Hytera MD78X, Service Manual

Page: 440 / 519

Circuit Description
VHF (136MHz
–
174MHz)
24
TX signal controls the switching transistor Q810, to further control the power supply (9 V 3 A) for the
PIN diodes D801 and D802. Quiescent bias current of the PIN diode is controlled by the resistors
R821 and R825. In this mode, D801 and D802 are turned on; RF signals are transmitted via the
antenna port through the low-pass filter (composed of C821, C816, C836, C837, C830, C838, L809,
L813, and L810).

In RX mode
TX signal controls the switching transistor Q810, to further control the power supply for the PIN
diodes D801 and D802, which are not applied with DC bias current. When the two PIN diodes are
turned off, D802 is turned on and signals feed into the RX path through the low pass filter (composed
of L871, C852, and C862).
Low-pass Filter
The low-pass filter consisting of inductors (L809, L813, and L810) and capacitors (C821, C816, C836,
C837, C830, and C838), is used to attenuate harmonic energy from the transmitter.
Directional Coupler
The directional coupler is composed of components such as microstrip directional coupler, D803 and
D804. It is used to detect both forward and reverse power of the transmitter. The forward power is
coupled to the diode D803, and the voltage is applied to the power control circuit (U801). Then U801
outputs the voltage VGG for controlling the gate voltage of Q805 and Q804, ensuring a constant power
output. The reverse power is coupled to D804. The voltage is applied to U802 and then fed into U302 for
detection.
Power Control
The TX power is controlled by power control chips U801. The forward power is applied to the directional
coupler, to output a voltage that can represent the forward power. The voltage together with the preset
voltage feeds to U801 to output a voltage VGG, which can control both gate voltage and gain of Q804
and Q805, ensuring a constant power output.
Thermal and Over-voltage Protection
The circuit comprises thermistor RT804 and resistor R884. The output voltage is proportional to the
detected temperature. Both the voltage used for temperature detection and the threshold voltage are fed
to the operational amplifier U804, to output a voltage signal that is proportional to the detected
temperature. The voltage is applied to software judgment via the diode, and then the preset voltage will
be subsequently changed to lower 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, the power control voltage VGG