| 摘要 |
PURPOSE:To lock a PCM signal automatically when it is unlocked and preventing it from being unlocked easily afterward by detecting whether the PCM signal is locked or not with a signal from a PLL loop circuit and varying the cutoff frequency of an LPF. CONSTITUTION:A PCM telemeter device inputs an input PCM signal to a phase- locked PLL9 and makes a phase comparison 2 with a signal obtained by dividing 5 the frequency of a voltage-controlled oscillator 4, and the phase difference signal is inputted to an LPF3. The signal from the LPF3 is detected by a lock detector 10 to judge whether the signal is locked or not. When the detector 10 judges that the signal is locked, the resistance value of the resistance R of the LPF3 is increased with time and a capture range CR and a lock range LR are narrowed down to make the signal harder to unlock. If the signal is unlocked owing to an instantaneous break of the input signal, the control over the detector 10 is released, and the resistance value of R is decreased to widen CR and LR, locking the signal speedily. |
