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CDM 625 comtech link problem. Help please ..

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Feb 14th, 2015 at 6:40pm  
Dear All

We are having issues with a CDM625 link, what is happening is I am locked to the carrier of the far remote which is a teleport but they cannot get my lock. The satellite operator sees my carrier at contract values and the teleport can also see it. If I put the values of my tx on to my rx I get  a lock. We have gone through the settings with the teleport and they match. What could be the cause of this?
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Reply #1 - Feb 14th, 2015 at 7:11pm  
Do you have a CDM 625 with the same options installed at each end ?

Does the teleport use a different frequency range.  e.g different LNB local oscillator, 70 or 140 MHz IF instead of L band ?

Can the teleport successfully receive its own transmit carrier ?

Please anyone with more ideas ?
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Reply #2 - Feb 15th, 2015 at 8:17am  
Hi,

CDM625 has many parameters related to the baseband. Can you please confirm if they cant get a lock on the RF level or is it the baseband that is not getting synchronized?

I would suggest to check two things:
1. Check the symbol rate of your Tx and of the teleport Rx. The symbol rate is the result of several factors, such as FEC, modulation, overhead, datarate. If one of those parameters is not matched, you will see a different symbol rate, and this will give you an indication where you should look for the problem.

2. Check for spectrum inversion and try to toggle it on or off.


Please let us know how did you manage eventually,
Nimrod
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Reply #3 - Feb 15th, 2015 at 4:15pm  
The teleport is using 140 MHz while we are on L band.

We compared all our settings and they look ok. The modem at my site can lock to itself if I mirror settings to the rx. Also all the test shows that my modem is working well. But we do not have a lock at the teleport. The teleport say if they do satellite loop they also get a lock on theirs.
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« Last Edit: Feb 15th, 2015 at 8:11pm by Admin1 »  
 
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Reply #4 - Feb 15th, 2015 at 8:11pm  
Nimrod makes some good suggestions.

The hub should try toggling the spectrum inversion. This would probably not stop the carrier locking, but would certainly disrupt FEC lock and framing lock.

If the hub can clearly see your carrier on a spectrum analyser on the 140 MHz band IF cable then they need to tune their receiver to the correct frequency within their IF band range.This frequency can only be calculated by someone at the hub who knows what RF centre frequency their down-converter is tuned to.

Knowing that the hub can successfully receive their own carrier, I would also consider trying to set up, temporarily, the same carrier parameters in both directions. i.e make your return link parameters the same as the outlink from the hub. This applies to everything except frequency, bit rate and power level settings.

If your return link carrier uses different parameters (other than frequency and bit rate) there is the possibility of incompatible options installed. CDM modems have many options, some you pay for and may then activate (FAST?), others need hardware changes.

Try getting someone else to check the parameters. It is easy to misread settings if you already know what you expect to see.

See: http://www.comtechefdata.com/files/datasheets/ds-cdm625.pdf

The handbooks are here, but beware hundreds of pages and multi-Mbyte download! http://www.comtechefdata.com/support/docs/satellitemodemdocs

Best regards, Eric
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