What is the relationship between phase noise of the local oscillator and receiver sensitivity?
Phase Noise Impact
Reciprocal mixing is the primary mechanism by which LO phase noise degrades receiver performance. A strong signal (blocker) at frequency fb mixes with the LO phase noise at offset fm = fb - fLO. The resulting noise appears at the IF centered on the desired signal frequency. The noise power density is proportional to the blocker power and the LO phase noise level. Thus, a strong blocker effectively raises the receiver noise floor through the LO phase noise sidebands.
The reciprocal mixing noise is: N_RM (dBm/Hz) = P_blocker (dBm) + L(fm) (dBc/Hz), where fm is the frequency offset between the blocker and the desired signal. For a blocker at -20 dBm and LO phase noise of -140 dBc/Hz at the blocker offset: N_RM = -20 + (-140) = -160 dBm/Hz. If the thermal noise floor is -174 dBm/Hz: reciprocal mixing noise exceeds thermal noise when N_RM > -174, which occurs when the blocker is stronger than P_blocker > -174 - L(fm).
For digitally modulated signals, phase noise also directly degrades the constellation quality. The LO phase noise causes the received constellation points to rotate randomly, increasing EVM. The phase noise requirement for a given modulation depends on the symbol rate and the constellation size: 256-QAM requires approximately 10 dB better integrated phase noise than QPSK.
Frequently Asked Questions
How good does the LO phase noise need to be?
Depends on the application. Cellular base station receivers: -110 to -120 dBc/Hz at 10 kHz offset. Military receivers: -120 to -140 dBc/Hz. Radar receivers: -140 to -160 dBc/Hz at 10 kHz. WiFi: -90 to -100 dBc/Hz is usually sufficient.
Does phase noise affect the transmitter?
Yes. LO phase noise in the transmitter creates out-of-band noise emissions (spectral regrowth) that appear as interference to receivers on adjacent channels. The transmitter LO phase noise specification is often more demanding than the receiver specification.
Can I filter out phase noise?
Phase noise within the signal bandwidth cannot be filtered; it is part of the signal. Phase noise outside the signal bandwidth can be rejected by the IF filter. This is why narrowband receivers (crystal filter IF) are less affected by LO phase noise than wideband receivers.