Kirisun FM540, Service Manual

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FM540 Service Manual
3.5 Principle of Frequency Synthesizer
Figure 3-4Diagram of Frequency Synthesizer
This radio applies PLL frequency synthesizer.The frequency synthesizer consists of reference oscillator,
voltage controlled oscillator (VCO), programmable frequency divider, phase comparator and low pass filter.
Q6, D1, D4, D5, D6 and other resistor-capacitor components compose TX VCO unit. D8 is the modulation
circuit of TX VCO.
Q12, D4, D16, D17, D18 and other resistor capacitor components compose RX VCO unit.
IC3 (MB15E03) is PLL integrated circuit, which contains programmable reference divider, programmable
divider, phase comparator, charge pump and so on. R54, C113, R55, R57, C117, R51, C109 and so on
compose loop filter.
The reference frequency is offered by X1 (TCXO, 12.8MHz).
The reference frequency from TCXO(temperature controlled crystal oscillator) is divided by the
programmable reference divider in IC3 to generate reference frequency of 5kHz or 6.25kHz(based on the
set channel frequency and controlled by MCU).
The oscillation frequency from VCO is sent to IC3 after being multiplied by second multiplier circuit, and will
then be compared to reference frequency after being divided by programmable divider to get error signal.
The error signal will be filtered by the low pass filter and sent to VCO to change the oscillation frequency.
When the VCO frequency reaches the set value, VCO is locked.
Loss of Lock Detection: when PLL loses its lock, IC3 pin14 outputs low electric level signal to MCU and
MCU controls the transmitter to forbid the transmitting and sound alert. When locked, IC3 pin14 outputs
high electric level.
15
PLL
DATA
LPF
Q12
RX VCO
PLL
DATA
Q1
BUFF AMP
12.8MHz
TCXO
X1
IC3
PLL IC
Q6
TX VCO
Q25,U2
SW
Q2
RF AMP
LPF
MCU
IC12
½ FREQ
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