Circuit Description; 1 Pll Frequency Synthesizer; Vco - HYT POWER446 Service Manual

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
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.
Q5
BUFFER
APC/TUNE
U605
1
Q6
1
D100
L101
D104
D101
D102
2SK508NV
L102
D103
12.8MHZ
LPF
1/2 CP OSC
EN
DATA
1/2
32/33 U202
CLK
17bitDIV
21bitDIV
Figure 1
7
POWER446 Service Manual
Q100
2SK508NV
TX-VCO
T/R-SW
Q101
RX-VCO