Low-distortion Audio-range Oscillator

[RNMarsh]

Originally Posted by HpW

For measurement the source or converter should deal in a higher league!

There are also some new ADC/DAC on the horizon as Arda Technologies - Home...


HpW

THD+N = -105db
About average these days. Have others in-house here that are equal. But, Thx- RNM

[1audio]

THD+N is not a good figure of merit when the distortion is so low. The noise in the measurement bandwidth need to be figured out before anything else. If the source noise (equivalent source resistance) is too high you won't get useful info with that number. The low voltage devices used today make this more of an issue. 140 dB below 1V in a 20 KHz (or 30 KHz using the traditional measurement) is a pretty small resistor. A high res FFT confuses this since the measurement bandwidth gets quite small and the noise in that band is very low. To get to -140 dBV in a 20 KHz band your source + input needs to be less than 30 Ohms assuming its noise free etc. A 50 Ohm generator won't get there. High voltage ADC's are really history. Most run on 5V or 3.3V so the input may be 2V max.

I think measuring the individual harmonics is the only meaningful way forward if this actually will accomplish anything. I think -120 dB is probably 40 to 60 dB beyond audibility in practice. However complex IM would be a different issue.

[davada]

Originally Posted by RNMarsh

BTW --just got a note - Victor hyper tuned a 1KHz oscillator that takes the H2 down to -150 ! That pretty much takes it out of the picture for my use !

Thx-RNMarsh

And what is he measuring that on???
At what level and load.

[scott wurcer]

Originally Posted by RNMarsh

I have to stand by what i saw.... took it to another with newer AP model and compared.... they saw same thing. Either mine has an problem or that is the way it is made.
But, -120 isnt good enough anyway. Marginal. I'm looking for at least -130. -140 would be more useful.

BTW --just got a note - Victor hyper tuned a 1KHz oscillator that takes the H2 down to -150 ! That pretty much takes it out of the picture for my use !

Thx-RNMarsh

The service manual is still available and it describes the operation of the fundamental notching in detail. Quite clever, they quadrature detect the residual fundamental and tune out both I and Q components. Maybe we are talking about different things, there was no A/D in 1985 that even remotely could do -100dB.

EDIT - I see, THD + N, I agree a useless metric.

[Samuel Groner]

I have to stand by what I saw... took it to another with newer AP model and compared... they saw same thing. Either mine has an problem or that is the way it is made.

Of course you can configure it such that the ADC sees the direct input signal, and you will then see excess distortion. If you post a screenshot of the settings you're using I might have a chance to point you in the right direction (some settings aren't obvious), or, as Scott suggests, consult the manual.

BTW--just got a note--Victor hyper tuned a 1 kHz oscillator that takes the H2 down to -150! That pretty much takes it out of the picture for my use!

My sincere apologies for repeating myself every 20 pages or so in this thread, but quoting direct analog distortion measurements at the -150 dB level is simply not credible at the current state of the art (unless someone gives a detailed, repeatable proof how he verified measurement integrity to this level, which is pretty unlikely).

But, -120 isnt good enough anyway. Marginal. I'm looking for at least -130 dB. -140 dB would be more useful.

As noted several pages back I've designed a fixed frequency passive notch filter PCB which is currently probably your best DIY bet. It should get you to -130 dB, and perhaps even to -140 dB. But it is cumbersome to use.

Samuel

[davada]

Originally Posted by 1audio

THD+N is not a good figure of merit when the distortion is so low. The noise in the measurement bandwidth need to be figured out before anything else. If the source noise (equivalent source resistance) is too high you won't get useful info with that number. The low voltage devices used today make this more of an issue. 140 dB below 1V in a 20 KHz (or 30 KHz using the traditional measurement) is a pretty small resistor. A high res FFT confuses this since the measurement bandwidth gets quite small and the noise in that band is very low. To get to -140 dBV in a 20 KHz band your source + input needs to be less than 30 Ohms assuming its noise free etc. A 50 Ohm generator won't get there. High voltage ADC's are really history. Most run on 5V or 3.3V so the input may be 2V max.

I think measuring the individual harmonics is the only meaningful way forward if this actually will accomplish anything. I think -120 dB is probably 40 to 60 dB beyond audibility in practice. However complex IM would be a different issue.

Perhaps we should do more in the digital domain. Borrow some of the techniques used in the ShibaSoku AD725D. The frequency of the input signal can be adjusted digitally to fall in the center of the bin and absolutely periodic. From what I understand if this condition is met windowing is not needed. Noise reduction can be done before it hits the FFT and all this can be done without lose from the original signal.

A modern ShibaSoku built from today's technology.

[richiem]

@ Samuel -- Yes, and the passive filter's shoving of the prominent harmonics further into the noise also complicates accurate measurements.

[davada]

Originally Posted by Samuel Groner

As noted several pages back I've designed a fixed frequency passive notch filter PCB which is currently probably your best DIY bet. It should get you to -130 dB, and perhaps even to -140 dB. But it is cumbersome to use.

Samuel

I spotted a few small boxes designed for EMI RF shielding which come with BNC connector
at each end. The size is quite limited. Do you suppose your notch filter can squeezed onto a small very board? The boxes are expensive but I think well suited for this purpose.

I what to do at least a couple of these for a reference notch filter.

[davada]

Originally Posted by richiem

@ Samuel -- Yes, and the passive filter's shoving of the prominent harmonics further into the noise also complicates accurate measurements.

Sure. Can we get the noise down or just the distortion up with out adding too much to it???

[scott wurcer]

Originally Posted by davada

Sure. Can we get the noise down or just the distortion up with out adding too much to it???

You can always reduce the FFT bin width by taking longer FFT's but if you want 140dB below 10dBV that's -130dBV or 316nV and even a 64K FFT is around 1Hz BW at 96K sampling IIRC. Even the passive notch penalty should not hurt.