Kirisun DP990, Руководство по обслуживанию, Страница: 8 / 76

DP990/DP995service manual
7
3.2.Tx Circuit
The TX circuit is mainly composed of three parts below.

RF Power Amplification Circuit
A carrier signal TX VCO generates is modulated and amplified before entering the Tx circuit, in which the signal
passes a Π-type resistive attenuator to achieve inter-stage isolation between the PA circuit and the TX VCO, enters
the pre-drive amplifiers (Q1404 + Q1403) for amplification and achieve isolation from the next-stage amplifiers,
enters the drive-PA (Q1406) for amplification to provide sufficient drive power signal to the final-PA (Q1407) for
ultimate power amplification. After multi-stage amplification, output impedance match is performed on the signal at
the output terminal of final power amplifier through a microstripe matcher to minimize output power loss because
of impedance match failure, and then the signal goes through the Tx/Rx switch and enters the low-pass filter.

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 circuit of automatic power control and temperature detection, the drain current from drive power amplifier
and final power amplifier converts to voltage through the sampling resistor and subtracter circuit composed of the
first operational amplifier, the voltage is compared at the second operational amplifier with the APC control voltage
the DAC outputs, and the error voltage output changes the Tx power through the grid bias voltage of the control
power amplifier transistors (in the drive stage and final stage) to change the Tx power.
The temperature transducer IC detects the surface temperature of the final power amplifier transistor, and converts it
to and output a DC voltage value, which is compared with the voltage value corresponding to the protection
temperature of the power amplifier transistor. In case of overtemperature, the bias voltage of the amplifier is
reduced to decrease the output power until the temperature becomes normal again, and then increase the voltage
again. This cycle repeats.
Note:
To ensure the best voice quality, temperature protection is disabled by default. You may enable it using the CPS.