FIG. 9 is a basic circuit diagram of a phase locked loop (PLL) circuit 30 using a VCO 20. In the VCO 20, an oscillation frequency fo of an output signal line 42 can be changed by a control voltage Vc input, that is, a control voltage line 46. Generally, in the PLL circuit 30, a phase frequency comparator 26 detects a phase difference between a signal (fo/N) obtained by dividing the oscillation frequency fo output from the VCO 20 by N with a variable frequency divider 22 and a signal (fi/D) obtained by dividing a reference frequency fi input from an input signal line 44 by D with a variable frequency divider 24. To eliminate this phase difference, the control voltage Vc is fed back to the VCO 20 through a charge pump 32.
In this feedback loop, however, if an initial value of control voltage Vc of the VCO 20 or an initial oscillation frequency of the VCO 20 is far from a target frequency ft whose minimum value is ft_L and whose maximum value is ft_H, it will cause a problem that a frequency of an output signal does not correspond with that of an input signal, that is, a problem of unlocking of the PLL. Therefore, a range of Vc (the minimum value is Vclamp_L and the maximum value is Vclamp_H) is controlled in a clamp circuit 34 or the like so that an initial value of the control voltage Vc is not far from the predetermined range. In a conventional method, voltage is corrected in a charge pump 32 so as to oscillate in the middle frequency of the predetermined frequency range when control voltage Vc is the middle of its voltage range.
However, as shown by a characteristic curve in FIG. 10, even if voltage is calibrated only with the charge pump 32, a clamp range (Vclamp_L.about.Vclamp_H) of the control voltage Vc does not necessarily cover a whole range of target frequency ft (ft_L.about.ft_H). In the circuit, only the charge pump 32 in the feedback loop can control oscillation frequency fo, therefore, if a characteristic curve of the VCO 20 goes into a too-fast zone or to too-slow zone due to such as process variation, VCO 20 can not oscillate in the required frequency.
As shown by a characteristic curve in FIG. 10, if the VCO 20 has a nonlinear characteristic, the VCO 20's response can be changed according to its operating point. Therefore, the locking time of the PLL varies widely. Moreover, the clamp voltage can be changed by itself. Therefore, considering all these factors, it is necessary to set the most suitable oscillation characteristic of the VCO 20 by adjusting an operating point at the maximum or the minimum frequency (ft_L, ft_H) of the target frequency ft.
Therefore, it is an object of the present invention to calibrate an oscillation frequency versus a control voltage characteristic in a VCO so that the VCO can oscillate in a target frequency securely within a control voltage range.