Circuit Description; Frequency Configuration; Receiver System; Rf Circuit, If Circuit - Kenwood NX-300 Service Manual

1. Overview
The NX-300 is a UHF portable transceiver designed to
operate in the frequency range of 450 to 520MHz (K,K3) or
400 to 470MHz (K2,K4). The unit consists of receiver, trans-
mitter, 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 (D606, D607, D709,
D711) and then the bandpass fi lter (L721, L722). 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 (Q705), and passed through the bandpass
fi lter (L713, L714, L715). The resulting signal is applied to
the fi rst mixer (Q703), 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
L612~L614
ANT
LPF
SW
D606,D607
D709,D711
IC108
ASIC
IC102
DSP

CIRCUIT DESCRIPTION

TX/RX: 450~520MHz (K,K3)
TX/RX: 400~470MHz (K2,K4)
1st MIX MCF
RF
SW AMP
58.05MHz
19.2MHz
391.95~
461.95MHz (K,K3)
VC
341.95~
TCXO
411.95MHz (K2,K4)
PA TX
AMP
450~520MHz (K,K3)
400~470MHz (K2,K4)
Fig. 1 Frequency confi guration
RF AMP
Q705
L721,L722 L713~L715
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 fi rst IF signal is passed through a four-pole mono-
lithic crystal fi lter (XF700) to reject adjacent channel signals.
The fi ltered fi rst IF signal is amplifi ed by the fi rst IF amplifi er
(Q701) and then applied to the lF system IC (IC701). 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
(CF700) 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 fi lter (CF701) and operational
amplifi er (IC700).
MCF IF AMP
1st MIX
XF700 Q701
Q703
BPF
2nd OSC
X500
VC
x3
TCXO
Q700
19.2MHz
IC413
IC408 IC409
SW
IC412
NX-300
SP
AF
AMP
MIC
MIC
AMP
CF701 IC700
IC701
BPF
MIX, AGC
BPF
CF700
INT. SP
EXT. SP
11