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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 are passed through the
low-pass filter (L2, L3, C1–C5, C117). The filtered signals are
applied to the
λ
⁄
4
type antenna switching circuit (D1–D4,
L4–L6, C6–C11).
The antenna switching circuit functions as a low-pass filter
while receiving. However, its impedance becomes very high
while D3 and D4 are turned ON (while transmitting). Thus,
transmit signals are blocked from entering the receiver cir-
cuits. The passed signals are then applied to the RF amplifi-
er 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 (Q1) and passed through the bandpass fil-
ter (FI4) 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
(Q3), and is then applied to the 2nd mixer circuit (IC1, 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 (Q3) is applied to the
2nd mixer section in the FM IF IC (IC1, 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 (X2) is used for the 2nd LO signal via the PLL IC
(IC2, pins 16, 17), and is applied to pin 1 of the FM IF IC
(IC1).
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 (IC1, pin 5), and is applied to the quadrature
detector section (IC1, 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; IC1, pin 9) are applied
to the MAIN unit via J300 pin 14.
The AF signal applied to the [VOL] control (MAIN unit; R406)
to control the audio level via the volume mute switch (MAIN
unit; Q316). The level controlled AF signals are applied to the
AF power amplifier (MAIN unit; IC305, pin 2) to drive an inter-
nal speaker (CHASSIS unit; SP1) via the [SP] jack (MAIN
unit; J301).
2nd
Mixer
16
Limiter
amp.
2nd IF filter
450 kHz
PLL IC
IC2
X2
21.25 MHz
X1
(21.25 MHz)
RSSI
IC1 TA31136FN
14
1st IF (21.7 MHz)
from the IF amplifier (Q3)
"SQLIN" signal to the CPU
(MAIN unit; IC301, pin 46)
11
10
9
8
7
5
3
AF signal "AF"
R+3
1
17
16
Active
filter
FI3
Noise
detector
FM
detector
C43
C42
C45
C48
C46
C47
R32
R27
R26
R34
R33
R28
R30
• 2ND IF AND DEMODULATOR CIRCUITS