Circuit Description; Overview; Frequency Configuration; Receiver System - Kenwood NX-800 Service Manual

NX-800

1. Overview

NX-800 is a UHF Mobile transceiver designed to operate in
the frequency range of 450 to 520MHz. The unit consists of
receiver, transmitter, phase-locked loop (PLL) frequency syn-
thesizer, base band parts, power supply, and control circuits.

3. Receiver System

3-1. RF circuit
The front-end circuit consists of former BPF (D212, D213),
RF amplifier Q210, and latter BPF (D207, D208, D209 and
D210).
The BPF covers frequency ranges 450 to 520MHz.
The latter BPF (D207, D208, D209 and D210) attenuates
the unwanted signals, and sends only the necessary signal to
the first mixer.
-118dBm
ANT
D212,D213
Buff AMP
RX
VCO
18

CIRCUIT DESCRIPTION

ANT
TX/RX:450~520MHz
1st MIX
ANT RF
SW AMP
58.05MHz
19.2MHz
VCTCXO
391.95~
461.95MHz
PA TX
AMP AMP
450~520MHz
Fig. 1
-120.5dBm -114.5dBm
RF AMP
Q210
D207~D210
BPF BPF
Buff AMP Buff AMP
Q9 Q10 Q208
0dBm

2. Frequency Configuration

The receiver is a double-conversion super heterodyne
using first intermediate frequency (IF) of 58.05MHz and sec-
ond IF of 450kHz. Incoming signals from the antenna are
mixed with the local signal from the PLL circuit to produce
the first 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 amplified and fed to the antenna.
CF
450kHz
MCF
MIX
IF AMP
x3
57.6MHz
PLL
VCO
Frequency configuration
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 first mixer DBM (IC204) to become a 58.05MHz first
intermediate frequency(IF) signal.
The first IF signal is fed through a monolithic crystal
filter(XF202) to further remove spurious signals.
-112.5dBm
1st MIX DBM
XF202
IC204
MCF
BPF
0dBm
Fig. 2
SP
AF
AMP
MIC
MIC
AMP
-104dBm
IF AMP IF AMP FM SYSTEM IC
Q205 Q204 IC202