Hytera MD652, Руководство по обслуживанию

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Circuit Description
VHF1 (136
‐
174 MHz)
24
diodes D7007 and D7008. Quiescent bias current of the PIN diode is controlled by the resistors R7111
and R7047. In this mode, D7007 is turned on; RF signals are sent to the low-pass filter (composed of
C7141, C7142, C7139, C7153, C7154, L7043, L7044 and L7045) and then transmitted via the antenna
port.
In RX mode, TX signal controls the switching transistor Q7007, to further disconnect the PIN diodes
D7007 and D7008, so as to supply power. When the two PIN diodes are disconnected, signals feed into
the RX path through the low-pass filter (composed of L7046, C7177 and C7178).
Low-pass Filter
The low-pass filter is composed of microstrip inductors and capacitors (C7139, C7153, C7136, C7152,
C7141, C7154, C7137 and C7140) and is used to suppress harmonic energy from the PA.
Directional Coupler
The directional coupler is composed of microstrip directional coupler, L7031, R7100, R7101, R7098,
L7038, R7097, R7099, R7096, etc. And it is used to detect both forward and reverse power of the
operating transmitter. The forward power is coupled to the diode D7006, and the voltage is input to the
power control circuit (U7002). Then the voltage VGG is output to control the gate voltage of pre-driver
and driver PA, ensuring a constant power output.
The directional coupler can adjust the TX power and detect the VSWR load. The reverse power is
coupled to D7004. The output voltage is then sent to U7002 for detection.
Output Power Control
The TX power is controlled by power control chips U7001 and U7002 of the transmitter. The forward
power is coupled to the directional coupler, to output a voltage that can represent the forward power. The
voltage together with the preset output voltage are sent to U7001 and U7002 to output a voltage VGG,
which can control both gate voltages and gains of Q7012 and Q7010, ensuring a constant power output.
Temperature Protection
The voltage division circuit consists of thermistor RT1, resistor R7049 and R7060. The output voltage is
proportional to the detected temperature. Both the voltage used for temperature detection and the
threshold voltage are fed into the operational amplifier U3001 to output a voltage signal that is in
proportion to the detected temperature. Afterwards, the voltage is sent to software for detection, and
then the preset voltage will be subsequently changed to reduce the TX power, and to protect the PA
from over-heating.
Pressure Pad Switch
The power control circuit includes a pressure pad switch SW1, which is controlled by the conductive
rubber part mounted on the upper cover. When the switch is turned off, VGG will become low, and no
power will be output from PA. Otherwise, PA will work normally.