Receiver Circuits - Icom IC-A24 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, L45, C3–C7, C210, C211). The filtered signals are
λ
applied to the / type antenna switching circuit (D8, D9).
4
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, L70,
C58, C60). The amplified signals are applied to the
1st mixer circuit (Q12) after out-of-band signals are
suppressed at the another tunable bandpass filter
(D14–D16, D44, L22, L23, C70–C79).
Varactor diodes are employed at the bandpass filters
(D13–D16, D44) that track the filters and are controlled by
the CPU (LOGIC unit; IC1) via the expander IC (IC17) using
bandpass filter control voltages (T1–T4). These 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 (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.
• RF AND IF CIRCUIT
ANT
LPF
AF sigunal
to the AF circuit
AM/FM
1
Switch
IC2
CIRCUIT DESCRIPTION
λ
/ type diode switching system.
4
D8, D9 D13
ANT RF
BPF
AMP
SW
AM signal
D18 Q18
AM
ANL
DET
6
7
9
FM signal
The signals from the RF circuit are mixed at the 1st mixer
(Q12) with a 1st LO signal (AM; 77.95–106.925 MHz,
FM; 131.6–133.225 MHz) coming from the VCO circuit to
produce a 30.05 MHz 1st IF 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
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, quadrature detector and s-meter detector circuit,
etc.
The 1st IF signal from the 1st IF amplifier (Q13) 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 cir-
cuit using the reference frequency (29.6 MHz) to produce a
450 kHz 2nd IF signal.
The 2nd IF signal from the 2nd mixer (IC1, 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 amplified again at the 2nd IF ampli-
fiers (Q16, Q17) and applied to the AM detector (Q18) to
demodulate 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).
Q11 D14–D16 Q12
BPF
1st Mixer
Q16, Q17
2nd lF
AMP
FM
DET
5
IC1
4 - 1
Fl1 lC1
Q13
16
XTAL
1st lF
BPF AMP
3
2nd lF
BPF
AMP
Q15 FI2
2nd Mixer