Phase-locked loop technology

Phase-locked loop technology


As early as 1932 the theory of the phase-locked technology was proposed, but it had not been widely used until 1940s. The full name of it is”Phase-Locked Loop” for short “PLL”. Phase is to achieve automatic control of a negative feedback system, which makes the phase of the oscillator frequency and the phase and frequency synchronization with the input signal

PLL include 3 important part: 

⑴ phase loop (or the phase comparator, denoted as PD or PC): the completion of the phase comparison unit for comparing the input signal and the phase reference signal between the output voltage is proportional to the phase of its two input signals at.

⑵ differential low pass filter (LPF):. is a linear circuit, its function is to filter the output voltage of the phase of the high frequency components from the effects are usually smoothed by the resistors, capacitors or inductors, etc., are sometimes also included. operational amplifier.

Figure 1 is a block diagram of the three parts consisting of the PLL , it works as follows: the input signal of the phase comparator as the standard, its frequency and phase sent from the VCO output signal is compared . If it is detected in any phase ( frequency ) within its working range difference , generates an error signal Ve (t), the phase error signal proportional to the input signal and the VCO output signal , the modulation of the AC component is generally DC level . A low-pass filter to filter out the AC component of the error signal , a signal Vd (t) to control the VCO, forced towards the VCO to reduce the phase / frequency error of the frequency of change of direction , the input reference signal and the VCO output signal any frequency or phase gradually decreases until 0, then we say the loop is locked.

   If the VCO output frequency is lower than the output amplitude of the frequency , the phase comparator positive input reference signal , by filtering to control the VCO, so that the frequency increases until the frequency and phase of the two signals is precisely synchronized . Conversely, if the VCO output frequency is higher than the input reference signal , the phase comparator output will decrease the VCO input frequency locked to the reference signal.

   The following detailed description of the capture process and track its status compared .

   Located in the VCO control signal is not input to the natural oscillation frequency Wo. After power on, if the phase comparator and the input signal frequency Wi Wo is very close to the output of the phase comparator both the beat frequency signal wave , because of the low frequency low- pass filter will pass , and then add as the input to the VCO control voltage , VCO by this beat frequency modulation , the center frequency is still Wo. And immediately return to the FM signal in the phase comparator , a signal at its output a DC component having been through the integral action is taken out of the low-pass filter , coupled to the input of the VCO , so that the shifted center frequency of the VCO . This offset is exactly the direction toward the direction of the input signal frequency Wi moving , so that the phase difference between the output of the comparator beat signal frequency becomes lower and lower , the phase difference of the DC component will be growing . This DC component is gradually increased after the low-pass filter to control the VCO, faster VCO oscillation frequency tends to Wi.

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