Kenwood TK-3312 Service Manual page 13

2-3. IF Amplifier Circuit
The first IF signal is passed through a four-pole mono-
lithic crystal filter (XF401) to remove the adjacent channel
signal. The filtered first IF signal is amplified by the first 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 first 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 fil-
ter (Wide: CF402, Narrow: CF401) to remove the adjacent
channel signal. The filtered second IF signal is amplified by
the limiting amplifier 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 filters CF401 (narrow).
The second IF signal always passes the ceramic filters
CF402 (Wide).
The wide and narrow switching data is output from the
MCU (IC820).
D401 and D402 are switched to ceramic filters 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.
Q404
WID/NAR
R423
Q401
IC401
+ -
R409
CF402
Fig. 3 Wide/Narrow switching circuit
CIRCUIT DESCRIPTION
R424
R426
CD401 50C
AFOUT
D401 D402
CF401
TK-3312/3317
2-5. Audio Amplifier Circuit
The demodulated signal from IC401 is sent to an AF am-
plifier through baseband IC (IC812), and is routed to an au-
dio power amplifier (IC815) where it is amplified 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 amplified
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 Amplifier 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
filter, low-pass filter 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 goes to the TCXO
modulation input.
MIC
D807,808
AGC
Q810,811
MIC_MT SW
Q809
Fig. 4 Microphone amplifier circuit
IC812
VCO
Baseband
IC X1
TCXO
LSD
OUT
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