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Message started by radiofm on Apr 24th, 2011 at 12:04pm

Title: interference below carrier
Post by radiofm on 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.

Title: Re: interference below carrier
Post by Eric Johnston on 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  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.

Title: Re: interference below carrier
Post by radiofm on 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.

Title: Re: interference below carrier
Post by Oasis Networks on 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,

Title: Re: interference below carrier
Post by radiofm on May 20th, 2011 at 7:11pm
Our operator changed the frequency on satellite for our carrier. Thank you all for your help !

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