If Shift Circuit; S-Meter Circuit; Squelch Circuit; Demodulator Circuits - Icom IC-706MKIIG Service Manual

3-1-9 IF SHIFT CIRCUIT (MAIN BOARD)
The IF shift circuit shifts the center frequency of IF signals to
electronically shift the center frequency.
The IF shift circuit shifts the 1st LO and BFO within ±1.2 kHz
in SSB/CW/RTTY modes or ±250 Hz in CW-N/RTTY-N
modes. As a result, the 2nd IF (also 1st IF) is shifted from
the center frequency of the 2nd IF filter (FI671, FI681 or
optional IF filters). This means 2nd IF signals do not pass
through the center of the 2nd IF filter. Therefore, the higher
or lower frequency components of the IF are cut out. Since
the BFO frequency is also shifted the same value as the 1st
IF, frequency is corrected at the detector.
In the IC-706MKIIG, the 1st LO frequency is shifted to
change the 2nd IF because a fixed 2nd LO frequency (60
MHz) is used. The 1st IF filter (FI671) and crystal filter
(FI561) have 15 kHz pass-band widths, and do not affect IF
shift operation.
3-1-10 AGC CIRCUIT (MAIN BOARD)
The AGC (Automatic Gain Control) circuit reduces IF ampli-
fier gain to keep the audio output at a constant level. The
receiver gain is determined by the voltage on the AGC line
(Q776 collector).
The 2nd IF signal from the buffer amplifier (Q751) is detect-
ed at the AGC detector (D771) and applied to the AGC
amplifier (IC811b). IC811b sets the receiver gain with the
[RF/SQL] control via the "RFGV" signal line.
When receiving strong signals, the detected voltage increas-
es and the AGC voltage decreases via the DC amplifier
(Q776). The AGC voltage is used for the bias voltage of the
transmit/receive switching PIN diodes (D521, D522, D572,
D573) to attenuate the received signals.
When AGC slow is selected, C775 and R778 are connected
in parallel to obtain appropriate AGC characteristics.
• AGC CIRCUIT
R772 D772
RFGV
(RF gain control)
R773
R774
2nd IF
Q771
AGC det.
signal
C771
D771
C772
8 V
IC811b
Q776, Q777
AGC
FAST
AGC
Amp.
amp.
AGC line
"SML"
S-meter signal
3-1-11 S-METER CIRCUIT (MAIN BOARD)
The S-meter circuit indicates the relative received signal
strength while receiving by utilizing the AGC voltage which
changes depending on the received signal strength.
The output voltage of the AGC amplifier (IC811b, pin 7) is
applied to the main CPU (IC2001, pin 36) as an S-meter sig-
nal via the analog switch (IC2101, pins 5, 3) as the "SML"
signal. The FM S-meter signal from the FM IF IC (IC791, pin
12) is also applied to the analog switch (IC2101, pin 5) via
the meter amplifier (Q774).
The S-meter signal from the main CPU (IC2001) is applied
to the sub CPU and is then displayed on the S-meter read-
out.
3-1-12 SQUELCH CIRCUIT (MAIN BOARD)
The squelch circuit mutes audio output when the S-meter
signal is lower than the [RF/SQL] control setting level.
The S-meter signal is applied to the main CPU (IC2001, pin
36) in SSB/CW/RTTY modes and is compared with the
threshold level set by the [RF/SQL] control. The [RF/SQL]
setting is picked up at the sub CPU (DISPLAY board; IC1,
pin 99). The main CPU compares the S-meter signal and
[RF/SQL] setting, and controls the AF selector switch
(IC861) to cut out AF signals via IC2122a.
In FM mode, a portion of the AF signals from the FM IF IC
(IC791, pin 9) are applied to the active filter section (pin 8)
where noise components above 20 kHz are amplified. The
signals are rectified at the noise detector section and then
output from pin 14. The noise squelch signal from pin 14 is
applied to the main CPU (IC2001, pin 31) via the analog
switch (IC2101, pins 14, 13) as the "NSQL" signal. The CPU
then controls the AF selector switch (IC861).
3-1-13 DEMODULATOR CIRCUITS (MAIN BOARD)
(1) SSB/CW/RTTY modes
The 2nd IF signals from the buffer amplifier (Q751) are
mixed with the BFO signal from the PLL unit at the product
detector (IC841, pin 6). The detected AF signals from IC841
(pin 1) are applied to the AF selector switch (IC861, pin 12).
(2) AM mode
The 2nd IF signals from the buffer amplifier (Q751) are
detected at the AM detector (D761). The detected AF signal
is applied to the AF selector switch (IC861, pin 15).
(3) FM/FM NARROW modes
The 2nd IF signals from the buffer amplifier (Q721) are
applied to the FM IF IC (IC791, pin 16) where the IF signals
are converted into 455 kHz IF signals. The signals pass
through FI791 and are applied to the quadrature detector
section. X791 is used for quadrature detector. The detected
AF signals from pin 9 are then applied to the AF selector
switch (IC861, pin 14) via the de-emphasis circuit (IC811a).
(4) WFM mode
The 2nd IF signals from the IF amplifier (Q611) are applied
to the WFM demodulator circuit (IC631, pins 2, 3) where the
IF signals are converted into AF signals. The detected AF
signals from pin 8 are then applied to the AF selector switch
(IC861, pin 11).
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