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Old 21st December 2003, 04:51 PM   #21
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Sy, I will tell you this:
I am making a series of fairly difficult measurements with the test equipment that I have. My ST was designed to work to approximiately -100dB, at measurement levels about .3V or more. With added FFT processing, I can separate the harmonics from the null residue and reduce the noise floor.
I CAN produce measurement artifacts, if I am not careful. For example, if I load the oscillator buffer excessively, I can begin to produce extra 2'nd and 3'rd harmonic distortion. However, when I find a 'clear' area of measurement, then I can make measurement comparisons.
Once I have a measurement set-up that seems OK with my reference cables, I make NO changes to the measurement equipment. I just change the cable by removing it from the connector and replacing it with another.
At this point, I can see differences between different cables, both of different lengths and the same length. I can also see differences with similar cables, but with different use patterns, such as the amount of signal passed through them over time.
I can also see minor distortion 'bumps' build up, if my reference cable's connectors and the the whole external assembly are not periodically cleaned with industrial purity isopropyl alcohol The 'bumps' go down or away, after cleaning.
I monitor the input of the analyzer with a 350MHz Tek 485 scope, running maximum bandwidth, given the test conditions. I see no oscillation on the scope.
At this point I can do no more to show anything.
If Bruno's results are again a null, then the comparison is ended.
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Old 21st December 2003, 05:05 PM   #22
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John, thanks for the additional details.

This may be asking a lot, and I won't at all take offense at a "no" answer (nor draw any inferences; lots of people just like their privacy), but would it be possible for you to demonstrate for me how the measurement is done? I'd like to do an informal gauge R&R by connecting and disconnecting the same cable, playing around with the dress of the cable, and so forth.

As I said, my own professional expertise is the physics of conductive materials, and coincidently, fourier transform techniques (I was the #2 guy in R&D at Nicolet back when they were the Big Guys in FFT instrumentation, which certainly dates me!). So this really seems to be up my alley. I've got no axes to grind, I just want to understand your measurements and what they really mean.

As recompense for your time, I'll happily cart along a bottle of something rare and vinous.
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Old 21st December 2003, 05:22 PM   #23
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One person I like to see participate here is Siegfried Linkwitz he was the BIG GUY who constructed FFT monsters at HP a very nice technician that can sweeping the fog away.

Another good techie Nelson Pass I will remember that he made a measure on cables years ago maybe this can be used as a reference here.
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Old 21st December 2003, 05:45 PM   #24
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Quote:
Originally posted by kamskoma
About test equipment, can I use a simple THD analyzer like the HP 332A to null out the fundamentals and put this signal into a soundcard like the DAL CardDeLuxe togetherwith a good FFT software, is this a good measurement device?.
Well, the graphs of the measurements John did some time ago were done by feeding the residual from his 1700B into the built in sound card in his Macintosh using a demo version of Mac The Scope software.

Quote:
What I think is, it may be very simple to make a reference measurement that will go for all comming measurements or?

Just measure the nulling device with a good soundcard Card DeLuxe then you have a very good an cheap instrument.

If not! where is the fault, and can I do it with even cheaper nulling device and soundcards.
The issue here isn't determining whether John is measuring distortion or not. He clearly is. The issue is whether the distortion he's measuring is in fact being produced by the cables or by his 1700B.

So far, using a system known to have lower distortion and greater resolution than what John's using, none of the high order distortion products that John routinely measures have shown up. John says he measures distortion with virtually every cable he's used, but even when measuring a good 20dB below where John's measuring, nothing has turned up.

The measurements that Bruno did had been dismissed by someone else because Bruno wasn't measuring the same cables as John. So this is an attempt to repeat the measurements with both John and Bruno measuring the same cables.

se
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Old 21st December 2003, 05:48 PM   #25
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Quote:
Originally posted by kamskoma
One person I like to see participate here is Siegfried Linkwitz he was the BIG GUY who constructed FFT monsters at HP a very nice technician that can sweeping the fog away.

Another good techie Nelson Pass I will remember that he made a measure on cables years ago maybe this can be used as a reference here.
If you think you can interest either of these gentlemen in doing low level distortion measurements on cables, that would be a plus. I know Nelson has an AP rig.

se
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Old 21st December 2003, 06:02 PM   #26
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Quote:
Originally posted by john curl
Frank, please quit putting your foot into this.
ouch!

Quote:
Originally posted by SY
(I was the #2 guy in R&D at Nicolet back when they were the Big Guys in FFT instrumentation, which certainly dates me!).
Wow! we have some seriesly knowledgeable folks here.

Quote:
Originally posted by Steve Eddy


Well, the graphs of the measurements John did some time ago were done by feeding the residual from his 1700B into the built in sound card in his Macintosh using a demo version of Mac The Scope software.


se
I didn't follow this thread as closely but two things here:

1) did John repeat the tests? maybe the sound card is faulty the time John was testing it?
2) did the distortion come from the source? what if John used a very short or better cable? Would John have seen anything materially different?

I think we are certainly making progress here, from anecdotes to concrete measurements. Keep up the good work, folks.
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Old 21st December 2003, 06:10 PM   #27
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Wow! we have some seriesly knowledgeable folks here.
Not compared to the #1 guy there at the time, a spooky-smart Cajun named Warren Vidrine. I was seriously lucky to be able to get a job working for him, and I've even forgiven him for introducing me to my first wife. We're still close friends, even after all these years (damn, was that really 20 years ago?!).

It's sobering for me to realize how sophisticated the stuff we were doing then seemed to be, yet it's all trivially easy these days with PCs and modern software.
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Old 21st December 2003, 07:43 PM   #28
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Originally posted by SY
It's sobering for me to realize how sophisticated the stuff we were doing then seemed to be, yet it's all trivially easy these days with PCs and modern software.

yeah. it is just that much easier to implement FFT in software than hardware, .

Thank God for data acquisition and computers,
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Old 21st December 2003, 10:57 PM   #29
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Kam, the HP 332 probably won't do it, BUT the HP339 should be OK. It is just a matter of distortion residual. I have enough problems, you would not not want to use something even older and compromised as far as residual distortion is concerned.
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Old 22nd December 2003, 12:06 AM   #30
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Kam, the HP 332 probably won't do it, BUT the HP339 should be OK. It is just a matter of distortion residual. I have enough problems, you would not not want to use something even older and compromised as far as residual distortion is concerned.
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