Circuit Description - Kenwood NX-820G Quick Start Manual

1. Overview
The NX-820 is a UHF Mobile transceiver designed to
operate in the frequency range of 400 to 470MHz. The unit
consists of a receiver, transmitter, phase-locked loop (PLL)
frequency synthesizer, base band parts, power supply, and
control circuits.
2. Frequency Confi guration
The receiver is a double-conversion super-heterodyne
using a fi rst intermediate frequency (IF) of 49.95MHz and a
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 49.95MHz. This is then mixed with the 50.4MHz
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.
ANT
TX /RX : 400~470MHz
1st MIX MCF
ANT RF
AMP
SW
49.95 MHz
16. 8 MHz × 3
TCXO
350.05~
420.05MHz
PA TX
AMP
AMP 400~470MHz
Fig.1 Frequency confi guration
ANT
D 110 , 111
L116 , 117
D107
ANT
LPF
SW
IC507
ASIC
IC502
DSP

CIRCUIT DESCRIPTION

CF
450kHz
SP
MIX AF
IF AMP AMP
50.4 MHz
MIC
PLL MIC
VCO AMP
L207,208
L215 , 216 ,
L209,211 1st MIX
Q 202
210
BPF BPF
RF AMP
1st PLL
OSC ( PLL)
IC711
IC 711
IC712
( 2 /2 )
( 1 / 2 )
VOL
Fig.2 Receiver System
NX-820(G)/820
3. Receiver System
3-1. RF circuit
An incoming RF signal from the antenna terminal is
passed through the antenna switch (D110, 111 and 107) and
then the bandpass fi lter (L215, 216 and 210). 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 (IC712). The signal is amplifi ed by an RF
amplifier (Q202), and passed through the bandpass filter
(L207, 208, 209 and 211). The resulting signal is applied to
the fi rst mixer (Q201) where it is mixed with the fi rst local
oscillator signal output from the frequency synthesizer to
produce the fi rst IF (49.95MHz).
3-2. IF circuit
The fi rst IF signal is passed through a four-pole mono-
lithic crystal fi lter (XF1) to reject the adjacent channel signal.
The fi ltered fi rst IF signal is amplifi ed by the fi rst IF amplifi er
(Q305) and then applied to the IF system IC (IC303). 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
50.4MHz of the second local oscillator output and produces
the second IF signal of 450kHz.
The second IF signal is passed through the ceramic fi lter
(CF1) to reject the adjacent channel signal. The fi ltered sec-
ond IF signal is amplifi ed by the AGC amplifi er.
The signal from the AGC amplifi er is input to an AD con-
verter in ASIC (IC507) through the ceramic fi lter (CF2).
Q 201
XF 1 Q 305
MCF IF AMP
X1 Q303
x 3
TCXO
16.8 MHz
IC714
SP
BPF
CF 2
IC 303
BPF
MIX , AGC
CF1
11