Kenwood FM Transceiver TK-481 Service Manual page 17

TK-480/481
The signal from microphone passes through the limitter
circuit in D11, and through the high-pass filter, the ALC cir-
cuit, the low-pass filter, the high-pass filter, and pre-empha-
sis/IDC circuit in IC12. When encoding DTMF, mute switch
(Q7) is turned OFF for muting the microphone input signal.
The signal passes through the D/A converter (IC4) for the
maximum deviation adjustment, and enters the summing
amplifier consisting of IC3 (2/2), and is mixed with the low
speed data from the CPU (IC15).
The output signal from the summing amplifier passes
through the D/A converter (IC4) again for the TA maximum
deviation adjustment,and the AF switch (Q21 is off in TX),
and goes to the VCO modulation input.
The other output signal from the summing amplifier
passes through the D/A converter (IC4) again for the BAL
adjustment,and the buffer amplifier (IC1 (2/2)), and goes to
the VCXO modulation input.
EXT.
MIC
MIC
16
CIRCUIT DESCRIPTION
IC12
Q300
D11
12
SW LIMIT HPF
MIC
IC4
Q301 D/A
I5 O5
MSW
Fig. 6 Microphone amplifier
Q11 Q12
From
Pre-DRIVE
DRIVE
T/R SW
AMP
AMP
(D5)
+B
R127
R128
R129
IC21
(1/2)
REF
VOL
(IC4)
Fig. 7 Drive and final amplifier and APC circuit
4-2. Drive and Final amplifier
The signal from the T/R switch (D5 is on) is amplified by
the pre-drive (Q11) and drive amplifier (Q12) to 50mW.
The output of the drive amplifier is amplified by the RF
power amplifier (IC30) to 2.5W (1W when the power is low).
The RF power amplifier consists of two stages MOS FET
transistor. The output of the RF power amplifier is then
passed through the harmonic filter (LPF) and antenna switch
(D7 is on) and applied to the antenna terminal.
4-3. APC circuit
The APC circuit always monitors the current flowing
through the RF power amplifier (IC30) and keeps a constant
current. The voltage drop at R127, R128 and R129 is caused
by the current flowing through the RF power amplifier and
this voltage is applied to the differential amplifier (IC21 1/2).
IC21 (2/2) compares the output voltage of IC21 (1/2) with
the reference voltage from IC4, and the output of IC21 (2/2)
controls the VGG of the RF power amplifier to make the
both voltages to same voltage.
The change of power high/low is carried out by the
change of the reference voltage. Q14,15 and 18 are turned
on in transmit and the APC circuit is active.
MUTE DTMF
Q7
SW
15 18 19 9
16
COMP LPF HPF
ALC
IC3 (2/2) IC4 Q21
SUM D/A AF
AMP I2 O2 MUTE
5R
LSD
IC4 IC1
D/A BUFF
I1 O1
AMP
IC30 D7
RF ANT
LPF
POWER AMP SW
VDD VGG
IC21
(2/2)
8
6
PRE EMP
IDC
IC14
VCO
X2
VCXO
ANT