(No.RA011<Rev.001>)1-11
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.
Fig.3 AF signal system
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.
Fig.4 Squelch circuit
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.
Fig.5 Transmitter system
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.
AF PA
IC719
IC714
MCU
(Build in Vocoder)
Baseband IC
IC709
E-Vol
IC715
FM IC
IC501
AMP
IC500
NAMPO
RSSI
SQIN
Q500
Noise DET
RSSI
Noise
AMP
Q502
IC501
FM IC
IC714
MCU
ANT
IC706
MIC
MIC
AMP
D712, Q701
SW
MCU
Q7
TX
VCO
Q11
BUFFER
AMP
Q12
RF
AMP
DRIVE
AMP
Q300
IC300
RF POWER
module
PLL IC
TCXO
16.8MHz
MIC key
input
IC709
Baseband
IC
IC714
X1
IC2
RF
AMP
Q2
Содержание NX-840
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