LO
Understanding Local Oscillators
The local oscillator is a critical component in any frequency conversion system. Its frequency determines which RF channel is selected, and its purity directly affects the quality of the converted signal. Phase noise on the LO transfers directly to the converted signal, degrading EVM, BER, and radar clutter rejection.
LO Requirements
- Phase noise: Must be low enough that it does not limit the system EVM or BER. For 64-QAM, LO phase noise should be below -100 dBc/Hz at 10 kHz offset.
- Power level: Must match the mixer LO drive requirement, typically +7 to +17 dBm for passive mixers.
- Spurious: Harmonics and sub-harmonics of the LO create unwanted mixer products. Should be < -40 dBc.
- Leakage: LO power that appears at the RF or IF port of the mixer must be minimized to avoid interfering with other systems.
LO Sources
- Frequency synthesizer: PLL-based. Tunable with low phase noise locked to a crystal reference.
- DRO: Fixed-frequency dielectric resonator oscillator. Very low phase noise.
- VCO + PLL: Voltage-controlled oscillator locked to a reference. Most common approach.
Upconversion: f_RF = f_IF + f_LO
LO power requirements (passive mixer):
Level 7 (+7 dBm), Level 13 (+13 dBm),
Level 17 (+17 dBm), Level 23 (+23 dBm)
Higher LO drive = better linearity (higher IP3)
Frequently Asked Questions
What is a local oscillator?
A local oscillator (LO) generates a reference frequency that drives a mixer for frequency conversion. In a receiver, the LO frequency determines which RF channel is translated to the IF for processing. The LO is the tuning element that selects the received frequency.
Why does LO phase noise matter?
Phase noise on the LO transfers directly to the converted signal through the mixing process. This phase noise degrades EVM in digital communication systems, increases BER, and reduces the ability of radar systems to detect small targets in the presence of clutter.
What determines the required LO power?
The mixer specifies the required LO drive level. Passive diode mixers need +7 to +23 dBm depending on design level. Higher LO drive generally provides better conversion loss, higher IP3 (linearity), and better port isolation, but requires more LO power.