Icom IC-2SA Service Manual page 7

ЗЕСТІОМ 4
CIRCUIT DESCRIPTION
4-1 RECEIVER CIRCUITS
4-1-1 ANTENNA SWITCHING CIRCUIT
(MIN AND APC UNITS)
Received signals enter the antenna connector and pass
through a low-pass filter (L2~L4, C21~C25). Тһе signals
are applied to the antenna switching circuit (D7, D9, L5,
L6, and C26~C28) and then to the RF UNIT via the RFIN
signal line. The antenna switching circuit employs a
two-stage A/4-type diode switching system.
The antenna switching circuit functions as a low-pass
filter while in receiving and becomes very high impedance
while in transmitting.
4-1-2 RF CIRCUIT (RF UNIT)
The signals from the antenna switching circuit pass
through a bandpass filter (L1, D2), and are applied to
the RF amplifier (Q1, Q2).
Amplified signals are reapplied to the other bandpass
filter (L2, L3, D4, D5) to suppress unwanted signals.
The signals are applied to the 1st mixer circuit (MIN
UNIT Q1).
D2, D4 and D5 are varactor diodes that track the band-
pass filters and are controlled by the PLL lock voltage.
These diodes tune the center frequency of the bandpass
filters for wide bandwidth reception and good image
response rejection.
4-1-3 1ST MIXER CIRCUIT (MIN UNIT)
The signals from the RF circuit are mixed with the 1st LO
signal from the PLL UNIT to produce a 30.875 MHz 1st
IF signal.
RECEIVER CIRCUIT BLOCK DIAGRAM
Ы NDER
ke Tana
ІСІ, IC2
СРУ ü BUSY
DET UNIT
4-1-4 1ST IF CIRCUIT (MIN AND DET UNITS)
After passing through the matching circuit (L1), the 1st IF
signal is applied to a pair of crystal filters (F11) to suppress
out-of-band signals. The 1st IF signal enters the DET
UNIT and is amplified at the IF amplifier (Q2) and then
applied to the 2nd mixer circuit.
4-1-5 2ND IF AND DEMODULATOR
CIRCUITS (DET UNIT)
The 1st IF signal from Q2 is applied to the 2nd mixer
section of IC1, and is mixed with the 2nd LO signal to be
converted to a 455 kHz 2nd IF signal.
IC1 contains the 2nd mixer, local oscillator, limiter
amplifier and quadrature detector circuits. Тһе local
oscillator section and X2 generate 30.42 MHz for the
2nd LO signal.
The 2nd IF signal from the 2nd mixer (IC1, pin 4) passes
through the ceramic filter, Fl1, where unwanted signals
are suppressed. Itis then amplified at the limiter amplifier
section (IC1, pin 6) and applied to the quadrature detector
section (IC1, pin 10 and ceramic discriminator X1) to
demodulate the 2nd IF signal into an AF signal.
AF signal output from IC1 pin 11 is applied to the squelch
circuit and de-emphasis circuit (R7, C24, C25). This de-
emphasis circuit is an integrated circuit with frequency
characteristics of —6 dB/octave. The resulting signal is
applied to the AF amp, optional tone squelch and optional
DTMF decoder circuits.
30.42 MHz
PLL UNIT