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19.2. RX Operation
Circuit Operation:
The receiving signal from antenna ( /4) is switched by the antenna switch (IC4), and led to RF
amplifiers (Q1) to be amplified. Passing through the SAW filter (446 MHz), the signal is mixed
with the 1st local at the 1st mixer (IC2) to produce the 1st IF frequency of 21.3 MHz. Then it is
amplified by IF amplifier and led to IF demodulation circuit (IC1) through crystal filter (21.3
MHz). In IF demodulation circuit, the signal is mixed with 2nd local frequency of 21.25 MHz to
produce the 2nd IF frequency (50 kHz). The FM of 2nd IF is detected by the demodulation circuit
without coils in IC, then the voice output is gained. Also the squelch signal is output by the
noise detection on demodulating. / The local frequency generation circuit consists of VCO (V1)
and PLL IC (IC5). In PLL circuit the signal which is lower than RX frequency by 21.3MHz is
generated as the 1st local frequency. The reference frequency of PLL employs 21.5MHz crystal
with good temperature characteristics. So it is employed as the 2nd local frequency. The data
from CPU is sent to PLL IC through the serial port, and according to this data PLL frequency is
set.
19.3. TX Operation
Circuit Operation:
The frequency generation circuit of transmitting signal consists of VCO (V2) and PLL IC (IC5).
The output from VCO is amplified at the buffer (IC7) and supplied to PLL IC, at the same time it
is further amplified at the buffer (Q11) to drive the final stage output (Q8). At the final stage the
power is amplified up to about 500 mW, and it is output to the antenna which is selected by the
antenna switch. The low-pass filter which decreases the frequencies outside the range is
provided between the antenna switch and antenna. The data from CPU is sent to PLL IC
through the serial port, and according to this data PLL frequency is set as well as the RX
operation.As for the modulation by the voice, the signal processed on CPU board is added to
VCO directly to perform the FM modulation.
19.4. Control of TX and RX
Function:
The types of operation are TX, RX and periodical RX, and they control ON/OFF of the power
supply to circuits. (Q12 and Q14 in RF block and Q13 on the CPU board side.
20. NEW CIRCUIT OPERATION
20.1. RX Operation
RF Board:
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