Definition
Phase noise L(f) = power spectral density of phase fluctuations L(f) [dBc/Hz] = 10·log₁₀(P_sideband(f_offset, 1 Hz BW) / P_carrier) Measured at a specific offset frequency f from carrier: e.g., −90 dBc/Hz at 1 kHz offset from 900 MHz VCO
Typical Phase Noise Levels
| Oscillator Type | Offset 1 kHz | Offset 100 kHz | Offset 1 MHz |
|---|---|---|---|
| Crystal (10 MHz TCXO) | −130 dBc/Hz | −155 dBc/Hz | −165 dBc/Hz |
| PLL synthesizer (900 MHz) | −90 dBc/Hz | −115 dBc/Hz | −130 dBc/Hz |
| LC-VCO (2.4 GHz CMOS) | −85 dBc/Hz | −110 dBc/Hz | −125 dBc/Hz |
| Ring oscillator (5 GHz) | −60 dBc/Hz | −90 dBc/Hz | −110 dBc/Hz |
Reciprocal Mixing
Phase noise of the local oscillator (LO) causes "reciprocal mixing" — a strong adjacent-channel signal mixes with the LO phase noise sidebands, creating an interferer at the IF frequency that degrades receiver sensitivity. Requirement:
L(Δf) + P_blocker < Noise Floor + SNR_min Example: 30 dBm blocker at 200 kHz offset, required SNR = 10 dB: P_noise_floor = −174 + NF + 10·log₁₀(BW) = −100 dBm Required L(200 kHz) < −100 − 10 − 30 = −140 dBc/Hz
Phase Noise vs S-Parameters
Phase noise is not directly related to S-parameters — S-parameters characterize linear frequency-domain behavior, while phase noise describes oscillator stability in the time domain (phase fluctuations). Phase noise is measured with a spectrum analyzer in phase noise measurement mode, not with a VNA.
RF View: RF View analyzes S-parameter characteristics of oscillator tank circuits and buffers. Load oscillator output buffer .s2p to verify gain and port match. For phase noise, use a spectrum analyzer with phase noise measurement option.