emad
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Cairo, Egypt
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City,
The phenomena you are seeing is NOT AM-to-FM noise coversion. The paper by Prof. Abidi and myself illustrates that this effect (AM-to-FM) is due to flicker noise from the current source. Since you don't have one, the whole analysis is that paper is of no use in your case. Let me illustrate more, switching devices cannot upconvert flicker noise to the oscillation frequency (please read Darabi's paper on mixers to see what I mean). Flicker noise shows at the oscillation frequency through frequency modulation rather than phase modulation. Interestingly, for an RF designer, phase and frequency noise are indistinguishable. Having that said, the amount of flicker noise in the PMOS and NMOS in your circuit has nothing to do with their relative contribution in the noise analysis.
The frequency noise modulation index is dependent to a great extent on the common mode point capacitance for example. (This was illustrated by Darabi for Mixers and hinted to by Rael for Oscillators). Therefore, if we assume an ideal current source, smaller switches will have more flicker noise but will have a much lower conversion gain. So, depending on the actual values you [u]might[/u] end up better off with smaller devices. The analysis is far from straightforward, that's why there has been no physically-based theory for flicker noise in oscillators).
Without sounding commercial, there is a new book on oscillators coming soon in the Designer's guide series. It is co-authored by myself, Jacob Rael, and Asad Abidi. The publisher is Springer Verlag. The analysis of flicker noise is detailed there.
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