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3V is supplied to TXP of Pin 11 of CN5, RX of pin 10 of CN5 becomes Low (0V), Q12 is turned
on, and the reception circuit operates.
1. The signal received at the antenna is input to pin 5 of antenna
switch IC4 through the low-pass filter.
2. The switching of TX/RX at IC4 are controlled by the voltage of the
transmission circuit. If the voltage of pin 4 of IC4 is Low (0V), and
pin 6 of IC4 is High (3V), it is switched to RX side.
3. The signal from pin 1 is amplified with RF amplifier (Q1) with
which the cascade is connected, and the signal out of the range
is attenuated by the SAW filter (F1).
4. Next, the amplified signal is input to pin 1 of mixer circuit (IC2),
and mixed with 1st local frequency input to pin 3 of IC2.
5. 1st local frequency is oscillated in VCO (V1), amplified by IC3, and
input to the mixer (IC2). Then the original signal becomes 21.3
kHz by this mixing. This signal is called 1st IF (intermediate
frequency) signal. Regarding the oscillation frequency of VCO, it
is controlled by PLL IC (IC5). The reference oscillation of X1
(21.25MHz) and the feed back signal from pin 4 of IC3 is input to
this IC. Then, pin 14 of IC5 outputs the signal to controll VCO (V1)
as a result of the comparison of these two signals to generate
proper VCO frequency. That is, if the VCO frequency is higher
than the ideal value, it is fed back to IC5 and IC5 outputs the
signal to reduce the VCO frequency (If the feed back signal is
lower than ideal value. IC5 outputs the signal to make VCO
frequency higher.). The information necessary for the comparison
process in IC5 is sent from the CPU as serial data.
6. The 1st IF signal output from pin 6 of IC2 passes through the
filter (F2).
7. The 1st IF signal and the 2nd local frequency [reference
oscillation frequency (21.25MHz) of X1] are mixed inside of IC1 (IF
IC). Then 2nd IF frequency (50 kHz) is generated.
8. The 2nd IF frequency passes through the band-pass filter,
amplified by IF Amp., detected by FM, then it is output from pin 15
of IC1 as AF signal.
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