2. CIRCUIT DESCRIPTIONS
TRANSMISSION UNIT
• PRE-AMPLIFIER
The pre-amplifier consists of Q807 &Q808. The VCO output signal is applied to pre-amplifier
(Q807 & Q808), to obtain a level above 14dBm, while operating in the frequency range of
135MHz to 174MHz. The T5V supplies a 5V to pre-amplifier Q810 & Q809 are turned on by
T5V. The TX5 supplies a 5V to Q807 & Q808.
• RF POWER AMPLIFIER
The output signal of pre-amplifier provided the drive stage Q801 (RD01MUS1) and power
amplifier Q802 (2SK3476 ), to obtain Rf power above 6 watts. Its current drain is less than
2000mA, while operating in the frequency range of 135MHz to 174MHz.
• ANTENNA SWITCH
Antenna switch circuit consists of two PIN diode (D801, D802), transistor(Q803), and
network(C810 L814 C832). In TX mode, TX5 supplies a High level to turn on Q803 & Q804,
5V bias is applied to the switch circuit to bias two diodes on.
The shunt diode(D802) shorts out the receiver port and a high impedance between L814
and harmonic filter.
In receive mode, the diodes are both off so it will form a low attenuation path between
antenna and receiver ports.
• HARMONIC FILTER
Harmonic filter consists of C807, C806, L803, C805, L802, C804, C803, L801 & C801 to
form three poles low pass filter to attenuate harmonic energy of the transmitter.
• APC (AUTO POWER CONTROL)
Transmitter with feedback voltage which controls driver stage(Q802) and final stage(Q801),
stabilize rf output power. The feedback voltage is formed by D803 and D805, to provide the
U801(B) amplifier and voltage reference(A). It controls two stages, Q801 &Q802, to get bias
voltage and forms an auto level control to stabilize transmitter output power.
• HIGH/LOW POWER CONTROL
PWM (pulse width modulation) which produce basically digital waveforms, the PWM level is
fed to RC filter which is composed of R888, R885, C841, C843 to change U801A pin 1
voltage for control RF power.
RECEPTION UNIT
• FRONT-END
RF signal is fed through antenna into low pass filter, which is composed of C801, C802,
L801, C803, C804, L802, C805, C806, L803, C807, then goes into TX/RX antenna switch,
which is composed of 2 PIN diode (D801, D802), transistor (Q803) and network (C810,
L814, C832). RF signal is coupling with C701 to varactor tuned band pass filter, which is
composed of L701, L702, C702, C703, C704 & D701,D702, to change varactor diode
(D701, D702) voltage to tune band pass filter shift band.
The control voltage of band pass filter is supplied from Q504, the voltage is controlled by
VCO. The output signal of band pass filter is provided with RF amplifier transistor (Q701), the
C710 coupling into second band pass filter, which is composed of L704, C711, C783, C712,
L705, C713, C714, C715, C784, L706, C716, D704,& D705. The Q504 is controlled by VCO.
The voltage of Q504 controls the second varactor tuned band pass filter (D704, D705) band
shift. The signal of RF amplifier and band pass filter are fed through C717 coupling into
double balance mixer. The signal comes from T701, matching with D706, D707, D708,&
D709. The first local signal comes from VCO output, then into local amplifier Q702 and low
pass filter, the Low Pass Filter is composed of C730, L710, C731, L711, C732, the signal
coupling with C733 into transformer (T702) matching with D706, D707, D708, D709, The
VCO using a high side injection the RF signal is down-converted to 45MHz IF signal , the
signal output from T701. The IF signal output comes from mixer through L715, C734 &
C735, coupling to RF signal attenuate and depress loop circuit, which is composed of L713,
C738, R715, L714, C736, C737, and then, through IF amplifier (Q703) applied to crystal filter to
provide necessary selectivity and intermodulation protection.
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