Local Oscillator Design
Understanding LO Design
The local oscillator is, along with the LNA, the most critical component in a receiver chain. Phase noise directly sets the close-in noise floor, spurious signals create false targets, and LO power level determines mixer conversion loss.
LO Requirements by Application
| Application | Phase Noise (10 kHz) | Spurs |
|---|---|---|
| Consumer Wi-Fi | -90 dBc/Hz | -50 dBc |
| Cellular base station | -110 dBc/Hz | -70 dBc |
| Military radar | -120 dBc/Hz | -80 dBc |
| SIGINT receiver | -130 dBc/Hz | -90 dBc |
LO Generation Methods
- PLL + VCO: Most common. Frequency agile. Phase noise set by VCO out-of-band and reference in-band.
- DDS + PLL: Fine resolution, fast switching. DDS provides resolution; PLL provides cleanup.
- Crystal oscillator + multiplier: Lowest phase noise for fixed frequency. Used in radar and high-performance receivers.
Frequently Asked Questions
What is an LO?
A local oscillator generates the signal that drives mixers for frequency conversion. LO quality (phase noise, spurious, frequency accuracy) directly determines receiver sensitivity and transmitter signal purity.
Why does LO phase noise matter?
LO phase noise mixes with large interfering signals through reciprocal mixing, raising the noise floor and desensitizing the receiver. Noise at 10 kHz offset from a -20 dBm interferer directly adds to the noise at the IF.
What LO power level is needed?
Typical mixer requirements: +7 dBm (standard level), +13 dBm (high-level for better IP3), +17 dBm (high-performance doubly-balanced). Insufficient LO power increases conversion loss and degrades mixer linearity.