the output signal at the desired frequency.
The method preferably further comprises the steps of sensing a frequency of the output signal, and comparing the sensed frequency of the output signal to the desired frequency. Based upon the sensed frequency of the output signal, the method preferably further includes the step of incrementing or decrementing a value of the digital control word. The above steps are preferably repeated until the sensed frequency of the output signal corresponds to the desired frequency. The digital control word producing the control signal providing the desired frequency of the output signal is preferably stored in a memory.
REFERENCES:
patent: 4827226 (1989-05-01), Connell
patent: 5030926 (1991-07-01), Walden
patent: 5117206 (1992-05-01), Imamura
patent: 5631609 (1997-05-01), Oka et al.
patent: 5648744 (1997-07-01), Prakash et al.
patent: 5777524 (1998-07-01), Wojewoda et al.
patent: 6100770 (2000-08-01), Litwin et al.
Alzu Ignacio Herrera
Peon Rogelio
Visee Maarten
Walden Robert William
Agere Systems Guardian Corp.
Allen Dyer Doppelt Milbrath & Gilchrist, P.A.
Grimm Siegfried H.
LandOfFree
Integrated oscillator circuit with a memory based frequency... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Integrated oscillator circuit with a memory based frequency..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Integrated oscillator circuit with a memory based frequency... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2542363