HYT POWER446, Service Manual

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POWER446 Service Manual
7
Circuit Description
1 PLL Frequency Synthesizer
The PLL circuit mainly provides the local oscillator signal for RX and RF carrier signal for TX. It is
composed of VCO and the baseband processor IC, allowing frequency tracking and channel switching
under the control of MCU signals.
1) PLL
MB95F108AK
WPFV-GE1
U605
1/2
17bitDIV
32/33
OSC
1/2
21bitDIV
U202
CP
RX-VCO
L102
D103
L101
APC/TUNE
T/R-SW
CLK
DATA
EN
Q101
2SK508NV
D102
BUFFER
Q6
1
2
3
D104
D101
D100
Q100
2SK508NV
12.8MHZ
TX-VCO
LPF
Q5
1
2
3
Figure 1
The step frequency of the PLL circuit is 5.0 KHz, 6.25 KHz, 10.0 KHz or 12.5 KHz. In U202, the 38.4MHz
reference oscillator signal is divided into 5.0 KHz, 6.25 KHz, 10.0 KHz or 12.5 KHz reference
frequencies via a fixed counter. The signal from the VCO enters U202 after processed by the buffer
Q102 and amplifier Q103, and then it is applied to the variable frequency-divider in PLL for division. After
division, the signal is compared with the reference frequency at a phase detector (PD) in PLL. The signal
from the PD passes through a low-pass filter and then is applied to the varactors (D100
、
D101
、
D102 and
D103) of VCO so as to control the output frequency.
2) VCO
The VCO section is realized by the three-point capacitor oscillator circuit.
The oscillation frequency of VCO is generated by Q100 in TX mode and by Q101 in RX mode
respectively.
U202 generates a control voltage via PD to control the varactors (D100 and D101 in TX mode; D102 and
D103 in RX mode), making the oscillation frequency of VCO consistent with the preset frequency of
MCU in a wide frequency range.