Kenwood NX-900 Service Manual page 18

NX-900
3-3. IF circuit
The fi rst IF signal is passed through a four-pole mono-
lithic crystal fi lter (XF202) to reject adjacent channel signal.
The fi ltered fi rst IF signal is amplifi ed by the fi rst IF amplifi er
(Q205,Q204) and is then applied to the IF system IC (IC202).
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
57.6MHz of the second local oscillator output and produces
the second IF signal of 450kHz.
ANT
IC704
3-4. Audio amplifi er circuit
Audio processing (high-pass filter, low-pass filter, de-
emphasized, etc.) at FM mode and decoding at NXDN mode
are processed by the DSP (IC717). The audio signal from
IC510, IC717 goes through the amplifi er (IC701). The signal
then goes through the mute switch (IC707), amplifi er (IC711),
and electronic volume control (IC719).
While busy, INAMT becomes High, turning the switch
(Q709) on and the Mute switch (Q710) off, and the signal is
fed to the AF Power Amp (IC715). While not busy, INAMT
becomes Low, turning the switch (Q709) off and the Mute
switch (Q710) on (there is no AF output).
18
CIRCUIT DESCRIPTION
RF/1st IF Circuit
X1
VC
TCXO
19.2MHz
IC711
(2/2)
IC510 IC701 IC707
SW
AMT
DSP
IC717
IC705 (2/2) Q702
FM_DET
IC202 BPF AMP
The second IF signal is passed through the ceramic fi lter
(CF201,CF202) to reject the adjacent channel signal. The fi l-
tered second IF signal is amplifi ed by the AGC amplifi er.
The signal from the AGC amplifier is input to the A/D
converter (IC510) through the ceramic fi lter (CF203) and op-
erational amplifi er (IC201 and IC704).
IC202
MIX, AGC
2nd OSC
Q11 Q203
BPF
x3
CF201
CF202
IC719 Q710 IC715
MUTE
VOL
SW
AF AMP
SW
/INAMT
Q709
Fig. 3
3-5. Squelch Circuit
The squelch circuit amplifies the demodulated noise
signal from IC202 after fi ltering through the BPF circuit. The
amplifi ed signal is then converted to a DC signal by the de-
tection circuit. The converted signal is fed back to IC510.
D703 IC705 (1/2)
ASQDET
RECT AMP
IC203 (1/2)
VAGC
AMP
IC203 (2/2)
RSSI
AMP
Fig. 4
CF203 IC201
BPF
INT. SP
EXT. SP
IC510