Receiver Circuits - Icom IC-V85 Service Manual

SECTION 4

4-1 RECEIVER CIRCUITS

• ANTENNA SWITCHING
The antenna switching circuit toggles the receive (RX) line and
transmit (TX) line.
The received signals from the antenna are passed through
a two-staged Low-Lass Filter (LPF; L1, L2, C1−C5) and
then applied to the antenna switch (D1, D2, D8, L15, L58,
C52, C76, C516).
While transmitting, the voltage on the T5V line is applied
to D1, D2 and D8, and these are ON. Thus the TX line is
connected to the antenna. Simultaneously, the RX line is
connected to the GND to prevent transmit signal entering.
While receiving, no voltage is applied to the D1, D2 and
D8, and these are OFF. Thus the TX line and the antenna
is disconnected to prevent received signals entering.
Simultaneously, the RX line is disconnected from the GND
and the received signals are passed through the LPF (L15,
L58, C52, C76, C516).
The fi ltered signals are applied to the RF circuits.
• RF CIRCUITS (RF UNIT)
RF circuits fi lter and amplify the received signals within the
frequency coverage.
The received signals from the antenna switch are passed
through a tuned Bandpass filter (BPF; D9, L16, C81) and
applied to the RF amplifier (Q12) that received signals are
amplified in low NF rate.
The pass band frequency of the tuned BPF (D9, L16, C81)
is adjusted by the tuning voltage "T1" from the D/A converter
(IC10, pin 2), to suppress unwanted signals for high selectivity.
The amplified received signals are passed through the
tuned BPF (D11, D12, L18, C92, C94, C96, C502), limiter
(D72) and the tuned BPF (D65, L57, C505, C507), then
applied to the 1st mixer (Q13).
• 1ST IF CIRCUITS (RF UNIT)
The received signals are converted into the 1st IF signal,
fi ltered and amplifi ed at the 1st IF circuits.
The signals from the RF circuits are applied to the 1st mixer
• 2nd IF AND DEMODULATOR CIRCUITS
24
D/A converter
(IC10)
Quadrature
23
detector
Demodulated signals
to the LOGIC UNIT
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CIRCUIT DESCRIPTION
8
7 5
Filter Noise
amp.
amp.
Limiter
amp.
RSSI
10 11
9
R5V
X3
4 - 1
(Q13) and converted into the fixed frequency of 1st IF signal
by mixing with the 1st Local Oscillator (LO) signals from the
VCO (Q76, D59−D61). By changing the 1st LO frequency,
only the desired frequency will pass through a pair of
crystal filters at the next stage of the mixer. By changing LO
frequency, the frequency of the 1st IF signal is fixed one for
the receiving frequency.
The converted 1st IF signal is passed through the BPF (FI1)
and limiter (D63), then amplified by the 1st IF amplifier (Q14).
The amplified 1st IF signal is applied to the IF IC (IC2, pin 16).
• 2ND IF CIRCUITS (RF UNIT)
The 1st IF signal is converted into the 2nd IF signal and
demodulated.
The 1st IF signal from the 1st IF circuits are converted into
the 2nd IF signal at the 2nd mixer section of the IF IC (IC2),
by mixing with the 2nd LO signal which is generated at
the reference frequency oscillator (X4) after being passed
through the PLL IC (IC19, pins 1, 2) and the tripler (Q81).
The converted 2nd IF signal is output from pin 3 and passed
through the BPF (FI2) to remove the sideband noise, then
applied to the 2nd IF amplifier section in IC2 (pin 5). The
amplified 2nd IF signal is then applied to the FM-detector
section. The demodulated AF signals are output from pin 9
and applied to the AF circuits.
• AF CIRCUITS (LOGIC UNIT)
The demodulated AF signals from the IF IC are amplified
and fi ltered at the AF circuits.
AF signals from the 2nd IF circuits are passed through the
the HPF (IC3, pins 6, 7), and LPF (IC3, pins 13, 14) via the
TX/RX AF switches (IC4, pins 1, 2; 10, 11). The switched
AF signals are passed through the D/A converter (RF UNIT;
IC10, pins 11, 12) for audio level adjustment.
The D/A converter is a level controller IC which adjusts
AF signal level according to the control signals ("DATA,"
"STROBE," "CLOCK") from the CPU (IC8).
The level adjusted AF signals are then applied to the AF power
amplifier (IC23, pin 4) to be amplified to obtain rated audio
output power. The power-amplified AF signals are output from
pin 10, and then drive internal or external speaker.
FI2
45.9 MHz
2nd IF filter
2
3
Noise
detector
Mixer
12 13
16
From the 1st IF amplifier (Q14)
"NOIS" signal to the CPU (LOGIC UNIT: IC8, pin 17)
"SD" signal to the CPU (LOGIC UNIT: IC8, pin 90)
Q81
2 PLL IC
3
(IC19)
1
15.3 MHz
X4
IF IC (IC2)