TC-508 Service Manual
15
BUSY
LO
AF
Figure 4
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. After
being filtered and amplified, the RF signals and first LO signals are mixed at Q502, and then the mixed
signal is filtered by crystal filter XF501 to get 38.85MHz differential signals as the first IF signal.
After being amplified by Q506, the IF signal goes to chip U608, where the signal will be mixed with
the second LO signal (38.4MHZ). Next, the signals will be filtered by CF501 to produce 450 KHz IF
signal. When the IF signals are processed by demodulation circuit (including U611 and CD501) to
generate audio signals, which are delivered to IC Q613 for amplification and output in two channels.
After simple filtering, one channel of signals is supplied 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
U610 (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.
2.4 MCU Control, Signal Processing and Audio Power Amplification
Circuit diagram of this section is shown as below:
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