Circuit Description; General Principle Diagram - HYT TC-508 Service Manual

TC-508 Service Manual

Circuit Description

1. General Principle Diagram

The general circuit is composed of TX circuit, RX circuit, power supply circuit, control circuit and etc. See
the block diagram below:
Figure 1
TX Section: The audio signal from MIC is sent to the MCU chip (U601) directly. The baseband signal
processing circuit in U601 converts the amplified audio signal to digital signal via A/D conversion, and
then converts the digital signal to analog audio signal via D/A conversion after all processing procedures
such as filtering, signal compression, encryption, pre-emphasis, TX gain control and amplitude limiting
are completed. Then the analog audio signal from MCU passes through the low-pass filter circuit and
directly goes to VCO for frequency modulation. The modulated HF carrier signal passes through the
buffer amplifiers (Q104 and Q406), and goes to the amplifier (Q401) through the RF switch (D404). Then
the amplified RF signal enters the low-pass filter circuit after it is further amplified by the pre-driver (Q402)
and final-stage power amplifier (Q403), and then it will be finally transmitted via antenna after ultra
harmonics is removed.
RX Section: RF signals received via the antenna are fed to the band-pass filter to filter out undesired
out-of-band interference signal. Then the remaining RF signals are amplified by LNA Q501. Afterwards,
the amplified signal goes through the bandpass filter again to filter out undesired out-of-band
interference signal. Next, the RF signals will be mixed with the first local oscillator signal at Q502. Then
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