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

[Attila Kinali]

On Tue, 15 Feb 2022 17:07:54 -0500
Joseph Gwinn <joegwinn at comcast.net> wrote:

> > That's an interesting noise model. And one that is oddly specific.
> > Did you see that in some application? If yes, could you explain a bit more?
> 
> When integrating large systems, people have a lot of trouble with 
> loose and/or broken coax connectors, especially under vibration, or 
> when a cable is moved for some unrelated reason.  I'm looking for a 
> simple way to detect from the outside if this is happening, without 
> disassembling everything (and introducing added problems).  Nor is 
> the problem necessarily obvious to the eye, even if it is all 
> disassembled.

Hmm... And what makes you think that phase noise would be the best
way to do so? Wouldn't looking at the impedance match at the sending
side, i.e. looking at the reflected wave be the better approach?
That should be a much better method and could potentially tell you
also where the problem is. (this is being used in large scale
fiber networks to detect breaks)

> >>  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 is the PN here? Phase noise? If so, then there is no AM PN.
> 
> Well, Rubiola uses AM PN and PM PN,

Does he? I am not aware of that. Quite to the contrary, in his
talks and presentations he does make sure that people do not
confuse AM and PM.

If you look at [1], specifically at slide 3 and 12 you will see
that he splits total noise on a signal into AM and PM noise.
I.e. total noise is the sum of AM and PM noise.

> and TimeLab says Phase Noise and 
> AM Noise, and others use other names.  Plain PN was always a 
> misnomer, as there was always an AM component, even though PM 
> generally dominated. 

I think what you mean here by PN is total noise, not phase noise.

> Except in laser systems, where AM PN is also 
> known as RIN (Relative Intensity Noise), and PM PN was basically 
> unmeasurable.  I like the AM PN and PM PN nomenclature because it is 
> precise and symmetric, from DC to daylight.

Ok.. there is some confusion here. RIN is just an other word for
amplitude noise. The reason why lasers have (had) such a high amplitude
noise is because, being oscillators with no active amplitude control,
that are based on a stochastic process with a high positive gain but only
a slow process to restore energy into the system, the fluctuations of
photon density in the cavity are basically uncontrolled and jump up
and down quite a lot. But because the photon density never sinks to zero,
the phase is preserved. But mind you, high AM noise (aka RIN) translates
to increased PM noise as well, due to AM-PM translation in the (non-linear)
gain medium (laser are very weird devices that are plagued by all kinds
of problems if you are looking too closely).

As for PM noise being unmeasureable, that's not true. It was and is
measurable. It was just that we did not have the tools to do so
commonly available until a few decades ago. To do phase noise
measurament of lasers, you need a fast photo diode to beat two
lasers against each other, then you analyse the output signal with
a phase noise analyser. Coincidentally, with the photo diode you
get both AM and PM noise.

Today, narrow line-width lasers are all phase noise (or rather
frequency noise) controlled, additional to being amplitude controlled.
The three major techniques for this are saturated absorption spectroscopy,
dichroic atomic vapor lock and Pound-Drever-Hall lock.
Sub-Hz line-widths are quite common and the best get down to ~50mHz.


			Attila Kinali

[1] "Basics of Phase Noise" by Enrico Rubiola, IFCS/PTTI 2005 tutorial
http://rubiola.org/pdf-slides/2005T-ifcs-basics.pdf
-- 
The driving force behind research is the question: "Why?"
There are things we don't understand and things we always 
wonder about. And that's why we do research.
		-- Kobayashi Makoto