Kenwood TH-K2AT Service Manual page 7

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 unw anted 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 M ute Circuit
A noise component is obtained by passing FM detection
output (FM IC pin 9) through an operational amplifier in the
9
AFM
49
IC8
SM
M PU
48
SQ
1st IF input
(38.85M Hz)
16
M IX
IC3
FM IC
Buffer
Local
OSC
1
2nd local OSC
(38.4M Hz)
AF Amplifier
The detected audio signal passes through an AF filter (Q21)
and enters an AF mute sw itch (Q20).
After passing through the AF mute sw itch (Q20), the signal
passes through an AF volume (VR201) and is amplified to a
specified output level w ith an AF amplifier (IC6).
The audio signal amplified w ith the AF amplifier (IC6) is
output through an internal speaker or an external speaker
jack (J4).
The beep tone and the DTM F signal are output from the
DTM F/BEEP terminal (pin 40) of the M PU (IC8), enters the AF
amplifier (IC6) and is output as a monitor tone.
CIRCUIT DESCRIPTION
SM V
5R
SQV
CD1
15 14 13 12 11 10
RSSI
Noise
Quadrature
com p
Rectifier
IF AM P
2 3 4 5 6
C173
CF1
450kHz
Fig. 3 IF, Squelch, M ute and S meter circuit
TH-K2AT/K2E/K2ET
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, w hich 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
M PU (IC8) and is compared w ith the reference voltage preset
in the M PU to control audio signal ON/OFF.
When "L" is output from the AFM terminal (pin 9) of the MPU,
the AF mute sw itch (Q20) is activated to mute the audio signal.
S M eter 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 M PU. Then the voltage is converted from analog to digital
in the M PU to control the S meter display on the LCD.
AF OUT
9
LPF
detector
Noise
AM P
Inverter
AM P
7 8
R135
C174
R126
R125
IC3
FM IC
9
AF OUT
VR201
AF VOLUM E
DTM F/BEEP
IC8
M PU
VOL IN
Q20
AF M UTE
SWITCH
Q21
AF FILTER
5R
Q21 Q20
AF AF M UTE
FILTER SWITCH
SPG
REM /TXD
IC6
AF AM P
SPK
AFO
40 SP
46
REM
Fig. 4 AF amplifier
J4
SP JACK
7