University of Twente Student Theses


Analysis and design of a low noise LC oscillator

Baveco, M.A. (2017) Analysis and design of a low noise LC oscillator.

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Abstract:During this master thesis project, research has been carried out in the field of LC oscillators. Oscillators are applied in many electrical systems (for example as a Voltage Controlled Oscillator (VCO) in communication systems for modulation, demodulation and filtering). The research goal is to give an analysis to suppress 1/f-noise in an LC oscillator and design an integrated circuit (IC) of it. The motivation for this research is based on a system currently developed by NXP and ICD: the ”single trim highly accurate frequency reference”. In that system, an LC oscillator is used from time to time as stable frequency reference to recalibrate other oscillators. Due to the relatively long calibration time that is required by the chosen architecture the phase noise is dominated by flicker noise and hence could form a bottleneck for system performance. Therefore, a low 1/f noise LC oscillator is required. Initially, a theoretical framework has been formulated that focuses on the phenomenon flicker noise in ICs, LC oscillators in general, phase noise and flicker phase noise suppression techniques. Results of that framework have been poured into the circuit design of low 1/f-noise LC oscillators. Two novel LC oscillators based on a chopping mechanism have also been designed and simulated. A standard class B LC oscillator formed the starting point for circuit design. Several oscillator circuits have been designed afterwards, from which the (improved) Tchamov oscillator performed best. It achieved a phase noise performance of -40 dBc/Hz at 100 Hz. Almost no flicker phase noise is upconverted in this oscillator, since the phase noise slope decays with -20 dB/decade indicating only white phase noise. It obtains a FoM of -192 dBc/Hz (at 1 MHz) which is 4 dBc/Hz away from the limit of -196 dBc/Hz (assuming T = 300 K, Qtank = 13). However, it should be noted that no measurement results are yet available of the Tchamov oscillator, raising suspicion. This is because the authors first did reply after having contact, but when it was asked why no measurements were yet available, no answer was given. It is recommended though to investigate the circuit further because the performance is outstanding so far. Another main finding of the research is that flicker noise forms no direct threat for the ”single trim highly accurate frequency reference” system of NXP and ICD. It was found that the accumulating period jitter of all designed oscillators remained below 1 period of the 5 GHz (TO = 0.2ns) LC oscillation frequency after even 200 µs oscillation time. Thus, the counting mechanism, which is part of the system of NXP and ICD, will make no error in counting periods during calibrating the main signal frequency to the LC oscillation frequency. The two novel LC oscillators are based on a chopping mechanism to reduce the flicker noise were developed. The first novel chopping LC oscillator achieved 3 dBc/Hz flicker phase noise improvement compared to its unchopped counterpart. The flicker phase noise is aimed to be reduced by flushing the transistor channel when VGS = 0V . It is expected that more improvement is feasible, since transistor models used for circuit simulation seem to lack for a large part the effect of flicker noise reduction due to channel flushing. The second novel chopping LC oscillator seemed to obtain eventually not really an improvement in flicker phase noise performance. It can be concluded that the first chopping LC oscillator reduces flicker phase noise in a novel way and could form a new research direction. To give an overall reflection on the posted research goal, one can conclude that an analysis to suppress 1/f-noise in LC oscillators has been formulated. Unfortunately, no IC lay-out has been made, however six LC oscillators have been designed.
Item Type:Essay (Master)
Faculty:EEMCS: Electrical Engineering, Mathematics and Computer Science
Subject:31 mathematics, 33 physics, 50 technical science in general, 51 materials science, 53 electrotechnology
Programme:Electrical Engineering MSc (60353)
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