[time-nuts] Re: Types of noise (was: Phase Station 53100A Questions)

[Joseph Gwinn]

On Sun, 13 Feb 2022 03:30:30 -0500, time-nuts-request at lists.febo.com 
wrote:
time-nuts Digest, Vol 214, Issue 15

Attila,

> Date: Sat, 12 Feb 2022 20:38:48 +0100
> From: Attila Kinali <attila at kinali.ch>
> Subject: [time-nuts] Types of noise (was: Phase Station 53100A
> 	Questions)
> To: Discussion of precise time and frequency measurement
> 	<time-nuts at lists.febo.com>
> Message-ID: <20220212203848.72783256d221001199dfd9cc at kinali.ch>
> 
> On Fri, 11 Feb 2022 18:25:05 -0500
> Joseph Gwinn <joegwinn at comcast.net> wrote:
> 
>> May not realize that thermal noise (additive) and phase 
>> noise (multiplicative) are not the same, and do not behave the same.
> 
> It seems like you are mixing up here quite a few different concepts:
> Phase noise vs amplitude noise, additive vs multiplicative noise,
> thermal vs other noise sources, white noise vs 1/f^a-noise.

You are right of course.  I was using shorthand.  

A better word than multiplicative is parametric, the varying 
parameters being path loss and path group delay.  This is as seen at 
the phase noise test set.


> All these are orthogonal to each other and you can pick and match them.
> I.e. Phase noise can be additive, 1/f^2-noise and thermal.

At the generator, certainly.  But the downstream PN test set may not 
be able to tell.  More later.


> Amplitude and phase noise are looking at noise from two different
> perspective. One is how large the variation of the peak of a sine
> wave is, the other is how much the zero crossing varies in time.
> Note that all natural noise sources will be both amplitude and
> phase noise. 

Hmm.  One case I'm interested in is where the path attenuation varies 
according to a random telegraph waveform, due to for instance a loose 
connector or cracked center conductor rattling under heavy 
vibration.  In this, the electrical length does not change.  While 
the source of the carrier whose PN is being measured will have some 
mixture of AM and PM characteristic of that source, the residual 
(added) PN will be characteristic of the transit damage encountered 
between source and PN test set.  So wouldn't this randomly varying 
attenuation yield mostly residual AM PN and little residual PM PN?  


>   What makes them one or the other is how we treat them
> in our system. E.g., amplitude noise is often relative easy to suppress
> with some AGC system. Or, passing through amplifiers amplitude noise
> and phase noise gets different "treatment" and thus the noise figure
> amplifier for each of them will end up being different.

I would suspect that an AGC would struggle with a random telegraph 
waveform, as it would amplitude modulate the telegraph modulation 
onto the the noise floor background.

 
> Whether noise is additive or multiplicative is not a property of the 
> noise itself, but a property where in the system it appears. E.g. while
> in school we always treat all noise sources in an amplifiers additive,
> once we get out and start engineering we add up noise figure values in dB.
> Which means we treat them as multiplicative noise. In reality most systems
> have both additive and multiplicative behaviour, but often one is dominant
> over the other. In RF systems, due to having multiple non-linear stages,
> noise is almost always dominantly multiplicative. 

Yes.


> Last but not least, the power spectral density or whether noise is white
> or 1/f^a is depends which of the noise sources is dominant at which
> frequency. Obviously, 1/f^a is the dominant one for low frequencies.
> While for white noise we have good models that explain them (either
> thermal/Johnson noise or shot noise, usually), we do not have a proper
> explanation for 1/f^a noise. Most seem to agree that at least 1/f^1 noise
> in semiconductors, the most likely cause is trapping of electrons (though
> that explanation has quite some problem). Please note that for very low
> frequencies what we declare to be noise in noise spectra is usually related
> to environmental effects, which I personally would not bunch together with
> "normal" noise processes, as their treatment/mitigation has to be done
> differently.

Yes.

Well, a key distinction has emerged, between the PN of a source, and 
of the residual PN imposed on a signal in transit.  This is quite 
useful.


Joe Gwinn