3-3 TRANSMITTER SECTION
RF POWER Amplifier
The transmitter parts of the TX-446 is designed to amplify the RF signal oscillated and modulated by the synthesizer to
approximately 500mW by the power transistor of Q407.
The transmitted signal of approximately 7mW, combined at the driver FET is supplied to the gate of the Q407 amplifier.
The transmitted signal amplifier to 500mW here passes the TX LPF of the second characteristics of the C418-C421, L408,
PCB_L1 and PCB_L2 and RX/TX switching takes place by the Q406. After this, the signal is provided to the antenna the
TX LPF of the first characteristics consisted of the C427, C428, C432, L412, L413 and L414.
Antenna Switching
Switching of the antenna between the transmitter and the receiver is accomplished by the antenna transmit/receive
switch consisting of diodes Q404 and Q406 in conjunction with C415 and R414. In the transmit mode, switched TX B+
is applied through RF choke L415, hard forward biasing the two diodes on. Q406 thus permits the flow of RF power
from output of the low-pass filter fed by the output amplifier to the output low-pass filter. Q404 shorts the receiver input
to C415, which is AC coupled to ground. L407, C416, C415 and R414 then function as a lumped constant quarter-wave
transmission line, thus presenting a high impedance to the RF output path, effectively isolating the receiver input and
transmitter output sections.
POWER Control
Output power is controlled by the microprocessor Q106, which is used as a differential amplifier and comparator.
Current is sensed by the voltage drop across R417.
When the radio transmitter mode, this voltage is compared to the one set by the 500mW.
The power output is then reduced or increased by varying the applied to the power amplifier Q407.
Transmitter Audio Circuits
The transmitter audio circuits consist of the audio processing circuits, the CTCSS circuits and the DCS circuits.
Audio Processing
Transmit speed audio is providing by either the internal electric microphone E201 or the external microphone.
The microphone audio is pre-emphasized by 6dB per octave by C540 and R557, and then signal amplification and
applied to Lo-pass filter Q507D, Q505D.
The gain is such that when a signal 20dB greater than limiting the peak-to-peak output.
Under these conditions, the MOD. ADJ. Pot RV501 configured as a four-pole active low-pass filter.
The resulting signal is then limited when respect to side band splatter, and has an 18dB per octave roll-off above 3KHz.
The audio is then applied to transmit VCO. By varying the voltage on the varactor diode Q308 at an audio rate.
The resonant frequency of VCO is varied. The result is an oscillator output that is frequency-modulated at the audio
frequency.
CTCSS Tone Encoder / Digital Code Squelch (DCS) Encoder
CTCSS signals and DCS signals are synthesized by microprocessor Q106 and appear as pulse waveform on I/O lines
Pin 4-8. These I/O lines are applied to a resistive digital-to-analog converter network (consisting of R128-R132) which
produce a pseudo-sine wave for CTCSS or a waveform for DCS at its output. The waveform is smoothed by low pass
filters Q507C to produce an acceptable sine wave output. The CTCSS tone signal is adjust to the proper level by
RV101. The DCS signal is adjust to the proper balance by RV503. The signal is then applied to the audio processing
circuit at R305 and to the TCXO circuit at X301.
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