Receiver Circuits - Icom IC-F24/S Service Manual Addendum

SECTION 4.

4-1 RECEIVER CIRCUITS

4-1-1 ANTENNA SWITCHING CIRCUIT
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 (L1, L2, L45, C1–C6,
C175). The filtered signals are passed through the
antenna switching circuit (D2, D5, L5) and then applied to
the RF circuit.
4-1-2 RF CIRCUIT
The RF circuit amplifies signals within the range of fre-
quency coverage and filters out-of-band signals.
The signals from the antenna switching circuit pass through
the bandpass filter (D3, D4, D7, D8, L7, L8, C21, C23,
C24). The filtered signals are amplified at the RF amplifier
(Q2) and then passed through the another bandpass filter
(D9, D10, C39, C40, C45) to suppress unwanted signals.
The filtered signals are applied to the 1st mixer circuit.
D3, D4, D7–D10 employ varactor diodes, that are con-
trolled by the CPU via the D/A converter (IC8), 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.
4-1-3 1ST MIXER AND 1ST IF CIRCUITS
The 1st mixer circuit converts the received signal into fixed
frequency of the 1st IF signal with the PLL output fre-
quency. 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
"SQLC" signal from the
D/A converter IC
(IC8, pin 2)
To D/A converter IC
(IC8, pin 1)
AF signal "DET"
to the AF circuit
CIRCUIT DESCRIPTION
λ
1
⁄ type
4
8 7
Noise
AMP
Active
filter
Limiter
AMP
FM
detector
9 10 11
X1
The RF signals from the bandpass filter are mixed with the
1st LO signals, where come from the RX VCO circuit via
the BPF (L12, L38, C49, C304, C305), at the 1st mixer cir-
cuit (Q3) 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 sig-
nal. The filtered signal is applied to the 2nd IF circuit after
being amplified at the 1st IF amplifier (Q4).
4-1-4 2ND IF AND DEMODULATOR CIRCUITS
The 2nd mixer circuit converts the 1st IF signal into a 2nd
IF signal. The 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 (Q4) is applied to
the 2nd mixer section of 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 (IC1) contains the 2nd mixer, 2nd local oscil-
lator, limiter amplifier, quadrature detector, active filter and
noise amplifier circuits. The 2nd LO signal (45.9 MHz) is
produced at the PLL circuit by tripling it's reference fre-
quency (15.3 MHz).
The 2nd IF signal from the 2nd mixer (IC1, pin 3) passes
through the ceramic filter (FI2) to remove unwanted het-
erodyned frequencies. It is then amplified at the limiter
amplifier section (IC1, pin 5) and applied to the quadrature
detector section (IC1, pins 10, 11) to demodulate the 2nd
IF signal into AF signals.
The demodulated AF signals are output from pin 9 (IC1) as
"DET" signal, and are then applied to the AF circuit.
2nd IF filter
450 kHz
FI2
5 3
Noise
detector
Mixer
Noise
comparator
RSSI
12 13
"NOIS" signal to the CPU (IC13, pin 53)
"RSSI" signal to the CPU (IC13, pin 63)
R5V
4 - 1
X2
Q19
15.3 MHz
45.9 MHz
×3
2
IC1 TA31136FN
16
1st IF signal from the IF amplifier (Q4)