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4.1 RECEIVING CIRCUIT OPERATION:
4.1.1 Antenna Switching Circuit:
A signal received at the antenna connector J1 goes to the antenna switching
circuit composed of pin diode Dl and D2 via the low pass filter.
4.1.2 High Frequency Amplifier Circuit:
After being amplified by transistors RF AMP1 (Q21) a nd RF AMP2 (Q22),the
high frequency signal which is sent from the antenna s witching circuit goes
through BPF to suppress the undesired signals . Then the high frequency signal
is added to the 1st mixer of the next step diode (D15, D16).
A local oscillating signal from PLL unit (134MHz range) is input to this 1st
mixer, mixed with the receiving signal to make the 1st intermediate frequency
(21.6MHz).
4.1.3 1st Intermediate Frequency Amplifier Circuit
After undesired signals have been removed by a pair of crystal filters (FIL101),
the 1st intermediate frequency signal generated in the 1st mixer is amplified a t
the 1st IF AMP (Q30).
4.1.4 2nd Intermediate Frequency Circuit:
The 1st intermediate frequency signal is added to IC307, which is composed of a
local oscillating circuit, a mixer circuit, a li miter amplifier circuit, a
remodulat i on circuit a nd a squelch c ircuit.
The 1st intermediate frequency signal is converted to 455KHz 2nd intermediate
frequency signal after being mixed with 21.145MHz signal whic h is output from
the local oscillating circuit in IC307 and a crystal oscillator (XT A L4 ). Undesired
signals are removed from this signal b y a c era mic filte r ( FIL3), a nd t he s ignal is
subjected to amplification in the limiter amplifier circ uit.
The amplified 2nd intermediate frequency signal is re-modulated by the re-
modulation circuit composed of a circuit in the IC307 and a ceramic
discriminate element (FIL4).
Then it is out put as a low fr e quency sig nal.