KX-TG2521RUT, KX-TGA251RUT — Panasonic Telephone Service Manual (repair manual). Page 6

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6
KX-TG2521RUT/KX-TGA251RUT
3 Specifications
Note:
• Design and specifications are subject to change without notice.
Note for Service:
• Operation range: Up to 300 m outdoors, Up to 50 m indoors, depending on the condition.
• Analog telephone connection: Telephone Line
7
KX-TG2521RUT/KX-TGA251RUT
4 Technical Descriptions
4.1.
Block Diagram (Base Unit)
Analog
Front
End
T
o TEL_LINE
Bridge
Rect
 D3D3
Hook Switch
Q3,Q4,Q5
Audio
Bell/Caller ID
Interface
CPU
18
16
23
20
21
19
24
BELL
HOOK
Off-Hook Line Voltage
25
A/D
D/A
ADPCM
Codec Filter
DSP
Speech Encoding
Speech Decoding
BMC
Burst Encoding
Burst Decoding
RF
PLL
5
3
77
75
9
10
EEPROM
SCL
SDA
57
58
44
To AC Adaptor
Limit
Resistor
Charge
Detector
CHARGE
CONTACT
(Optional)
IC2
Q9
Q8
2.4V
Reg.
1.8V
Reg.
3.0 V
2.4 V
1.8 V
BBIC
IC401
IC7
KX-TG2521 BLOCK DIAGRAM 
(BASE UNIT)
MOD/DEMOD
Charge
Pump
43
2.5V
X
T
A
L
X
1
X1
1
0.3
68
M
H
z
IC801
R
X
n
R
X
p
T
X
p
T
X
n
P
on
P
S
E
L
V
D
D
-P
A
D
R
V
4
3
6
7
T
X
on
R
X
on
D801
6
72
AN
T1
AN
T2
9
8
AN
T1
ANT2
R
F
 P
A
ANT1
AN
T2
78
73
74
80
L
1
L
2
D
C
P
D
C
M
2
FLASH MEMORY
IC421
42
3.0V
54 
55
56 
53
TA
M-C
Sn
SP
I-DI
SP
I-DO
SP
ICL
K
IC3
8
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
9
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>
10
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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