Hi Joe,
On 2022-02-21 20:52, Joseph Gwinn wrote:
time-nuts Digest, Vol 214, Issue 22
On Sun, 20 Feb 2022 03:30:27 -0500, [email protected]
wrote:
Message: 5
Date: Sun, 20 Feb 2022 01:13:50 +0100
From: Magnus Danielson <[email protected]>
Subject: [time-nuts] Re: Types of noise (was: Phase Station 53100A
Questions)
To: [email protected]
Message-ID: <[email protected]>
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Hi,
On 2022-02-20 00:08, Joseph Gwinn wrote:
Message: 14
Date: Sat, 19 Feb 2022 01:12:05 +0100
From: Magnus Danielson <[email protected]>
Subject: [time-nuts] Re: Types of noise (was: Phase Station 53100A
Questions)
To: [email protected]
Message-ID: <[email protected]>
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Dear Joe,
On 2022-02-13 23:31, Joseph Gwinn wrote:
On Sun, 13 Feb 2022 03:30:30 -0500, [email protected]
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.
The experience was very clearly that it gave very direct indication of
what needed to be fixed. The story was that they had some 50+ machines
to work through in a few weeks at the customer. The japanese engineer
brough with him some extra tools and they used these to diagnose, fix
and then verify all the machines, and it was very efficient approach.
The take-away is that it makes sense and it's essentially doing the
phase-measurement and spectrum as we do, and was able to detect small
variations, and being trained on them it pin-points the issue.
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.
Which makes sense.
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.
Well, it may be a good enough approximation. I just want to avoid an
assumption turning into it's own "truth".
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.
An alternative approach there is to transmitt a suitable offset
frequency, and mix-products will occurr, as one essentially do the 2
frequency PIM test. and 2f1-f2 and 2f2-f1 frequencies will occurr among
others. It all depends on what is practical.
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.
Well built, good signal integrity, EMC, EMI etc. fit well together in
basic approach with multiple benefits.
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.
You can look up the NIST T&F archive for how to analyze it.
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.
Which is why I propose to really use the toolset of both genres to
combine them to meet the actual need of the application.
Cheers,
Magnus
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