Kenwood NX-740H Service Manual page 11

2.4.2.4 AF Signal System
The detection signal from FM IC (IC501) goes to the baseband IC (IC709) DISC input (pin 16) after pass through IC500 for character-
izing the signal.
• Analog signal
The demodulated AF signal pass through IC709 with the signal processing and output at AUDIO (pin26) to the E-Vol before the
Audio amplifier (IC719). The AF signal from IC719 switches between the internal speaker and speaker jack (J701) output.
• NXDN (Digital signal)
The recovered 4L-FSK signal obtained from IC501 is decoded in the baseband IC (IC709).
Decoded signal is fed into microprocessor (IC714) and converted to PCM audio signal, and return to baseband IC (IC709) for con-
verting to audio signal.
This audio signal will output at AUDIO (pin26) to the E-Vol before the Audio amplifier (IC719).
The AF signal from IC719 switches between the internal speaker and speaker jack (J701) output.
2.4.2.5 Squelch Circuit
Part of the AF signal goes a rectifier circuit to produce a DC voltage corresponding to the noise level after filtering and amplification.
This DC voltage send to MCU (IC714) SQIN input pin.
The MCU controls squelch according to the voltage (SQIN) level.
The signal from the RSSI pin of IC501 is monitored. The electric field strength of the receive signal can be known before the SQIN
voltage is input to the MCU, and the scan stop speed is improved.
2.4.3 Transmitter System
2.4.3.1 Outline
The transmitter circuit produces and amplifies the desired frequency directly. It FM-modulates the carrier signal by means of a varicap
diode.
MIC IC706 D712, Q701
MIC
AMP
MIC key
input
2.4.3.2 Power Amplifier Circuit
The transmit output signal from the VCO passes through the transmission/reception selection diode (D16) and amplified by Q300.
The amplified signal goes to the RF power module (IC300) through a low-pass filter.
The lowpass filter removes unwanted high-frequency harmonic components, and the resulting signal is goes the antenna terminal.
IC501 IC500
FM IC AMP
(Build in Vocoder)
Fig.3 AF signal system
NAMPO
IC501
FM IC
RSSI
Fig.4 Squelch circuit
IC709
Baseband
SW
IC
TCXO
MCU
16.8MHz
IC714 X1
Fig.5 Transmitter system
IC709 IC715 IC719
Baseband IC E-Vol AF PA
MCU
IC714
Q500
Q502
Noise DET
Noise SQIN
AMP
RSSI
Q7 Q11
TX BUFF
VCO AMP
RF
PLL IC
AMP
IC2 Q2
IC714
MCU
Q12 Q300 IC300
RF DRIVE RF POWER
AMP AMP module
(No.RA010<Rev.001>)1-11
ANT