Gentlemen,

I would like to know, what is the actual difference between C/N and S/N ?

I always note that upon system monitoring these values are not exactly the same, e.g. when I have C/N 7 dB my SNR could be 7,4 dB

Can please someone enlighten me on this one?

Many Thanks,

Xband

It depends on how you make the measurements.

How do you measure the carrier or signal ?  

The preferred method is to turn off the modulation and measure an unmodulated CW carrier. Just use a narrow filter and measure the level.  Being a sinewave the measurement of the peak value may be used to accurately determine the RMS power value.

If the wanted carrier is modulated, measurement is much more difficult and uncertain.   PSK modulation with scrambled data produces a noise-like spectrum.   One (accurate) way to measure this is to use a thermocouple power meter.  This works well for measuring the output of a BUC or HPA where there are no other signals present.   Other ways, such as spectrum analysers, have big problems due to the bandwidth and shape of the filter and the responsiveness of the detector device.   Best results are obtained by using MARKER NOISE ON function.  This will compensate for the filter and detector and give a result in dBm/Hz.   This works both for the modulated carrier and the noise measurements either side.

Comparing the CW carrier power, measured with the spectrum anlayser in its normal mode, and the noise floor using the MARKER NOISE ON function (in dBm/Hz) gives good results.

The question arises.. What is the bandwidth to be used when determining the total carrier power ?.  You will get slightly different answers.

Be aware that when you put a marker on top of a received modulated carrier you are measuring the wanted carrier plus the noise floor underneath.  

So, examples..

If you see a hump that is 3 dB above the noise floor the actual wanted carrier is in fact the same level as the noise, i.c. C/N=0dB.  

If the noise is 1 milli-watt ( 0 dBm) and the signal 4  milli-watt (+6 dBm) then the total power is 5  milli-watt (+7 dBm). You see a 7 dB high hump, the (C+N)/N = 7 dB, C/N = 6 dB.
 
Remember, what is important is does the system work and do you have an acceptable margin to cope with operational system degradations ?

The terms "carrier" and "signal" normally mean the same thing.  An exception is in the case of FM television where the term "signal" is used to refer to the baseband 0 - 5 MHz wide video signal.  Spread spectrum operation may also lead to the use of different meanings.  A spread signal may have a C/N of - 12 dB and, when de-spread, a C/N of +6 dB.

Best regards, Eric.