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Design >> RF Design >> confusion about phase noise
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Message started by dog1 on Sep 12^th, 2016, 2:57am

Title: confusion about phase noise
Post by dog1 on Sep 12^th, 2016, 2:57am

Hello everyone,

I have a confusion about the basics of phase noise.

I am reading the Razavi's book RF Microelectronics, 2nd Edition. And in Chapter 8.7.3, it says that a for noise around a oscillatory waveform, half of the energy goes into AM side band while the other goes into PM sideband. I think the derivation there also apply to any periodical waveform with dominant 1st harmonic, since it only require a noise around w0. This is confusing. Does this mean that for phase noise simulation of periodical signal, I can always just simulate the total noise spectrum around w0, and divide it by two, regardless of the waveform of the periodical signal and what kind of noise there is? Then why do we need "modulated" type as cadence Spectrum pnoise simulation option? Why don't we just always use "source" type? I also remember that for hard switching circuit, the noise power is mostly PM, but then this contradicts my understanding of the previous statement.

From my understanding, the contradiction seems to arose from the fact that the previous statement is made without consideration for the phase correlation between noise slots in the spectrum. Is that correct?

Thanks

CHEN

Title: Re: confusion about phase noise
Post by Ken Kundert on Sep 12^th, 2016, 11:05am

Adding stationary noise to a sinusoid results in a signal with equal amounts or AM and PM noise. This is described in An Introduction to Cyclostationary Noise. However, that is not the situation in an oscillator. All practical oscillators have some mechanism that controls their output amplitude, and this mechanism acts to suppress any AM component in the noise. Thus, noise in an oscillators is almost purely phase modulated.

Generally when simulating phase noise in oscillators, you do use the 'sources' method, which computes the time-average noise. There is no need to understand the amount of AM and PM noise separately because the AM component is generally negligible.

The 'modulated' method of computing noise is usually only used on driven circuits, such as modulators.

-Ken

Title: Re: confusion about phase noise
Post by dog1 on Sep 19^th, 2016, 2:12pm

Hello Ken,

Thanks very much for your fast reply. It is now clearer to me.

BR

CHEN :)