(No.RA073<Rev.001>)1-19
2.4
CIRCUIT DESCRIPTION
2.4.1 Overview
The NX-5700HB is a VHF Analog FM & Digital Mobile transceiver designed to operate in the frequency range of 136 to 174MHz. The
unit consists of a receiver, a transmitter, a phase-locked loop (PLL) frequency synthesizer, a digital control unit, and a power supply
circuit.
2.4.2 Frequency Configuration
The receiver is a double-conversion super-heterodyne using first intermediate frequency (IF) of 49.95MHz and second IF of 2.25MHz.
Incoming signals from the antenna are mixed with the local signal from the VCO/PLL circuit to produce the first IF of 49.95MHz. This
is then mixed with the 47.7MHz second local oscillator output to produce the 2.25MHz second IF. The transmit signal frequency is
generated by the PLL and the TX VCO, and modulated by the signal from the Baseband Circuit. It is then amplified and fed to the
antenna.
Fig.1 Frequency configuration
2.4.3 Receiver System
2.4.3.1
RF Circuit
The receive signal from antenna switch (D305, D306, D307, D308, D309 and D310) is amplified by a RF amplifier (Q502) and passes
through the band-pass filter (L516, L517, L508, L509, L511 and L514) to remove unwanted signals. The signal is then fed to the 1st
mixer (IC500).
2.4.3.2
IF Circuit
The first IF signal is passed through a four-pole monolithic crystal filter (XF600) to reject adjacent channel signal. The filtered first IF
signal is amplified by the first IF amplifier (Q604 and Q603) and then applied to the IF system IC (IC600). The IF system IC provides
a second mixer, second PLL, AGC and A/D converter.
The second mixer mixes the first IF signal with the 47.7MHz of second local oscillator output and produces the second IF signal of
2.25MHz.
The second IF signal is then be fed into an A/D converter, generates the “I” and “Q” data. This data is in the form of SSI (Serial
Synchronous Interface), and sent to the MPU/DSP (IC706).
2.4.3.3
Audio Amplifier Circuit
Audio processing (high-pass filter, low-pass filter, de-emphasized and so on) at Analog FM mode and decoding at Digital mode are
processed by DSP. SSI signal from DSP is converted to audio signal at CODEC (IC902). The signal goes to connector for external
speaker (CN9) through the amplifier (IC903), electronic volume control (IC719), the amplifier (IC906), Changeover switch (IC912) and
audio amplifier (IC911).
Fig.2 RF and IF circuit
ANT
P
S
z
H
M
7
.
7
4
z
H
M
5
9
.
9
4
X
I
M
t
s
1
19.2MHz
185.95~223.95MHz
MIC
136~174MHz
ANT
SW
PA
AMP
RF
AMP
TX
AMP
MCF
MIX
IF Circuit
TCXO
RX
VCO
2nd
VCO
PLL
TX
VCO
Baseband
Circuit
CODEC
AF
AMP
TX/RX: 136~174MHz
ANT
D305,306
Q600
D307,308
Q502
L508,509
D309,310
L517
L516
RF AMP
L514,511
SSI
IC600
IF AMP
IF AMP
MCF
RX VCO/PLL
IC903
IC719
IC906
IC706
IC902
External Speaker
ANT
LPF
ANT
SW
BPF
BPF
BPF
IF IC
2nd
VCO
DSP
CODEC
IC911
CN9
VOL
IC912
SW
IC500
XF600
Q604
Q603
Содержание NX-5700HB
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