If Circuit; Audio Amplifier Circuit; Squelch Circuit - Kenwood NX-800 Service Manual

3-3. IF circuit

The first IF signal is passed through a four-pole monolithic
crystal filter (XF202) to reject adjacent channel signal. The
filtered first IF signal is amplified by the first IF amplifier
(Q204, Q205) and then applied to the lF system IC (IC202).
The IF system IC provides a second mixer, AGC amplifier,
and RSSI (Received Signal Strength Indicator).
The second mixer mixes the first IF signal with the
57.6MHz of second local oscillator output and produces the
second IF signal of 450kHz.
ANT
IC704

3-4. Audio amplifier circuit

Audio processing (high-pass filter, low-pass filter, de-
emphasized and so on) at FM mode and decoding at NXDN
mode are processed by DSP. Audio signal from IC510, IC502
goes through the amplifier (IC701). The signal then goes
through mute switch (IC707), amplifier (IC711), and elec-
tronic volume control (IC712).
While busy, INAMT becomes High, turn switch (Q709) on
and Mute switch (Q710) off, and signal is fed to AF Power
Amp (IC715). While Non-busy, INAMT is become Low, turn
switch (Q709) off and Mute switch (Q710) on, then there is
not AF output.
CIRCUIT DESCRIPTION
RF Circuit (Front end)
IC711
(2/2)
IC701 IC707
IC510 SW
AMT
IC502
DSP
The second IF signal is passed through the ceramic filter
(CF201, CF202) to reject the adjacent channel signal. The
filtered second IF signal is amplified by AGC amplifier.
The signal from AGC amplifier is input to AD converter
(IC510) through ceramic filter (CF203) and operational ampli-
fier (IC201 and IC704).
IC202
MIX. AGC
2nd OSC
X1 Q203
BPF
VC
x3
CF201
TCXO
CF202
19.2MHZ
IC712 Q710
MUTE
VOL
SW
INAMT SW
Q709
Fig. 3

3-5. Squelch Circuit

It amplifies the demodulated noise signal from IC202 after
filtering through BPF circuit. Then, the amplified signal is con-
verted to DC signal by the detection circuit. The converted
signal is fed back to IC510.
IC705(2/2)
FMDET
IC202 BPF
NX-800
CF203 IC201
BPF
IC715
INT. SP
AF AMP
EXT. SP
Q702 D703 IC705(1/2)
AMP RECT AMP
IC203(1/2)
AMP
IC203(2/2)
AMP
Fig. 4
ASQDET
IC510
VAGC
RSSI
19