Hytera PD502, Руководство по обслуживанию, Страница: 28 / 136

VHF (136-174 MHz) Circuit Description
22
Receiver Back-end
The first IF signal (44.85 MHz) output by the IF amplifier goes into AD9864 (U6001) via Pin 47, where
the signal is converted to the second IF signal (2.25 MHz). Then the signal is converted to digital signal
via ADC sampling, and output via the SSI interface. Finally, the digital signal is sent to the DSP
(OMAP5912) for demodulation.
AD9864 employs reference frequency of 19.2 MHz and shares the crystal with OMAP. The second LO
VCO comprises an oscillator, a varactor and some other components, to provide the LO signal
(47.1/42.6 MHz). The 18 MHz clock frequency is generated by the LC resonance loop.
5.2.3
FGU
The FGU is composed of VCO and PLL. It is the core module of the whole TX-RX system. This circuit
provides accurate carrier frequency during transmission, and stable LO signal during reception. It plays
a pivotal role and determines the performance of the system.