DP405 Service Manual
3.2. TX Circuit
The TX circuit is mainly composed of three parts below:
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 ).
3.3. RX Principle
The RX circuit is mainly composed of RF low pass filter, RX/TX switch, low noise amplifier, RDA1846S,
LT1901 and audio power amplifier.
RX Circuit Front-end
The signal from antenna is sent to Q507 3SK318 and low noise amplifier (LNA) which is made of peripheral
components though RX/TX switch for amplification, and the amplified signal will then be sent to U500
（RDA1846S）for demodulation and output.
RX Circuit Back-end
The analogue/digital signal which is demodulated and output by RDA1846S, after completing demodulation
and processing of digital signal through DSP in LT1901, 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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