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CIRCUIT DESCRIPTION
4. SIGNALING
The DET output signal is filtered by a 300Hz Low-Pass
fitter (LPF) consisting of 1/2 of I1C5 (NJM2904M)
and
1/2 of IC6 (NJIM2904M) to eliminate the audible frequen-
cy signals from the recovered audio. The filtered signal
is then amplified to approximately 3V p-p in 1/2 of iC6
and is applied to the analog signal input terminal of the
main MPU. D1 (DAN202K) is provided to limit the max-
The DET output signal is also sampled by the control
unit through 71/2 of IC4 and 1/2 of Q2. This signal de-
termines the presence
of an incoming signal during
SCAN operation.
In the transmit mode, the QT or DOT signals are sent
out from the D/A (Digital to Analog) converter output pin
of the MPU. A post Low-Pass filter is provided to remove
the
quantizing
noise
from
the
generated
signal.
imum input level applied to the MPU. The output signal
(Figure 12)
from 1/2 of IC5 above is further processed by the other
1/2
of
IC5,
waveform
shaped
by
1/2
of
IC4
(NJM4588M) and level translated by 1/2 of Q2 (FMG2)
to a O to 5V square wave format for DOT decoding at
the MPU. The MPU controls the receiver audio circuit
according to the presence or absence of the QT or DOT
signal.
I
| IF UNIT
|
104
| TX-RX UNIT
| +e
OET
+
6.RF
-—
|
Lod
CONTROL
UNIT
a
|
IC6
1/2
1c5
1/2
C
5
2
5 +AF
z
6
6] _LPE
Icé
72
Pwe
"AAA
3
2
a
IcS
V2
1
el Fay
IC4 1/2
ALPE
2
COMPARATOR
[|
cq
1/2
COMPARATOR
AMA
VV
COMPARATOR
FsQ CoP
5V
a2 v2
SINS
bt—o
COMPARATOR
MAIN
p-COM
1c J
AAh—@-
AAA
Vv
WY
|
QT/O0T TRANSMIT
| sto
Fig. 12 Signaling
34
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