1. Field of the Invention
The present invention relates generally to electrical circuits and, more particularly, to voltage-controlled oscillators.
2. Related Art
Frequency synthesizers, such as a multi-rate fractional-N frequency synthesizer, are well known. In a typical frequency synthesizer, a frequency adjustment is required when the maximum or minimum frequency of the voltage-controlled oscillator (VCO) is not sufficient to synthesize the desired frequency. For example, a low-frequency gain VCO should have its center frequency adjusted close to the target value. Otherwise, the limited voltage range of the VCO control voltage is insufficient to compensate for variations in frequency due to such factors as temperature, supply voltage variations, and aging.
A common technique to adjust the center frequency of a VCO on an integrated circuit is to laser trim certain circuit elements, such as a capacitor or a resistor, that control the VCO frequency to adjust the VCO center frequency to within the frequency-capture range. One drawback of physical trimming is that the process is expensive and time consuming because the process must be performed on each integrated circuit. Another drawback is that physical trimming permanently limits the range of frequencies. For example, the target frequency of a programmable frequency synthesizer will vary depending on the application. If the VCO frequency is adjusted by trimming to the center frequency of the frequency range that the synthesizer ID supports, the low-frequency gain VCO will not be capable of varying its frequency to the extent necessary to reach the limits of its frequency-range due to its limited control voltage range. Thus, physical trimming permanently constrains the range of frequencies that can be synthesized to less than what could is possibly be generated by the synthesizer.
A VCO frequency auto-calibrating system is disclosed herein. The VCO frequency auto-calibrating system performs frequency self-calibration of its VCO by use of a VCO frequency range detector, monotonicity check circuit, and a lock detector. The VCO frequency range detector detects when the VCO frequency is out of fine-tuning range and provides the appropriate frequency adjustment. The lock detector and the monotonicity check circuit determine when lock has occurred and provide the appropriate VCO frequency adjustment using the VCO""s coarse control voltage so that the VCO""s fine control voltage is approximately at zero volts (i.e., the VCO""s fine-tuning frequency range is centered approximately around a target frequency). Physical trimming is thus eliminated by use of the on-chip trimming circuitry that adjusts the VCO frequency when it is out of range by executing adjustments based on the signals received from the VCO frequency range detector.
In accordance with one embodiment of the present invention, a voltage-controlled oscillator frequency calibration system includes a phase-locked loop circuit having a voltage-controlled oscillator, a frequency range detector, a counter, and a digital-to-analog converter. The frequency range detector determines when a target frequency is out of fine-tuning frequency range and provides a first output signal to tune the voltage-controlled oscillator. The counter counts up or down based on a value of the first output signal from the frequency range detector and provides a counter output signal. The digital-to-analog converter receives the counter output signal and adjusts a coarse-tuning control of the voltage-controlled oscillator based on a value of the counter output signal.
The system may further comprise a lock detector that provides a first output signal indicating if the phase-locked loop circuit is locked. A shift register provides a plurality of clock pulses when the first output signal from the lock detector is asserted. A monotonicity check circuit receives at least a first clock pulse from the plurality of clock pulses from the shift register and provides a first output signal to the counter to count up or down to adjust the coarse tuning control of the voltage-controlled oscillator to approximately center the fine-tuning frequency range of the phase-locked loop circuit around the target frequency.
In accordance with another embodiment of the present invention, a method of calibrating a phase-locked loop circuit includes monitoring the phase-locked loop circuit to determine if a target frequency is within a fine-tuning frequency range; adjusting a coarse tuning of the phase-locked loop circuit if the target frequency is outside of the fine-tuning frequency range; monitoring the phase-locked loop circuit to determine if the fine-tuning frequency range is centered around the target frequency; and adjusting the coarse tuning of the phase-locked loop circuit if the fine-tuning frequency range is not centered approximately around the target frequency.
A more complete understanding of embodiments of the present invention will be afforded to those skilled in the art, as well as a realization of additional advantages thereof, by a consideration of the following detailed description of one or more embodiments. Reference will be made to the appended sheets of drawings that will first be described briefly.