Satellite Internet Forum.
Please note that this forum is now READ ONLY and closed for login and changes
The new forum is here Space and Satellite Communications forum

Welcome, Guest.
Welcome to this satellite broadband discussion forum.  Wherever you are and whatever your problem we are here to help each other. Connecting to the internet via satellite is not always easy but is critically important to those in remote places or with poor terrestrial infrastructure.  Both service providers and customers are encouraged to contribute.  Read the Forum rules.
      Satellite internet forum          

interference below carrier

(Read 2523 times)
Ex Member

Apr 24th, 2011 at 12:04pm  

i have a question about real C/N level measured on spectrum analyzer.

I have space for plannded TV carrier - but as i can see there is interference on this space. the real C/N for this carrier will be c/n 1 or c/n 2 ?


Best Regards.
Back to top
IP Logged
Eric Johnston
Senior Member

Personal text from Profile,
Options, Top line

Posts: 2107
Reply #1 - Apr 24th, 2011 at 12:31pm  
C/N 2

The measurement you are making above is strictly (C+N)/N   In your example this makes negligible difference. See http://www.satsig.net/lnb/ebno-calculator.htm  If you consider the interference "carrier", its (C+N)/N = 2 dB and its C/N is -2.3 dB. The interference is actually 2.3 dB below your noise floor.  Think: If the interference was the same level as your noise floor you would see a 3 dB high hump.

Leave the spectrum analyser in max hold for a while to see if there are brief bursts of higher level interference that might cause intermittent error bursts in your transmission.

Why not find the source(s) of the interference and try to get it removed ?  Check your own receive system first. Is your polarisation correctly adjusted ?  Do you have receive sidelobes aimed at the adjacent satellies ?  Is the interference visible at other receive sites ? Does the interference change exactly as you would expect if you mispoint your dish slightly ?

Anyone with an operational carrier should exploit any outages by closely examining the noise floor under the carrier to see if interference exists.

Anyone with TDMA return link can see interference in real time by just watching the return link spectrum with its intermittent wanted bursts. Try using normal freq sweep display and max hold as well as zero span to look for TDMA burst interference.
Best regards, Eric.
Back to top
« Last Edit: Apr 25th, 2011 at 8:20am by Admin1 »  
IP Logged
Ex Member

Reply #2 - Apr 25th, 2011 at 8:53am  
Thank you for explanation !

So in this example C/N for receiver will be:

- about 2 dB interference
- (C+N)/N over interference =  11 dB
- real C/N for the receiver = 10.64 dB


tommorow i will try some tests with rx antenna.

Best Regards.
Back to top
IP Logged
Oasis Networks
Ex Member

Reply #3 - Apr 25th, 2011 at 9:31am  
In addition to all the excellent advices of Eric, I would like to add one grain more. I think you may contact the satellite operator you purchase the segment from, and ask them for screenshot of the space taken from their earth station, as well in max hold and normal sweep. It will allow you not only to find out whether your earth station has some problem (With polarity, side lobes, local interferences etc.), but will also allow you to request to replace the segment in the future, in case there is an interference that can not be eliminated.

Kind Regards,
Back to top
IP Logged
Ex Member

Reply #4 - May 20th, 2011 at 7:11pm  
Our operator changed the frequency on satellite for our carrier. Thank you all for your help !
Back to top
IP Logged