Wide/Narrow Switching Circuit; Af Signal System; Squelch Circuit - Kenwood TK-7108 Service Manual

TK-7108

■ Wide/Narrow Switching Circuit

The Wide port (pin 65) and Narrow port (pin 64) of the CPU
is used to switch between ceramic filters. When the Wide
port is high, the ceramic filter SW diodes (D303, D302) cause
CF301 to turn on to receive a Wide signal.
When the Narrow port is high, the ceramic filter SW di-
odes (D303, D302) cause CF302 to turn on to receive a Nar-
row signal.

■ AF Signal System

The detection signal from IF IC (IC301) goes to D/A con-
verter (IC161) to adjust the gain and is output to AF filter
(IC251) for characterizing the signal. The AF signal output
from IC251 and the DTMF signal, BEEP signal are summed
and the resulting signal goes to the D/A converter (IC161).
The AFO output level is adjusted by the D/A converter. The
signal output from the D/A converter is input to the audio
power amplifier (IC252). The AF signal from IC252 switches
between the internal speaker and speaker jack (J1) output.

■ Squelch Circuit

The detection output from the FM IF IC (IC301) passes
through a noise amplifier (Q301) to detect noise. A voltage is
applied to the CPU (IC101). The CPU controls squelch ac-
cording to the voltage (SQIN) level. The signal from the RSSI
pin of IC301 is monitored. The electric field strength of the
receive signal can be known before the SQIN voltage is input
to the CPU, and the scan stop speed is improved.
26
CIRCUIT DESCRIPTION /
IF_IN
CF301
(Wide)
CF302
(Narrow)
D303
Narrow
IC101 64pin
Fig. 3 Wide/Narrow switching circuit /
IC301 IC161
W/NO
(EVOL2)
D/A
IF IC
CONV.
Fig. 4 AF signal system /
IC301
AFO
IF
SYSTEM
RSSI
Fig. 5 Squelch circuit /
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