1. Field of the Invention
The invention relates to a wide-range voltage-controlled oscillator, and in particular to a wide-range and low-power consumption voltage-controlled oscillator applied in digital circuits and communication systems with high efficiency and low power consumption.
2. Description of the Related Art
Regardless of whether used in digital circuits or communication systems, a voltage-controlled oscillator plays a key role. In a digital circuit, the voltage-controlled oscillator is used to provide a required clock signal; and in a communication system, the voltage-controlled oscillator can provide an oscillation frequency for serving as a carrier wave or for use of a local oscillator when modulating. Meanwhile, the voltage-controlled oscillator is also a key device for both phase-locked circuits and frequency synthesizers.
In general, an oscillator is formed by an LC charge/discharge circuit. An oscillation frequency is generated by the LC charge/discharge circuit for use of above-stated circuits. Recently, since the integrity of ICs has made great progress, inductors used for ICs are no longer suitable for use in ICs manufacture.
For a better stability of an output frequency from an oscillator of a chip, a ring oscillator is usually adopted because it is hard to control the properties of inductors and the inductors use extra area.
FIG. 1 shows a conventional wide-range phase-locked loop. Reference symbol f.sub.ref represents a reference frequency, f.sub.out represents an output frequency, S.sub.o and S.sub.1 represent selecting signals, V.sub.c represents a control voltage. The reference frequency f.sub.ref is transmitted from an external circuit (not shown) to a phase detector 100. The reference frequency f.sub.ref is converted into the control voltage V.sub.c after passing through a charge pump 102 and a loop filter 104. The control voltage Vc and the selecting signals S.sub.0 and S.sub.1 together are inputted to a wide-range voltage-controlled oscillator 106. At this time, the wide-range voltage controlled oscillator 106 outputs the output frequency f.sub.out to a required circuit (not shown). Furthermore, a divider 108 receives the output frequency f.sub.out coming from the wide-range voltage-controlled oscillator 106 and then outputs a corresponding frequency by dividing the output frequency f.sub.out with a certain multiple. Using the phase detector 100, the corresponding frequency is compared to the reference frequency f.sub.ref thereby to determine whether the output frequency f.sub.out is a required frequency.
FIG. 2 shows a ring voltage-controlled oscillator 200, having an odd number of inverters 202 electrically connected in series. That is, the output terminal of each inverter 202 is electrically connected to the input terminal of its next inverter 220, and the like, and the output terminal of a last inverter 202 is electrically connected to the input terminal of a first inverter, thereby forming a closed loop. With a time delay caused by each inverter 202, an oscillation frequency can be generated. The period of the oscillation frequency is dependent on the entire time delay of the inverters 202.
Moreover, there is a voltage control load electrically connected between the output terminal of each inverter 202 and ground. The voltage control load consists of a voltage control resistor VCR204 and a capacitor C206 electrically connected in series. By adjusting the control voltage V.sub.c on the voltage control resistor VCR204, the charge/discharge time between two adjacent inverters 202 can be changed thereby to control the oscillation frequency. The time delay can be approximately equal to a RC time constant, wherein the C is the capacitance of the capacitor C206 and R is an equivalent resistance of the voltage control resistor VCR204. The equivalent resistance is dependent on the sizes of transistors inside the inverters 202.
The period of the output oscillation frequency is positively proportional to the RC time constant. In other words, the oscillation frequency is inversely proportional to the RC time constant. Therefore, the power consumption of the ring voltage-controlled oscillator 200 can be expressed by: EQU P=C*V.sup.2 *f
Wherein, C is the capacitance of the capacitor C206, V is a power supply voltage and f is an oscillation frequency. If the oscillation frequency f is replaced with the RC time constant, the power consumption can be obtained by: EQU P=K.sub.p *(C*V.sup.2)*(1/RC)=K.sub.p *V.sup.2 /R
Wherein, K.sub.p is a constant, representing the number of the inverters 202. It can be known from the above-stated equation that the power consumption of the ring voltage-controlled oscillator 200 is a constant once the sizes of transistors is decided, and is independent of the oscillation frequency.
Generally, to increase the range of the output frequency of the oscillator, the ring oscillator must generate and transmit a high-frequency clock signal to a frequency synthesizer, and in response to the high-frequency clock, the frequency synthesizer can generate a clock having various frequencies. Furthermore, the wide range of output frequency can also be obtained by increasing the number of the inverters. However, Increasing the number of the inverters results in more power consumption.
As to the stability of the output frequency, the oscillation frequency of the ring oscillator is adjusted during time change in a transient region. Therefore, the ring oscillator will be easily affected by noises and temperatures, resulting in a poor stability on the output signal.