Icom ic-r20 Service Manual page 58

• B-BAND CIRCUIT
The RF signals from the band switch (D32, D36, D143 and
D147) are amplified at the RF amplifier (IC18, pins 1, 4), and
are mixed with 1st LO signals at the 1st mixer circuit (IC15,
pin 1) to produce a 266.7 MHz 1st IF signal. The 1st IF sig-
nal is output from pin 6, and passed through the bandpass
filter (FI1) to suppress unwanted harmonic components. The
filtered 1st IF signal is applied to the 2nd mixer circuit.
The 1st LO signals are generated at the 1st B_VCO circuit
and are applied to the 1st mixer (IC15, pin 3) directly.
4-1-5 2ND MIXER AND 2ND IF CIRCUITS (RF UNIT)
The 2nd mixer circuit converts the 1st IF signal to a 2nd IF
signal.
• A-BAND CIRCUIT
The filtered 429.1 MHz 1st IF signal from the bandpass filter
(FI4) is mixed with the 2nd LO signal at the 2nd mixer circuit
(IC10, pin 1, 6) to produce a 19.65 MHz 2nd IF signal.
In case of AM/CW/SSB/Narrow FM mode, the 2nd IF signal
passes through the mode switch (D58), crystal bandpass fil-
ter (FI3) and another mode switch (D60).
In case of WFM mode, the 2nd IF signal passes through the
mode switch (D59), low-pass filter (C202, C745, R207 and
R536) and another mode switch (D61).
The filtered 2nd IF signal is amplified at the 2nd IF amplifier
(Q5), and is then applied to the demodulator circuit.
• B-BAND CIRCUIT
The filtered 266.7 MHz 1st IF signal from the bandpass filter
(FI1) is mixed with the 2nd LO signal at the 2nd mixer circuit
(IC16, pin 1, 6) to produce a 19.65 MHz 2nd IF signal.
In case of AM/CW/SSB/Narrow FM mode, the 2nd IF signal
passes through the mode switch (D130), crystal bandpass
filter (FI7) and another mode switch (D132).
In case of WFM mode, the 2nd IF signal passes through the
mode switch (D131), low-pass filter (C755, C756, R428,
R429 and R537) and another mode switch (D133).
The filtered 2nd IF signal is amplified at the 2nd IF amplifier
(Q542), and is then applied to the demodulator circuit.
• 1ST, 2ND MIXER AND 2ND IF CIRCUITS
1st LO signal from
the 1STAVCO board
RF signals from
the A-BAND circuit
1st LO signal from
the 1STBVCO board
RF signals from
the B-BAND circuit
2nd LO signal from
the 2NDCVCO board
BPF
1st mixer 2nd mixer
IC1 IC10
2nd LO signal from
the 2NDDVCO board
BPF
1st mixer 2nd mixer
IC15 IC16
4-1-6 3RD MIXER AND DEMODULATOR CIRCUITS
(RF UNIT)
The 3rd mixer circuit converts the 2nd IF signal to a 3rd IF
signal. The demodulator circuit converts the 3rd IF signal
into AF signals.
• A-BAND CIRCUIT
The 19.65 MHz 2nd IF signal from the 2nd IF amplifier (Q5)
is applied to the 3rd mixer section of the FM IF IC (IC2, pin
16) and is then mixed with the 3rd LO signal for conversion
into a 450 kHz 3rd IF signal.
IC2 contains the 3rd mixer, limiter amplifier, quadrature
detector and S-meter detector, etc. A frequency from the
PLL reference oscillator is used for the 3rd LO signal (19.20
MHz).
(1) FM MODE
The 3rd IF signal is output from the FM IF IC (IC2, pin 3),
and passes through the mode switch (D109) and ceramic
bandpass filter (FI2). The filtered signal passes through the
mode switch (D112), and is then fed back and amplified at
the limiter amplifier section (pin 5). The signal is demodulat-
ed to AF signals at the quadrature detector section (pins 10,
11) and detector coil (L21). The demodulated AF signals are
output from pin 9 and are applied to the AF switch (IC12, pin
11) via the "A_DETFM" line, and are then applied to the AF
circuit (LOGIC unit) as "A_DET" signal.
(2) WFM MODE
The 3rd IF signal is output from the FM IF IC (IC2, pin 3),
and passes through the mode switch (D108) and low-pass
filter (L99, C686 and C687). The filtered signal passes
through the mode switch (D111), and is then fed back to the
limiter amplifier section (pin 5). The amplified signal is
demodulated at the quadrature detector section (pins 10 and
11) and detector coil (L21). The AF signals are output from
pin 9 and are applied to the AF switch (IC12, pin 11) via the
"A_DETFM" line, and are then applied to the AF circuit
(LOGIC unit) as "A_DET" signal.
By connecting R55 to R54 in parallel, the output character-
istics of pin 12, "RSSI", change gradually. Therefore, the FM
IF IC can detect WFM components.
BAND BAND
Xtal
SW BPF SW
FI3
LPF
BAND BAND
Xtal
SW BPF SW
FI7
LPF
4 - 3
Q5
2nd IF signal
RF
A-BAND circuit
Q542
RF 2nd IF signal
B-BAND circuit