Icom IC-7400 Service Manual page 10

4-1-4 1ST MIXER AND 1ST IF CIRCUITS (RF UNIT)
The 1st mixer circuit mixes the received signals with the 1st
LO signal to convert the received signals into a fixed 64.455
MHz 1st IF signal.
The signals from the pre-amplifier circuit, or signals which
bypass the pre-amplifier circuit, are applied to the 1st mixer
(Q211, Q212) and mixed with the 1st LO signal (64.485–
238.455 MHz) coming from the 1st LO PLL circuit via the LO
amplifier (Q301) and low-pass filter.
The mixed 1st IF signal is passed through the MCFs (Mono-
lithic Crystal Filter: FI231a/b) to suppress out-of-band sig-
nals. The filtered signal is amplified at the IF amplifier (Q241),
and then applied to the 2nd mixer circuit.
When the bandscope function is activated, the 1st LO
sweeping signal is supplied to the 1st mixer circuit.
4-1-5 2ND MIXER CIRCUIT (RF UNIT)
The 2nd mixer circuit converts the 1st IF signal into the 2nd
IF frequency by mixing with the 2nd LO signal (64.00 MHz).
The 1st IF signal from the IF amplifier (Q241) is applied and
converted into a 455 kHz 2nd IF signal at the 2nd mixer cir-
cuit (D261) by mixing with a fixed 64 MHz 2nd LO signal.
The 2nd IF signal is passed through the low-pass filter (L263,
C262–C264) to suppress undesired signals such as the 2nd
LO signal, and the applied to the MAIN unit via J261.
The 2nd LO signal is a doubled reference frequency, gener-
ated at the reference oscillator (X1901: 32.0 MHz) and dou-
bled at Q1903 and Q1904.
4-1-6 NOISE BLANKER CIRCUIT (MAIN UNIT)
The noise blanker circuit detects pulse-type noise, and turns
OFF the signal line when the noise appears.
The 2nd IF signal from the RF unit is applied to the noise
blanker gate (D252, D253). A portion of the 2nd IF signal
from RF unit is amplified at the noise amplifiers
(Q200–Q203), and is then detected at the noise detector
(D200) to convert the noise components to DC voltages.
The signal is then applied to the noise blanker switch (Q206,
Q207). At the moment detected voltage exceeds Q206's
threshold level, Q207 outputs a blanking signal to close the
noise blanker gate (D252, D253). The PLL unlock signal are
also applied to Q207 to control the noise blanker gate.
Some DC voltage from the noise detector circuit is fed back
to the noise amplifiers (Q200, Q201) via the DC amplifiers
• DSP receiver circuit
IC651b/a
3rd IF
DRIF
Differential
signal
converter
36 kHz
MAIN unit DSP unit
IC301
DSP IC
IC501 CODEC IC
A/D Level
converter converter
4 - 2
(Q204, Q205). The DC amplifiers function as an AGC circuit
to reduce average noise. Therefore, the noise blanker func-
tion shuts off pulse-type noise only.
4-1-7 2ND IF CIRCUIT (MAIN UNIT)
The 2nd IF circuit amplifies and filters the 2nd IF signal, and
applies the 2nd IF signal to the 3rd mixer circuit.
The 2nd IF signal from the noise blanker gate (D252, D253)
is amplified at the 2nd IF amplifier (Q250) and passed
through the ceramic filters (FI271, FI272). The filtered signal
is applied to the 3rd mixer circuit.
4-1-8 3RD MIXER AND 3RD IF CIRCUITS
(MAIN UNIT)
The 3rd mixer circuit mixes the 2nd IF signal with the 3rd LO
signal to obtain the 3rd IF (36 kHz) signal.
The 2nd IF signal from the ceramic filters (FI271, FI272) is
applied to the 3rd mixer circuit (IC280, pin 1). The 3rd LO
signal from the 3rd LO PLL circuit is also applied to the 3rd
mixer (IC280, pin 5). The mixed signal is output from pin 6.
The 3rd IF signal is passed through the low-pass filter
(IC1460a) and amplified at the 3rd IF amplifier (IC1460b).
The filtered and amplified signal is then applied to the DSP
unit via DRIF line.
4-1-9 DSP RECEIVER CIRCUIT (DSP UNIT)
The DSP (Digital Signal Processor) circuit enables digital IF
filter, digital noise reduction, PSN (Phase Shift Network)/
Low Power/Phase demodulation, digital automatic notch,
and etc.
The 36 kHz 3rd IF signal from the 3rd IF amplifier (MAIN
unit; IC1460b) is amplified at the differential amplifiers
(IC651a/b), and is then applied to the A/D converter section
in the CODEC IC (IC501). At the same time, the converted
signal is level-shifted 5V to 3.3 V in the IC (IC501).
The level-shifted signal is applied to the DSP IC (IC301) for
the digital IF filter, demodulator, automatic notch and noise
reduction, etc.
The output signal from the DSP IC is applied to the D/A con-
verter section in the CODEC IC (IC501) to convert into the
analog audio signals. Also the signals are level-shifted 3.3 V
to 5V at the level converter section in the IC (IC501).
The level-shifted audio signals are passed through the
active filter (IC701a), and then applied to the MAIN unit via
J901 (pin 17) as the DRAF signal.
D/A Level
converter converter
IC701a
AF
DRAF
Aactive
signals
filter
MAIN unit