HYT TC-618, Service Manual

Page: 12 / 110

TC-618/626 Service Manual
11
Circuit Description
Realization Methods of Basic Functional Modules
1. PLL Frequency Synthesizer
The PLL circuit mainly provides the local oscillator signal for Rx and RF carri er signal for Tx. It
is composed of VCO and baseband processor, 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 5.0KHz, 6.25KHz, 10.0KHz or 12.5KHz (selectable). Therefore,
the 38.4MHz reference oscillator signal is divided into 5.0KHz, 6.25 KHz, 10.0 KHz or 12.5 KHz
reference frequencies via a fixed counter in the PLL (at U202). Meantime, the signal from the VCO goes
through the buffer Q102 and amplifier Q103, and then it feeds to the PLL (at U202), where it is divided by
a variable frequency divider. In the phase detector (PD) of PLL, the new signal derived from frequency
division is compared with the reference frequencies. After passing through a low-pass filter, the signal
from the phase detector goes to the varactors (D100, D101, D102 and D103) of VCO so as to control its
output frequency.
2) VCO
The VCO section is realized by the oscillator circuit of three-point capacitance.
In Tx mode, the oscillator frequency of VCO is generated by Q100; in Rx mode, it is generated by Q101.
U202 generates, via the phase detector, a control voltage to control varactors (D100 and D101 in Tx
mode; D102 and D103 in Rx mode), making the oscillator frequency of VCO consistent with the preset
frequency of MCU within a wide frequency range.