| 摘要 |
PURPOSE:To eliminate the need to adjust a detection block by composing the circuit of a 2nd LPF which controls the oscillation frequency of a 1st VCO, a detecting circuit which synchronously detects an upper and a lower video signal by using the oscillation output signal of the 1st VCO, etc. CONSTITUTION:A 1st phase comparator 13 and a 1st LPF 14 are provided and the phase of a 2nd VCO 15 is controlled. Its phase control result is fed back to the 1st VCO 12 through a 2nd phase comparator 17 and a 2nd LPF 18 to adjust the phase of the 1st VCO 12. The phase relation between the two inputs of the 1st phase comparator becomes accurately 90 deg. through the operation of the loop and the phase quantities of 45 deg. phase shifting circuits 20 and 21 are added to this phase relation to adjust the phase relation between the two inputs of the detecting circuit 19 to accurately 180 deg.. This circuit can, therefore, detect a video signal without adjusting the free-run frequencies of the VCOs. |
