Simple noise modulating functions (NMFs) for MOSFET thermal noise and low-frequency 1/f-noise are presented for the use with the time-varying impulse sensitivity function (ISF) theory of oscillator phase noise. The NMFs are employed to analyze the phase noise of a single-ended CMOS inverter ring oscillator with either voltage or current control. The refined ISF theory predicts a 5 to 8 dB phase noise reduction when current control is used instead of voltage control. The phase noise reduction is verified by measurement. As the theory with neglected NMFs does not predict any phase noise reduction, it is shown that adequate NMFs are essential when explaining oscillator phase noise within the ISF theory and that the neglect of the NMFs will mostly result in misleading results.
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