Low-gain, low-jitter voltage controlled oscillator circuit
First Claim
1. A voltage-controlled oscillator circuit comprising:
- a ring-oscillator comprising a serial chain of inverter circuits to provide an oscillating output signal having a desired frequency;
a first adjustable current source coupled to the ring oscillator inverter circuits to provide biasing current so as to establish oscillation of the output signal at approximately a selected center frequency, wherein the first adjustable current source is disposed within a bias feedback loop; and
a second adjustable current source also coupled to the ring oscillator inverter circuits to adjust the biasing current for tuning the output signal frequency in response to a control input signal.
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Abstract
A low-gain, low-jitter VCO circuit implemented in CMOS provides center frequency adjustment to overcome process variations. Further, noise immunity is improved by using a separate feedback loop to provide the nominal current biasing for the oscillator chain. This feedback loop coarsely sets the center frequency. The actual control of the oscillation frequency is achieved by a second current source, whose output is added to the nominal bias current to provide a total bias to the oscillator. This second current source "fine tunes" the oscillator frequency responsive to a control signal. Because two separate current sources are used, the circuit can realize a high oscillation frequency with a low VCO gain. Another feature provides for adjusting the center frequency in response to a digital input word provided via external pins, or from internal logic or memory. The center frequency thus can be calibrated by measurement at the time of manufacture, or changes later by the end user or by other circuitry.
43 Citations
21 Claims
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1. A voltage-controlled oscillator circuit comprising:
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a ring-oscillator comprising a serial chain of inverter circuits to provide an oscillating output signal having a desired frequency; a first adjustable current source coupled to the ring oscillator inverter circuits to provide biasing current so as to establish oscillation of the output signal at approximately a selected center frequency, wherein the first adjustable current source is disposed within a bias feedback loop; and a second adjustable current source also coupled to the ring oscillator inverter circuits to adjust the biasing current for tuning the output signal frequency in response to a control input signal. - View Dependent Claims (2, 3, 4)
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5. A voltage-controlled oscillator circuit comprising:
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a ring-oscillator comprising a serial chain of inverter circuits to provide an oscillating output signal having a desired frequency; means having a selected loop gain for biasing the inverter circuits so as to establish a nominal center frequency of the output signal; and control means for varying the biasing applied to the inverter circuits so as to tune the frequency of the output signal in response to a control input signal. - View Dependent Claims (6, 7, 8, 9, 10, 11)
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12. A method of controlling an output signal frequency of a ring oscillator comprising the steps of:
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establishing a predetermined nominal center frequency of oscillation of the ring oscillator by providing a bias feedback loop current to the ring oscillator using a first current source; and controllably tuning the output signal frequency by providing a control current to the ring oscillator using a second current source, wherein the second current source but not the first current source is variable in response to a frequency control signal. - View Dependent Claims (13, 14, 15, 16, 17, 18, 19, 20, 21)
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Specification