8
TK-2107
TK-260
:K, K2
CIRCUIT DESCRIPTION
3) VCO and RF amplifier
The transmit signal obtained from the VCO buffer amplifier
Q100, is amplified by Q101. This amplified signal is passed to
the power amplifier, Q102 and Q105, which consists of a 2-
stage FET amplifier and is capable of producing up to 5W of
RF power. (See Fig.7)
Fig. 7 APC system
AMP
Q101
DRIVE AMP
Q102
5T
B
Q103, Q104
B SW
DET
IC100
APC
5T
Q109
SW
Q106
SW
APC
5T
Q108
SW
RX
D102,103
ANT SW
FINAL AMP
Q105
ANT SW
D101
LPF
ANT
5T
SW
Q107
TH100
4) ANT switch and LPF
The RF amplifier output signal is passed through a low-
pass filter network and a transmit/receive switching circuit
before it is passed to the antenna terminal. The transmit/
receive switching circuit is comprised of D101, D102 and
D103. D102 and D103 turned on (conductive) in transmit
mode and off (isolated) in receive mode.
5) APC
The automatic power control (APC) circuit stabilizes the
transmitter output power at a predetermined level by sensing
the drain current of the final amplifier Field Effect Transistor
(FET) . The voltage comparator, IC100 (2/2), compares the
voltage obtained from the above drain current with a reference
voltage which is set using the microprocessor. An APC
voltage proportional to the difference between the sensed
voltage and the reference voltage appears at the output of
IC100 (1/2). This output voltage controls the gate of the FET
power amplifier, which keeps the transmitter output power
constant. The transmitter output power can be varied by the
microprocessor which in turn changes the reference voltage
and hence, the output power.
6) Terminal protection circuit
When the thermistor (TH100) reaches about 80˚C, the
protection circuit turns on Q107 to protect transmitting final
amplifier (Q105) from the over heating.
5. Power supply
A 5V reference power supply [5M] for the control circuit is
derived from an internal battery. This reference is used to
provide a 5V supply in transmit mode [5T], a 5V supply in
receive mode [5R], and a 5V supply common in both modes
[5C] based on the control signal sent from the microprocessor.
6. Control system
The IC403 CPU operates at 7.37MHz . This oscillator has a
circuit that shifts the frequency according to the EEPROM
data.
