Circuit Description - Kenwood NX-200 Service Manual

1. Overview
The NX-200(G) 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.
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
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 (IC703). The signal is ampli-
fied by an RF amplifier (Q204), and passed through the
bandpass fi lter (L212, L215). The resulting signal is applied
to the fi rst mixer (Q203), where it is mixed with the fi rst lo-
cal oscillator signal output from the frequency synthesizer to
produce the fi rst IF (58.05MHz).
ANT
L109,L110,
L113
ANT
SW
D104,D105
D209,D211
IC108
ASIC
IC102
DSP

CIRCUIT DESCRIPTION

ANT
TX/RX: 136~174MHz
1st MIX MCF
ANT RF
SW AMP
58.05MHz
19.2MHz
VC
TCXO
194.05~232.05MHz
PA TX
AMP AMP
136~174MHz
Fig. 1 Frequency confi guration
RF AMP
Q204
L220,L224 L212,L215
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 us-
ing the fi rst 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 generated 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 fi rst 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 fi ltered
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 fi lter (CF201) and operational
amplifi er (IC203).
MCF IF AMP
1st MIX
XF200 Q202
Q203
2nd OSC
X1
VC
x3
TCXO
Q201
19.2MHz
IC413
IC408 IC409
SW
IC412
NX-200(G)
SP
AF
AMP
MIC
MIC
AMP
CF201 IC203
IC202
MIX, AGC
CF200
INT. SP
EXT. SP
13