PT6808 Service Manual
signal. During the transmission, the power level of VCO’s pin3 becomes low level, and VCO
generates the frequency of 350-390MHz.
The output of VCO is amplified by buffer amplifier (Q16), and then enters to Pin5 of PLLIC.
Meanwhile, the output of VCO is also amplified by buffer amplifier (Q54), and then enters to the
next level according to T/R switch (D25, D24).
PLL IC is consisted of by programmable divider, reference drive, phase comparator and loads.
PLL IC is level N synthesizer, and operates by the signal of 40, 50 or 60 KHz at level 8 channel. The
input signal coming from PLL IC’ pin5 and pin 8 is divided into 50 KHz, and is compared in phase
comparator. The pulse output signal of phase comparator supplies pressure to load, and is sent to DC
signal of LPF after being transformed. The DC signal is sent to pin 1 of VCO and is locked to
maintain the stability of VCO frequency.
DT (pin 75), CP (pin 19) and EP (pin 47) of microprocessor’ outputs PLL data. If the channel is
changed, the radio changes from transmitting to receiving or the voice is reversed, the data is input to
PLL IC.
4.6 Control circuit
The control circuit is consisted of by microprocessor (IC19) and its peripheral circuit. The
microprocessor controls TX-RX unit, the data sending to the display unit and data sending from the
display unit. IC19 performs the following functions:
1) Switch between TX and RX by the PTT output signal.
2) Read system data, group data, frequency data and programming data from memory circuit.
3) Send frequency programming data to PLL.
4) Turn on/off the squelch by the DC voltage of squelch circuit.
5) Control the audio circuit by inputting decoding data.
6) Send audio and encoding data.
4.6.1 Memory circuit
The memory circuit is consisted of by CPU (IC19) and flash memory (IC20). The capacity of the
flash memory is 2M bits, including the radio control program of CPU and operating feature data.
This program can be written by the external equipment.
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Flash memory
Note: The data saved in the flash memory are FPU (KPG-62D CPS) program, security code
(MPT serial number) and firmware programming (user mode, test mode and tuning mode, etc.).
If the flash memory is changed, the data must be re-imported.
