Panasonic KX-TC2000SNB, Service Manual

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19.5. Telephone Line Interface
Function:
- Bell signal detection
- ON/OFF hook and pulse dial circuit
- Side tone circuit
Bell signal detection and OFF HOOK circuit:
In the idle mode, Q103 is open to cut the DC loop current and decrease the ring load. When ring
voltage appears at the Tip (T)
and Ring (R) leads (When the telephone rings), the AC ring voltage is transferred as follows:
JK1 (3): T L6 R184 R144 IC3D (12,13 14) C97 R148 Q20 IC2 (24) [BELL]
JK1 (2): R L4 R183 R152
When the CPU (DSP) detects a ring signal and press the TALK Key on the handset. Q6 turns on
and then RY1 turns on,
thus providing an off-hook condition (active DC current flow through the circuit) and the
following signal flow is for the loop
current.
T R160 L4 D2 RY1 R107 T7 R116 D5 L6 R161 R [OFF HOOK]
ON HOOK Circuit:
Q6 is open, RY1 disconnected as to cut the DC loop current and to cut the voice signal. The unit
is consequently in an on-hook
condition.
Pulse Dial Circuit:
CPU (Hook) turns RY1 ON/OFF to make the pulse dialing via Q6.
Make/Break ratio when dialing with the Handset: 40%:60%
Side Tone Circuit:
Basically this circuit prevents the TX signal from feeding back to RX signal.
As for this unit, TX signal feed back from Q11 is canceled by the cancellor circuit of AGC.
19.6. DTMF
The DTMF signal is generated by five outputs of IC2 (CPU) and output to telephone line through
the filter of IC3A which is a lowpass filter as shown below.
The combination of five outputs “1”, “0” makes 32 (=2 5) level signals and makes smooth
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