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Old 30th April 2012, 05:47 PM   #161
1audio is offline 1audio  United States
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Originally Posted by pol098 View Post
I used the oscillator to help with designing and building a very cheap distortion analyser with an unusual technique, working at zero frequency - much easier to get a bandwidth of a few Hertz than at higher frequency where you require crystal filters. I think it worked quite well; I took out a provisional patent for some of the techniques but never quite completed the project, deciding there was no market, or at any rate it wouldn't help me much in making a living. I have the prototype, but all paperwork, including circuit, has gone; I don't even know if I would remember how to use it. The idea came from a suggestion in M G Scroggie's Radio Laboratory Handbook. It did give readings, harmonic by harmonic, from the low-distortion oscillator - it was sensitive enough. Nowadays you'd probably do better with a good computer sound card and software to generate waves and analyse them.
Your design seems very close the ShibaSoku 725 distortion analyzer, which seems to hold the current record for lowest residual distortion. Certainly their oscillators are exceptional, possibly 20 dB better distortion than my KH4400.

I think its an excellent solution and will outperform a soundcard. All of this is academic given real world performance of real circuits.

For the state variable oscillator you learn a lot looking at existing solutions. The KH4402 and its older all discrete variants all have selectable time constants for the AGC loop that track the frequency range. The similar circuits in the Boonton 1120 use the microcontroller to manage AGC time constants and a lot of other aspects, including PLL'ing the frequency.

If interested I can post the schematics of the relevant circuits.
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Old 30th April 2012, 05:59 PM   #162
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Quote:
Originally Posted by 1audio View Post
Your design seems very close the ShibaSoku 725 distortion analyzer, which seems to hold the current record for lowest residual distortion. Certainly their oscillators are exceptional, possibly 20 dB better distortion than my KH4400.

I think its an excellent solution and will outperform a soundcard. All of this is academic given real world performance of real circuits.

For the state variable oscillator you learn a lot looking at existing solutions. The KH4402 and its older all discrete variants all have selectable time constants for the AGC loop that track the frequency range. The similar circuits in the Boonton 1120 use the microcontroller to manage AGC time constants and a lot of other aspects, including PLL'ing the frequency.

If interested I can post the schematics of the relevant circuits.
Selectable time constants in the oscillator AGC really do improve matters a lot. That's what I used in my distortion analyzer that I did about 30 years ago. Each decade range gets its own time constant. I also used speedup diodes in the AGC loop, which also helped helped tremendously with settling time for a given AGC filter bandwidth.

I also used range-dependent residual filtering to further improve performance by limiting the residual bandwidth to 10 times that of the fundamental frequency. The slectable residual filters also cut some of the frequency band below the fundamental. Some analyzers have a fixed 80 kHz HF cutoff, which unfortunately obscures upper harmonics for 20 kHz THD.

What is the residual of the ShibaSoku 725 at 20kHz and with what residual bandwidth?

Cheers,
Bob
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Old 30th April 2012, 06:18 PM   #163
davada is offline davada  Canada
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[QUOTE=If interested I can post the schematics of the relevant circuits.[/QUOTE]

Hi Audio1,

I'd like to have a look at the ShibaSoku 725 distortion analyzer. If you can post anything on that.

Cheers,

David.
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Old 1st May 2012, 06:57 AM   #164
1audio is offline 1audio  United States
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I don't have the schematics for the 725, I have not been motivated to buy the manual yet since its all working.

What I was offering were schematics for the Boonton and the KH4400.

The residual distortion of the AG15C is rated as less than -120 dB (.0001%) and I measured it with the 725 as more like .00006 or less but at that point I'm not sure what I have since it measures differently, more like a lock in amplifier than a conventional distortion analyzer.

Looking at the output of the AG16 I have with an AKM AK5394a demo board there are no distortion artifacts above -126 dB at 12 KHz on the attached plot. This demo board has JRC5534's on its input. Its very useful to have a source this clean when you have questions about intrinsic distortion.
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File Type: jpg ShibasokuAG16 at 12 KHz.JPG (107.0 KB, 673 views)
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Old 1st May 2012, 07:06 AM   #165
davada is offline davada  Canada
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I don't have the schematics for the 725, I have not been motivated to buy the manual yet since its all working.

What I was offering were schematics for the Boonton and the KH4400.

The residual distortion of the AG15C is rated as less than -120 dB (.0001%) and I measured it with the 725 as more like .00006 or less but at that point I'm not sure what I have since it measures differently, more like a lock in amplifier than a conventional distortion analyzer.

Looking at the output of the AG16 I have with an AKM AK5394a demo board there are no distortion artifacts above -126 dB at 12 KHz on the attached plot. This demo board has JRC5534's on its input. Its very useful to have a source this clean when you have questions about intrinsic distortion.
Very nice. I'll do some digging for docs.

David.
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Old 2nd May 2012, 02:44 PM   #166
pol098 is offline pol098  United Kingdom
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Thanks very much for the response to my posting:
Quote:
Originally Posted by 1audio View Post
Your design seems very close the ShibaSoku 725...

If interested I can post the schematics of the relevant circuits.
Unfortunately I'm quite out of it; this was done many years ago, and I don't even remember enough to discuss details sensibly. I see others have asked for details, so I'll have a look at them, but don't expect to have anything useful to contribute.
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Old 2nd May 2012, 05:28 PM   #167
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Quote:
Originally Posted by 1audio View Post
Your design seems very close the ShibaSoku 725 distortion analyzer, which seems to hold the current record for lowest residual distortion. Certainly their oscillators are exceptional, possibly 20 dB better distortion than my KH4400.
Mixing down to DC? Our ShibaSoku is MIA, I wonder if metrology still has the manual in a file somewhere. I sort of gave up on this stuff when I realized that the null technique (Cordell or similar) gives me most of the information I need. Measuring passives to -140dBc is purely academic to me.
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Last edited by scott wurcer; 2nd May 2012 at 05:32 PM.
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Old 2nd May 2012, 05:46 PM   #168
1audio is offline 1audio  United States
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Originally Posted by scott wurcer View Post
Mixing down to DC? Our ShibaSoku is MIA, I wonder if metrology still has the manual in a file somewhere. I sort of gave up on this stuff when I realized that the null technique (Cordell or similar) gives me most of the information I need. Measuring passives to -140dBc is purely academic to me.
Geeze, I hope it isn't the one I bought surplus a few years ago.

I use a Boonton 1120 on the bench since its faster to use and the performance is adequate to anything in the real world.

I use the Radiometer Copenhagen CLT-1 to measure passives since -140 dB is not nearly enough. -165 dB is just barely enough for most parts. Bad ones light up but good film resistors get close to -160 easily, wirewound and metal foil are essentially perfect. Look elsewhere for these problems (the thermal issues are significant however.)

If you do find manuals for the 725 and the associated oscillators, there are several, I'm very interested.
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Old 7th May 2012, 08:16 AM   #169
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Here is a oscillator with THD bellow -140 dB.
Low distortion oscillator tests measurement circuits - 2012-05-03 04:33:52 | Test & Measurement World
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Old 17th May 2012, 06:57 AM   #170
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Can anyone explain the purpose of the BAV99 diode that goes to "NC"?

I was also puzzled by the 18V zener diodes on the 15 volt supply lines. Perhaps they are only there as overvoltage protection?

Terry
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