Circuit Description - Kenwood NX-203 Service Manual

1. Overview
The NX-203 is a VHF portable transceiver designed to
operate in the frequency range of 136 to 174MHz. The unit
consists of receiver, transmitter, phase-locked loop (PLL)
frequency synthesizer, base band parts, power supply, and
control circuits.
ANT
ANT
AMP
3. Receiver System
3-1. RF Circuit
An incoming RF signal from the antenna terminal is
passed through the antenna switch (D104, D105, D209 and
D211) and then the bandpass fi lter (L220, L224). The band-
pass fi lter is adjusted by a variable capacitor. The input volt-
age to the variable capacitor is regulated by the voltage out-
put from the D/A converter (IC703). The signal is amplifi ed
by an RF amplifi er (Q204), and passed through the bandpass
fi lter (L210, L215). The resulting signal is applied to the fi rst
mixer (Q203), where it is mixed with the fi rst local oscillator
signal output from the frequency synthesizer to produce the
fi rst IF (58.05MHz).
ANT
L109,L110,
L113
ANT
LPF
SW
D104,D105
D209,D211
IC108
ASIC
IC102
DSP

CIRCUIT DESCRIPTION

TX/RX: 136~174MHz
1st MIX MCF
RF
SW AMP
58.05MHz
19.2MHz
VC
TCXO
194.05~232.05MHz
PA TX
AMP
136~174MHz
Fig. 1 Frequency confi guration
RF AMP
Q204
L220,L224 L210,L215
BPF BPF
1st PLL OSC (PLL)
IC704 IC707 IC703
SW VOL
IC713
(2/2)
AMT
Fig. 2 RF and IF circuit
2. Frequency Confi guration
The receiver is a double-conversion superheterodyne
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.
CF
450kHz
MIX
IF AMP
x3
57.6MHz
PLL
VCO
3-2. IF Circuit
The first IF signal is passed through a four-pole mono-
lithic crystal fi lter (XF200) to reject adjacent channel signals.
The fi ltered fi rst IF signal is amplifi ed by the fi rst IF amplifi er
(Q202) and then applied to the lF system IC (IC202). 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
57 .6MHz of second local oscillator output and produces the
second IF signal of 450kHz.
The second IF signal is passed through the ceramic fi lter
(CF200) to reject the adjacent channel signal. The filtered
second IF signal is amplifi ed by the AGC amplifi er.
The signal from the AGC amplifi er is input to the ASIC
(IC108) through the ceramic filter (CF201) and operational
amplifi er (IC203).
MCF IF AMP
1st MIX
XF200 Q202
Q203
BPF
2nd OSC
X1
VC
x3
TCXO
Q201
19.2MHz
IC413
IC408 IC409
SW
IC412
NX-203
SP
AF
AMP
MIC
MIC
AMP
CF201 IC203
IC202
BPF
MIX, AGC
BPF
CF200
INT. SP
EXT. SP
11