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LOCK_DETECTOR

2017-01-08 08:02  
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LOCK_DETECTOR
This PLL lock indicator not only can detect a 'locked or off-of-lock condition, but also even if a single pulse or transition has been missed.When being sampled by the flip-flop, if the V signal leads the input reference signal R and the rising edge of /i is lost, the D signal will remain hign throughout the interval, allowing the flip-flop to be clocked high (Fig. 26-2A). If the R signal leads the V signal when the transition is missed, the rising edge of the Tsignal will trigger the D signal of the phase detector, causing the LED to blink (Fig. 26-2B).A lock detector often is used with a phase-locked loop (PLL) or synthesizer to indicate when the loop is phase-locked with an input signal. This circuit can be helpful, but single cycle skips usu-ally frill go undetected because of the presence of the low-pass filter.