Filter
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TH-K2AT/K2E/K2ET
7
CIRCUIT DESCRIPTION
■ IF Circuit
The first IF signal (38.85 MHz) amplified by the IF amplifier (Q24)
and the second IF signal (38.4 MHz) generated by tripling the 12.8
MHz reference oscillator frequency of the TCXO (X1) by Q23, are
mixed in the FM IC to produce a second IF signal (450 kHz) (Lower
heterodyne).
The second IF signal passes through a ceramic filter (CF1)
to remove unwanted components.
The second IF signal passing through the ceramic filter (CF1)
passes through the IF amplifier in the FM IC again and is
detected to produced an audio signal.
■ Squelch and Mute Circuit
A noise component is obtained by passing FM detection
output (FM IC pin 9) through an operational amplifier in the
CF1
450kHz
CD1
AF OUT
AFM
SM
SQ
IC8
MPU
49
9
48
SMV
SQV
16 15 14 13 12 11 10 9
1 2 3 4 5 6 7 8
MIX
Local
OSC
Buffer
RSSI
Quadrature
detector
LPF
Noise
AMP
IF AMP
Inverter
AMP
1st IF input
(38.85MHz)
IC3
FM IC
2nd local OSC
(38.4MHz)
5R
Q21
AF FILTER
Q20
AF MUTE
SWITCH
VOL IN
R135
C173 C174
R125
R126
Noise
comp
Rectifier
5R
SP
AF OUT
AF
FILTER
Q21
AF MUTE
SWITCH
Q20
IC6
AF AMP
IC3
FM IC
VR201
AF VOLUME
DTMF/BEEP
REM
IC8
MPU
SPK
AFO
SPG
REM/TXD
40
46
9
J4
SP JACK
Fig. 3 IF, Squelch, Mute and S meter circuit
Fig. 4 AF amplifier
FM IC and band-pass filter consisting of R125, R126, R135,
C173, C174. The noise component is rectified in the FM IC to
produce a DC voltage, which is output from the N-REC terminal
(pin 14) of the FM IC as squelch voltage.
The squelch voltage enters the SQ terminal (pin 48) of the
MPU (IC8) and is compared with the reference voltage preset
in the MPU to control audio signal ON/OFF.
When "L" is output from the AFM terminal (pin 9) of the MPU,
the AF mute switch (Q20) is activated to mute the audio signal.
■ S Meter Circuit
The S meter voltage is output from the RSSI terminal (pin
12) of the FM IC (IC3) and input to the SM terminal (pin 49) of
the MPU. Then the voltage is converted from analog to digital
in the MPU to control the S meter display on the LCD.
■ AF Amplifier
The detected audio signal passes through an AF filter (Q21)
and enters an AF mute switch (Q20).
After passing through the AF mute switch (Q20), the signal
passes through an AF volume (VR201) and is amplified to a
specified output level with an AF amplifier (IC6).
The audio signal amplified with the AF amplifier (IC6) is
output through an internal speaker or an external speaker
jack (J4).
The beep tone and the DTMF signal are output from the
DTMF/BEEP terminal (pin 40) of the MPU (IC8), enters the AF
amplifier (IC6) and is output as a monitor tone.