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C/N and (C+N)/N

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Ex Member
Ex Member

Nov 25th, 2009 at 7:26am  

 Please help. May i know the difference between (Co + No)/No and C/N? how do you convert from one another and relationship?
 Please correct me if im i looking at C/N on a spectrum analyzer when i have my marker noise on from the noise floor to the top of the carrier?

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« Last Edit: Nov 25th, 2009 at 12:36pm by Admin1 »  
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Eric Johnston
Senior Member

Posts: 2109
Reply #1 - Nov 25th, 2009 at 8:00am  
Make sure the spectrum analyser resolution bandwidth is significantly less then the carrier bandwidth.

Use this page EbNo calculator to calculate the C/N starting with (C+N)/N.  Just set FEC=1 and Bits/Symbol=1.

When your marker is on top of the carrier you are measuring C+N.   When your marker is on the noise floor you are measuring N

The difference between the two is (C+N)/N.

If you set "Marker Noise function ON" you are measuring (Co+No) or No.  The analyser works out the power per Hz knowing what is the noise bandwidth of the internal resolution bandwidth filter (ie. - 10 * log(-3dB bandwidth in Hz) - 0.5 dB).  It also corrects for noise not being like a sinewave (+2.5 dB).  Normally an analyser detector measures peak voltage and displays RMS value, assuming a sinewave input.   Noise has a different peak to mean ratio from sinewave so display value is wrong unless you do "MKR NOISE ON".

A scrambled digital carrier is "noise like".

If you see a 3 dB high hump in the noise floor the carrier level and the noise level are the same and add together to make twice, so C/N=0 dB and (C+N)/N = 3 dB.

If you see a 20 dB (or higher) high hump in the noise floor then  C/N is about the same as (C+N)/N.

For intermediate values use the calculator.

Read more: HP application Note 150 (3.4 Mbytes pdf. print out 67 pages, pages 31 - 33 refer)

Best regards, Eric.
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« Last Edit: Nov 25th, 2009 at 11:09am by Eric Johnston »  
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Ex Member
Ex Member

Reply #2 - Nov 25th, 2009 at 9:16am  
hi eric, 

many thanks.  will check
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