Icom IC-80AD Service Manual page 11

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.
• RX AF CIRCUITS
LOGIC UNIT MAIN UNIT
[SP]
IC400
SP
CHASSIS
2
6
AF
AMP
AFON
POWER AMP
CONTROLLER
Q400-402
IC503
LINEAR
CODEC
DSP
CODEC
IC506
CPU
20,21
IC1
MODEM
IC501
LPF
IC504B
BUFF
11 7 6
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.
NOISE SQUELCH DIAGRAM
To RX AF circuits
9 10
DAC
IC600
From IF IC
Noise filter
(IC100, Pin16)
IC500A
IC600
IC501
2
3
M ODE
SW
AF
1
FIL 6
14 15
16
17
IC500D
Q552
11
D/A AF M ODE
SW
FIL
4 - 3
7
Noise
8
AMP
Com-
Noise
parator
detector
IC100
<DV MODE>
FM-demodulated AF signals from
IF IC (IC100)
4
<FM/AM/ MODE>
AM/FM-demodulated AF signals from
AM mode SW (IC500A)/FM mode SW (IC500D)
8
<WFM MODE>
FM-demodulated AF signals from
WFM mode SW (IC500C)
"NOIS"
13