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NX-900
17
CIRCUIT DESCRIPTION
1. Overview
NX-900 is an 800MHz Mobile transceiver designed to
operate in the frequency range of 806 to 870MHz. The unit
consists of a receiver, transmitter, phase-locked loop (PLL)
frequency synthesizer, baseband parts, power supply, and
control circuits.
2. Frequency Confi guration
The receiver is a double-conversion super heterodyne
using the first intermediate frequency (IF) of 58.05MHz
and the second IF of 450kHz. Incoming signals from the
antenna are mixed with the local signal from the PLL circuit
to produce the fi rst IF of 58.05MHz. This is then mixed with
the 57.6MHz second local oscillator output to produce the
450kHz second IF. The transmit signal frequency is gener-
ated by the PLL VCO, and modulated by the signal from the
DSP. It is then amplifi ed and fed to the antenna.
x3
ANT
SW
RF
AMP
2nd Mixer
IF AMP
PLL
VCO
ANT
58.05MHz
19.2MHz
x2
792.95~
811.95MHz
806~825MHz (TX)
851~870MHz (RX/TX)
806~825MHz
851~870MHz
MCF
VC
TCXO
Baseband Circuit
CF
450kHz
AF
AMP
PA
AMP
TX
AMP
MIC
AMP
TX/RX
SW
1st Mixer
SP
MIC
Fig. 1 Frequency confi guration
Fig. 2
3. Receiver System
3-1. RF circuit
The front-end circuit consists of the former BPF (L906),
RF amplifi er (Q2), and latter BPF (L907).
The BPF attenuates the unwanted signals, and sends
only the necessary signal to the fi rst mixer.
3-2. First Mixer
The signal from the BPF is heterodyned with the first
local oscillator signal from the PLL frequency synthesizer
circuit at the fi rst mixer (IC716) to become a 58.05MHz fi rst
intermediate frequency (IF) signal.
The fi rst IF signal is fed through a monolithic crystal fi lter
(XF202) to further remove spurious signals.
RF AMP
Q2
AMP
Q420
BPF
ANT
IF AMP
Q205
IF AMP
Q204
RX
VCO
AMP
Q10
Buff AMP
Q9
BPF
RF AMP
Q208
LPF
1st MIX
IC716
L906
BPF
L907
MCF
XF202
FM SYSTEM IC
IC202
Summary of Contents for NEXEDGE NX-900
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