Circuit Description - Kenwood NX-900 Service Manual

1. Overview
NX-900 is an 800MHz Mobile transceiver designed to
operate in the frequency range of 806 to 870MHz. The unit
consists of a receiver, transmitter, phase-locked loop (PLL)
frequency synthesizer, baseband parts, power supply, and
control circuits.
ANT
ANT
SW
PA
AMP
3. Receiver System
3-1. RF circuit
The front-end circuit consists of the former BPF (L906),
RF amplifi er (Q2), and latter BPF (L907).
The BPF attenuates the unwanted signals, and sends
only the necessary signal to the fi rst mixer.
Buff AMP AMP
Q9 Q10
RX
VCO

CIRCUIT DESCRIPTION

806~825MHz (TX)
851~870MHz (RX/TX)
1st Mixer
RF
MCF
AMP
58.05MHz
792.95~
19.2MHz
811.95MHz
VC
TCXO
TX TX/RX
x2
AMP SW
806~825MHz
851~870MHz
Fig. 1 Frequency confi guration
ANT
RF AMP
L906 Q2
BPF
AMP RF AMP
Q420 Q208
BPF
2. Frequency Confi guration
The receiver is a double-conversion super heterodyne
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
2nd Mixer
IF AMP
x3
PLL
VCO
3-2. First Mixer
The signal from the BPF is heterodyned with the first
local oscillator signal from the PLL frequency synthesizer
circuit at the fi rst mixer (IC716) to become a 58.05MHz fi rst
intermediate frequency (IF) signal.
The fi rst IF signal is fed through a monolithic crystal fi lter
(XF202) to further remove spurious signals.
1st MIX
L907
IC716 XF202
BPF MCF
LPF
Fig. 2
NX-900
SP
AF
AMP
MIC
MIC
AMP
IF AMP IF AMP FM SYSTEM IC
Q205 Q204 IC202
17