NX-320
13
3-2. IF circuit
The fi rst IF signal is passed through a four-pole monolithic
crystal fi lter (XF1) to reject adjacent channel signals. The fi l-
tered fi rst IF signal is amplifi ed by the fi rst IF amplifi er (Q303)
and then applied to the IF system IC (IC302). The IF system
IC provides a second mixer, AGC amplifi er, and RSSI (Re-
ceived Signal Strength Indicator).
The second mixer mixes the first IF signal with the
50.4MHz of the second local oscillator output and produces
the second IF signal of 450kHz.
The second IF signal is passed through the ceramic fi lter
(CF1) to reject the adjacent channel signal. The fi ltered sec-
ond IF signal is amplifi ed by the AGC amplifi er.
The signal from the AGC amplifier is input to the ASIC
(IC610) through the ceramic fi lter (CF2).
3-3. Audio amplifi er 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 the DSP. The audio signal from
IC610 and IC603 goes through the amplifier (IC904). The
signal then goes through an electronic volume control (IC4),
and AF amplifi er (IC901).
While AFSW is High, the power supply of audio power
amplifi er (IC901) is activated, and the output to the speaker
turns ON.
3-4. Squelch circuit
It amplifi es the demodulated noise signal from IC610 af-
ter fi ltering through the LPF and HPF circuit. The amplifi ed
signal is then converted to a DC signal by the detection cir-
cuit. The converted signal is fed back to IC610.
Fig. 3 Squelch circuit
DET
IC610
IC1 (1/2)
ASQAPC
ASQDET
ASQ
LPF
IC1 (2/2)
HPF
Q3
AMP
D2
RECT
IC304 (1/2)
AMP
4. Transmitter System
4-1. Audio band circuit
The signal from the microphone is amplifi ed by IC2 (2/2)
and limited by the AGC circuit which is composed of D3,
D4, Q4 and Q5. IC2 works as an anti-aliasing LPF fi lter.
4-2. Baseband circuit
The audio signal output from the baseband circuit is con-
verted to digital data with a sampling frequency of 48kHz.
This digital data is sent to the DSP (IC603), and voice signals
of 300Hz or lower and frequencies of 3kHz or higher are cut
off and the audio range of 300Hz to 3kHz is extracted. The
audio signal is then pre-emphasized in FM mode and syn-
thesized with the signals, such as QT and DQT, as required,
and is then output from IC610. In Digital mode, the audio
signal is converted to the 4-Level FSK baseband signal and
output from IC610. The DTMF and MSK baseband signals
are also generated by the DSP and output by IC610
The LPF (IC6) works as a smoothing fi lter. The level out-
put according to the transmit carrier is fi ne-adjusted accord-
ing to each modulation method.
Fig. 4 Audio band and Baseband circuit
AMP
AGC
IC2 (2/2)
D3,D4
Q4,Q5
LPF
VCO
to TX stage
PLL IC
IC403
IC6 (1/2)
IC610
IC603
DSP
MIC
CIRCUIT DESCRIPTION
