What is LO?
A LO (Local Oscillator) is an electronic circuit that generates a stable sinusoidal signal at a specific frequency, used in radio transceivers and test equipment for frequency conversion (up-conversion and down-conversion). The LO is a fundamental building block in every RF system — from 5G base stations and smartphones to spectrum analysers and signal generators. LO performance — particularly phase noise, frequency accuracy, and spurious content — directly impacts system performance metrics such as EVM, receiver sensitivity, and adjacent channel selectivity.
How Does LO Work?
In a transmitter, the LO signal is mixed with the baseband (or IF) signal to up-convert it to the desired RF frequency for transmission. In a receiver, the LO mixes with the incoming RF signal to down-convert it to baseband or IF for processing. The mixing process produces sum and difference frequencies, and filters select the desired output. In 5G NR, LO design is particularly challenging at FR2 (mmWave) frequencies where achieving low phase noise at 24–52 GHz requires advanced PLL (Phase-Locked Loop) architectures. LO phase noise directly causes CPE and ICI in OFDM systems, degrading EVM and throughput.
Use Cases
5G NR transceiver design (gNB and UE), mmWave front-end module design, test equipment (spectrum analyser, signal generator) architecture, frequency synthesizer design for multi-band operation, and phase noise optimization for high-order modulation support.
3GPP / Standards Reference
3GPP TS 38.104 (NR BS radio transmission — phase noise requirements), applicable IEEE/ETSI standards for oscillator specifications
Related Terms
CPE | ICI | EVM | FR2 | mmWave
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This glossary entry is part of the 5GWorldPro Complete 5G Glossary. To go deeper into 5G architecture and technology, explore our 5G Training courses.
