KX-TG2521RUT, KX-TGA251RUT — Panasonic Telephone Service Manual (repair manual). Page 8

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KX-TG2521RUT/KX-TGA251RUT
4.2.
Circuit Operation (Base Unit)
4.2.1.
Outline
Base Unit consists of the following ICs as shown in Block Diagram (Base Unit) (P.7).
• DECT BBIC (Base Band IC): IC7
- Handling all the audio, signal and data processing needed in a DECT base unit
- Controlling the DECT specific physical layer and radio section (Burst Module Controller section)
- ADPCM code filter for speech encoding and speech decoding (DSP section)
- Echo-cancellation and Echo-suppression (DSP section)
- Any tones (tone, sidetone, ringing tone, etc.) generation (DSP section)
- DTMF receiver (DSP section)
- Clock Generation for RF Module
- ADC, DAC, timer, and power control circuitry
- PLL Oscillator
- Detector 
- Compress/Expander
- First Mixer
- All interfaces (ex: RF Power Amp, EEPROM, LED, Analog Front End, etc.)
• RF Power Amp.: IC801
- Amplifier for transmission and reception
• EEPROM: IC401
- Temporary operating parameters (for RF, etc.)
• FLASH MEMORY: IC421
- Voice Prompt (TAM) D/L Area
- ICM/OGM Recording Area
• Additionally,
- Power Supply Circuit (+3.0 V, +2.4 V, +1.8 V output)
- Crystal Circuit (10.368 MHz)
- Charge Circuit
- Telephone Line Interface Circuit
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KX-TG2521RUT/KX-TGA251RUT
4.2.2.
Power Supply Circuit
The power is supplied to the DECT BBIC, RF Module, EEPROM and Charge Contact from AC Adaptor (+6.5 V) as shown in
Fig.101. The power supply is as follows;
• DECT BBIC (IC7): 
DC Jack (+6.5 V) 
→ IC2 → IC7
DC Jack (+6.5 V) 
→ IC2 → Q9 → IC7
DC Jack (+6.5 V) 
→ IC2 → Q8 → IC7
• RF Power Amp. (IC801): 
DC Jack (+6.5 V) 
→ IC2 → IC801 (Power AMP)
• EEPROM (IC401): 
DC Jack (+6.5 V) 
→ IC2 → IC7→ IC401
• FLASH MEMORY (IC421): 
DC Jack (+6.5 V) 
→ IC2→ IC7→ IC421
• Charge Contact (TP16): 
DC Jack (+6.5 V) 
→ R56 → R55 → D22→ TP16
<Fig.101>
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KX-TG2521RUT/KX-TGA251RUT
4.2.3.
Telephone Line Interface
<Function>
• Bell signal detection
• Clip signal detection
• ON/OFF hook circuit
Bell & Clip (: Calling Line Identification Presentation: Caller ID) signal detection:
In the standby mode, Q3 is open to cut the DC loop current and decrease the ring load.
When ring voltage appears at the L1T (A) and L1R (B) leads (when the telephone rings), the AC ring voltage is transferred as
follows;
• B 
→ L2 → C4 → R6 → R33 → IC7 Pin 21 (CID INp) 
• A 
→ L1 → C3 → R4 → R35 → IC7 Pin 20 (CID INn)
ON/OFF hook circuit:
In the standby mode, Q3 is open, and connected as to cut the DC loop current and to cut the voice signal. The unit is
consequently in an on-hook condition
When IC7 detects a ring signal or press the TALK Key onto the handset, Q4 turns on and then Q3 turns on, thus providing an
off-hook condition (DC current flows through the circuit) and the following signal flow makes the loop current.
• B 
→ L2 →D3 → Q3 → Q5 → R21 → R22 → D3 → L1 →A [OFF HOOK]
4.2.4.
Transmitter/Receiver
• Audio Circuits and DTMF tone signal circuits.
Base Unit and Handset mainly consist of RF Module and DECT BBIC. 
Base Unit and Handset transmit/receive voice signal and data signal through the antenna on carrier frequency.
Signal Path:
*Refer to Signal Route (P.13).
4.2.4.1.
Transmitter Block
The voice signal input from the TEL LINE interface goes to RF Power Amp. (IC801) through DECT BBIC (IC7) as shown in
Block Diagram (Base Unit) (P.7)
The voice signal passes through the analog part of IC7 where it is amplified and converted to a digital audio stream signal. The
burst switch controller processes this stream performing encryption and scrambling, adding the various other fields to produce
the GAP (Generic Access Profile) standard DECT frame, assigning to a time slot and channel etc.
In IC7, the carrier frequency is changing, and frequency modulated RF signal is generated.In IC801,RF signal is amplified, and
radiated from antenna. Handset detects the voice signal or data signal in the circuit same as the following explanation of
Receiver Block.
4.2.4.2.
Receiver Block
The signal of 1900 MHz band (1881.792 MHz ~ 1897.344 MHz) which is input from antenna is input to IC7 as shown in Block
Diagram (Base Unit)
 (P.7).
In IC7, the signal of 1900 MHz band is downconverted to 864 kHz signal and demodulated, as GAP (Generic Access Profile)
standard DECT frames. It passes through the decoding section burst switch controller where it separates out the frame
information and performs de-encryption and de-scrambling as required. It then goes to the DSP section where it is turned back
into analog audio. This is amplified by the analog front end, and goes to the TEL LINE Interface.
4.2.5.
Pulse Dialling
During pulse dialling the hookswitch (Q3, Q4) is used to generate the pulses using the HOOK control signal, which is set high
during pulses. To force the line impedance low during the “pause” intervals between dial pulses, the PULSE_DIAL signal turns
on Q2.
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KX-TG2521RUT/KX-TGA251RUT
4.3.
Block Diagram (Handset)
SPEAKER
RECEIVER
MIC
CHARGE
CIRCUIT
Q4, Q9, R7Q4,
Q9,
R7
CHARGE
CONTACTS
CHARGE(+)
CHARGE(-)
CHARGE_CTRL
CHARGE
EEPROM
SCL
SDA
BATTERY
TERMINAL
D7
R45
VBAT
BATT+
BATT-
1.8 V
Q2
1.8 V
LCD
CD
RESET
CSB
SDA
 SCL
CPU
Analog
Front
End
D/A
A/D
33
35
15
16
19
20
32
31
57
58
 36
13
43
42
63
54
55
66
ADPCM
Codec
Filter
DSP
Speech
Decoding
Speech
Encoding
BMC
Burst
Decoding
RF PA
ANT1
IC801
Burst
Encoding
RF
PLL
MOD/DEMOD
5
3
4
3
6
7
8
9
2
77
78
75
RXn
RXon
ANT1
DA801
RXp
TXp
TXn
Pon
PSEL
TXon
VDD-PADRY
9
10
XTAL
X1
10.368
MHz
BATTERY
44
ON SWITCH
KEYPAD
Charge
Pump
ROWS
COLUMNS
CP3.0V
CP4.0V
48, 49, 50, 51
22, 23, 24, 25, 26
BBIC
IC1
IC3
 56
52
73
74
80
LDO_CTRL
KX-TGA251 BLOCK DIAGRAM (HANDSET)
LCD-BACK  LIGHT
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KX-TG2521RUT/KX-TGA251RUT
4.4.
Circuit Operation (Handset)
4.4.1.
Outline
Handset consists of the following ICs as shown in Block Diagram (Handset) (P.11).
• DECT BBIC (Base Band IC): IC1
- All data signals (forming/analyzing ACK or CMD signal)
- All interfaces (ex: Key, Detector Circuit, Charge, DC/DC Converter, EEPROM, LCD, RF Power Amp.)
- PLL Oscillator
- Detector
- Compress/Expander
- Reception
• RF Power Amp: IC801
- Amplifier for transmission
• EEPROM: IC3
- Temporary operating parameters (for RF, etc.)
4.4.2.
Power Supply Circuit/Reset Circuit
Circuit Operation:
When power on the Handset, the voltage is as follows;
BATTERY(2.2 V ~ 2.6 V: BATT+) 
→ F1 → Q2 (1.8 V), IC1-43pin (2.5V)
The Reset signal generates IC1 (61 pin) and 1.8 V.
4.4.3.
Charge Circuit
Circuit Operation:
When charging the handset on the Base Unit, the charge current is as follows;
DC+(6.5 V) 
→ R56 → R55 → D22 → CHARGE+(Base) → CHARGE+(Handset) → Q4 → D7→ F1 → BATTERY+... Battery... 
BATTERY- 
→ R45 → GND → CHARGE-(Handset)→ CHARGE-(Base) → GND → DC-(GND)
In this way, the BBIC on Handset detects the fact that the battery is charged.
The charge current is controlled by switching Q9 of Handset.
Refer to Fig.101 in Power Supply Circuit (P.9).
4.4.4.
Battery Low/Power Down Detector
Circuit Operation:
“Battery Low” and “Power Down” are detected by BBIC which check the voltage from battery.
The detected voltage is as follows;
• Battery Low
Battery voltage: V(Batt)   2.25 V ± 50 mV
The BBIC detects this level and "
" starts flashing.
• Power Down
Battery voltage: V(Batt)   2.0 V ± 50 mV
The BBIC detects this level and power down.
4.4.5.
Speakerphone
The hands-free loudspeaker at SP+ and SP- is used to generate the ring alarm.

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