Circuit Description; Frequency Configuration; Receiver System; Audio Amplifier Circuit - Kenwood NX-5800 Service Manual

2.4

CIRCUIT DESCRIPTION

2.4.1 Overview
The NX-5800 is a UHF Analog FM & Digital Mobile transceiver designed to operate in the frequency range of 450 to 520MHz (F, K)
or 380 to 470MHz (F2, K2). The unit consists of a receiver, a transmitter, a phase-locked loop (PLL) frequency synthesizer, a digital
control unit, and a power supply circuit.

2.4.2 Frequency Configuration

The receiver is a double-conversion super-heterodyne using first intermediate frequency (IF) of 49.95MHz and second IF of 2.25MHz.
Incoming signals from the antenna are mixed with the local signal from the VCO/PLL circuit to produce the first IF of 49.95MHz. This
is then mixed with the 47.7MHz second local oscillator output to produce the 2.25MHz second IF. The transmit signal frequency is
generated by the TX VCO, and modulated by the signal from the DSP. It is then amplified and fed to the antenna.
ANT
TX/RX: 450~520MHz(F,K)
TX/RX: 380~470MHz(F2,K2)
ANT
SW AMP
19.2MHz
TCXO
PA
AMP AMP

2.4.3 Receiver System

2.4.3.1 RF circuit
The receive signal from antenna switch (D310, D311, D312, D313, D314 and D315) is amplified by a RF amplifier (Q502) and passes
through the band-pass filter (L511, L510, L509 and L508) to remove unwanted signals. The signal is then fed to the 1st mixer (IC500).
2.4.3.2 IF circuit
The first IF signal is passed through a four-pole monolithic crystal filter (XF600) to reject adjacent channel signal. The filtered first IF
signal is amplified by the first IF amplifier (Q604 and Q603) and then applied to the IF system IC (IC600). The IF system IC provides
a second mixer, second PLL, AGC and A/D converter.
The second mixer mixes the first IF signal with the 47.7MHz of second local oscillator output and produces the second IF signal of
2.25MHz.
The second IF signal is then be fed into an A/D converter, generates the I and Q data. This data is in the form of SSI (Serial Synchro-
nous Interface), and sent to the DSP (IC706).
2.4.3.3

Audio amplifier circuit

Audio processing (high-pass filter, low-pass filter, de-emphasized and so on) at Analog FM mode and decoding at Digital mode are
processed by DSP. SSI signal from DSP is converted to audio signal at IC902. The signal goes to internal speaker and connector for
external speaker (J900) through the amplifier (IC903), electronic volume control (IC719) and audio amplifier (IC911).
ANT
D310,311
D312,313
D314,315
ANT
ANT
LPF
SW
DSP
IC706
1st MIX 49.95MHz
RF
MCF
400.05~470.05MHz(F,K)
330.05~420.05MHz(F2,K2)
TX
TX
VCO
450~520MHz(F,K)
380~470MHz(F2,K2)
Fig.1 Frequency configuration
L515
L516
Q502
L517
BPF
RF AMP
CODEC
IC902
Fig.2 RF and IF circuit
2nd
VCO
47.7MHz
MIX
IF Circuit
Baseband
Circuit
PLL
RX
VCO
L508,509
L510,511 IC500 XF600
BPF BPF
MCF
RX VCO/PLL
VOL
IC903 IC719 IC911
SP
AF
AMP
CODEC
MIC
2nd
Q600
VCO
Q604 Q603
IF IC
IF AMP IF AMP IC600
Internal Speaker
External Speaker
J900
(No.RA017<Rev.002>)1-15
SSI