Output Power Control; Thermal And Over-Voltage Protections; Pressure Pad Switch; Receiver Circuit - Hytera MD78X Service Manual

VHF (136-174MHz 25W) Information
The directional coupler can adjust the TX power and detect the VSWR load. The reverse power is
coupled to D804. The voltage is applied to U802 and then feeds into APC for detection.
14.1.8

Output Power Control

The TX power is controlled by 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.
14.1.9

Thermal and Over-voltage Protections

The circuit consists of 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 into the operational amplifier U803, to output a voltage signal that is in proportion to the detected
temperature. Afterwards, the voltage is applied to software for judgment, and then the preset voltage
will be subsequently changed to reduce the TX power, and to protect the PA from over-heating.
14.1.10

Pressure Pad Switch

The power control circuit includes a pressure pad switch SW1, which is controlled by the conductive
rubber part mounted on the top cover. If the switch is turned off, VGG will become low, and no power
will be output from PA. Otherwise, PA will work normally.
14.2

Receiver Circuit

The receiver utilizes double conversion superheterodyne technique. The first IF is 44.85MHz and the
second IF is 2.25MHz. The first local oscillator signal is from the PLL circuit U100, and the second LO
signal (47.1MHz) is generated by the IF processor circuit U701. The major units are BPF, LNA, mixer,
IF filter, IF amplifier and IF processor.
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Service Manual