and RF amplifier. The 21.4 MHz IF then is mixed here with second mixer and converted into 2nd
Immediate Frequency (IF) 450 kHz. The 2nd IF passes through a ceramic filter F5 to filter out the
residue unwanted signal at pin 5 of U1 (BA4116) output this final IF signal and the Audio signal is
output at pin 9 of U1 (BA4116).
The audio signal is fed through a volume control VR3 and finally amplified by Audio amplifier U11
(NJM2070) and heard in the speaker.
The squelch control is also controlled by U1 (BA4116). The second IF passes through U1 (BA4116)
internal squelch control R90, C141, C142 form as a squelch amplifier. The ceramic filter produces a
squelch signal (RF noise). Pin 14 of U1 sends the digital squelch control signal to the CPU mute the
audio speaker path. Pin 12 of U1 output a RSSI level to the CPU.
z Low Voltage Detection
The battery voltage, divided by R148, R149 is input to U7 Pin 11 for voltage level sample.
z PLL (Phase Lock Loop) Circuit:
The receiver and transmitter both share the same PLL (Phase Lock Loop) Circuit to produce the
carrier or the receive frequency. The local oscillator consists of a fundamental frequency oscillator X2
20.95MHz and U6 (GP214D). A phase Lock Loop (PLL) U6 (KB8825), TX VCO Q14 and RX VCO
Q13. The fundamental frequency is frequency divided by U6 and a 12.5 kHz signal is produced.
When the VCO frequency applied to and frequency divided by U6 produces a frequency comparable
to 12.5 kHz, PLL will control the VCO. When these two frequencies are matched, a constant control
voltage is output from PLL to lock VCO in desired frequency. The PLL also will output a lock
indication to MCU to indicate the PLL is in frequency lock state.
z Memory Backup
U14 is an EEPROM AT24C16 which acts as a memory backup for the working channel code and the
system parameters. Every time when the unit is switched on, the MCU will reset the system, clear
the RAM, and recall in the memory from the EEPROM to refresh the RAM in CPU U7.
z ATIS(Automatic Transmitter Identification System) only for EU
ATIS circuit consists of U5(1/4),U4(4/4) and CPU U7, when receiving the AF signal from U1 Pin 9 via
U5(1/4) then pass to CPU U7 Pin 43, when transmitting AITS PWM signal form U7 Pin 45 output then
via U4(4/4) pass to TX VCO Varicap D11 then mixes with the carrier to form the modulated signal.
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