Robust phase-locked loop (RPLL) method proposes the phase frequency and amplitude of frequency varying signals very instantly and robustly. Even in hostile conditions such as variable frequency, phase jump like that this method can approximately estimate the correct data for PLL. SRF-PLL has the simplest structure as compared to all of the techniques discussed in this paper. The only drawback of this method is it assumes the input signal balanced and sinusoidal. There sometimes it suffers an error or has distorted output. These errors can be reduced using a low-pass filter in the loop. The APLL has been considered as operational at 4, 5 and 10 MHz with capability at working at different frequencies at this range. This method has a controlled startup scheme. The design was tested and fabricated in 2007 which resulted in good stability. It is good to make phase-locked loop when there is only two samples to filter. The chances of errors are less; therefore, efficiency is good.
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