Circuit Description; Frequency Configuration; Receiver System; Rf Circuit/If Circuit - Kenwood NX-720 Service Manual

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, baseband 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
TX/RX: 136~174MHz
1st MCF
MIX 49.95MHz
ANT RF
SW AMP
16.8MHz
TCXO
185.95~
223.95MHz
POWER TX
AMP AMP
136~174MHz
Fig.1 Frequency confi guration
ANT
D107,
D110,D111
L116,L117
ANT
LPF
SW
IC 507
IC 502

CIRCUIT DESCRIPTION

CF 450kHz
SP
AF
IF SYSTEM
PA
X3
50.4MHz
MIC
MIC
PLL/VCO
AMP
L207,L209
Q202
L215,L216
L210,L220
RF AMP
BPF
BPF
OSC (VCO/PLL)
IC 711
IC 711
(1/2)
(2/2)
ASIC
DSP
Fig.2

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).
Q201
XF1 Q305
1st MIX
MCF IF AMP
X1 Q303
1st local
TCXO x3
16.8MHz
IC 712
IC 714
VOL
Receiver System
NX-720(G)/720
CF2
IC303
BPF
MIX,AGC
CF1
SP
11