Receiver Circuits - Icom IC-F43GT Service Manual

SECTION 5

5-1 RECEIVER CIRCUITS

5-1-1 ANTENNA SWITCHING CIRCUIT (PA UNIT)
The antenna switching circuit functions as a low-pass fi lter
while receiving and a resonator circuit while transmitting.
This circuit does not allow transmit signals to enter the
receiver circuits. Received signals enter the antenna
connector (CHASSIS; J1) and pass through the low-pass
filter (ANT unit; L801, L802, C803, C806). The filtered
signals are passed through the 1/4 type antenna switching
circuit (D701, D704, L712) and then applied to the RF circuit.
5-1-2 RF CIRCUIT (MAIN UNIT)
The RF circuit amplifi es signals within the range of frequency
coverage and fi lters out-of-band signals.
The signals from the antenna switching circuit pass through
the two-stage tunable bandpass fi lters (D19, D20, D24, D25,
L7, L8, C27, C369). The fi ltered signals are amplifi ed at the
RF amplifier (Q5) and then passed through the another
twostage tunable bandpass fi lters (D14, D15, C39, C45) to
suppress unwanted signals. The fi ltered signals are applied
to the 1st mixer circuit. D14, D15, D19, D20, D24 and D25
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.
5-1-3 1ST MIXER AND 1ST IF CIRCUITS (MAIN UNIT)
The 1st mixer circuit converts the received signal into fi xed
frequency of the 1st IF signal with the PLL output frequency.
By changing the PLL frequency, only the desired frequency
passes through a crystal filter at the next stage of the 1st
mixer.
• 2ND IF AND DEMODULATOR CIRCUITS
"SQ LC" signal from the
D/A converter IC
(IC12, pin 2)
AF signal "DET"
CIRCUIT DESCRIPTION
8 7
Noise
amp.
Active
filter
Limiter
amp.
FM
detector
9 10 11
X1
The RF signals from the bandpass fi lter are mixed with the
1st LO signals, where come from the RX VCO circuit via the
low-pass filter (L46, C396, C397), at the 1st mixer circuit
(Q6) to produce a 46.35 MHz 1st IF signal. The 1st IF signal
is passed through a monolithic fi lter (FI1) in order to obtain
selection capability and to pass only the desired signals. The
filtered signal is applied to the 2nd IF circuit after being
amplifi ed at the 1st IF amplifi er (Q7).
5-1-4 2ND IF AND DEMODULATOR CIRCUITS
(MAIN UNIT)
The 2nd mixer circuit converts the 1st IF signal into a 2nd
IF signal. The double-conversion superheterodyne system
(which convert receive signals twice) improves the image
rejection ratio and obtains stable receiver gain.
The 1st IF signal from the IF amplifi er (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, 2nd
local oscillator, limiter amplifi er, 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).
2nd IF filter
450 kHz
FI2
5 3
Noise
detector
Mixer
Noise
comparator
RSSI
12 13
"NOIS" signal to the CPU (IC22, pin 75)
"RSSI" signal to the CPU ( IC22, pin 50)
R5V
5 - 1
X2
Q22
15.3 MHz
45.9 MHz
×3
2
IC9 TA31136FN
16
1st IF from the IF amplifier (Q7)