Kirisun PT6800, Service Manual

PT6800 Service Manual

4

Figure 4.3 Principle of Power Amplifier and Antenna Switch 

      The modulated RF signal from VCO is amplified in Q2, Q4, 

Q59, Q5, and Q3, and is routed to Q1 for power amplification.   

   Grid bias of Q1 and Q3 is controlled by the APC circuit. 

Through changing the grid bias voltage, the Tx output power 

can be controlled conveniently. 

APC (Automatic Power Control) Circuit 

   R57, R65, and R66 are used to test the power amplification 

current. IC3A is the sampling amplifier for the power 

amplification current. IC3B is the power comparator amplifier.   

      If the Tx output power is too high, the power amplification 

current and IC3A output will increase; IC3B output voltage will 

decrease, so the bias voltage of Q1 and Q3 will also decrease, 

which causes the Tx output power to be lowered, and vice versa. 

Thus the output power of Tx can keep stable under different 

working conditions.   

      MCU can set the power through changing the voltage input 

to IC3B.   

4.5 Principle of Frequency Synthesizer 

Figure 4.4 Frequency Synthesizer 

      The radio adopts PLL type frequency synthesizer. 

   The frequency synthesizer consists of reference oscillator, 

voltage control oscillator (VCO), programmable divider, phase 

comparator, and low pass filter.   

      Tx VCO unit consists of Q6, D2, D4, D5, and D6. D9 is the 

modulation circuit of Tx VCO. 

      Rx VCO unit consists of Q11, D11, D12, D13, and D14.   

      IC2 (ADF4111) is the PLL integrated circuit, which consists 

of programmable reference divider, programmable divider, 

phase comparator, and charge pump. 

      The low pass filter consists of R43 and C96. 

   The reference frequency is provided by X1 (TCXO, 

12.8MHz). 

   The reference frequency from TCXO (Temperature 

Controlled Crystal Oscillator) is divided by the programmable 

reference divider in IC2 to produce reference frequency of 

5KHz or 6.25KHz (determined by the preset channel frequency 

and is controlled by MCU). 

   The oscillation frequency from VCO goes to IC2 where it is 

divided by the programmable divider and is then compared 

with the reference frequency to obtain the error signal. The 

signal is then filtered by a low pass filter, and is routed to VCO 

to change the oscillation frequency of the VCO, enabling the 

frequency to reach the set value. Then the VCO is locked.   

   Unlock detection: When PLL is unlocked, Pin 14 of IC2 

will output the low level signal to MCU. Then MCU prohibits 

the Tx from transmitting and makes an alert tone.   

      Q13 and other relevant circuits are used to switch the low 

pass filters.   

4.6 Audio Processing Circuit 

Figure 4.5 Audio Processing   

      IC6 (AK2346) is a special voice processing chip, which is 

provided with functions of amplification, filtering, 

pre-emphasis, de-emphasis, scrambling, companding, MSK 

MODEM, etc. Most of the radio’s voice processing is done by 

IC6. 

MIC Signal Processing 

   The voice signal coming from the MIC passes through the 

AGC circuit, and goes to IC24. One branch of the signal is 

routed to IC24B (VOX circuit) for amplification, and then goes 

to D35, where it is processed and turned into DC voltage, and 

then sent to MCU for processing. The other branch of the signal 

is routed to IC24A for amplification, and then goes to pin3 of 

IC6, where it is amplified, filtered, companded (optional), 

pre-emphasized, scrambled (optional), and limited, and then is 

output from pin7 of IC6. After that, the resulting signal is sent 

to VCO for modulation together with CTCSS/DCS. The 2T/5T 

signal is input to pin6 of IC6. After being limited and amplified, 

it is then output from pin7 of IC6, and is routed to VCO for 

modulation. 

RX Audio Signal Processing 

      Voice signal output from IC4 is divided into two parts. One 

branch of the signal is routed to the low pass circuit consists of 

IC5 and other components, where it is amplified and filtered (to 

remove the voice signal) to obtain more purified CTCSS/DCS