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

[Joseph Gwinn]

time-nuts Digest, Vol 214, Issue 20

> 
> Message: 11
> Date: Fri, 18 Feb 2022 22:42:22 +0100
> From: Attila Kinali <attila at kinali.ch>
> Subject: [time-nuts] Re: Types of noise (was: Phase Station 53100A
> 	Questions)
> To: Discussion of precise time and frequency measurement
> 	<time-nuts at lists.febo.com>
> Message-ID: <20220218224222.d23061788e7bdf5b6c60fe10 at kinali.ch>
> Content-Type: text/plain; charset=US-ASCII
> 
> 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)

The question was not what way is best.  The intent is debugging, not 
precision measurement, and I will already have the 53100A hooked up 
for residual measuring phase noise.  The hope is to have a way to do 
a quick test of the mechanical integrity of the system under test, so 
we don't spend too much time chasing our tails.


>>>>  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.

It's true that he does that.  In the recently published "The 
Companion of the Enrico's Chart for Phase Noise and Two-Sample 
Variances", E. Rubiola, F. Vernotte, 18 January 2022, they adopt the 
AM and PM distinction.  And yes the sum of AM and PM is the total 
noise.  

I suppose we could split a hair here about how to handle added 
thermal noise versus noise modulated onto the RF carrier, but I lump 
thermal noise in as well, unless there is a reason to do otherwise.

.<https://www.semanticscholar.org/paper/The-Companion-of-the-Enrico's-Chart-for-Phase-Noise-Rubiola-Vernotte/b0b068ee5c9e566ef1d7f150ec76c17bebfd3e24>


>> 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.

If no distinction is made, yes.  In the days when power amplifier 
involved vacuum tubes, transmitters were very noisy, and PM usually 
dominated.  While AM could be measured, nobody much worried about it 
back then.

 
>> 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. 

Yes, specifically AM 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).

True.

> As for PM noise being unmeasurable, 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
> measurement of lasers, you need a fast photo diode to beat two
> lasers against each other, then you analyze the output signal with
> a phase noise analyzer. 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.

All true, of course.  When I said _unmeasurable_, I meant with the 
tools of the day.


> 			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
> 


> 
> Message: 14
> Date: Sat, 19 Feb 2022 01:12:05 +0100
> From: Magnus Danielson <magnus at rubidium.se>
> Subject: [time-nuts] Re: Types of noise (was: Phase Station 53100A
> 	Questions)
> To: time-nuts at lists.febo.com
> Message-ID: <c265e897-898a-3bb7-f522-52e282378523 at rubidium.se>
> Content-Type: text/plain; charset=UTF-8; format=flowed
> 
> Dear Joe,
> 
> On 2022-02-13 23:31, Joseph Gwinn wrote:
>> 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?
> 
> Actually, measure vibration impact like this have a long tradition and 
> is indeed possible.

I thought as much.  Can you cite any articles on this?

 
> It may or may not be an effective method thought. As suggested by 
> others, TDR may very well be more effective method to locate impedance 
> errors. Could be that they add good information for different errors.

TDR units may have some difficulties with an unstable contact under 
vibration.  When one has determined that there is a problem somewhere 
using the 53100A is when the TDR equipment comes out, if the root 
cause isn't obvious on inspection.


> Also, recall that erroneous connectors can create passive 
> intermodulation distortion (PIM), which is readily measured using the 
> two-tone method.

The signal levels are pretty low for PIM to be important.  And the 
connectors are generally gold plated.  A cracked copper conductor 
could in theory do PIM, but I have not seen this.  Even if it is 
happening, so long as the AM component jumps, it will serve to warn 
the experimenter.


> I would use a wealth of methods to attempt different techniques and see 
> what they excel at and not.

Yes.


> I would not assume the random telegraph waveform variation. I would 
> rather learn from reality the types of variations you see.

Random telegraph keying is likely when a loose contact is driven with 
random vibration.  If the vibration is instead a sine wave, some kind 
of square-wave keying is more likely.  And so on.  Random telegraph 
keyed waveform seemed representative.

 
> I think you should consider two different phases, detection of problem 
> and location of problem. When it comes to location finding, TDR excel 
> at that. AM measurements as well as PIM is relevant for detection of 
> problem as well as verification.

Yes, but with a pre-scan before phase-noise tests are run.

 
> I would recommend you to look at the updated IEEE Std 1193 when it comes 
> out. There is improved examples and references in it that may be of 
> interest to you.

Will do.  The prior version is well-thumbed now.

 
> It may be beneficial to stick accelerometers here and there to pick up 
> the vibrations, so it can be correlated to the measured noise, at it 
> could help to locate the source of the noise and thus help with locating 
> where, more or less which engine that was causing it.

We do usually have nearby accelerometers, but no direct way to 
correlate PN modulation waveform with vibration waveform.   It's 
something to think about though, as it could point directly at the 
culprit.


Joe Gwinn