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 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
CF1
450kHz
CD1
AF OUT
AFM
SM
SQ
IC8
M PU
49
9
48
SM V
SQV
16
15
14
13
12
11
10
9
1
2
3
4
5
6
7
8
M IX
Local
OSC
Buffer
RSSI
Quadrature
detector
LPF
Noise
AM P
IF AM P
Inverter
AM P
1st IF input
(38.85M Hz)
IC3
FM IC
2nd local OSC
(38.4M Hz)
5R
Q21
AF FILTER
Q20
AF M UTE
SWITCH
VOL IN
R135
C173
C174
R125
R126
Noise
com p
Rectifier
5R
SP
AF OUT
AF
FILTER
Q21
AF M UTE
SWITCH
Q20
IC6
AF AM P
IC3
FM IC
VR201
AF VOLUM E
DTM F/BEEP
REM
IC8
M PU
SPK
AFO
SPG
REM /TXD
40
46
9
J4
SP JACK
Fig. 3 IF, Squelch, M ute 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, 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 M PU,
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 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.
