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.
| 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 |
- Performance verification: confirm specifications against the application requirements before finalizing the design
- Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
- Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
- Interface compatibility: verify impedance, connector type, and mechanical form factor match the system architecture
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.