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SCPC errors on stabilized Seatel C-band

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Ex Member
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Dec 17th, 2009 at 9:12am  
Hi,

Im troubleshooting a stabilized C-band system. My SNR is always good (9 dB), but I always get many SCPC errors.


We have made sure that the problem is from our system, not from the hub. Before I start replacing everything in the chain do you have any advice to give me ?

Rx line

It uses a modem which in turn connects to modulator box which combines Rx RF with Antenna control commands then this in turns goes via Heliax to another modulator box which is directly connected to the LNB which also has a Radar filter and two C-band filters on it.

Any advice might save me lots of time !

Thanks

Xband
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« Last Edit: Jan 19th, 2010 at 8:40am by N/A »  
 
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Reply #1 - Dec 17th, 2009 at 11:58am  
Hi xband

So you have an LNB, some filters, modulator, heliax, splitter/combiner, modulator, iDirect satellite router.

I assume the 'modulators' could be up/down convertors or something else to make the IFL run work? Please give us more info?

Obviously this is the only site on this network experiencing the SCPC errors?
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TDMAMike
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Reply #2 - Dec 17th, 2009 at 12:00pm  
Sounds like you have receiving issues:

Questions:

1. Do the errors exist when the raydome is removed? If you have not removed it to check, I suggest you do. †Pull it off, have the hub re-assess (or do it yourself with telnet).

2. If you havent already done so, consider swapping your LNB.

3. †Even though your problem appears to be rx related, is the tx snr (as observed by the line card) looking good? †Read: Is the system meeting the UCP for the inroute? †The reason I ask, is you may have some mild RF leakage in/around the system that could be causing problems with your receive signal.  Literally, interfering with yourself. †



Bottomline, you want to isolate those errors or your throughput will suffer (which I am sure you are aware of).

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Regards, &&&&M
 
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Reply #3 - Dec 17th, 2009 at 1:22pm  
Thank you for your reply

It is impossible to remove the dome as itís a 2,8 meter dish

I will start by replacing the LNB and take it from there

My uplink is very good, Tx SNR is around 11 dB

The modulators combine

950-2150 KHz L-Band Rx IF
700 KHz SCPC signal antenna control
1,1 MHz M&C diagnostics
1,5 MHz BUC M&C

These all go through the same heliax cable

I might have to replace these aswell

Thanks

Xband


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Eric Johnston
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Reply #4 - Dec 17th, 2009 at 3:06pm  
When do the errors occur ?

Are they associated with nearby shipping activities, radar etc.?

Are they associated with own ship activities, crane movements, funnel blockage, radar and radio transmissions.? †Nearby helicopter or aircraft movements ? Harbour vehicles ?

Are they associated with tracking movements of the SeaTel antenna ?

A spectrum analyser in peak hold mode is good for recording short interference bursts. Look at all frequencies from DC upwards in the LNB cable and the cable to the demod.

Does the DC supply voltage to the LNB suffer brief voltage drop when something else happens ? This can cause a brief phase/frequency error burst in the receive carrier.

Best regards, Eric.
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Reply #5 - Dec 17th, 2009 at 4:43pm  
Thanks for your reply

I will troubleshoot and investigate further at first light

Interference and blockage are not responsible as the vessel keeps changing positions and locations every day.

These are some results I get now from I-monitor

Avg C/N down 9.84 dB
Avg C/N up 9.20 dB
Avg Tx power -13.26 dBm
TDM lost 841
Rx input power -37 dBm
Digital Rx power 20.31 dB
FLL DAC 0x515
Rx COF -6159
Symbol offsets 5185
Freq offsets 4454

SCPC errors

6612
134
7022
243
4334
169
129
8120
890
9120
12000
6115

etc
.......

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Reply #6 - Dec 18th, 2009 at 1:49am  
Try to run a test with the modem close to the antenna using a short cable and also I try without filter. If still problems you definitely need an spectrum analyzer to analyze the Rx
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TDMAMike
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Reply #7 - Dec 18th, 2009 at 2:14am  
Is this a new antenna? †Read: Has it been used previously?

Would definitely look for a voltage drop as Eric mentions above. †

You mentioned above that you have isolated it to your node specifically, I am assuming there are other customers residing under the downstream carrier you are using - and they are having no issues.  (Just checking). 

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Regards, &&&&M
 
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ScotlandTDMA
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Reply #8 - Jan 5th, 2010 at 10:40pm  
Xband

We are using many of these Cband seatel units the 9797 antennas, the major issues to look for are interference on the cables and also at the feed

are you able to look at the LNB pedestal with a spectrum anylzer?

If not when you are looking at the AGC on the DAC is it varying much (more than 50 counts)??

At the antenna is there a filter installed between the feed and the LNB? what type of LNB are you using?

Are you within 5 metres of a Radar or are your cables running along side any other cables?

Cheers

ScotlandTDMA
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YIM  
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Reply #9 - Jan 21st, 2010 at 9:01pm  
Itís a new antenna 3 year old baby, these antennas work great for 15-20 years. There is no voltage drop as I checked. Yes, itís the only vessel experiencing this issue.

I replaced the LNB, itís a Norsat 3220 C-band PLL LNB , Input 3.4-4.2 GHz, LO Freq 5.15 GHz, Stability +/- 10 KHz, Temp 20 K, Gain 60 dB

I added a second filter, a A1 microwave filter.

Itís not a radar issue as system worked fine before.

I'm starting to suspect there might be a problem with the Heliax cable run, thatís gonna be a mess as we can't replace cables unless vessel gets to a dry dock.

Can I ask, what is the theory behind SCPC errors ? In exact theorical terms ?

I-direct uses TDM inbound /TDMA outbound from the remore right ?

So where does the SCPC term come into play ?

I truly appreciate your advice

Thanks,

Xband
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Reply #10 - Jan 22nd, 2010 at 11:23am  
The received carrier at the remote site is a continuous carrier (called TDM or SCPC). †Problems at the hub transmit and interference into the satellite will affect all remote sites.

Problems noted just at your site are due to some problem at your site, antenna, tracking, local interference, LNB connectors etc.

I would try to identify when these error bursts occur and if anything can be seen on spectrum analyser at those instants.

If you operate the spectrum analyser with resolution bandwidth approx same as the carrier, zero span, and slow sweep say 60 seconds, does the level drop or increase when error bursts occur. †Try slow and fast video bandwidth and you can probably see the fast tracking spinner working and longer term slow changes and drop outs due to funnel blockage etc. †Level variations should match AGC DAC readings.

Fast sweep, wide span and peak hold can capture bursts of interference.

Can you induce the error by waggling cables etc ? Connector fault perhaps ?

Regarding C band interference from terrestrial wireless services. These wireless services operate at the lower end of the band and are so powerful they can completely overload the LNB, even if their frequency is a long way away from the wanted satellite carrier. Using old style 3.7-4.2 GHz LNBs may help, as may special front end filters - but don't be optimistic. †C band is almost unuseable in many Africa and Middle East towns. If this is the problem then it should reduce as you move further offshore.

Best regards, Eric.
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Scout
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Reply #11 - Jan 22nd, 2010 at 9:23pm  
Quote:
TDM lost 841

Symbol offsets 5185
Freq offsets 4454

SCPC errors

6612
134
7022
243
4334
169
129
8120
890
9120
12000
6115

etc
.......



Hi,
OK so I see a few issues here, the SCPC errors are the count of single bit errors on the downstream in a 15 second period, you are seeing huge numbers of single bit errors sometimes.  If this is just a single site seeing this, we can isolate to that location, so Rx interference, poor/faulty equipment, bad connections.  Now you have high symbol offset values, that could be due to the movement of the vessel so may not be a concern, but just watch to see how rapidly the Symbol and Frequency offsets alter, are they jumping by thousands from one reporting period to the next, so do they alter by 1000+ each 15 seconds?  are they in a pattern?  do you have huge fluctuations in Frequency offset value?

I am concerned about the TDM lost value of 841.  that is more of an issue than your SCPC errors.  TDM lost is a count of the total number of times since the remote was powered up that it has lost the downstream carrier completely.  how long has that modem been powered up to get a count of 841?  that is potentially more concerning than the SCPC errors.  could be caused by the same thing....bad cable? 
I really think you need to have an engineer take a close look at the whole installation, is the antenna tracking correctly, is the GPS system working correctly and providing info to Antenna and modem correctly, are all cables terminated and fitted correctly, any combiners/splitters used should have all open ports terminated correctly. 
Lots of things to check, but it is common to see high SCPC with high TDM lost in situations of poor installations.
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Ex Member
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Reply #12 - Jan 27th, 2010 at 11:07pm  
Gentlemen !

I replaced Netmodem, SCPC antenna communications modulator boxes, lnb , filter, sealed all the feed with foil and now SCPC erros have disappeared !

No SCPC or TDM error has been reported for the last 5 days

Appreciate everyone's advice

Thanks!

Xband
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