### PLL Oscillator for Medium Wave Application Circuit

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**

**This is the figure of the 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.

scraper site

ReplyDelete