Tuesday, November 2, 2010

What is Phase Locked Loop (PLL)??

Many kind of information of PLL (Phase Locked Loop) which is mean close circuit with locked by phase. Sometimes it’s a bit difficult of the meaning consume for non expert like us. I’d just understand what PLL is, how to work and application in the circuit after many years later. It cause of my brain is not really good for low accepting maybe. J
I try to share for a simple explanation about PLL. In order to you get a bit comprehension about  how and what for the PLL purpose. The block as like below.
Figure 1. PLL Block Diagram
Works as signal reference for signal detector, usually use X-tal as reference frequency.
Reference Divider:
Divide oscillation reference from X-tal. The purpose is for divide oscillation signal with 128 / 80 / 64 / 50 / 28 / 24 divider. This block can be fixed to the one of the divider in the circuit.
Phase Detector:
This block would detect of different phase between two input and going to supply signal to Charge pump block. Phase detector output is going to 0 if the reference signal is same as feedback signal.
Charge Pump:
This block is working as like DC/DC converter. It works for increase and decrease signal level are received from phase detector. Signal produced are still in frequency signal.
LPF (Low pass filter) is working to make a limitation of signal input. More high frequency received from charge pump, the output will down in level. And it’s use also for changing signal into direct current to the VCO. Because of VCO requirement is DC voltage.
VCO is Voltage Controlled Oscillator. It means this block are works to produce frequency which is controlled by input voltage. Output frequency is proportional with the input.
Programmable divider:
The function of programmable divider is divide of frequency feedback from VCO which is continued to phase detector block. Divider system is programmable via data line as we call I2C. For more explanation of I2C, it would be discuss at next time. The purpose is  divider system can make output VCO is suit to the frequency as we want.
There are a very simple explanation about how PLL’s working. PLL block as usually used in tuning frequency in tuner system television.


  1. This is intersting that you explain then. But I still don't understand how PLL work, flow; bcause of my lack brain to understand it. I just use it into next circuit. thanks

  2. Thanks of your comment.

    This PLL is usually used in mixer block which is applied in television or radio tuner application. it's for matching broadcasting frequency as like PAL, SECAM or NTSC system in television broadcasting. In order to the output frequency is stable in one frequency.
    For an example:
    Channel received is 210.25MHz (BBC channel).
    Broadcasting is PAL. IF (Intermediate Frequency) is 38.9MHz.
    so the VCO is have to be produce 210.25+38.9MHz= 249.15MHz of oscillator.
    the result, PLL block will set and search 249.15MHz at the output.