Receiver Circuit - Hytera DMR Service Manual

Service Manual
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.
17.1.10 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.

17.2 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 PLL circuit U701. The major units are BPF, LNA, mixer, IF filter, IF
amplifier and IF processor.
Figure 17-2 Diagram of Receiver Circuit
17.2.1 Receiver Front-end
RF signals feed to the low-pass filter to remove out-of-band signals, and are applied to the two-stage
electrically tunable band-pass filter to get useful signals. After passing through the RF band-pass filter
and LNA (Q6102), the RF signals together with the first LO signal feed to the mixer for the first frequency
conversion, to generate the first IF signal (44.85MHz). The mixer that employs the passive diode can
ensure good dynamic range and port isolation. The LO signal (3-5dBm) from the VCO feeds to Q6103
to get a gain (17dB). The first LO signals (44.85MHz) pass through the crystal filter (Z6100) to remove
out-of-band spurious signals, and then feed to the two-stage IF amplifier circuit (composed of Q6133 and
Q6134) for amplification. Finally the amplified signals go to the IF IC for processing
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