DP580 Service Manual
4.2.Tx Circuit
The Tx channel circuit includes an RF power amplification circuit, a low pass filter circuit and an automatic
power control circuit.
RF Power Amplification Circuit
Transmitting signals TX LO are buffered and amplified by the Q8 and Q9, enter the pre-drive amplifier (Q10)
for amplification, and output strong enough drive power signals, and then power is amplified by the
end-level amplifier (Q11). After multiple levels of power amplification, transmitting signals through a
microstrip to finish output impedance match at the output end of Q11, to reduce the loss of output power
because of impedance mismatch, and enter the low-pass filter through a TX/RX switch (composed of D14,
D15, D16 and D17).
Low Pass Filter Circuit
The low-pass filter for inhibiting harmonic wave is high-order Chebyshev filter composed of a lumped
parameter inductance and a capacitance. Under the precondition of certain in-band fluctuation performance,
the filter may strengthen the inhibition of out-of-band harmonic wave and spurious signals. Emitting signals
after wave filtering is radiated through the antenna.
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 of the IC13, and the error voltage that is output will change
the transmitting power through the grid bias voltage of the control tube power amplifier (including the drive
level and final level).
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