Circuit Description; Receiver Circuits - Icom IC-A23 Service Manual

SECTION 4

4-1 RECEIVER CIRCUITS

4-1-1 ANTENNA SWITCHING CIRCUIT (RF UNIT)
The antenna switching circuit functions as a low-pass filter
while receiving. However, its impedance becomes very high
while D8 and D9 are turned ON. Thus transmit signals are
blocked from entering the receiver circuits. The antenna
switching circuit employs a ⁄
Received signals are passed through the low-pass filter
(L1–L3, C3–C7). The filtered signals are applied to the ⁄
type antenna switching circuit (D8, D9).
The passed signals are then applied to the RF amplifier cir-
cuit.
4-1-2 RF CIRCUIT (RF 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 are amplified
at the RF amplifier (Q11) after passing through the tunable
bandpass filter (D13, L18, C58, C60). The amplified signals
are applied to the 1st mixer circuit (Q12) after out-of-band
signals are suppressed at the tunable bandpass filter
(D14–D16, L22, L23, C70–C79).
Varactor diodes are employed at the bandpass filters
(D13–D16) that track the filters and are controlled by the
CPU (LOGIC unit; IC1) via the expander IC (IC4) using
bandpass filter control voltage. These diodes tune the cen-
ter frequency of an RF passband for wide bandwidth receiv-
ing and good image response rejection.
4-1-3 1ST MIXER AND 1ST IF CIRCUITS (RF UNIT)
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 signals from the RF circuit are mixed at the 1st mixer
(Q12) with a 1st LO signal (AM; 78.85–107.825 MHz, FM;
132.45–134.125 MHz) coming from the VCO circuit to pro-
duce a 30.05 MHz 1st IF signal.
• RF CIRCUIT
ANT
LPF
AF signal
to the AF circuit

CIRCUIT DESCRIPTION

type diode switching system.
4
D8, D9 D13
ANT
BPF
SW
D18
AM signal
ANL
IC2
AM/FM
Switch
FM signal
The 1st IF signal is applied to a crystal filter (FI1) to sup-
press out-of-band signals. The filtered 1st IF signal is
applied to the 1st IF amplifier (Q13), then applied to the 2nd
mixer circuit (IC1, pin 16).
4-1-4 2ND IF AND DEMODULATOR CIRCUITS
(RF UNIT)
The 2nd mixer circuit converts the 1st IF signal into a 2nd IF
signal. A double conversion superheterodyne system (which
4
converts receive signals twice) improves the image rejection
ratio and obtains stable receiver gain.
The IF IC contains the 2nd local oscillator, 2nd mixer, limiter
amplifier, guadrature detector and s-meter detector circuit,
etc.
The 1st IF signal from the 1st IF amplifier is applied to the
2nd mixer section of the IF IC (IC1, pin 16), and is mixed
with a 29.6 MHz 2nd LO signal generated at the PLL circuit
using the reference frequency (29.6 MHz) to produce a 450
kHz 2nd IF signal.
The 2nd IF signal from the 2nd mixer (IC2, pin 3) passes
through a ceramic filter (FI2) to remove unwanted hetero-
dyned frequencies. The filtered signal is amplified at the IF
amplifier (Q15), and is then applied to the AM detector cir-
cuit or FM detector circuit respectively.
(1) AM DETECTOR CIRCUIT
The amplified signal is then amplified at the 2nd IF amplifiers
(Q16, Q17) and applied to the AM detector (Q18) to demod-
ulate the 2nd IF signal into AF signals.
The demodulated AF signals are applied to the AM/FM
switch (IC2, pin 6) via the ANL circuit (D18).
(2) FM DETECTOR CIRCUIT
The amplified signal is then amplified at the limiter amplifier
(IC1, pin 5) and applied to the quadrature detector (IC1, pins
10, 11) to demodulate the 2nd IF signal into AF signals.
The demodulated AF signals are output from pin 9 of the IC1
and are applied to the AM/FM switch (IC2, pin 7).
Q11 D14, D15, D16 Q12
RF
BPF
AMP
1st Mixer
Q16, Q17
Q18
AM
2nd IF
DET
AMP
FM
DET
IC1
4 - 1
FI1
Q13
XTAL
1st IF
BPF
AMP
2nd IF
2nd IF
BPF
AMP
AMP
FI2
Q15
IC1
IC1
2nd Mixer