Icom IC-F33GS Service Manual - Section 5
4 - 1
SECTION 5
CIRCUIT DESCRIPTION
1ST IF CIRCUITS
The 1st IF circuits consist of 1st mixer, 1st IF fi lter and 1st IF
amplifi er, and converts the received signals into the 1st IF
signal, fi lters to remove unwanted signals and amplifi es.
The RF signals from the bandpass filter are mixed with the
1st LO signals, where come from the RX VCO circuit, at
the 1st mixer circuit (Q6) to produce a 46.35 MHz 1st IF
signal. The 1st IF signal is passed through a monolithic filter
(FI1) to suppress out-of-band signals. The filtered signal is
applied to the 2nd IF circuit after being amplified at the 1st
IF amplifier (Q7).
2nd IF AND FM DEMODULATOR CIRCUITS
The 2nd IF circuits consist of 2nd mixer, 2nd IF filter, 2nd
IF amplifier, and converts the 1st IF signal from the 1st IF
circuits into the 2nd IF signal, filters to remove unwanted
signals and amplifi es.
An FM IF detector IC consists of whole of the 2nd IF circuits
and FM demodulator circuit.
The 1st IF signal from the IF amplifier (Q7) is applied to
the 2nd mixer section of the FM IF IC (IC9, pin 16), and is
mixed with the 2nd LO signal to be converted into a 450
kHz 2nd IF signal.
The FM IF IC (IC9) contains the 2nd mixer, limiter amplifier,
quadrature detector, active filter and noise amplifier circuits.
A 2nd LO signal (45.9 MHz) is produced at the PLL circuit
by tripling it’s reference frequency 15.3 MHz).
RX AF CIRCUITS
The AF circuits consist of AF filters, AF amplifier, AF
power amplifi er, etc., and amplify, fi lter the AF signals FM-
demodulated by the FM IF detector IC.
The AF selector changes the destination of the AF signals.
The AF signals from the FM IF IC (IC9, pin 9) are amplified
at the AF amplifier section in the base band IC (IC14, pin
23), and are then applied to the high-pass filter and low-
pass filter section of it.
5-1 RECEIVER CIRCUITS
RF CIRCUITS
RF circuits consist of RF fi lters, antenna switch, preamplifi er,
etc., and extracts and amplifi es the signal which is desired to
receive.
An antenna switch toggles RX line and TX line by PIN
diodes.
While receiving, the TX line and the antenna is disconnected
to prevent received signals entering. The RX line is
disconnected from the GND simultaneously, and an
LPF which guides received signals to the RX circuits is
composed.
While transmitting, serial-connected PIN diodes are ON,
thus the TX line is connected to the antenna, and the RX
line is connected to the GND simultaneously to prevent
transmit signal entering.
The tuned-BPF is tuned to a desired frequency to receive by
variable capacitor, and extracts the signal which is desired
to receive.
The signals from the antenna switching circuit pass through
the two-stage tunable bandpass filters (D19, D24, L7, L8,
C27, C369). The filtered signals are amplified at the RF
amplifier (Q5) and then passed through the another two-
stage tunable bandpass filters (D14, D15, L11, C39, C45) to
suppress unwanted signals. The filtered signals are applied
to the 1st mixer circuit.
Received signals enter the antenna connector (CHASSIS;
J1) and pass through the low-pass filter (ANT unit; L801,
L802, C803). The filtered signals are passed through the 1/4
λ type antenna switching circuit (D701, D704, D706) and
then applied to the RF circuit.
D14, D15, D19 and D24 employ varactor diodes, that are
controlled by the CPU via the D/A converter (IC12), to track
the bandpass filter. These varactor diodes tune the center
frequency of an RF passband for wide bandwidth receiving
and good image response rejection.
Mixer
16
Limiter
amp.
2nd IF filter
450 kHz
X2
15.3 MHz
45.9 MHz
IC9 TA31136FN
12
1st IF from the IF amplifier (Q7)
"RSSI" signal to the CPU ( IC22, pin 50)
11
10
9
8
7
5
AF signal "DET"
"SQLC" signal from the
D/A converter IC
(IC12, pin 2)
R5V
X1
2
Active
filter
Noise
detector
FM
detector
13
"NOIS" signal to the CPU (IC22, pin 75)
RSSI
Noise
amp.
Noise
comparator
×3
Q22
FI2
3
• 2nd IF AND FM DEMODULATOR CIRCUITS