DP480 Service Manual
8
⚫ RF Power Amplification Circuit
RDA1846S outputs RF signal from pin 18 and enters first-level pre-drive amplifier (Q502) for initial
amplification; then it enters second-level pre-drive amplifier (Q503) and drive amplifier (Q501) for further
power amplification, ensuring sufficient drive power signal can be offered to final amplifier for ultimate power
amplification; after amplification by multiple amplifiers, the transmitted signal will complete the output
impedance match at the output terminal of final power amplifier through a section of microstripe, so as to
minimize output power loss from impedance match failure; afterwards, the transmitted signal will enter the
low pass filter through TX/RX switch.
⚫ Low Pass Filter Circuit
The low pass filter which suppresses harmonic wave is an advanced Chebyshev filter composed of lumped
parameter inductor and capacitor. On a certain basis of in-band fluctuation, the filter can improve
suppression to out-of-band harmonic wave and spurious signal.
⚫ Automatic Power Control Circuit (APC)
In the automatic power control circuit, the drain current from drive power amplifier and final power amplifier
completes switch from current to voltage through the sampling resistor and subtraction circuit which is made
of the first operational amplifier; the voltage will be compared with the APC control voltage output from DAC
at the second operational amplifier, and the error voltage that is output will change the transmitting power
through the grid bias voltage of the control power amplifier tube (including the drive level and final level).
4.5.RX Circuit
The RX circuit is mainly composed of RF low pass filter, RX/TX switch, low noise amplifier, AT1846S,
HS8861 and audio power amplifier.
⚫ RX Circuit Front-end
The signal from antenna is sent though RX/TX switches (D500, D501, D502 and D503) to the U500
(AT1846S) for demodulation and output.
⚫ RX Circuit Back-end
The analogue/digital signal which is demodulated and output by AT1846S, after completing demodulation
and processing of digital signal through DSP in HS8861, will be input to digital audio processor module for
further processing, and then be switched to analogue voice signal through DAC of codec; the analogue
voice signal will finally be amplified by external audio amplifier to drive the speaker.
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