PLL oscillator is used to get very stable frequency woth very low distortion of sine wave output. The stability is equal to crystal-based oscillator, and the low distortion level of the sine wave output is equal to LC based oscillator. The method of PLL control in PLL oscillator is done by adjusting the frequency of sine wave VCO (voltage controlled oscillaltor). The VCO is an LC based oscillator, but the capacitor is not a fixed one, but it’s a varactor, a variable capacitor which its capacitance value is variable depends on the applied voltage. The control of this voltage change the frequency of the VCO. This voltage control is coming from a phase difference detector that detect the phase difference between the crystal reference clock and the VCO output. If the frequency of the VCO output is different then the error signal from the phase difference detector will try to correct the VCO until the VCO has the same frequency and the phase difference is slipped at certain level to produce the voltage control that maintain the VCO frequency.
Multiplying PLL Oscillator’s Output Frequency
To get the frequency higher than the PLL oscillator’s internal reference clock, we just need to insert a frequency divider between the VCO and the phase difference detector. Using this method, now the PLL control system will maintain the divided VCO output frequency to be same with the reference, so the PLL Oscillator that come directly from the VCO has the frequency of the reference frequency multiplied by the division value of the programmed divider.
PLL Oscillator Circuit
Simple PLL oscillator circuit can be constructed using MC145106 IC, and it need only external VCO to produce a sine wave output. The crystal frequency is 4608 kHz for European medium wave frequency spacing standart, and 5120 for USA. The power supply must be regulated and well decoupled.