TK-2140
8
CIRCUIT DESCRIPTION
Fig. 6 Squelch and RSSI voltage vs ANT input level
Fig. 5 Squelch circuit
12
DET
BPF
AMP
BPF
AMP
DET
RSSI
NOISE
AMP
IC309
CPU
IF AMP
7
91
93
IC200 : FM IF IC
Q208
IC201
(2/2)
D201
SQ close
SQ open
Preset
value
ANT input level
SQ voltage
ANT input level
RSSI voltage
Preset balue
4. Transmitter System
4-1. Microphone amplifier
The signal from the internal microphone goes through the
mute switch (Q301).
When the SP-MIC is not attached, the microphone switching
terminal (MSW) on the universal connector becomes High, and
mute switch (Q301) is turned ON. When the SP-MIC is
attached, MSW is connected to GND at inside of SP-MIC. For
this reason, Q301 is turned OFF, the internal microphone is
muted, and only the input of the external microphone is supplied
to the microphone amplifier.
The signal from microphone passes through the limitter
circuit in D501 and amplified by IC501 (1/2), and Mic mute
switch (Q502 is off in TX) and through the high-pass filter, the
ALC circuit, the low-pass filter, the high-pass filter, and pre-
emphasis/IDC circuit in IC500 on the small board. When
encoding DTMF, mute switch (Q501) is turned OFF for muting
the microphone input signal.
The signal passes through the D/A converter (IC307) for
the maximum deviation adjustment, and enters the summing
amplifier consisting of IC305 (1/2), and is mixed with the low
speed data from the CPU (IC309).
3-5. Squelch circuit
The output from IC200 enters FM IC again, then passed
through a band-pass filter. And passed through a band-pass
filter IC202 (2/2). The noise component output from IC202 (2/
2) is amplified by Q208 and rectified by D201 to produce a DC
voltage corresponding to the noise level. The DC voltage is
sent to the analog port of the CPU (IC309). And IC200 outputs
a DC voltage (RSSI) corresponding to the input of the IF
amplifier. The CPU reads the RSSI signal via pin 93.
IC309 determines whether to output sounds from the
speaker by comparing the input voltage of pin 91 and pin 93
with the preset value.
The output signal from the summing amplifier passes
through the D/A converter (IC307) again and goes to the VCO
modulation input.
The other output signal from the summing amplifier passes
through the D/A converter (IC307) again for the BAL
adjustment, and the buffer amplifier (IC302 : 2/2), and goes to
the VCXO modulation input.
Fig. 7 Microphone amplifier
12
HPF
LPF
HPF
IDC
PRE
EMP
ALC
COMP
SW
LIMIT
SW
MIC
Q301
D501
AMP
IC501
MIC
EXT.
MIC
Q305
IC500
15
16
18
19
Q500
MUTE
DTMF
9
8
6
D/A
D/A
IC307
IC307
IC302
I5
O5
I1
O1
D/A
IC307
I2
O2
LSD
DI9
IC305 (1/2)
SUM
AMP
BUFF
AMP
VCXO
VCO
A1
X1
MIC
MUTE
Q502
PTT
MSW
4-2. Drive and Final amplifier
The signal from the T/R switch (D101 is on) is amplified by
drive amplifier (Q100 and Q101) to +15~17dBm.
The output of the drive amplifier is amplified by the RF power
amplifier (Q106) to 5.0W (1W when the power is low). The RF
power amplifier is MOS FET transistor. The output of the RF
power amplifier is then passed through the harmonic filter (LPF)
and antenna switch (D102,103 are on) and applied to the
antenna terminal.
Fig. 8 Drive and final amplifier and APC circuit
From
T/R SW
(D9)
Pre-DRIVE
AMP
DRIVE
AMP
RF
POWER AMP
LPF
ANT
SW
D102,103
ANT
VGG
Q100
Pre-DRIVE
AMP
Q101
Q103
Q106
VDD
R119
R120
R125
+B
IC100
(1/2)
IC100
(2/2)
REF
VOL
(IC8)
4-3. APC circuit
The APC circuit always monitors the current flowing through
the drive amplifier (Q103) and the RF power amplifier (Q106)
and keeps a constant current. The voltage drop at R119, R120
and R125 is caused by the current flowing through the RF
power amplifier and this voltage is applied to the differential
amplifier (IC100 1/2).
IC100(2/2) compares the output voltage of IC100(1/2) with
the reference voltage from IC307, and the output of IC100(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. Q105,107 and 108 are turned on in
transmit and the APC circuit is active.
