What is the phase noise requirement for the LO in a digital communication system with a given modulation?
Phase Noise and Modulation
Phase noise directly rotates the received constellation points, increasing the EVM. The EVM contribution from phase noise is approximately: EVM_PN (dB) ≈ 10·log10(2 × ∫ L(fm) dfm), where the integral is the integrated phase noise power over the signal bandwidth. This integral gives the variance of the phase error in radians².
| Parameter | Passive Diode | Active FET | Subharmonic |
|---|---|---|---|
| Conversion Loss/Gain | 5-9 dB loss | 0-10 dB gain | 8-12 dB loss |
| LO Drive Level | +7 to +17 dBm | -5 to +5 dBm | +5 to +13 dBm |
| IP3 (typical) | +15 to +30 dBm | +5 to +20 dBm | +10 to +20 dBm |
| Noise Figure | 5-9 dB (= conv. loss) | 8-15 dB | 9-14 dB |
| LO-RF Isolation | 25-45 dB | 15-35 dB | 20-40 dB |
Frequently Asked Questions
How do I convert from dBc/Hz to RMS degrees?
Integrate the SSB phase noise L(fm) over the offset frequency range of interest: θ_RMS = √(2 × ∫ 10^(L(fm)/10) dfm). For a flat phase noise of -100 dBc/Hz integrated over 1 MHz: θ_RMS = √(2 × 10^(-10) × 10^6) = √(2×10^-4) = 0.014 rad = 0.8°.
Which offset frequencies matter most?
For narrowband signals: close-in phase noise (1 Hz to 10 kHz offset) dominates. For wideband signals: far-out phase noise (100 kHz to 10 MHz offset) contributes more because of the wider integration bandwidth. OFDM systems are particularly sensitive at offsets corresponding to the subcarrier spacing (15-240 kHz for 5G NR).
Does the receiver correct for phase noise?
Partially. Phase tracking in the receiver's equalizer can track and correct slow phase variations (low-frequency phase noise). Fast phase noise (high-frequency offset) cannot be tracked and appears as residual EVM. Common phase error (CPE) correction removes the mean phase offset per OFDM symbol but not the inter-carrier interference.