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SECTION 4
4-1 RECEIVER CIRCUITS
4-1-1 ANTENNA SWITCHING CIRCUIT (RF UNIT)
The antenna switching circuit functions as a low-pass filter
while receiving and as resonator circuit while transmitting.
The circuit does not allow transmit signals to enter receiver
circuits.
Received signals from the antenna connector pass through
the low-pass fileter (L15–L17, C72–C77, C79) and antenna
switching circuit (D11, D12). The filtered signals are then
applied to the RF amplifier circuit (Q13).
4-1-2 RF AND 1ST MIXER CIRCUITS (RF UNIT)
The 1st mixer circuit converts the received signals to a fixed
frequency of the 1st IF signal with a PLL output frequency.
By changing the PLL frequency, only the desired frequency
will be passed through a pair of crystal filters at the next
stage of the 1st mixer.
The signals from the antenna switching circuit are passed
through the tunable bandpass filter (D13, D14) and amplified
at the RF amplifier (Q13). The amplified signals are passed
through another tunable bandpass fileter (D15–D18), and
then applied to the 1st mixer circuit (Q14).
The filtered signals are mixed at the 1st mixer (Q14) with a
1st LO signal coming from the PLL circuit to produce a 21.7
MHz 1st IF signal. The 1st IF signal is passed through a pair
of crystal filters (FI1, FI2) and is then amplified at the IF
amplifier (Q15).
2ND IF AND DEMODULATOR CIRCUITS
R4
AF signal "DET"
CIRCUIT DESCRIPTION
C4
C3
R5
8
7
R3
Active
C2
filter
FM
detector
10
9
C10
R7
C12
C13
X1
450 kHz
4-1-3 2ND IF AND DEMODULATOR CIRCUITS
(MAIN AND RF UNITS)
The 2nd mixer circuit converts the 1st IF signal to a 2nd IF
signal. A double convertion superheterodyne system (which
converts receive signal twice) improves the image rejection
and obtain stable receiver gain.
The 1st IF signal is applied to a 2nd mixer section of the FM
IF IC (MAIN unit; IC1, pin 16). The signal is then mixed with
a 2nd LO signal for conversion into a 450 kHz 2nd IF signal.
IC1 contains the 2nd mixer, limiter amplifier, quadrature
detector and active filter circuits. A 21.25 MHz 2nd LO signal
is produced at the PLL circuit using the reference frequency.
The 2nd IF signal from the 2nd mixer (MAIN unit; IC1, pin 3)
passes through ceramic filters (MAIN unit; FI1 and FI2) to
remove unwanted heterodyned frequencies. It is then ampli-
fied at the limiter amplifier section (MAIN unit; IC1, pin 5)
and applied to the quadrature detector section (MAIN unit;
IC1, pin 10 and 11) to demodulate the 2nd IF signal into AF
signals.
4-1-4 AF CIRCUIT (MAIN UNIT)
AF signals from the FM IF IC (IC1, pin 9) are fed to the
optional voice scrambler unit to demodulate scrambled
audio or are bypassed around the unit via the analog switch
(IC5).
The AF signals (detected signals) are passed through the
analog switch (IC5, pins 8 and 9) and are then applied to the
active low-pass filter (IC4c, pins 9 and 8).
The filtered AF signals are applied to and adjusted audio
level at the [VOL] control (VR unit; R1). The level controlled
signals are passed through another analog switch (IC3, Q3)
which is controlled by "SPSEL" signal from the CPU (IC9,
pin 83). The passed signals are applied to the AF power
amplifier (IC2), and then output to the internal speaker or
[EXT SP] jack.
2nd IF filter
450 kHz
FI1, FI2
5
3
Noise
detector
2nd
Mixer
IF amp.
Noise
comp.
11
12
R6
R1
R4V
4 - 1
MAIN unit
RF unit
17
16
PLL IC
2
IC1
X1
21.25 MHz
IC1 TA31136FN(D)
16
14
"IF" (1st IF signal: 21.7 MHz)
from RF unit, Q15
"NOIS" signal to the CPU (pin 99)
"SD" signal to the CPU (pin 98)
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