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D22, D23. The first IF (Immediate Frequency) 21.4 MHz is produced. This IF is passed through a coil T2
and a pair of crystal filter F1, F2 to further filter out other unwanted signals. The first IF then is amplified
by Q11 and the IF amplifier IC7 (BA4116FN). (IC7 is an integrated IF amplifier which consists of a local
oscillator, a demodulator, a second mixer, squelch control circuit, and IF amplifier). The 21.4 MHz IF then
is mixed here with second mixer and converted into 2nd Immediate Frequency (IF) 455 kHz. The 2nd IF
passes through a ceramic filter F4 to filter out the residue unwanted signal at pin 5 of IC7 (BA4416FN)
output this final IF signal and The demodulated AF signal is output at pin 9 of IC7 (BA4416FN).
The demodulated AF signal is amplified by IC8B. the amplified audio then passes through a volume
control VR201 through de-emphasis circuit feeding IC10A, IC10B.and finally amplified by Audio amplifier
IC11 (LA4629) and heard in the speaker.
The squelch control signal also produced by IC7 (BA4116FN), the rectified noise signal is output by Pin
14 of the IC7 then signal is amplified by IC8A. Pass through the network by composition of R45 R38 R44
and C171 sends the digital squelch control signal to the MCU to mute the audio speaker path and to
indicate the RX station on LCD. R214.R217 and VR203 form a variable resistor, which correspond to the
squelch level.
!" Local oscillator PLL (Phase Lock Loop) Circuit:
The receiver and transmitter both share the same PLL (Phase Lock Loop) Circuitry to produce the carrier
frequency or the receiver local oscillation frequency. The local oscillator consists of a fundamental
frequency oscillator Q301 and A phase Lock Loop (PLL) IC16 (TB31202) When Rx 5Vvoltage is supply,
the VCO will produce receiver local oscillator frequency. The high stability frequency is determined by
crystal X2 (20.950 MHz) and as the PLL reference oscillator. This signal is frequency-divided by IC16 and
a 12.5 kHz signal is produced. When the VCO frequency applied to IC16 pin1 and frequency-divided by
IC2 produces a frequency comparable to 12.5 kHz, PLL IC pin 3 will output a PD voltage to control the
VCO. When these two frequencies are matched, a constant control voltage is output from PLL to lock
VCO in desired frequency. Otherwise the PLL IC pin 4 will also output a unlock indication to MCU to
indicate that the PLL is in the frequency unlock state.
!" Memory Backup
IC201 is an EEPROM AT24C64, 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 the memory from the EEPROM to refresh the RAM in MCU IC203.
!" Low Voltage Detection
The battery voltage divided byR250 and R251 through R254 is input to IC203 for voltage level detection.
If the battery voltage drops below 9.5 V, LCD will indicate the battery is in low state.
!" Local/distance circuit
Local/distance circuits are composed of Q16,R195,R94,C125.L17,D17,R93. When local SW is on high
level, receiver sensitive is high otherwise is on low.
!" Wx alert circuit
The 1050Hz tone of receiver through IC6A composed band-pass filter and IC6B amplifier form square
wave signal. To fed IC203 pin 97produce weather alert.
!" Large LCD
All message through series bus from MCU to display driver IC204 and will be displayed through a
1.5"1"×1" in 4×12 characters dot matrix LCD display.
TEKCOM Industries Limited
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