| 摘要 |
PURPOSE:To quicken the switching of the frequency by providing a 2nd phase frequency comparator and a 2nd variable frequency divider in addition to a 1st phase frequency comparator being the component of a phase locked loop, and switching the time constant of a loop filter depending on the output of the 2nd phase frequency comparator. CONSTITUTION:The output of a voltage controlled oscillator 4 is inputted to a 1st variable frequency divider 5, in which the signal is frequency-divided according to a frequency division number represented by a frequency division number 8, the output of a reference frequency oscillator 2 is frequency-divided at a fixed frequency divider 3 and the frequency division outputs are compared by a 1st phase frequency comparator 1. On the other hand, the output of the voltage controlled oscillator 4 is inputted simultaneously to a 2nd variable frequency divider 10, in which the signal is frequency- divided according to a frequency division number represented by a frequency division data input 11 and the result is compared with the output from the reference frequency oscillator 2 being frequency-divided at the fixed frequency divider 3 at the 2nd phase frequency comparator 9. When the deviation of both the outputs is smaller than a prescribed value, the time constant of a loop filter 7 is switched. Thus, the switching timing of the time constant of the loop filter is quickened to reduce the lock time of the phase locked loop. |
