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10 MHz reference
17th Dec, 2017 at 6:43am
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Dear friends,

I would like to know what 10 MHz reference in satellite modem is and what does it do ?
does it supply 10MHz reference of BUC/LNB when they support external reference ???
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Re: 10 MHz reference
Reply #1 - 18th Dec, 2017 at 1:54pm
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The outdoor transmit module, BUC or Block Up Converter converts cable frequency signals (normally less than 2 GHz) up to satellite uplink frequencies around 6, 14 or 30 GHz. It is very important that the frequency of the transmitted signals to the satellite are correct.  For burst mode TDMA signals, it makes it easier for the teleport hub receive demodulator to lock onto each burst quickly soon after it arrives.

The BUC has, as its input, signals from the cable. These are mixed with a local oscillator frequency in order to generate the actual uplink signals to the satellite. The local oscillator frequency must be accurate and stable and this is achieved using a 10 MHz reference signal sent up the cable from the indoor modem.  The BUC local oscillator is of the phase lock loop (PLL) type so that its output accuracy relates to the accuracy of the supplied 10 MHz reference.

Inside the modem may be a temperature controlled reference 10 MHz oscillator, which is designed to be very stable. In SCPC systems this may be free running after initial frequency adjustment at the factory or after a few hours or days warming up. In TDMA systems the teleport hub may be able to remotely adjust the frequency slightly to keep all the remote sites together.

At a teleport hub, the big outlink carrier frequency will be controlled by a redundant pair of 10 MHz atomic rhubidium standards or similar. An accurate 10 MHz may also be derived from GPS receivers.

LNBs that are described at "External Reference PLL" type do need 10 MHz supplied from the modem.

LNBs that are described at "Internal Reference PLL" type have their own 'fairly stable' oscillator internally and do not need 10 MHz supplied from the modem.

PLL LNBs are good for systems receiving small bandwidth carriers (e.g 60 kHz wide) where frequncy error would allow the receiver to lock to the wrong carrier or never find the wanted carrier. 

DRO LNBs are cheaper and unstable in frequency (e.g. up to +/- 2 MHz) and are suitable when the receiver is searching for carriers of say 10 MHz to 30 MHz bandwidth.

BUCs always need a 10 MHz reference, normally supplied from the indoor modem.

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