Circuit Description; Receiver Circuits - Icom IC-A110 Service Manual

SECTION 3

3-1 RECEIVER CIRCUITS

3-1-1 ANTENNA SWITCHING CIRCUIT
The antenna switching circuit functions as a low-pass filter
while receiving. However, its impedance becomes very high
while D1 and D2 are turned ON. Thus transmit signals are
blocked from entering the receiver circuits. The antenna
switching circuit employs a
The passed signals are then applied to the RF amplifier cir-
cuit.
Received signals are passed through the low-pass filter (L2,
L3, L48, C1–C6, C330, C331). The filtered signals are
λ
applied to the ⁄ type antenna switching circuit (D1, D2).
4
3-1-2 RF CIRCUIT
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 are amplified
at the RF amplifier (Q1) after passing through the RX atten-
uator (D38, L52, R295), bandpass filter (L27, L8, C10,
C164, C165, C21) and two-stage tunable bandpass filters
(the first filter is consisted of D6, L9, C342, C405 and the
second filter is consisted of D37, L49, C23, C406). The
amplified signals are applied to the 1st mixer circuit (Q2,
gate 1) after out-of-band signals are suppressed at the
another two-stage tunable bandpass filters (D7, L10, C28,
C407 and D8, L11, C61, C553).
The tunable bandpass filters (D6–D8, D37) which employ
varactor diodes, track the filters and are controlled by the
PLL IC (IC2) via the tune buffer amplifier (Q79) using
"TUNE" signal. These diodes tune the center frequency of
an RF passband for wide bandwidth receiving and good
image response rejection.
• Receiver construction
118.000–136.975 MHz
D1–D3
Ant SW
LPF
to AF circuit

CIRCUIT DESCRIPTION

λ
⁄ type diode switching system.
4
D38
ATT
BPF
AM
DET
3-1-3 1ST MIXER AND 1ST IF CIRCUITS
The 1st mixer circuit converts the received signal into a fixed
frequency of the 1st IF signal with a PLL output frequency.
By changing the PLL frequency, only the desired frequency
will pass through a crystal filter at the next stage of the 1st
mixer.
The filtered signals from the RF circuit are mixed at the 1st
mixer (Q2) with a 1st LO signal coming from the VCO circuit
to produce a 38.85 MHz 1st IF signal.
The 1st IF signal is applied to a pair of crystal filter [Wide
mode: FI1, Narrow mode: FI2] to suppress out-of-band sig-
nals. The filtered 1st IF signal is applied to the IF amplifier
(Q3), then applied to the 2nd mixer circuit (IC40, pin 16).
3-1-4 2ND MIXER AND 2ND IF CIRCUITS
The 2nd mixer circuit converts the 1st IF signal into a 2nd IF
signal. A 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 is applied to the 2nd
mixer section of the IF IC (IC2, pin 16), and is mixed with the
38.4 MHz 2nd LO signal to be converted into a 450 kHz 2nd
IF signal.
A 2nd LO signal (38.4 MHz) is produced at the PLL circuit by
tripling it's reference frequency (12.8 MHz).
The 2nd IF signal from the 2nd mixer (IC2, pin 3) passes
through a ceramic filter [Wide mode: FL3, Narrow mode:
FI4] to remove unwanted heterodyned frequencies. It is then
amplified at the 2nd IF amplifiers (Q4–Q6).
D6,
D7,
D37 D8
Q1
Tuned Tuned
RF
BPF AMP BPF
"TUNE" signal
IF IF
AMP AMP
Q4,
Q6
Q5
3 - 1
FI1 [Wide],
FI2 [Narrow]
1st mixer
38.85 MHz
Q3
Q2
XTAL
IF
BPF AMP
1st LO signal
79.15–98.125 MHz
from VCO circuit
12.8 MHz signal
from TCXO (X1)
Tripler
3
Q83, Q84
2nd LO signal
38.4 MHz
450 kHz
XTAL
2nd IF
BPF IC
FI3 [Wide],
FI4 [Narrow]