Kenwood NXR-1800 Service Manual page 14

2.5.3 Receiver System
2.5.3.1 RF circuit
The receive signal from RX ANT is amplified by a RF amplifier (LNA: IC703) and passes through the variable capacitor tuned band-
pass filters (BPF) to attenuate unwanted signals. The signal passing the band-pass filters (BPF) is heterodyned by the first mixer (RX
1st MIX: IC702) with the first local oscillator signal from the RX PLL frequency synthesizer circuit to produce a 45.00 MHz first
intermediate frequency signal (first IF signal).
2.5.3.2 IF circuit
The first IF signal is amplified by a first IF amplifier (IF AMP: IC701) and is passed through a four-pole monolithic crystal filter (MCF:
XF700) to reject adjacent channel signal. The filtered first IF signal is amplified by the first IF amplifiers (IF AMP: Q701 and Q700) and
fed to the IF system IC (IF IC: IC700). The IF system IC provides a second mixer, second PLL, AGC, and A/D converter. Second mixer
mixes the first IF signal with the 42.6MHz of second local oscillator (2nd VCO) output and produces the second IF 2.4MHz signal. The
second IF signal is then fed into an A/D converter and generates the I and Q data. This data is in the form of SSI (Serial Synchronous
Interface) and fed to the DSP built into the MPU/DSP (IC33) via FPGA (IC60).
2.5.3.3 RX Audio circuit
Audio processing (high-pass filter, low-pass filter, de-emphasized and so on) in Analog FM mode and decoding in Digital mode are
processed by MPU/DSP (IC33). SSI signal from MPU/DSP is converted to audio signal by CODEC ICs (IC42, IC670).
In "Repeat" mode, the detected audio signal is converted to the modulation signal for TX VCO and TX PLL. It is fed to "VCO_MOD"
terminal of TX VCO from CODEC (IC670), and also fed to "PLL_MOD" terminal of TX PLL (IC2000) from FPGA (IC60). At the same
time, the detected RX audio signal can be monitored by USB Headset Speaker-Microphone from USB connector (J9).
In "Duplex/Simplex" mode (except DMR), the detected RX audio signal can be monitored only by USB Headset Speaker-Microphone.
Also, the detected RX audio signal is fed to RA terminal and RD terminal in CONTROL I/O connector (D-SUB 25-pin J4) from CODEC
(IC42).
RX ANT
2.5.4 Transmitter System
2.5.4.1 TX Audio circuit
In "Repeat" mode, the detected RX audio signal is converted to the TX modulation signal. Refer to "2.5.3.3 RX Audio circuit".
In "Duplex/Simplex" mode (except DMR), the voice signal from USB Headset Speaker-Microphone can be modulated for transmission.
It is fed to MPU/DSP (IC33) and is converted to the modulation signal for TX VCO and TX PLL. It is transferred to "VCO_MOD" terminal
of TX VCO from CODEC (IC670), and also transferred to "PLL_MOD" terminal of TX PLL (IC2000) from FPGA (IC60). Also, the signal
from TA terminal and TD terminal in CONTROL I/O connector (D-SUB 25-pin J4) can be modulated when transmit by external PTT
switch in CONTROL I/O connector (J4).
2.5.4.2 Baseband circuit
The audio signal is processed by MPU/DSP (IC33). Voice signals of 300Hz or lower and frequencies of 3kHz or higher are cut off and
an audio range 300Hz to 3kHz is extracted. The audio signal is filtering processed then synthesized with the signals, such as QT and
DQT, as required, and is transferred to TX PLL Frequency Synthesizer block.
In Digital mode, the audio signal is converted to the 4-Level FSK baseband signal and is transferred to TX PLL Frequency Synthesizer
block. The output level is fine-adjusted according to each modulation system.
2.5.4.3 Drive and Final amplifier
The signal from the TX PLL frequency synthesizer circuit is amplified by pre-drive amplifier (PRE DRV AMP: IC3023). The output
signal of the pre-drive amplifier is amplified by the pre-drive amplifier 2 (PRE DRV AMP2: Q3067), drive amplifier (DRV AMP: Q3065)
and final amplifier (FINAL AMP: Q3069) to 40W. The signal amplified by final stage Q3069 passes through CM coupler circuit and
harmonic filter (LPF) circuit, then it is fed to the TX ANT.
1-14 (No.RA094<Rev.002>)
RX 1st MIX
IC703 IC702
BPF BPF
LNA
RX VCO/RX PLL
IC60
FPGA
IC670
CODEC
MPU/DSP
CODEC
IC33
IC42
Fig.2 Receiver system
XF700 Q701 Q700
IC701
BPF
IF AMP MCF IF AMP IF AMP
TX PLL_MOD
TX VCO_MOD
RA, RD
TA, TD
USB Headset Speaker-Microphone
42.6MHz
2nd
VCO
SSI
IF IC
IC700