I have searched google but I couldn't get a clear picture about this CNR. CNR looks to be something that is measured at the input of a RF system (In satellite communication I see this term a lot). C/N is basically a RF domain measurement (pre-detected measurements)
SNR is a baseband domain measurement (post detected measurements on baseband signals). But they both look to be related in some way.  SNR (in this case) depends on the symbol period as the matched filter would average the noise for a time period Ts whereas CNR is something that is measured before matched filter so that there will be no averaging of the noise and hence I believe CNR will be smaller compared to SNR, right? Do you know the exact relation between them?

Thanks in advance.

raja.cedt wrote on Mar 25^th, 2013, 12:47pm:


Thanks. Is there any exact relation between CNR and SNR? Some Mathematical relation?

CNR appears to be a spot noise comparison between the carrier power and the noise floor. This only works well for CW and/or really narrowband modulated carriers where the total signal power fits withing the resolution BW of the spectrum analyzer (or FFT). It is a measure and comparison of power, using the same resolution BW, at two different frequencies.

SNR compares the signal power over the modulation BW (i.e. detected signal) to the integrated noise + interference power over the same BW.

The problem with CNR is that the resolution BW (RBW) needs to be  defined or else the noise floor power measurement will be different for different RBW settings.

Well, I guess it depends on what is important to the system. A really narrowband or CW signal will have the same power no matter what the resolution bandwidth is, but a widerband signal spectral density will scale with the RBW.

Put another way, if you had a CW signal then the CNR would change with RBW because the noise power measured is a function of the RBW while the CW power measured is independent of RBW. However, if everything is normalized to dBc/Hz then the RBW dependency is removed.

If you changed the RBW to the signal BW then you are effectively measuring SNR if the only interference is the noise floor and the noise outside of the channel is the same across the channel.