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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.
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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.
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Large LCD
All message through series bus from MCU to display driver IC401 and will be displayed through a
34
×
52mm 4
×
12 characters dot matrix LCD display.
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Local/distance circuit
Local/distance circuits
are composed of Q16, R195 and R88. When local SW is on high level, receiver
sensitive is high otherwise is on low.
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2nd Receiver
2nd receiver circuit is as figure. RF input is from C101 of main PCB, the received RF signal then passes
through a high RF transformer L505 and is amplified by RF amplifier Q501, L506, L507, L508, C15, C18,
C20 form the band pass filter. Then passes through a high RF transformer L509 is mixed with the first local
oscillation frequency by the mixer D501, D502, D503, D504. The first IF (Immediate Frequency) 17.9 MHz
is produced. This IF signal is passed through a transformer coil L510 and a pair of crystal filter F1, F2 to
further filter. The first IF then is amplified by Q503 and the IF amplifier IC501. IC501 is an integrated IF
amplifier which consists of a
2
nd
local oscillator (18.355Mhz),a demodulator, a second mixer, squelch
control circuit, and IF amplifier circuits). The 17.9MHz 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 F503 to
filter out the residue unwanted signal at pin 5 of IC501 output this final IF signal and The demodulated FSK
signal is output at pin 9 of IC501 through R517 C543 pass J501.J4 fed in IC15 pin 13 of the main PCB.
The squelch control is also controlled by IC501. The audio signal passes through the low pass filter R514,
R515, C541 and IC501 internal squelch control R516, and C542 that form as a squelch amplifier to
produce a squelch signal (noise) from pin11 of IC501 output, this signal is then amplified by Q505 and
regulated by D505 to produce a direct current voltage as a control voltage then it is fed throughR526and
pin14 to IC501 from pin13 output the busy signal.
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WX Alert Circuit (only USA)
The 1050Hz tone of receiver through IC6A composed band-pass filter and IC6B amplifier form square
