Receiver Circuits - Icom IC-F43GT Service Manual

SECTION 4

4-1 RECEIVER CIRCUITS

4-1-1 ANTENNA SWITCHING CIRCUIT (PA UNIT)
The antenna switching circuit functions as a low-pass filter
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
λ
the ⁄ type antenna switching circuit (D701, D704, L712)
4
and then applied to the RF circuit.
4-1-2 RF CIRCUIT (MAIN 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 pass through
the two-stage tunable bandpass filters (D19, D20, D24, D25,
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, C39, C45) to sup-
press unwanted signals. The filtered 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.
• 2ND IF 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
4-1-3 1ST MIXER AND 1ST IF CIRCUITS
(MAIN UNIT)
The 1st mixer circuit converts the received signal into fixed
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.
The RF signals from the bandpass filter 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 filter (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
amplified at the 1st IF amplifier (Q7).
4-1-4 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 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, 2nd local oscil-
lator, limiter amplifier, quadrature detector, active filter and
noise amplifier circuits. A 2nd LO signal (45.9 MHz) is pro-
duced 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
4 - 1
X2
Q22
15.3 MHz
45.9 MHz
×3
2
IC9 TA31136FN
16
1st IF from the IF amplifier (Q7)