Panasonic KX-FL612CX Service Manual page 162

KX-FL612CX / KX-FL612CX-S
7.4.5. LINE AMPLIFIER AND SIDE TONE CIRCUIT
1. Circuit Operation
The reception signal output from the line transformer T201 is input to pin (2) of IC204 via C232, R236 and then the signal is
amplified at pin (1) of IC204 and sent to the reception system.
The transmission signal goes through C224, R232 and enters IC204-pin (6), where the signal is amplified. Then, it is output
from pin (7) of IC204 and transmitted to T201 via L230, C237, R244, R243. Without a side tone circuit, the transmission signal
will return to the reception amplifier via R236, C232. When the side tone circuit is active, the signal output from IC204 pin (7)
passes through L230, C237, R241, R233, C233, C225, C223, and R229 and goes into the amplifier IC204 pin (3). This circuit
is used to cancel the transmission return signal.
The TX signal is output to the circuit analog the route from the IC204 7 pin→L230→C237→R244→R243→T201→TEL LINE.
However, if balance is lost in the bridge, a voltage occurs between the IC207 6 pin and 5 pin and a side tone results, because
the balance cannot be maintained completely at all frequencies in the audio range some side tone always occur.
7.4.6. LOW PASS FILTER AND NOTCH FILTER
1. Function
This low pass filter and notch filter attenuates the account signal from the commutator to eliminate influence on the conversation
and communication.
2. Circuit Operation
This low pass filter is an eight-order active filter, and the cutoff frequency is approximately 4 kHz.
NOTCH FILTER has filter pole of account signal frequency.
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