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

[Magnus Danielson]

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 
80thies to diagnoze professional quarter inch tape machines.

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.

Regardless, it's fundamentally the same principle involved.

>   
>> 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.

>   
>> 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.

>   
>> 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.

Cheers,
Magnus