The increasing demand for higher data rates has driven greater interest in communication systems that operate at higher frequencies, especially in the millimeter-wave (mmW) range, due to the significantly larger available bandwidth. To facilitate communication systems, information must be upconverted to, and downconverted from, the desired transmission frequencies using local frequency synthesizers known as phase-locked loops (PLLs). The spectral purity, or phase noise, of the local frequency synthesizer is one of the most critical parameters determining the fidelity of data transmission or the signal-to-noise ratio (SNR) of the communication system.This PhD research focuses on the design of oscillators, which are pivotal components influencing the spectral purity of local frequency synthesizers and, as a result, the SNR of the entire communication system. The primary focus of this work is on LC and transformercoupled oscillators, which store energy in the form of fluctuating electric and magnetic fields, offering superior phase noise performance. The prototypes are fabricated using advanced semiconductor nodes, such as 28 nm CMOS and 22 nm FDSOI.Phase noise caused by thermal perturbations is strongly dependent on the amount of energy stored in the resonator. The higher the energy in the resonator, the better the phase noise performance. This PhD research primarily focuses on techniques to enhance tank energy for better phase noise performance. In the first work, energy enhancement was achieved through the coupling or synchronization of multiple oscillators. The aim was to directly synthesize a reliable oscillator in 28-nm CMOS for a 60 GHz communication system while simplifying LO distribution. A unit NMOS-only stacked oscillator was proposed to ensure reliable operation under nominal supply conditions. Four-unit oscillators were coupled in a specific configuration to simplify LO distribution across four remote ends of the chip. This configuration can be utilized to enable power-efficient millimeterwave (mmW) local oscillator (LO) distribution to both the transmitter (TX) and receiver (RX) mixers. Additionally, it can be adapted for distributing LO signals to various antenna paths in beamforming systems.In the second work, a passive voltage-boosting technique was introduced to enhance tank energy by increasing the voltage swing across the main energy-storing capacitors, thereby improving phase noise performance. Due to the limited quality factor of passive components in direct millimeter-wave (mmW) synthesis, a lower frequency range of 13-16 GHz was selected for implementation. The objective of this work was to generate a local oscillator (LO) signal with reduced phase noise to enable 64-QAM modulation at D-Band frequencies. The prototype single-core oscillator achieved performance metrics close to state-of-the-art results in advanced technology nodes and was comparable to even multicore oscillators. During the course of this work, it was observed that frequency instability caused by flicker noise was significantly higher in measurements than in simulations. An investigation is also presented, which could shed new light on the mechanisms of flicker noise up-conversion.
Balamurali, S 2025, 'Design of low-phase noise VCOs for mmWave communications', Vrije Universiteit Brussel.
Balamurali, S. (2025). Design of low-phase noise VCOs for mmWave communications. [PhD Thesis, Vrije Universiteit Brussel]. Crazy Copy Center Productions.
@phdthesis{2f230da476304926a2fd2472d4e06fb8,
title = "Design of low-phase noise VCOs for mmWave communications",
abstract = "The increasing demand for higher data rates has driven greater interest in communication systems that operate at higher frequencies, especially in the millimeter-wave (mmW) range, due to the significantly larger available bandwidth. To facilitate communication systems, information must be upconverted to, and downconverted from, the desired transmission frequencies using local frequency synthesizers known as phase-locked loops (PLLs). The spectral purity, or phase noise, of the local frequency synthesizer is one of the most critical parameters determining the fidelity of data transmission or the signal-to-noise ratio (SNR) of the communication system.This PhD research focuses on the design of oscillators, which are pivotal components influencing the spectral purity of local frequency synthesizers and, as a result, the SNR of the entire communication system. The primary focus of this work is on LC and transformercoupled oscillators, which store energy in the form of fluctuating electric and magnetic fields, offering superior phase noise performance. The prototypes are fabricated using advanced semiconductor nodes, such as 28 nm CMOS and 22 nm FDSOI.Phase noise caused by thermal perturbations is strongly dependent on the amount of energy stored in the resonator. The higher the energy in the resonator, the better the phase noise performance. This PhD research primarily focuses on techniques to enhance tank energy for better phase noise performance. In the first work, energy enhancement was achieved through the coupling or synchronization of multiple oscillators. The aim was to directly synthesize a reliable oscillator in 28-nm CMOS for a 60 GHz communication system while simplifying LO distribution. A unit NMOS-only stacked oscillator was proposed to ensure reliable operation under nominal supply conditions. Four-unit oscillators were coupled in a specific configuration to simplify LO distribution across four remote ends of the chip. This configuration can be utilized to enable power-efficient millimeterwave (mmW) local oscillator (LO) distribution to both the transmitter (TX) and receiver (RX) mixers. Additionally, it can be adapted for distributing LO signals to various antenna paths in beamforming systems.In the second work, a passive voltage-boosting technique was introduced to enhance tank energy by increasing the voltage swing across the main energy-storing capacitors, thereby improving phase noise performance. Due to the limited quality factor of passive components in direct millimeter-wave (mmW) synthesis, a lower frequency range of 13-16 GHz was selected for implementation. The objective of this work was to generate a local oscillator (LO) signal with reduced phase noise to enable 64-QAM modulation at D-Band frequencies. The prototype single-core oscillator achieved performance metrics close to state-of-the-art results in advanced technology nodes and was comparable to even multicore oscillators. During the course of this work, it was observed that frequency instability caused by flicker noise was significantly higher in measurements than in simulations. An investigation is also presented, which could shed new light on the mechanisms of flicker noise up-conversion.",
author = "Sriram Balamurali",
year = "2025",
language = "English",
isbn = "9789464948875",
publisher = "Crazy Copy Center Productions",
address = "Belgium",
school = "Vrije Universiteit Brussel",
}