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WimRFP said: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.
WimRFP said:Hello,
I know understand your application. It is when receiving a weak signal, while on the adjacent channel a strong signal is present. Due to the phase noise of the LO, signal from the adjacent channel leaks into the IF where your wanted signal is.
Assume Padj = RF power of signal in adjacent channel, Prf is power in wanted channel
Power that appears into your IF (assuming 0dB insertion loss for the mixer) from adjacent channel:
PIFn = Padj*(phase noise)*BW
Phase noise in W/Hz @offset, BW = bandwidth of IF (Hz).
For good reception you need a certain S/N ratio, so the wanted RF signal must be at least above:
PIF = Prf > (S/N ratio)*PIFn This results in:
Prf > (S/N ratio)*Padj*(phase noise)*BW in dB's:
dBPrf > dBSNR + dBPadj + dBphnoise + 10log(BW)
So:
dBph.noise < dBprf - dBSNR - dBpadj - 10log(BW)
This equals your formula, Sdesired = dBprf, Sblocker = dBpadj.