Icom IC-756PROII Service Manual page 8

3-1-5 1ST IF CIRCUIT (RF-A UNIT)
The 1st IF circuit filters and amplifies the 1st IF signal. The
1st IF signal combined at L653 is applied to a pair of MCF
(Monolithic Crystal Filter; FI711a/b) to suppress out-of-band
signals.
The level of converted 1st IF signal is adjusted at the PIN
attenuators (D531–D533, D535 or D631–D633, D635) con-
trolled by the [BAL] controller for the dualwatch function. The
signal is applied to the 1st IF amplifier (Q551 or Q651) and
then combined at L653.
The combined signal passes through the 3 dB attenuator
(R711–R713) and MCFs (FI711a/b). The signal is amplified
at the 1st IF amplifier (Q721). The amplified signal is then
applied to the 2nd mixer circuit.
3-1-6 2ND MIXER CIRCUIT (RF-A UNIT)
The 2nd mixer circuit mixes the amplified 1st IF signal and
2nd LO signal (64.00 MHz) for conversion into the 2nd IF
signal.
The 1st IF signal from the 1st IF amplifier (Q721) is convert-
ed into a 455 kHz 2nd IF signal at the 2nd mixer circuit
(D941).
The 2nd IF signal is applied to the noise blanker gate (MAIN-
A unit) via the J741.
3-1-7 NOISE BLANKER CIRCUIT (MAIN-A 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-A unit is applied to the noise
blanker gate (D112, D116). A portion of the 2nd IF signal
from RF-A unit is amplified at the noise amplifiers
(Q271–Q273, Q279), and is then detected at the noise
detector (D271) to convert the noise components to DC volt-
ages.
The signal is then applied to the noise blanker switch (Q276,
Q278). At the moment the detected voltage exceeds Q276's
threshold level, Q278 outputs a blanking signal to close the
noise blanker gate (D113, D114). The PLL unlock signal are
also applied to Q278, to control the noise blanker gate.
• DSP receiver circuit
IC2291
3rd IF 5
DRIF
6
signal
1
36 kHz 7
T/R switch
MAIN-A unit DSP-A board
"TXS" signal
"TXS" signal
IC2301b/a IC2321 IC2051
Differential A/D Level
converter converter converter
IC2372x
IC2401
13
IC2372y
14
15
12 1
11
10
"TXS" signal
Some DC voltage from the noise detector circuit is fed back
to the noise amplifiers (Q271–Q273) via the DC amplifiers
(Q274, Q275). The DC amplifiers function as an AGC circuit
to reduce average noise. Therefore, the noise blanker func-
tion shuts off pulse-type noise only.
3-1-8 2ND IF CIRCUIT (MAIN-A 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 (D113, D114)
is amplified at the 2nd IF amplifier (Q141) and passed
through the ceramic filter (FI111). The filtered signal is
applied to the 3rd mixer circuit.
3-1-9 3RD MIXER AND 3RD IF CIRCUITS
(MAIN-A UNIT)
The 3rd mixer circuit mixes the 2nd IF signal and the 3rd LO
signal to obtain the 3rd IF (36 kHz) signal.
The 2nd IF signal from the ceramic filter (FI111) is applied to
the 3rd mixer circuit (IC151, pin 1). The 3rd LO signal from
the PLL unit is applied to the 3rd mixer (IC151, pin 5). The
mixed signal is output from pin 6.
The 3rd IF signal is passed through the low-pass filter
(IC201a) and amplified at the 3rd IF amplifier (IC201b). The
filtered and amplified signal is then applied to the DSP-A
board via DRIF line.
3-1-10 DSP RECEIVER CIRCUIT (DSP-A BOARD)
The DSP (Digital Signal Processor) circuit enables digital IF
filter, digital noise reduction, digital 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-A
unit, IC201b) is amplified at the differential amplifiers
(IC2301a/b) after being passed through the T/R switch
(IC2291), and is then applied to the A/D converter (IC2321).
The coverted signal is level shifted 5V to 3.3 V at the level
converter (IC2051).
IC2001 IC2052
Level
DSP IC
converter
IC2372z
5
IC2473
4
LPF
3 1
9
3 - 2
IC2351
D/A
converter
IC2441a IC2381b
Mixer
DRAF
4
HPF
amplifier
7
MAIN-A unit
AF
signals