If Circuit; Audio Amplifier Circuit; Squelch Circuit - Kenwood NX-700 Service Manual

NX-700/700H

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
(Q204, Q205) and then applied to the lF 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 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 and so on) at FM mode and decoding at NXDN
mode are processed by DSP. Audio signal from IC510,
IC502 goes through the amplifi er (IC701). The signal then
goes through mute switch (IC707), amplifier (IC711), and
electronic volume control (IC712).
While busy, INAMT becomes High, turn switch (Q709)
on and Mute switch (Q710) off, and signal is fed to AF Pow-
er Amp (IC715). While Non-busy, INAMT is become Low,
turn switch (Q709) off and Mute switch (Q710) on, then
there is not AF output.
18
CIRCUIT DESCRIPTION
RF Circuit (Front end)
IC711
(2/2)
IC701 IC707
IC510 SW
AMT
DSP
IC502
IC705 (2/2) Q702
FMDET
IC202 BPF AMP
The second IF signal is passed through the ceramic fi lter
(CF201, CF202) to reject the adjacent channel signal. The
fi ltered second IF signal is amplifi ed by AGC amplifi er.
The signal from AGC amplifi er is input to AD converter
(IC510) through ceramic fi lter (CF203) and operational ampli-
fi er (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 IC510
after fi ltering through BPF circuit. Then, the amplifi ed signal
is converted to DC signal by the detection circuit. The con-
verted 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
IC715
INT. SP
AF AMP
EXT. SP
IC510