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Old 30th March 2013, 06:42 AM   #2011
RNMarsh is offline RNMarsh  United States
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Originally Posted by 1audio View Post
. With .6 Hz bandwidth the noise floor is about -160 dB. Translating that to meaningful SNR or equivalent nV/RtHz shall we say "is an exercise left to the reader".
Ok. Thats what I said the noise floor was awhile back... about -160dB. Check.

For my purposes at this time, I am finding ways to lower noise and thd of oscillators.
Both a clean source and an analyzer that can be as good as the source are needed.

Im looking forward to seeing what davada can do to get his -145dB even lower.
The add-on interface is very welcome and great that it doesnt affect the thd/noise issue.

This is all good for DIY.

Thx-RNMarsh

Last edited by RNMarsh; 30th March 2013 at 06:48 AM.
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Old 30th March 2013, 12:29 PM   #2012
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Originally Posted by RNMarsh View Post
I am looking at the notched output of residual harmonics from the analyzer monitor output. I am using an analog swept wave filter type analyzer - HP-3580A.

The monitor output is scaled to the analyzer range selected with a normalized output level of 1v rms = full scale (fs) of the range selected. The range is on its lowest from a ShibaSoku AD725D analyzer which is -110dB (.0003% fs). The noise floor from this monitor port is about -160dB re fs.

The dynamic range of the HP is >85dB on its 0db/1v range.

The signal level from the generator being measured is 7v rms.


Thx-RNMarsh
Hi Richard, thanks.

I also like and trust my HP 3580A. I use it to get below the noise of the distortion analyzer as you do by feeding it the residual output from the analyzer. In my case it is my THD analyzer rather than the ShibaSoku.

With such an arrangement, the distortion floor of looking at an incredibly good oscillator will be the distortion floor of the analyzer. The same thing holds true for inferring the oscillator noise. In other words, the analyzer must be substantially better than the oscillator under test, both in regard to THD and noise.

In my case, my distortion analyzer section is made with virtually the same kind of circuitry as the oscillator, using SV architectures. Indeed, the amplitude and frequency autotune circuits use the same kind of JFET control elements. So, to first order, my analyzer section tends to have similar THD and noise limitations as the oscillator. Indeed, when I did my last round of upgrades about a year ago, I seem to recall ending up believing that the analyzer section was the limiting factor.

Which brings me to this question. You have shown some stunning results with your oscillators using the arrangement you described, with the ShibaSoku as the analyzer. I know that the ShibaSoku has impressive specs, but is it really better than the oscillators you are measuring, both for THD and noise?

Am I missing something here?

Cheers,
Bob
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Old 30th March 2013, 05:21 PM   #2013
RNMarsh is offline RNMarsh  United States
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Originally Posted by Bob Cordell View Post
Hi Richard, thanks.

With such an arrangement, the distortion floor of looking at an incredibly good oscillator will be the distortion floor of the analyzer. The same thing holds true for inferring the oscillator noise. In other words, the analyzer must be substantially better than the oscillator under test, both in regard to THD and noise.

I seem to recall ending up believing that the analyzer section was the limiting factor.

Which brings me to this question. You have shown some stunning results with your oscillators using the arrangement you described, with the ShibaSoku as the analyzer. I know that the ShibaSoku has impressive specs, but is it really better than the oscillators you are measuring, both for THD and noise?

Am I missing something here?

Cheers,
Bob

You arent missing anything re noise and the analyzer. I cant go below the noise floor of the analyzer monitor output, naturally. It become a natural limitation using this Fast Track method and would only work with a very good analyzer system... If I had to, I would rent the best analyzer I could to do the oscillator upgrades. Otherwise it is a very slow process of single tone measurements back and forth sort of thing. Which ultimately would be the final solution after the present method.

The oscillators and analyzers of the past thought none would ever need to read below .001-2% and left it there. Today with SOTA opamps and ADC/DAC systems, measuring with the old tools isnt good enough and measuring with lab cludges is too slow. So its upgrade time. The topologies are Ok... some better than others but the general upgrades with best parts now available can and has done wonders.

This morning, I recieved anothr opamp and when i used it (at 1KHz) with existing new opamps was able to drop both 2H and 3H into the generator noise floor at -155dB. BUT it was higher than the other opamps tried at 10KHz.

The generator noise is close to the analyzer noise of -160db so the accuracy is not so good anymore on the noise part but still fine for getting the THd lower iinto the noise - what ever it is.

Clearly with the opamp changes this morning, the two opamps' harmonics are doing their own cancelling -- phase shifts no doubt at work (one bipolar and one fet input opamp). This is another clue as to how to cancel harmonics once the opamps are as good as they can be. And, if the Twin-T R/C values were optimized for the newest opamps it would help more. But its a fast and good process to get to the bottom line.
:-)
Thx-RNMarsh

Last edited by RNMarsh; 30th March 2013 at 05:32 PM.
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Old 30th March 2013, 06:38 PM   #2014
1audio is online now 1audio  United States
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What is the equivalent input noise of the analyzer? If the noises are near 5 nV/rtHz or less you won't find significant improvements in noise easily. The input attenuator will be the roadblock. I did notice the noise floor is lower for a 1V input than a 10V input suggesting the attenuator is out of the circuit at 1V.
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Old 30th March 2013, 06:46 PM   #2015
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I will tell you to NEVER connect an audio oscillator (at Any Freq.) to an audio power amp, in an attempt to drive a 170w A/B amp, to 200W. It will Crack the output transistors open.

BTDT

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Old 30th March 2013, 06:54 PM   #2016
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It seems you had a problem. I do that regularly without issues but I make sure everything is OK first. From your description your amp had a stability issue when clipping.
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Old 30th March 2013, 07:13 PM   #2017
RNMarsh is offline RNMarsh  United States
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Quote:
Originally Posted by 1audio View Post
What is the equivalent input noise of the analyzer? If the noises are near 5 nV/rtHz or less you won't find significant improvements in noise easily. The input attenuator will be the roadblock. I did notice the noise floor is lower for a 1V input than a 10V input suggesting the attenuator is out of the circuit at 1V.
Already very significant improvement has been made. Now, however, as I get under 10dB from the test system noise, it will be harder. Therefore -> My reason to go in and lower the noise of the ShibaSoku which I started to do. And, I know where i can get 6-12 db improvement. waiting for Ic's to arrive. At that point I can go ahead and get a few more dB's from the generator and thats about it for this go-around.

The system is optimized at 1v input but the noise floor is still higher than it has to be compared with newer opamps. It isnt sufficiently lower at 1v to make me Not want to decrease it further.

Thx-Richard

Last edited by RNMarsh; 30th March 2013 at 07:24 PM.
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Old 30th March 2013, 07:34 PM   #2018
1audio is online now 1audio  United States
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If the front end of the 725 isn't quiet enough you won't get there. That is the first place to quantify.A quick check on this one indicates that the input noise is -110 dBV in a 100 KHz bandwidth. That would be the equivalent of 5000 Ohms. (Useful tool for this- Thermal Noise Calculator ) And that translates into 8 nV/RT Hz or -160 dBV in a 1 Hz bandwidth.

Without the circuit diagrams we are limited in what we can do to improve this. My guess it that the input attenuator and protection circuits are the source of the limitation. The input transistors are probably significantly quieter than this.
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Old 30th March 2013, 09:26 PM   #2019
RNMarsh is offline RNMarsh  United States
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I am about to start looking at the analyzer some more. The input card looked like it was all discrete but a further look will reveill more when I remove it. The filters are always noisy for some reason... Will get into it more soon. Without schematic it i slow going but still doable for certain basic things - especially analog portions.

Have you disconnected the protection to see if noise drops? Shorted the input pot/attenuator to see if noise drops/changes as you think it might?

Thx-RNMarsh

Last edited by RNMarsh; 30th March 2013 at 09:29 PM.
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Old 30th March 2013, 10:02 PM   #2020
RNMarsh is offline RNMarsh  United States
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yep its all discrete analog design to get the performance they felt was needed. Only one opamp IC (5534 - maybe servo?) to replace. All the input switching is done with hermetically sealed relay contacts.... Will be looking at in more detail-- Notice the K389. Wonder what the topology is?

-Richard

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Last edited by RNMarsh; 30th March 2013 at 10:05 PM. Reason: low noise/distortion
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