TC-508 Service Manual
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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
the newly generated signals will be processed by crystal filter XF501 to produce 38.85M the first IF
signal. After being amplified, the IF signal goes via Q506 to the chip U611, where the IF signal will be
mixed with the second local oscillator for the second time to produce the 450KHz second IF signal. After
filtered by 450KHZ ceramic filter, U611 demodulates the IF signal to output audio signal, which will be
delivered to Q613 for amplification and output in two channels. After simple filtering, one of the channels
is fed to MCU U601 for CTCSS/CDCSS decoding; for the other channel, it will go through two stages of
low-pass filter first, and then it goes to high-pass filter (consisting of 4 operational amplifiers) for audio
processing. Finally, the signals will be sent to audio power amplifier U302 for amplification so as to drive
the speaker.
Power Supply Section: the 7.4V battery voltage is converted to 5V via DC-DC module U604, and
further converted via LDO U602 to 3V including two channels: VCC and VDD. VCC powers CPU and
VDD powers PLL circuit. The 5V voltage will be converted via LDO U606 to 4.5V including two channels:
VT and V-SAVE. V-SAVE powers VCO circuit and VT powers TX circuit. Finally, V-SAVE together Q604
and Q305 are used to control VR, which powers RX circuit.
2. Realization Methods of Basic Functional Modules
2.1 PLL Frequency Synthesizer
The PLL circuit mainly provides the local oscillator signal for RX and RF carrier signal for
TX. It is composed of VCO and PLL chip, allowing frequency tracking and channel switching
under the control of MCU signals.
1) PLL
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