4 - 2
• ABOVE 230 MHz RF CIRCUIT
(230 MHz–550 MHz, EXCEPT 430 MHZ–440 MHz)
The signals from the attenuator circuit pass through the tun-
able bandpass filter (D36, L37, L38, C242, C253, C261).
The filtered signals are amplified at the above 230MHz RF
amplifier (Q40) and are then enter another 2-stage tunable
bandpass filter (D25, D30, C205, C212) to suppress
unwanted signals and improve the selectivity. The filtered
signals are applied to the RF amplifier (Q36). The amplified
signals pass through the 400 MHz RX switching circuit
(D17), and then applied to the 400 MHz 1st mixer circuit
(IC12).
• ABOVE 800 MHz RF CIRCUIT (810 MHz–1000 MHz)
The signals from the attenuator circuit pass through the tun-
able bandpass filter (D37, L39, C243, C254, C260, C269).
The filtered signals are amplified at the above 800 MHz RF
amplifier (Q41) and are then enter another tunable band-
pass filter (D26, D31, L27, L31, C188, C197, C206, C213,
C223) to suppress unwanted signals and improve the selec-
tivity. The filtered signals are applied to the RF amplifier
(Q37). The amplified signals pass through the 400 MHz RX
switching circuit (D18), and then applied to the 1st mixer cir-
cuit (IC12) same as 400 MHz RF circuit.
The tunable bandpass filters (D11–D13, D15) employ var-
actor diodes to tune the center frequency of the RF pass-
band for wide bandwidth receiving and good image
response rejection.
4-1-5 VHF 1ST MIXER CIRCUIT (MAIN UNIT)
The 1st mixer circuit converts the received signal to a fixed
frequency of the 1st IF signal with a 1st LO frequency.
• VHF 1ST MIXER CIRCUIT
The signals from the VHF RF circuit are mixed with the 1st
LO signal at the 1st mixer circuit (Q32) to produce a 46.05
MHz 1st IF signal. The 1st IF signal passes through the RX
switching circuit (D8), and are then applied to the 1st IF cir-
cuit.
• UHF (430 MHz–440 MHz) 1ST MIXER CIRCUIT
The signals from the UHF RF circuit are mixed with the 1st
LO signal at the 1st mixer circuit (Q35) to produce a 46.05
MHz 1st IF signal. The 1st IF signal passes through the RX
switching circuit (D9), and are then applied to the 1st IF cir-
cuit.
• ABOVE 230 MHz (EXCEPT 430 MHz–440 MHz) 1ST
MIXER CIRCUIT
The signals from the above 230 MHz and above 800 MHz
RF circuit are mixed with the 1st LO signal at the 1st mixer
circuit (IC12) to produce a 46.05 MHz 1st IF signal. The 1st
IF signal passes through the RX switching circuit (D10), and
are then applied to the 1st IF circuit.
The output 46.05 MHz 1st IF signals from the each RX
switching circuits is applied to the crystal bandpass filter
(FI3), and then passed through the limiter circuit (D6). The
signal is amplified at the IF amplifier (Q18), and then applied
to the 2nd mixer circuit (IC8).
4-1-7 2ND IF AND DEMODULATOR CIRCUITS
(MAIN UNIT)
The 2nd mixer circuit converts the 1st IF signal to a 2nd IF
signal. A double superheterodyne system (which converts
receive signals twice) improves the image rejection ratio and
obtains stable receiver gain.
The FM IF IC (IC8) contains the 2nd local oscillator, 2nd
mixer, limiter amplifier, quadrature detector, and noise
detector circuits, etc.
The 1st IF signal from the IF amplifier is applied to the 2nd
mixer section of IC8 (pin 16), and is mixed with a 45.6 MHz
2nd LO signal generated by the reference oscillator circuit
(Q2, Q3, D1, X1) to produce a 450 kHz 2nd IF signal.
The 2nd IF signal from the 2nd mixer passes through the
ceramic bandpass filter (passes through FI2 during wide
channel spacing selection, or passes through FI1 during
narrow channel spacing selection), where unwanted signals
are suppressed. The ceramic filters (FI1, FI2) are switched
by IC9 and IC10 that is controlled by “WN_SEL” signal from
the MAIN CPU (IC505, pin 99).
The filtered signal is applied to the AM or FM detector circuit
separately.
W/N selector
FI2
FI1
AM
DET
AM/FM
selector
2nd IF filter
450 kHz
Q13
Q16
IC9,
IC10,
Q21
Noise
detector
Q2,
Q7
Limiter
amp.
FM
detector
Active
filter
AF signals
("DET" signal)
R5
X2 Discriminator
RSSI
Mixer
45.6 MHz
1st IF from the
IF amplifier (Q18)
"RSSI" signal to the CPU
"SQL" signal to the CPU
8
7
5
BPF
3
2
3
16
12
14
11
10
9
IC8
TA31136FN
X1
15.2 MHz
• 2ND IF AND DEMODULATOR CIRCUITS