Icom IC-7800 Service Manual Addendum page 347

3-1-5 1ST IF CIRCUIT
(RXPLL-A AND RXPLL-B UNITS)
The 1st IF circuit filters and amplifies the 1st IF signal. The
1st IF signal from the 1st mixer circuit is applied to an MCF
(Monolithic Crystal Filter) to suppress out-of-band signals.
• RX-A OR RX-ONLY CIRCUITS (RXPLL-A UNIT)
The filtered 1st IF signal from the low-pass filter passes
through the one of the three crystal bandpass (roofing) filters
(FI1401: bandwidth is 15 kHz, FI402: bandwidth is 6 kHz or
FI403: bandwidth is 3 kHz) via the FIL selectors (IC1402,
IC1403). The filtered signal is amplified at the 1st IF ampli-
fiers (Q1401–Q1403) via the low-pass filter (L1405, L1406,
C1420, C1421). The amplified 1st IF signal is applied to the
2nd mixer circuit.
• RX-B CIRCUIT (RXPLL-B UNIT)
The filtered 1st IF signals from the low-pass filter passes
through the one of the three crystal bandpass (roofing) filters
(FI1401: bandwidth is 15 kHz, FI1402: bandwidth is 6 kHz or
FI1403: bandwidth is 3 kHz) via the FIL selectors (IC1402,
IC1403). The filtered signal is amplified at the 1st IF ampli-
fiers (Q1401–Q1403) via the low-pass filter (L1405, L1406,
C1420, C1421), and is then applied to the 2nd mixer circuit.
3-1-6 2ND MIXER CIRCUIT
(RXPLL-A AND RXPLL-B UNITS)
The 2nd mixer circuit mixes the amplified 1st IF signal and
2nd LO signal for conversion into the 2nd IF signal.
• RX-A OR RX-ONLY CIRCUITS (RXPLL-A UNIT)
The 1st IF signal from the 1st IF amplifiers (Q1401–Q1403)
is applied to the 2nd mixer circuit (IC1501, pin 4, IC1502, pin
4) respectively. The applied signals are mixed with 2nd LO
signal from the RXPLL-A circuit to convert into 36 kHz 2nd
IF signal.
The 2nd IF signal from the 2nd mixer circuit (IC1501, pin 5)
is amplified at the buffer amplifier (Q1501), and is then
applied to the 2nd IF amplifier (IC1503, pin 3). The signal is
applied to the 90 degrees transphasor (IC1505, pin 6).
• DSP CIRCUIT (RECEIVER)
ADS signal
IC331
5
IF_IN
3rd IF
6
signal
1
36 kHz
7
switch
RXPLL-A/B
units
"AGC DET" signal from
RXPLL-A/B units
"AGC OUT" signal to
RXPLL-A/B units
IC341,
IC371
A/D
converter
IC301
(IC401)
Low-pass
filter
AGC
IC501
The 2nd IF signal from the other 2nd mixer circuit (IC1502,
pin 5) is amplified at the buffer amplifier (Q1502), and is then
applied to the 2nd IF amplifier (IC1504, pin 3). The signal is
applied to the 90 degrees transphasor (IC1505, pin 2).
The signals from the 90 degrees transphasor (IC1505, pins
1, 7) are applied to the compositor (IC1505, pins 9, 10). The
signal is amplified at other 2nd IF amplifier (IC1506), and is
then applied to the DSP-A board to demodulate 2nd IF sig-
nal.
A portion of 2nd IF signal from the compositor (IC1505, pin
8) is applied to the AGC detector circuit.
The 2nd mixer circuit has 2 mixers and 90 degrees
transphasor to improve image rejection ratio.
• RX-B CIRCUIT (RXPLL-B UNIT)
The 1st IF signal from the 1st IF amplifiers (Q1401–Q1403)
is applied to the 2nd mixer circuit (IC1501, pin 4, IC1502, pin
4) respectively. The applied signals are mixed with 2nd LO
signal from the RX-B circuit to convert into 36 kHz 2nd IF
signal.
The 2nd IF signal from the 2nd mixer circuit (IC1501, pin 5)
is amplified at the buffer amplifier (Q1501), and is then
applied to the 2nd IF amplifier (IC1503, pin 3). The signal is
applied to the 90 degrees transphasor (IC1505, pin 6).
The 2nd IF signal from the other 2nd mixer circuit (IC1502,
pin 5) is amplified at the buffer amplifier (Q1502), and is then
applied to the 2nd IF amplifier (IC1504, pin 3). The signal is
applied to the 90 degrees transphasor (IC1505, pin 2).
The signals from the 90 degrees transphasor (IC1505, pins
1, 7) are applied to the compositor (IC1505, pins 9, 10). The
signal is amplified at other 2nd IF amplifier (IC1506), and is
then applied to the DSP-B board to demodulate 2nd IF sig-
nal.
A portion of 2nd IF signal from the compositor (IC1505, pin
8) is applied to the AGC detector circuit.
The 2nd mixer circuit has 2 mixers and 90 degrees
transphasor to improve image rejection ratio.
DSP-A/B units
D/A
DSP IC
converter
(IC1)
(IC551)
D/A
IC771
3 - 3
IC601,
IC651
IC701
to AF amplifier circuit
Low-pass
filter