A Thorough Look At The Clocking Circuit Of Dm500s

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Since there are many OEM version of Dreambox on the market now, some of them do not give out the enough data which can be used to repair. Here is the data about the clocking circuit of DM500S. The clocking circuit is a significant part of the satellite receiver. The receiver can run in a normal way only the clocking circuit works well. The DVB system usually uses a 27MHz system clock. It is mainly used to give out an exact time for the decoding system. Except the 252MHz CPU clock, all the time is provided by the PLL circuit of the 27MHz clocking chip.

1.#The restore function of the PCR clock. In order to keep the clocking frequency, the digital receiver usually has function of PCR clock restoration.
2.#The realization of the PCR clock restoration. The DVB system has a VCO (Voltage Controlled Oscillator) PLL (Phase Locked Loop) in the 27MHz clocking circuit; this hardware is used to restore the time settings of the circuit. Some of the satellite receivers may use the software to VCXO (Voltage Control X Tal Oscillator); however, the VCXO is the best choice.
3.#VCXO chip. There are four kinds of chips mainly used in satellite receivers. The first is the PLL502 02 chips. This chip is produced by Phase Link in USA. The second is the T73227 chip. This chip is produced by TLSI and it is packaged with SOIC or MSOP. The third is the PLL502 02 chips with the 13.5 MHz crystal oscillator. The forth way is the T73227 chip and the 27 MHz crystal oscillator.
4.#Problems. If the clocking circuit has a problem, it could be the circuit is off or the frequency is not exactly 27 MHz. If the first happens, the machine wont work anymore or will be system halted. If the later problem is happened, the picture will become vague and the audio and video may not be synchronous.
5.#Way to solve the problem. If the system has the problems above, you have to change the units on the system board. First you should change the crystal oscillator. Try to use the crystal oscillator that recommended by the manufacturer. If the VCXO chip is broken, also try to change it with the recommended unit. You can get the VCXO crystal oscillator on the broken VCD or DVD player; remember to stall it using the original way.


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