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Old 5th April 2013, 08:58 PM   #2091
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The bottom line is you can always measure the noise floor of an amplifier in the absence of any signal. Why try to do it at the same time?
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Old 5th April 2013, 10:39 PM   #2092
RNMarsh is offline RNMarsh  United States
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because I am not really interested in measuring the noise floor of an amplifier at this time; Just the harmonics of an oscillator that I wish to reduce the THD/harmonics. It happens that the better parts and opamps are also lower noise than the original ones. And, that helps continue the pursuit of lowering the harmonics of the oscillator.

Next up -- another oscillator: The VP-7723A just arrived and its oscillator looks promising at THD of .0001% (1v/1KHz) on the ShibaSoku 725. The VP7723a also includes an analyzer so I can compare against the SibaSoku 725. This VP-7723a was from Hitech&lab in Japan. Thru the monitor of the ShibaSoku 725 again 2H is -133 and 3H is -150. Noted the noise floor is at -160 now... so the 7723a gen is a little quieter than the KH4402B.

VP7723A 1Khz.JPG

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Old 6th April 2013, 12:23 AM   #2093
RNMarsh is offline RNMarsh  United States
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The question comes up... couldnt you just use an FFT to do the harmonic measuring and a seperate sensitive ac voltmeter to measure noise and accomplish the same thing?
Yes.

IF they dont have artifacts, dont have software issues, dont cost the earth to own and dont have enough bits, etc etc etc. There is a lot of potential for low cost FFT but they arent ready for Prime Time, yet.

So I am making good with what I have and what is easily and readily available as used test equipment. Another versatile instrument I am looking for would be a good high speed transient capture instrument with sufficient resolution -- then import the data into software.

-RNM

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Old 6th April 2013, 12:37 AM   #2094
davada is offline davada  Canada
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I do wish these FFT software packages would allow us to plugin our own windowing, filters etc. Or at least allow the data to be dumped to a file for further analysis. I looked over that paper that Samuel linked to a while back. There is some pretty impressive work done there.
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Old 6th April 2013, 06:54 AM   #2095
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Quote:
The bottom line is you can always measure the noise floor of an amplifier in the absence of any signal. Why try to do it at the same time?
Personally I don't use a THD+N function primarily to measure distortion and noise at the same time. But I find viewing the time domain residual on a scope to be tremenduously helpful for diagnosing unexpected distortion issues--the temporal alignement, or the possibility to see distortion changes in real time (which rules out using the average function of the scope to average noise in the time domain), gives numerous clues. These considerations apply particularly for converter testing, where various noise-like artefacts (from improper dithering, gain ranging etc.) may only show up with signal stimulus. So there is certainly some benefit from also improving the THD+N of a generator.

Quote:
Another versatile instrument I am looking for would be a good high speed transient capture instrument with sufficient resolution -- then import the data into software.
I'm not quite sure why it needs to be "high speed" (which implies for me MHz or even GHz) for what we're doing. A decent pro audio interface (e.g. RME, Motu, Benchmark) should cover most needs.

I could design a very good DYI ADC box (and I might do it once), but there's just so much time left.

Samuel
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Old 6th April 2013, 03:10 PM   #2096
1audio is offline 1audio  United States
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This is a picture of the waveform at the distortion analysis output of the 725 for the 590AR and my KH2200 function generator. There is around 70 dB difference in level between the two pictures. The lower trace is the sync output.

If you understand what you are looking at you can infer where the distortion is coming from (crossover, too much/too little gain at a certain part of the wave etc.). Tweaking the diode array in the KH2200 would be really easy with this.
Attached Images
File Type: jpg 590ARdist.jpg (849.2 KB, 163 views)
File Type: jpg KH2200dist.jpg (959.6 KB, 159 views)
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Old 6th April 2013, 04:56 PM   #2097
davada is offline davada  Canada
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Quote:
Originally Posted by 1audio View Post
This is a picture of the waveform at the distortion analysis output of the 725 for the 590AR and my KH2200 function generator. There is around 70 dB difference in level between the two pictures. The lower trace is the sync output.

If you understand what you are looking at you can infer where the distortion is coming from (crossover, too much/too little gain at a certain part of the wave etc.). Tweaking the diode array in the KH2200 would be really easy with this.
Those old TEK scope just keep on going.
Is this an impulse test?
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Old 6th April 2013, 05:07 PM   #2098
1audio is offline 1audio  United States
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The Teks are a real value. I have spares since they are so cheap around here, compared to the hassle of repair.

This is similar to the monitor output with the noise removed. The digital part does a synchronous sampling and seems to only see harmonics. The lower trace is a sync for the scope
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Old 6th April 2013, 05:17 PM   #2099
davada is offline davada  Canada
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Originally Posted by 1audio View Post
The Teks are a real value. I have spares since they are so cheap around here, compared to the hassle of repair.

This is similar to the monitor output with the noise removed. The digital part does a synchronous sampling and seems to only see harmonics. The lower trace is a sync for the scope
From the patent it's hard to tell. I thought the harmonics were separated using BP filters after reconstruction, The digital deals with the noise and reconstruction is at a single frequency. Different interpretations of the patent.
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Old 6th April 2013, 09:25 PM   #2100
RNMarsh is offline RNMarsh  United States
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Quote:
Originally Posted by Samuel Groner View Post
These considerations apply particularly for converter testing, where various noise-like artefacts (from improper dithering, gain ranging etc.) may only show up with signal stimulus. So there is certainly some benefit from also improving the THD+N of a generator.

I could design a very good DYI ADC box (and I might do it once), but there's just so much time left.

Samuel
Pls do design one... PLS !! We need one. You'll get plenty of help. Many of us are in the final quarter of the game before the buzzer. Getting down our experience is something important... Now we have the world watching and listening via the WWW/Internet. A once in a life-time opportunity.
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Thx-RNMarsh
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