Icom IC-80AD Service Manual - page 11
4 - 3
RX AF CIRCUITS (LOGIC UNIT)
• FM/W-FM/AM MODE
The AF signals from the FM/AM demodulator circuits are
passed through the mode SW (MAIN UNIT: IC500D/C/A)
and one of the AF filters whose audio frequency response
is set as stable for each RX mode (FM/AM or WFM).
FM/AM-demodulated AF signals are filtered by IC501, and
FM-demodulated AF signals (W-FM mode) are filtered by
Q552.
The filtered AF signals are passed through the D/A
converter (MAIN UNIT: IC600, pins 15, 14; 16, 17) for level
adjustment. The level-adjusted AF signals are entered to the
LOGIC UNIT, and applied to the AF power AMP (IC400) to
obtain audio output power. The power-amplified AF signals
are applied to the internal speaker or output from the [SP]
jack.
• DV MODE
The FM-demodulated signals from the IF IC (MAIN UNIT:
IC100) are applied to the modem (IC501) via the LPF and
buffer (IC504B), to be converted into the DV data. The DV
data is applied to the CPU (IC1), and converted into the
AMBE signals. The AMBE signal is then applied to the DSP
CODEC IC (IC506) and decoded. The decoded AMBE
signals are converted into the analog audio signal by liner
CODEC IC (IC503). The converted AF signals are passed
through the mode SW (MAIN UNIT: IC500B) and AF filter
(MAIN UNIT: IC501A).
The filtered AF signals are passed through the D/A
converter (MAIN UNIT: IC600, pins 15, 14; 16, 17) for level
adjustment. The level-adjusted AF signals are entered to the
LOGIC UNIT, and applied to the AF power AMP (IC400) to
obtain audio output power. The power-amplified AF signals
are applied to the internal speaker or output from the [SP]
jack.
SP
AF
AMP
BUFF
LPF
CPU
Q400-402
POWER AMP
CONTROLLER
MODEM
DSP
CODEC
LINEAR
CODEC
IC600
IC400
LOGIC UNIT MAIN UNIT
CHASSIS
[SP]
IC503
IC506
IC504B
IC501
IC1
IC501
SW
SW
FIL
IC500A
M ODE
IC500D
M ODE
AM/FM-demodulated AF signals from
AM mode SW (IC500A)/FM mode SW (IC500D)
FM-demodulated AF signals from
WFM mode SW (IC500C)
FM-demodulated AF signals from
IF IC (IC100)
AF
D/A
AFON
AF
FIL
Q552
15
4
3
8
11
2
6
1
14
6
7
11
20,21
16
17
2
6
DAC
Noise
AMP
Noise filter
From IF IC
(IC100, Pin16)
To RX AF circuits
Noise
detector
Com-
parator
NOISE SQUELCH DIAGRAM
“NOIS”
IC600
IC100
9
8
7
13
10
SQUELCH CIRCUIT
The squelch circuit cuts off the AF output signals when
no RF signals are received. Detecting noise components
(approx. 30 kHz signals) in the demodulated AF signals, the
squelch circuit stops audio signals being emitted.
A portion of FM-demodulated AF signal from the IF IC (MAIN
UNIT: IC100) is passed through the DAC (MAIN UNIT:
IC600) for level (=threshold) adjustment. The level-adjusted
AF signals are passed through the noise filter (IC100, pins 7,
8 and R111–113, C121, 122) to filter the noise components
(approx. 30 kHz signals) only. The noise components are
rectified to produce DC voltage corresponding to the noise
level.
If the noise level is higher than the preset one, the internal
comparator set the "NOISE" signal to the CPU to "High",
then the CPU turns the "AFON" signal which controlls the
AF power AMP (L: IC400) to "Low," to inactivate the AF
power AMP (L: IC400). At the same time, the CPU turns the
"RMUTE" signal which controlls the AF mute SW (L: Q403,
404) to "Low," to cut-off the RX AF line.
• RX AF CIRCUITS