Kenwood NX-840, Service Manual

Page: 14 / 54

1-14 (No.RA011<Rev.001>)
2.4.5.3 Key Matrix Circuit
The front panel has function keys. Each of them is connected to
a cross point of a matrix of the KMI1 to KMO3 ports of the micro-
processor.
The KMO1 to KMO3 ports are always high, while the KMI1 to
KMI3 ports are always low.
The microprocessor monitors the status of the KMI1 to KMO3
ports. If the state of one of the ports changes, the microprocessor
assumes that the key at the matrix point corresponding to that
port has been pressed.
Fig.11 Key matrix circuit
2.4.6 Signaling Circuit
2.4.6.1
Encode
(1) Low-speed data (QT, DQT)
Low-speed data is output from pin 40 (LSDO) of the MCU (IC714).
The signal passes through a low-pass CR filter. The signal is mixed with the audio signal and goes to the VCO and TCXO (X1)
modulation input after signal processing in the baseband IC (IC709).
(2) High-speed data (2-tone)
High-speed data (HSD) is output from pin 41 (HSDO) of the MCU.
The signal passes through a low-pass CR filter. TX deviation making an adjustment by microprocessor is applied to the base-
band IC (IC709).
The signal is mixed with the audio signal and goes to the VCO and TCXO.
The side tone is audio ouput of baseband IC(IC709) at the same time to audio power amplifier and then to the speaker.
(3) MSK / DTMF
MSK and DTMF signal is self genarated by the baseband IC (IC709).
The TX deviation adjustment is done by the output gain of baseband IC (IC709), and is routed to the VCO.
When encoding MSK/DTMF, the microphone-input signal is muted.
Fig.12 Encode
2.4.6.2 Decode
(1) Low-speed data (QT, DQT)
The demodulated signal from the FM IC (IC501) will input to baseband IC(IC701) to remove frequency above 300Hz.
The signal is input to pin 26 (LSDI) of the MCU.
The MCU digitizes this signal, performs processing such as DC restoration, and decodes the signal.
(2) High-speed data (2-tone)
The demodulated signal from the FM IC (IC501) is amplified by baseband IC and passes through a band-pass filter in IC709 to
remove frequency of 3kHz or above and 300Hz or below.
The MCU digitizes this signal anddecodes the signal after receive the signal at pin 27(HSDI).
(3) MSK / DTMF
The demodulated signal from theFM IC(IC501) will input to baseband IC(IC709), then the baseband IC will decode and send the
decode information to MCU by the data line.
The MCU then processes the decoded information.
Fig.13 Decode
IC714
MCU
KMO1
KMO2
KMO3
KMI3
KMI2
KMI1
B
CH UP
VOL DN
A
VOL UP
S
C
CH DN
<
>
VCO
IC709
Baseband IC
DTMF/
MSK
IC1
X1
TCXO PLL
TCXO
MOD
VCO
MOD
QT/DQT
LSDO
2-TONE
HSDO
IC714
MCU
IC709
Baseband IC
DTMF/MSK
decode
IC714
MCU
QT/DQT
LSDI
DTMF/MSK
decode data
REPLAY data
2-TONE
HSDI
IC501
FM IC