NX-720H(G)/720H
11
CIRCUIT DESCRIPTION
Fig.1 Frequency confi guration
1. Overview
The NX-720 is a VHF Mobile transceiver designed to
operate in the frequency range of 136 to 174MHz. The unit
consists of a receiver, transmitter, phase-locked loop (PLL)
frequency synthesizer, base band parts, power supply, and
control circuits.
2. Frequency Confi guration
The receiver is a double-conversion super-heterodyne
using a fi rst intermediate frequency (IF) of 49.95MHz and a
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 49.95MHz. This is then mixed with the 50.4MHz
second local oscillator output to produce the 450kHz second
IF. The transmit signal frequency is generated by the PLL
VCO, and modulated by the signal from the DSP. It is then
amplifi ed and fed to the antenna.
ANT
SW
RF
AMP
1st
MIX
AF
PA
TCXO
MIC
AMP
TX
AMP
POWER
AMP
CF 450kHz
MCF
4
9
.
9
5MHz
IF SYSTEM
PLL/VCO
16.8MHz
ANT
SP
MIC
TX/RX:136~174kHz
X3
50.4MHz
Base band Circuit
185.
9
5~
223.
9
5MHz
136~174MHz
3. Receiver System
3-1. RF circuit
An incoming RF signal from the antenna terminal is
passed through the antenna switch (D110, 111 and 107)
and then the bandpass fi lter (L215, 216). The bandpass fi lter
is adjusted by a variable capacitor. The input voltage to the
variable capacitor is regulated by the voltage output from the
D/A converter (IC712). The signal is amplifi ed by an RF am-
plifi er (Q202), and passed through the bandpass fi lter (L207,
209, 210 and 220). The resulting signal is applied to the fi rst
mixer (Q201) where it is mixed with the fi rst local oscillator
signal output from the frequency synthesizer to produce the
fi rst IF (49.95MHz).
3-2. IF circuit
The fi rst IF signal is passed through a four-pole mono-
lithic crystal fi lter (XF1) to reject the adjacent channel signal.
The fi ltered fi rst IF signal is amplifi ed by the fi rst IF amplifi er
(Q305) and then applied to the IF system IC (IC303). The
IF system IC provides a second mixer, AGC amplifi er , and
RSSI (Received Signal Strength Indicator).
The second mixer mixes the first IF signal with the
50.4MHz of the second local oscillator output and produces
the second IF signal of 450kHz.
The second IF signal is passed through the ceramic fi lter
(CF1) to reject the adjacent channel signal. The fi ltered sec-
ond IF signal is amplifi ed by the AGC amplifi er.
The signal from the AGC amplifi er is input to an AD con-
verter in ASIC (IC507) through the ceramic fi lter (CF2).
ANT
BPF
Q202
RF AMP
Q305
IF AMP
Q201
1st MIX
XF1
MCF
D107,
D110,D111
ANT
SW
TCXO
IC303
MIX,AGC
1st local
OSC (VCO/PLL)
CF1
LPF
L116,L117
L215,L216
BPF
L207,L20
9
L210,L220
16.8MHz
Q303
x3
2nd OSC
X1
BPF
CF2
IC 507
ASIC
IC 711
(1/2)
IC 502
DSP
IC 711
(2/2)
VOL
IC 712
IC 714
SP
Fig.2 Receiver System
Summary of Contents for Nexedge NX-720H
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