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How RCM modules work in iDirect 5IF chassis

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Apr 25th, 2013 at 10:45am  
Dear Eric

Kindly explain why the is of 10 MHz reference reference  and how RCM modules work in 5IF chassis.

Martin
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« Last Edit: Apr 25th, 2013 at 4:47pm by N/A »  
 
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Eric Johnston
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Reply #1 - Apr 25th, 2013 at 7:52pm  
I am not an expert on iDirect so can't say what an RCM module is of how it might work in a particular chassis.  Hopefully someone else will answer your question.

Regarding 10 MHz reference:

Typically a hub teleport will have a master 10 MHz reference source and distribution network, cabled all around the site distributing an accurate 10 MHz signal to all modems, up and down converters etc, and to test equipment such as spectrum analysers.

The master 10 MHz reference may be an atomic standard or a temperature controlled crystal oscillator, probably as a redundant pair with auto-switch over on failure.  Often such an oscillator is combined with a GPS (off-air) based reference, but note that the GPS signal will be lost at times.

I use, for testing purposes, a off-air long wave radio receiver, tuned to a local BBC radio station at 198 kHz, whose signal frequency is linked to a national frequency standard.

An iDirect hub may have the possibility of an external 10 MHz reference being fed into it to make sure that all its frequencies are correct. If your iDirect hub is using an internal reference it would be a good idea to check and adjust this at installation, after a month and then every 6 months or so by reference to some  standard.

A BUC normally has a 10 MHz reference supplied to it by the transmit modem.  In the software options configuration of the transmit modem the 10 MHz reference to the BUC cable must be turned ON.

LNBs come with various oscillator options.  An LNB with DRO oscillator has very poor stability and is suitable only for large high symbol rate carriers.  An LNB with an internal crystal oscillator is very much better but there is some frequency drift due to temperature changes and with time.  An LNB with an external reference has a frequency accuracy the same as the station master reference.   In the software options configuration of the receive modem the 10 MHz reference on the LNB cable must be turned ON.

Best regards, Eric.
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Reply #2 - Apr 26th, 2013 at 6:51am  
Dear Eric

Many thanks for the detailed info. I am still not clear about why this 10Mhz is needed by BUC and LNB?

Martin
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Eric Johnston
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Reply #3 - Apr 26th, 2013 at 10:45am  
A BUC has a local oscillator, used for the up frequency conversion. For example, 13050 MHz, to get from L band (950-1450 MHz) to Ku band (14000-14500 MHz).  

To achieve high frequency accuracy the local oscillator is a phase locked loop (PLL) type using a very high accuracy low frequency, 10 MHz, reference supply.

The highly accurate 10 MHz reference is supplied up the cable from the transmit modem.

LNBs:
LNBs come with several possible frequency accuracies:
DRO : very poor frequency accuracy but good for receiving large carriers.
PLL type with internal xtal reference : fair accuracy, tend to drift with temperature and time.
PLL type with external reference : accuracy same as reference supplied.  Reference supplied is of excellent accuracy, based on indoor, temperature controlled equipment, atomic standard, GPS etc.

TDMA VSAT return links (remote sites to teleport hub) require extreme frequency accuracy to ensure that the bursts can be locked on extremely quickly by the burst demod. The burst receiver must slightly adjust its frequency, adjust the gain level, adjust the symbol rate and start distinguishing 1s and 0s very quickly.  The 10 MHz reference used at a remote site may derive its accuracy from the symbol rate of the outlink carrier from the hub.
   
Best regards, Eric.
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Reply #4 - Apr 28th, 2013 at 8:11am  
Dear Eric

Thanks for your detailed explanation.

Martin
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Reply #5 - Jul 5th, 2013 at 5:11pm  
It's been a while since I logged in to this site Smiley
The RCM is iDirect's Hub Reference Clock Module, it provides the reference for the iDirect Hub Line cards. 

iDirect's hub has a back-plane that all the line cards connect to, the RCM feeds a 10MHz reference and the cards lock on to the timing in groups.

The RCM can be fed by an external station clock or use it's own internal source, the RCM has a hot standby in the hub as well.

iDirect's 5IF hub does not provide the clock reference to the hub RF equipment.  The 4 slot chassis is capable of providing BUC power and reference.
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