NX-800
19
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.
CIRCUIT DESCRIPTION
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.
IC715Q710
AF AMP
INAMT
IC711
(2/2)
MUTE
SW
Q709
SW
SW
IC202
MIX. AGC
IC701 IC707IC704
IC510
IC502
DSP
IC201
IC712
AMT
VOL
RF Circuit (Front end)
CF203
BPF
Q203
2nd OSC
X1
19.2MHZ
VC
TCXO x3 CF201
CF202
BPF
INT. SP
EXT. SP
ANT
IC202 IC510
IC705(2/2)
FMDET ASQDET
VAGC
RSSI
BPF
Q702
AMP
D703
RECT
IC705(1/2)
AMP
IC203(1/2)
AMP
IC203(2/2)
AMP
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.
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).
Fig. 3Fig. 4