TK-2317
13
2-3. IF Amplifi er Circuit
The fi rst IF signal is passed through a four-pole mono-
lithic crystal fi lter (XF401) to remove the adjacent channel
signal. The fi ltered fi rst IF signal is amplifi ed by the fi rst IF
amplifier (Q405) and is then applied to the IF system IC
(IC401).
The IF system IC provides a second mixer, limiting
amplifier, quadrature detector and RSSI (Received Signal
Strength Indicator). The second mixer mixes the fi rst IF sig-
nal with the 50.4MHz signal of the second local oscillator
output (Q403) and produces the second IF signal of 450kHz.
The second IF signal is passed through the ceramic fi l-
ter (Wide: CF402, Narrow: CF401) to remove the adjacent
channel signal. The fi ltered second IF signal is amplifi ed by
the limiting amplifi er and demodulated by the quadrature de-
tector with the ceramic discriminator (CD401). The demodu-
lated signal is routed to the audio circuit.
2-4. Wide/Narrow Switching Circuit
Wide and narrow settings can be made for each channel
by switching the ceramic fi lters CF401 (narrow).
The second IF signal always passes the ceramic fi lters
CF402 (Wide).
The wide and narrow switching data is output from the
MCU (IC820).
D401 and D402 are switched to ceramic fi lters when a
narrow mode is selected.
Q404 turns on/off with the Narrow and the IC401 detector
output level is changed to maintain a constant output level
during wide or narrow signals.
IC401
WID/NAR
D401
IFOUT
MXO
IFI
QU
A
D
AFOUT
C404
R406
C402
C414
R408
R411
R415
D402
CF401
CF402
C424
R423
R424
Q401
R426
50C
AFOUT
CD401
Q404
R40
9
IC402
+
-
R471
Fig. 3 Wide/Narrow switching circuit
2-5. Audio Amplifi er Circuit
The demodulated signal from IC401 is sent to an AF am-
plifi er through baseband IC (IC812), and is routed to an au-
dio power amplifi er (IC815) where it is amplifi ed and output
to the speaker.
2-6. Squelch Circuit
Part of the AF signal from the IC401 enters the FM sys-
tem IC (IC401) again, and the noise component is amplifi ed
and rectified by a filter and an amplifier to produce a DC
voltage corresponding to the noise level.
The DC signal from the FM IC goes to the analog port of
the MCU (IC820). IC820 determines whether or not to output
sounds from the speaker by checking if the input voltage is
higher or lower than the preset value.
To output sounds from the speaker, IC820 sends a high
signal to the AF_CONT line and turns IC815 on through
Q813, Q814, Q818 and Q819.
3. Transmitter System
3-1. Microphone Amplifi er Circuit
The signal from the microphone is limited by the AGC
circuit, which is composed of D807, D808, Q810, Q811 and
the mute switch (Q809). IC812 is composed of a high-pass
fi lter, low-pass fi lter and pre-emphasis/IDC circuit.
The signal from the microphone and the low speed data
from the MCU (IC820) enter the baseband IC (IC812) and
pass through each path and are mixed inside the IC.
The output signal from the baseband IC goes to the VCO
modulation input. The other output signal passes through the
amplifi er (IC3) and goes to the TCXO modulation input.
AMP
IC3
AGC
SW
D807,808
Q810,811
MIC_MT
X1
TCXO
VCO
LSD
OUT
IC812
Q80
9
MIC
Baseband
IC
Fig. 4 Microphone amplifi er circuit
CIRCUIT DESCRIPTION
