TC-508 Service Manual
17
and transmit/receive data to/from PC.
2) Signal Processing Circuit
a. TX Baseband Processing
The audio signals input from MIC are divided into two parts. One part enters U601, and is then
converted into digital signal by analog-to-digital conversion (ADC). Afterwards, the converted signal will
go through a series of processing including low-pass filtering, AGC, companding, high-pass filtering, and
pre-emphasizing, and then it will be output as baseband signal after digital-to-analog conversion (DAC).
The baseband signal is processed by the active low-pass filter and then sent to VCO for modulation; for
the other part, it will go through VOX circuit (Q611 and D616) and then sent to MCU for VOX processing.
b. RX Baseband Processing
The demodulated IF signals with CTCSS/CDCSS signaling enter U601 for decoding. After ADC, the
IF signals will be converted into digital signals, which go to low-pass filter for filtering and then are
divided into two parts. One part is processed by CTCSS/CDCSS decoder, and the other is processed by
high-pass filter and then is subject to de-emphasis, decryption, signal expansion and DTMF
demodulation and etc, and finally digital-to-analog conversion to output audio signal.
3) Audio Amplifier
The audio signal output from U601 is amplified by the audio power amplifier (U302) to drive the
speaker directly.
2.5 Power Supply Processing
Block diagram of power supply circuit is shown as below:
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