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PLL Oscillator for Medium Wave Frequency

2016-08-16 06:48  
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What is a PLL oscillator?

PLLstands for phase locked loop, and this means a control method by comparation of the controlled plant or system to a reference clock or oscillator, that the plant is controlled to synchronize the clock.PLLhas been widely implemented in motor control application, to make the motor rotates at very precise speed, even at very precise phase. WhenPLLis implemented to make aPLL oscillator, the common goal is to get very precise oascillation as we seen in thecrystal oscillator, but with very low distortion of sine wave output as we see in LC based oscillator circuit.

pll oscillator circuit schematic diagram

How PLL Oscillator Works

The block diagram of aPLL Oscillatorshould contain a stable reference crystal clock, a phase error detector, and a voltage controlled oscillator (VCO). The phase error detector will detect the difference of the reference phase and the VCO output phase,? and the output will be proportional to the phase difference. This phase difference signal is then used to control the VCO, if the frequency of VCO output is not equal to the reference, the phase error signal will try to correct the VCO frequency by feeding more voltage to the VCO until the phase difference is equal to a level to maintain the VCO frequency to be same with the reference clock.

Frequency Multiplication of PLL Oscillator

By adding a divider between the VCO output and the phase difference detector, now the PLL will maintain the divided VCO frequency to be equal eith the reference, and it means the VCO output will be maintained at X times the reference, where x is the division of the divider inside the loop. Using this way, thePLL oscillatoris capable to obtain very precise frequency steps at the output by programming the divider.

PLL Oscillator Circuit

The schematic diagram of the circuit shown in the above figure is simple since the phase detector and the divider is equipped in a single chip MC145106  IC, and only VCO is required as the extrernal circuit. The crystal (xtal) is 4608 kHz for Europe broadcasting standard, and 5120 kHz for USA. The test point TP at pin 5 gives the center frequency of th crystal, and can be used for calibration purpose.


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