Hello,
Phase noise is often expressed as the noise density, PSD [W/Hz], referenced to the carrier power [W] at a certain frequency offset from the carrier.
Phase noise, Side band noise = (PSD (at offset) ) / (carrier power) [1/Hz]
When you convert this to decibels, you get dBc/Hz.
The "c" denotes that the carrier power is taken as reference.
So when you have a spectrum of a Local Oscillator:
Pcar = 1dBm, noise power at 10 kHz offset = -80dBm with an RBW setting of 300Hz.
Noise power = PSD*BW (assuming flat Power Spectral Density within the BW).
You measured noise power -80 dBm, to find the PSD, you have to subtract 10*log(300) = 24.8 dB. So
PSD = -80dBm - 24.8dBHz = -104.8 dBm/Hz (at 10 kHz from carrier).
Now Phase Noise = -104.8 dBm/Hz – 1 dBm = -105.8dBc/Hz @ 10kHz offset.
This definition doesn't make any difference between amplitude and phase noise as it is just a ratio of powers.