Icom IC-91A Service Manual page 10

5-1-3 2ND IF AND DEMODULATOR CIRCUITS
(MAIN UNIT)
The 1st IF signal is converted into the 2nd IF signal and
demodulated in the IF IC. The IF IC contains 2nd mixer, limiter
amplifier, quadrature detector, etc. in its package.
5-1-3-1 2ND IF CIRCUITS
- A_BAND (0.495–999 MHz) -
• FM/AM mode
The 1st IF signal from the IF amplifier (Q100) is applied to the 2nd
mixer in the A_BAND IF IC (IC100, pin 16), and converted into the
450 kHz 2nd IF signal by being mixed with the 61.2 MHz 2nd LO
signal from the reference frequency oscillator (X450) quadrupled
by the quadruplicater (Q452).
The converted 2nd IF signal is output from pin 3, and passed
through the 2nd IF filter (FI100) via the mode switch (D102) to
suppress sideband noise.
In FM mode, the filtered 2nd IF signal is applied to the limiter
amplifier (IC100, pin 5) via the mode switch (D103).
In AM mode, the filtered 2nd IF signal is applied to the IF amplifier
(Q103) via the mode switch (D103).
• WFM mode
The 2nd IF signal from the IF amplifier (Q100) is applied to the 3rd
mixer in the A_BAND IF IC (IC100, pin 16), and converted into the
1.95 MHz 3rd IF signal by being mixed with the 15.3 MHz 2nd LO
signal from the reference frequency oscillator (X450).
The converted IF signal is output from pin 3, and applied to the
limiter amplifier (IC100, pin 5) via the mode switches (D102,
D103).
• A_BAND 2ND IF AND DEMODULATOR CIRCUITS
2nd IF signal to the AM demodulator (Q104, Q105)
FM-demodulated signals
to the AF amplifier circuits
Q103
8
7
Noise
Amplifier
Limitter
Quadrature Amplifier
Detector
10 11
9
MAINR3A
(+3V)
X100
Mode switch
Q101, Q102
L101
5-1-3-2 DEMODULATOR CIRCUITS
• FM/WFM mode
The amplified IF signal from the limiter amplifier in the IF IC is
FM-demodulated at the quadrature detector (IC100, pins, 10, 11,
X100) and output from pin 9. The demodulated AF signals are
applied to the AF amplifier circuits.
• AM mode
The amplified IF signal from the IF amplifier (Q103) is applied to
the AM demodulator (Q104, Q105).
The demodulated AF signals are applied to the AF amplifier
circuits.
- B_BAND (118–174 MHz, 350–470 MHz) -
The 1st IF signal from the 1st IF amplifier (Q200) is applied to the
2nd mixer in the B_BAND IF IC (IC200, pin 16), and converted into
the 2nd IF signal by being mixed with the 2nd LO signal from the
reference frequency oscillator (X450) tripled by the tripler (Q451).
The converted 2nd IF signal is output from pin 3, and passed
through the 2nd IF filter (FM/AM mode: FI200; FM-N mode: FI201)
via the mode switch (D202) to suppress sideband noise.
• FM/FM-N mode
The filtered 2nd IF signal is applied to the limiter amplifier in the
B_BAND IF IC (IC200, pin 5) via the mode switch (D203). The
amplified 2nd IF signal is FM-demodulated at the quadrature
detector (IC200, pins, 10, 11, X200) and output from pin 9. The
demodulated AF signals are applied to the AF amplifier circuits.
• AM mode
The 2nd IF signal is passed through the FI200 and applied to the
AM demodulator circuit (Q203, Q204) via the IF amplifier (Q202).
The demodulated AF signals are applied to the AF amplifier
circuits.
(FM/AM)
BPF
D102
D103
FI100
15.3/61.2 MHz
2nd LO signal
R117
5 3 2
Noise
Detector
RSSI
12 13
16
"ANOIS" signal to the CPU (LOGIC UNIT; IC1, pin 26)
"S-LV" signal to the CPU (LOGIC UNIT; IC1, pin 48)
5 - 3
D450
Q450
X4
Q452
A_BAND IF IC (IC100)
1st IF signal from the IF amplifier (Q100)
TCXO
X450