What is diffence between C/N and (C+N)/N

Make sure the spectrum analyser resolution bandwidth is 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 the N only.

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

If you set "Marker Noise function ON" you are using the noise marker to measuring either (Co+No) or No. The analyser works out the power per Hz (noise power spectral density) knowing what is the noise bandwidth of the internal resolution bandwidth filter (ie. - 10 * log(-3dB bandwidth in Hz) - 0.5 dB). The spectrum analyser internal calculation 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, unmodulated carrier, input. Noise has a different peak to mean ratio from sinewave, so the normal 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 they add equally together to make twice the power, so C/N=0 dB means (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.

It occurs to me that you might also be interested in detecting and measuring carriers that are below the noise floor, with negative C/N ratios.

If you look at an apparantly 'empty' part of the spectrum you may see small humps, possibly as low as a fraction of dB above the noise floor, especially if you have the video averaging set to high value.

These small humps may be interference, for example from carriers on the other polarisation or picked up via antenna sidelobes from a nearby satellite.  You can research these. First check your transmitter is OFF!. Then look on the opposite polarisation and record the spectrum, similarly repoint your dish towards adjacent satellites and record the spectrums there.

A small hump, less than 3 dB high, represents a carrier (typically interference) that is below the noise floor and it will have negative C/N ratio.

Beware that the hump may be caused by a rather narrow wanted carrier (such as CW beacon) so look with your narrowest resolution bandwidth filters.

Examples:

3 dB high hump C/N=0 dB
1 dB high hump C/N=-5.86 dB
0.2 dB high hump C/N=-13.3 dB
0.1 dB high hump C/N=-16.3 dB

Use the calulator at https://www.satsig.net/lnb/ebno-calculator.htm with FEC=1 and Bits/symbol=1