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Basic Wide Range Phase Locked Loop (PLL)

2017-01-08 08:07  
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A phase-locked-loop (PLL) is a servo system, or, in other words, a feedback loop that operates with frequencies and phases. PLL’s are well known to be quite useful (powerful, in fact) in communications systems, where they can pluck tiny signals out of large noises. Here, however, we will discuss a new kind of PLL which cannot work with low-level signals immersed in noise, but has a new set of advantages, instead. It does require a clean noise-free input frequency such as a square wave or pulse train. Here is a schematic drawing:


Basic Wide Range Phase Locked Loop circuit Basic Wide Range Phase Locked Loop (PLL)

The basic circuit shown in Figure has all the functional blocks of a standard PLL. The frequency and phase detection do not consist of a quadrature detector, but of a standard dual-D flip-flop. When the frequency input is larger than F2, Q1 will be forced high a majority of the time, and provide a positive error signal (via CR3, 4, 5, and 6) to the integrator. If F input and F2 are the same, but the rising edges of F input lead the rising edges of F2, the duty cycle of Q1=HI will be proportional to the phase error. Thus, the error signal fed to the integrator will decrease to nearly zero, when the loop has achieved phase-lock, and the phase error between FIN and F2 is zero. Actually, in this condition, Q1 will put out 30 nanosecond positive pulses, at the same time that Q2 puts out 30 nanosecond negative pulses, and the net effect as seen by the integrator is zero net charge. The 30 nanosecond pulses at Q1 and Q2 enable both flip-flops to be CLEARED, and prepared for the next cycle. This phase-detector action is substantially the same as that of an MC4044  Phase-Detector, but the MM74C74  is cheaper and uses less power. It is fast enough for frequencies below 1 MHz. (At higher frequencies, a DM74S74 can be used similarly, with very low delays.)

Source circuit : National Semicondutor Application


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