TP660 Service Manual
3.1.3. Frequency Synthesizer Circuit
Figure 3-4 Frequency Synthesizer Circuit Diagram
16.8MHz
TCXO
U601
2SK72310
LPF
X601
Loop
Filter
TX VCO
RX VCO
LD
CS
DATA
PS
MOD_Q
MOD_I
RF
Frequency synthesizer circuit is composed of VCO and PLL, and it is the kernel module of the whole TRx
system. When transmitting, the circuit supplies accurate carrier frequency; when receiving, it supplies
stable local oscillation signal. The circuit directly influences some of the important functions in the system.
•
PLL Working Principle
The 16.8MHz frequency generated by the reference crystal oscillator enters the frequency divider of PLL
chip, and becomes reference frequency (i.e. step frequency f1). The frequency generated by VCO passes
LPF for filtering out the second harmonic wave and enters PLL chip for frequency division and gets
frequency f2. The frequency f2 compared with f1 on phase difference in phase comparator, producing
continuous pulse current. When the pulse current passes the loop filter, it starts to accumulate RC and
converts to CV voltage. The CV voltage is sent to the VCO varactor, directly controlling and adjusting the
VCO output frequency until CV becomes constant. PLL will be currently locked, and the stable frequency
output from VCO passes two buffer amplifiers before entering the TRx access.
•
The Working Principle of Voltage Controlled Oscillator
The voltage controlled oscillator applies oscillation mode of three point capacitor. It changes the control
voltage of varactor to get different output frequency. Rx VCO is composed of oscillator circuit and Q603.
Tx VCO is composed of oscillator circuit and Q601, supplying carrier wave of TX signal.
3.2. Power Section
Figure 3-5 Power Structure Diagram
Page 6 of 65
Summary of Contents for TP660
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