Panasonic UF-890 Service Manual page 236

6.2.6 Line Monitor Circuit
The Line Monitor Circuit consists of an operational amplifier (IC100), analog master (IC91) and its
peripheral circuits. Its function is to monitor the dial tone, DTMF tone, response signals, etc. over the
speaker. It also sounds the output of the key touch tones, alarm tones, etc. from the panel CPU over the
speaker. The received signal from the Ain (M) passes through an AGC circuit and is conditioned by the
Analog Front-End DSP (IC90) and is then input to the Analog SW2 for volume control. The signal is then
input to the Speaker Amplifier (IC100, Q91, 92), where it is amplified to a level sufficient to drive the
speaker. The key touch tones and Buzzer Signals from the panel are input to the Analog SW2 for volume
control and then input to the Speaker Amplifier. The monitor tone from the phone line and the buzzer tone
from the panel can be adjusted from the Control Panel.
HYSIG
HYBSR
Speaker Amplifier
IC100, Q91, 92
Speaker
RX signal
LPF Fc: 7.8 KHz
TX signal
LPF Fc: 7.8 KHz
Analog Master IC91
Analog SW1
Analog SW2
Buzzer Signal
from Panel
236
Analog Front-End DSP
(IC90)
Ain(M)
AGC
Aout(M)
Aout(V)