DIY Analog-to-Digital Converter project.Audio measurements tool

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oscillator.

Hello,

jackinnj, below how looks my version of the LT oscillator.

Ghg, i'm not sure that active notch filter allow to improve measurement resolution of THD.
I have built one, using two AD797, and finally, it seem to add THD...
Fully passive notch filter is also poor, impedance is too high and it's low Q factor give high attenuation at harmonics frequency.


Left, the new AA5381 ADC working, at right the oscillator and it's differential outputs.
oscillator_02.jpg


Inside the oscillator...
oscillator_03.jpg


Left, my previous ADC (with AKM5394).At center the new AA5381 and at
right the oscillator again.
oscillator_01.jpg



Frex.
 
results.

Hello,

Thank jackinnj, yours is also nice ! :)
I have noticed that the DC source of the oscillator must be quiet for very low HD. Bad line rejection issue could add intermodulation around Fosc.
To solve it, very good PSU or battery powering is required.
Using metal housing is not a problem if is it connected to ground (of oscillator).Metal case improve line and EMI rejection.
I use for mine with a JSR06 low noise PSU.

Now, all my measurements results on the AA5381 ADC, and some others with my older ADC to compare.
Hello,
 

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  • AA5381_10kHzTHD_96kHz_4Vpp_se.pdf
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  • EADCAKM_10kHzTHD_192kHz_4Vpp_diff.pdf
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  • EADCAKM_noisefloor_48kHz_0dBFS.pdf
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You both did a fine job on these 10kHz super oscillators, I am jealous ...
Any boards left, you layout kings ???

>frex: Ghg, i'm not sure that active notch filter allow to improve measurement
>resolution of THD.I have built one, using two AD797, and finally, it seem to add THD...

Well, the AD797 is a beast, I never managed to get it 100% stable on the breadboard and was too lazy for doing a good pcb layout,
but just take an AD712 and all is fine. Take a look at the plot below, a standard active twin T dual opamp twin T, gives 60dB attenuation,
THD is at the same level as the related "sweet spot" FFT of the TI eval board.
A standard soundcard without this notch would be worse by 20dB.

>Fully passive notch filter is also poor, impedance is too high and it's low Q factor >give high attenuation at harmonics frequency.

Works too, use just a +20dB FET buffer and take the insertion loss in account.
 

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You both did a fine job on these 10kHz super oscillators, I am jealous ...
Any boards left, you layout kings ???

One of the guys at big semi bought all the boards I had made -- I don't know if they ever got them working!

Same thing happened when I built the phase-meter from an Intersil Application Note -- the semi companies will sell their development modules for a fraction of the engineering time and money which goes into them in the hopes of selling tens of thousands of parts.

AD797 seems to like a copper plane underneath it -- if you don't want to lay out a PCB consider "dead bug" style development. Development boards are OK, but really fall down in low noise circuits as they pick up all kinds of radiated energy. With the AD797, it's important as well to read the entire datasheet!
 
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ghg,

The PCB of my oscillator is not professional. It's home made.
If you are interested, i can send you Gerber files to build it.
(you can read/print it with any Gerber reader like GCprevue or others).
(Maybe if enough DIYers are also interested, it could be a good idea to get professional PCB from PCBcart or others ? )

For the notch filter, i will try to work on it this week with others OPAMPs.
Like says ikoflexer , could you post or send your schematic of notch filter ?

Frex
 

iko

Ex-Moderator
Joined 2008
Before seeing this discussion I was thinking of building the attached. Not exactly with that opamp... but something like that. Haven't thought about the 20dB buffer/amp yet.
 

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TWIN T notch filter

1. passive + 20dB amp:

EDN -- 11.10.94 Oscillator keeps THD below 1 ppm Fig. 2

2. active:
Distortion Analyser Fig. 4


@frex: Gerber would be fine, thanks a lot.

@ikoflexer:
>Gary, do you mean a fet input opamp based +20dB buffer after the notchfilter? The buffer/amp would also have to be very low thd+n, no?
Imagine 0dBV to the notch, after the notch you will get app. -60dBV.
Todays opamps show about/or less than -100db THD ( 0dBV input,1kHz,+20dB gain ).
At an input of -60dBV we have app. -160dB opamp selfdistortion.
 

iko

Ex-Moderator
Joined 2008
Lest you wield the iron in vain, run a monte-carlo simulation to see how the depth of those notches holds up when the component values don't have 23bit-precise matching. Also 100k resistors at the opamp inputs won't help the distortion performance:p

Nah, you saved me from wasting my time, thanks so much. Could you please point me to an opamp based notch filter schematic which is not sensitive to well matched RC values? The 100k nasties I can fix.
 
Could you please point me to an opamp based notch filter schematic which is not sensitive to well matched RC values?

If your aim is to get better distortion measurements, you could do a lot worse than Bob Cordell's distortion analyser (full schematics and explanations available at his website). In order to get better than, say a 60dB notch, the filter's centre frequency needs to be auto-tuned within a control loop. Bob's design does this very elegantly, you can just lift his auto-tuned notch filter without building the whole project.

Cordell Audio: Papers: Build a THD Analyzer

If you just want a precise notch circuit which is insensitive to component values, I don't have a clue:eek:
 
A good notch filter is also sensitive to layout, not just component values!

Texas Instruments has a filter utility which is free to download "FilterPro" -- it will allow you to see the effects of using "exact" values, and resistors from E12 to E192, caps E6 to E24.

I'll say this 'til I'm blue in the face -- the HP3581 has one of the best filter systems you're ever going to see for audio -- I just picked up another HP3581 for $49 a few weeks ago. It's RMS detector is better than any unit you'll find in a hand-held DVM. Of course, unless you're Hercules you're not going to hand-hold a '3581!
 

iko

Ex-Moderator
Joined 2008
A good notch filter is also sensitive to layout, not just component values!

Texas Instruments has a filter utility which is free to download "FilterPro" -- it will allow you to see the effects of using "exact" values, and resistors from E12 to E192, caps E6 to E24.

Yep, that's the utility I used.

I'll say this 'til I'm blue in the face -- the HP3581 has one of the best filter systems you're ever going to see for audio -- I just picked up another HP3581 for $49 a few weeks ago. It's RMS detector is better than any unit you'll find in a hand-held DVM. Of course, unless you're Hercules you're not going to hand-hold a '3581!

What do you mean, option filters? I do have an HP 3581a.
 

iko

Ex-Moderator
Joined 2008
Oh, but not a notch filter to null the fundamental. Perhaps you're saying use the band filter to isolate and measure individually the 2nd, then 3rd, etc. harmonics? But I could do the same with the hp 3585a, simply choose the start and stop frequency around the harmonic number(s) that I want to measure. Then the dynamic range would apply only to those harmonics. Or am I delusional here?
 
Hello,

After some advice of Ghg, i have build a twin-T notch filter at 10kHz, to try to measure the THD of the ultra-pure oscillator from the AN67 of Linear Tech.
For that, i use a small shielded box with 0.1% metal resistors and 1nF 1% low drift mica capacitors.After the passive notch filter, i add an OPA134
low THD OPAMP as follower. Is it powered with clean linear +/-12DC supply.
I used Spectraplus for plot the filter response in frequency domain.
As you can see in this first file : 10k_NotchFilter.pdf , the notch filter allow approximately -68dB attenuation at 10kHz.
This performance is very good for non trimmed filter.

Then, i make a measurement of oscillator spectrum with this notch filter inserted between the oscillator and the ADC.
For this purpose, i use my old ADC (AKM AK5394 IC).

You can see the resulting spectrum here : THDwithNotch.pdf
The THD (not THD+N!) measured by Spectraplus is 0.035% !
Only the second and third harmonic are visible, at levels of -155 and -160dBFS (here 0dBFS is 7.07Vrms or 20Vpp).
In green on the same plot, you can see also the notch filter response.

To calculate the real THD, you must add some gain for the harmonics, corresponding to the notch response.
The notch filter attenuate H2 to about 9dB, H3 about 5dB and H4 about 3dB.
The input level at of the signal at notch filter input is -14dBFS, so attenuation at 10kHz is about 68dB.
After correction of the level of harmonics, and correction of the level of fundamental, the recalculated THD is about 0.000039%.
It is corresponding to -168dBFS !

To be sure that the ADC doesn't introduce THD at this input level, i have also build a -70dB passive attenuator.
I have inserted this attenuator between oscillator and ADC, and was made same spectrum measurement (With this attenuator,
the ADC input level become same as notch filter, but all harmonics are attenuated by 70dB).
The resulting spectrum is THDwith70dBAtt_lin.pdf show clearly no harmonics contain.
This allow to say that previous THD measurement with notch filter is valid.

I really think that it will be extremely difficult to build better oscillator than that !


Frex.


Note:
I have send files (including Gerber,bom,schematics..) to DIYaudio user for building this oscillator.
I think that if many people are interested to build it it could be a good idea to make a group buy for the PCB.
The price of it can be very low if 10 or more is ordered.
Please let me your opinion..
 
Note:
I have send files (including Gerber,bom,schematics..) to DIYaudio user for building this oscillator.
I think that if many people are interested to build it it could be a good idea to make a group buy for the PCB.
The price of it can be very low if 10 or more is ordered.
Please let me your opinion..
Thank you for the files. If you decide to make boards, I'd be happy to buy one (or two, if that will help you get to ten). It's not every day you get the opportunity to put together a 0.000039% distortion oscillator :)

The best I've been able to do is about 0.0025% with a simple circuit from Elektor.
 

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