Ring oscillator having an externally adjustable variable...

Oscillators – Ring oscillators

Reexamination Certificate

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Details

C331S034000, C331S17700V

Reexamination Certificate

active

06211744

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a ring oscillator and, more particularly, to a ring oscillator having a variable frequency.
2. Description of the Related Art
Ring oscillators are widely used in electronic equipment such as computers, televisions, videocassette recorders (VCRs) and the like. Typically, a ring oscillator comprises a series of discrete components including transistors, capacitors, etc. For a predetermined combination of discrete components, a conventional ring oscillator only produces a fixed frequency. That is, once assembled, the frequency of the oscillating signal generated by a ring oscillator cannot be externally adjusted. The result is that a conventional ring oscillator can only be used in a limited manner in electronic equipment.
Accordingly, a need remains for a ring oscillator having an externally adjustable variable frequency.
SUMMARY OF THE INVENTION
An object of the invention is to overcome problems associated with prior art ring oscillators.
Another object of the invention is to provide a ring oscillator having a variable frequency capable of use in different application circuits. The ring oscillator according to the present invention comprises an oscillating circuit for producing an oscillating pulse signal and a frequency adjusting circuit coupled to the oscillating circuit for varying a frequency of the oscillating pulse signal. The oscillating pulse signal includes a constant duty cycle.
The oscillating circuit comprises a current supply means for supplying a current and a charging/discharging means coupled to the current supply means for generating a first and a second node voltages by charging and discharging a first and second capacitors with the current responsive to an internal voltage. A switching means is coupled to the charging/discharging means for selectively providing either the first or a second node voltage as a switching means output signal responsive to a control signal. An inverting means including a first inverter for inverting the switching means output signal. The frequency adjusting circuit varies the frequency of the oscillating pulse signal by adjusting a charge/discharge time of the first capacitor.
The frequency adjusting circuit comprises a reference voltage means for supplying a reference voltage, a voltage comparator for comparing the reference voltage with an externally input voltage and adjusting the charge/discharge time of the first capacitor, and a current mirror coupled to the differential amplifier for adjusting a charge/discharge current. The frequency adjusting circuit may also comprise a current mirror having first and second NMOS transistors, a third NMOS transistor connected in parallel to the first NMOS transistor, and a first PMOS transistor coupled to the first NMOS transistor.


REFERENCES:
patent: 4570131 (1986-02-01), Lingstaedt
patent: 4723114 (1988-02-01), D'Arrigo et al.
patent: 4904960 (1990-02-01), Izadinia et al.
patent: 5036300 (1991-07-01), Nicolai
patent: 5153534 (1992-10-01), Ruetz
patent: 5175512 (1992-12-01), Self
patent: 5508664 (1996-04-01), Rizzo
patent: 5710527 (1998-01-01), Yaguchi et al.

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