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4 - 1
SECTION 4 CIRCUIT DESCRIPTION
4-1 RECEIVER CIRCUITS
4-1-1 ANTENNA SWITCHING CIRCUIT (RF UNIT)
Received signals from the antenna connector are passed
through the low-pass filter (L5, L6, C8–C12). The filtered sig-
nals are applied to the
λ
⁄4 type antenna switching circuit (D7,
D101, D102, L4, L206, C209, C210).
The antenna switching circuit functions as a low-pass filter
while receiving. However, its impedance becomes very high
while D101 and D102 are turned ON (while transmitting).
Thus transmit signals are blocked from entering the receiver
circuits. The passed signals are then applied to the RF ampli-
fier circuit.
4-1-2 RF CIRCUIT (RF UNIT)
The RF circuit amplifies signals within the range of frequen-
cy coverage and filters out-of-band signals.
The signals from the antenna switching circuit are amplified
at the RF amplifier (Q3) and passed through the bandpass fil-
ter (FI1) to suppress out-of-band signals. The filtered signals
are applied to the 1st mixer circuit (Q2).
4-1-3 1st MIXER AND 1st IF 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 desired signals will be
passed through a crystal filter at the next stage of the 1st
mixer.
The signals from the bandpass filter are mixed at the 1st
mixer circuit (Q2) with a 1st LO signal coming from the VCO
circuit to produce a 21.7 MHz 1st IF signal.The 1st IF signal
is applied to a crystal filter (FI2) to suppress out-of-band sig-
nals. The filtered 1st IF signal is applied to the IF amplifier
(Q1), then applied to the 2nd mixer circuit (IC2, pin 16).
4-1-4 2nd MIXER AND DEMODULATOR CIRCUITS
(RF UNIT)
The 2nd mixer circuit converts the 1st IF signal to a 2nd IF
signal. A double conversion superheterodyne system (which
converts receive signals twice) improves the image rejection
ratio and obtains stable receiver gain.
The 1st IF signal from the IF amplifier (Q1) is applied to the
2nd mixer section in the FM IF IC (IC2, pin 16), and is mixed
with the 2nd LO signal to be converted into a 450 kHz 2nd IF
signal.
The FM IF IC contains a 2nd mixer, quadrature detector,
noise amplifier and a limiter amplifier, etc. The PLL reference
oscillator (X1) is used for the 2nd LO signal via the PLL IC
(IC1, pins 11, 9), and is applied to pin 1 of the FM IF IC (IC2).
The mixed 2nd IF signal is output from pin 3 and passed
through the ceramic bandpass filter (FI3) to remove unwant-
ed heterodyne frequencies. It is then amplified at the limiter
amplifier section (IC2, pin 5) and applied to the quadrature
detector section (IC2, pins 10, 11) to demodulate the 2nd IF
signal into AF signals.
4-1-5 AF CIRCUIT (RF AND MAIN UNITS)
AF signals from the FM IF IC (RF unit; IC2, pin 9) are passed
through the high-pass filter (RF unit; Q15, Q16) to remove
CTCSS signals then applied to the MAIN unit via J2 pin 10 as
the “VOL” signal.
The “VOL” signal (AF signals) from the RF unit is applied to
the [VOL] control (MAIN unit; R58) to control the audio level
via the volume mute switch (Q23). The level controlled AF
signals are applied to the AF power amplifier (IC9, pin 2) to
drive an internal speaker (SP1) via the [SP] jack (J1).
• 2nd IF AND DEMODULATOR CIRCUITS
Mixer
16
Limiter
amp.
2nd IF filter
450 kHz
PLL IC
IC1
X1
21.25 MHz
X2
(21.25 MHz)
RSSI
IC2 TA31136FN
14
1st IF (21.7 MHz)
from Q1
"SQL" signal to the CPU pin 59
11109
87 5 3
AF signal "AF"
R+3
1
9
11
Active
filter
FI3
Noise
detector
FM
detector
C74
C75
C76
C67
C77
C78
R51
R52
R53
R57
R58