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

[Joseph Gwinn]

time-nuts Digest, Vol 214, Issue 22
On Sun, 20 Feb 2022 03:30:27 -0500, time-nuts-request at lists.febo.com 
wrote:

> 
> Message: 5
> Date: Sun, 20 Feb 2022 01:13:50 +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: <9b7416ef-1ead-72eb-1010-0bf355c80cb5 at rubidium.se>
> Content-Type: text/plain; charset=UTF-8; format=flowed
> 
> Hi,
> 
> On 2022-02-20 00:08, Joseph Gwinn wrote:
>> 
>>> 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,
>>>> 
>>>> 
>>>>> 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?
> 
> Well, in the audio industry, wow and flutter measurements have been 
> taken a step further to use such a recording and then analyze the 
> side-band and use that to identify which wheel etc. I have not seen an 
> article about it, but I've been told about it being used in the late 
> 1980s to diagnose professional quarter inch tape machines.

I had not heard of this approach, but it certainly makes sense.  No 
gear teeth in such systems, but an eccentric rubber wheel would leave 
a signature for sure. I would guess that a modulation time series 
would show the wobble quite clearly, allowing its period to be read 
directly.


> You will also find that similar have FFT spectrum analyzers been used 
> for quite some time for similar mechanical analysis to diagnose large 
> machinery and pin-point which cog-wheel or whatever is having an issue. 
> Often used with vibration sensors. I know that HP featured it in a few 
> of their catalogs etc.

I have read many articles in IEEE Instrumentation and Measurement 
Society publications on diagnosing geared machinery for (impending) 
bearing and/or gear failure by looking for tones whose frequencies 
are in the same rational-number angular-speed ratios as the various 
parts of that gear train. 

 
> Regardless, it's fundamentally the same principle involved.

Yes.

 
>>> 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.
> Impedance variations when they exists will be measureable, and smoothed 
> out by the TDR for sure, but location bump should be clear enough when 
> detectable.
>>> 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 was suggesting it as an active diagnosis approach if your signals does 
> not provide suitable PIM products.
>> 
>> 
>>> 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.
> 
> Rather than random noise, yet to he correlated noise is what you mean. 
> Not all of that is random in occurrence.

Yes.  But if we are using only waveforms and spectra, random 
telegraph keying is a good summary.

 
>>> 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.
> 
> The dynamics of modern phase-measurement kits is amazing, so yes.
> 
> BTW When you do indeed have PIM, AM-to-PM and PM-to-AM conversion is not 
> unheard off.

Oh, yes.  The traditional worst case is a powerful radar on a ship - 
every rusty joint (like hatch door hinges) chimes in. You get 
everything, and the carrier third harmonic to boot.  Typically, the 
field team finds culprits using a sniffer tuned to the radar's 3rd 
harmonic.  The field fix involves welding a flexible copper or steel 
wire jumper around every hinge, and so on.  

You also see these welded copper wire jumpers in steel-frame 
buildings, to carry lightning-strike currents to ground, bypassing 
bolted structural joints.  In this case, the welds are generally made 
using copper thermite.


>>> 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.
> 
> Indeed. You also get a time-difference for which the correlation occurs 
> in, which with a bit of triangulation could help in the pin-pointing. To 
> get a useful signal for that, just a lock-up to the signal could be 
> useful as the phase-detector output will for sure have that signal, and 
> conveying this over to a cross-correlation measurement should be fairly 
> straight-forward. Almost feel like going into the lab and set it up.

Hmm.  In a time-series display, if one locks to one of the signals, 
perhaps the loudest or cleanest, all coherent and thus correlated 
signals will also stand still, thus revealing their common heritage.  
A wide window could be needed to see all patterns, unwrapped.  This 
has the advantage of not requiring prior knowledge of the underlying 
ratios, but allowing the ratios to be computed from the data.  
Comparison with the gear-train details, will likely yield exactly one 
solution, disentangling the data into cause and effect.

The people diagnosing geared machinery do much the same, except that 
the data is plotted normalized to the drive shaft angle versus time.

I'm sure that the folk developing low phase noise oscillators also 
have their bag of tricks.  Comparing notes may be useful.

Joe Gwinn