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Old 16th September 2012, 04:12 PM   #401
Frex is offline Frex  France
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Hello,

Trioflex, could you explain me how you could measure the 1ppm THD of an oscillator with with an ADC (AD7760) given for -105dB (5.6ppm) ?...
You can use a stepped passive attenuator to see how the THD of your ADC
move xith input level.

To measure very low level of THD you will need to use a twin-tee notch filter, reducing drastically the level of signal. Then, you ADC will produce no (or very low) spurious and you show the spectrum of the residual THD of the oscillator.
(This has been already fully explain on the thread in my signature).

FRex
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Old 16th September 2012, 04:36 PM   #402
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Well if you look at my FFT plots then you see that H2 and H3 are at least 20dB over the ADC noise floor, hence when those spurs would disappear measured THD I think should already be at or below 1 ppm. This with direct measurement and no notch filter used. Or put it that way, AD7760 if used with best performance configuration should be able to get a little bit better results compared to what we have. We are however already very close to the figures and FFT plots that ADI is giving in the datasheet.

Please understand this: I am testing the ADC not the oscillator, I am assuming as of all measurements done by different 3rd parties, that is if operated properly Victors oscillator has H2 and H3 below the levels I am measuring right now (if ADC and diff amplifier are good to maximum IC specs).


2 Frex: you do not happen to have any 10KHz LT super oscillator PCB's overleft?
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Old 16th September 2012, 05:18 PM   #403
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Victor has a few 10kHz oscillators up on eBay right now. I bought one of them.
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Old 16th September 2012, 06:42 PM   #404
Frex is offline Frex  France
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At this time, bare PCB of AN67 10kHz oscillator are available for sale.
Regards.

Frex.
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Old 17th September 2012, 02:08 PM   #405
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Quote:
Originally Posted by Frex View Post
At this time, bare PCB of AN67 10kHz oscillator are available for sale.
Regards.

Frex.
Looks great. I'm already committed to Victor's stuff though. I just ordered a Par-Metal series 14 box (RFI proof to -70dB).
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Old 17th September 2012, 02:45 PM   #406
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Quote:
Originally Posted by dirkwright View Post
Looks great. I'm already committed to Victor's stuff though. I just ordered a Par-Metal series 14 box (RFI proof to -70dB).
Hm, I did buy some candy from the market for 7EUR.

Placing Victors oscillator in the candy box (BOX not connected to common ground!) removed all spurs of 50 and 100Hz from the spectrum.

Will be drilling holes tomorrow to assemble the generators properly
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Old 17th September 2012, 03:34 PM   #407
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Quote:
Originally Posted by trioflex View Post
Hm, I did buy some candy from the market for 7EUR.

Placing Victors oscillator in the candy box (BOX not connected to common ground!) removed all spurs of 50 and 100Hz from the spectrum.

Will be drilling holes tomorrow to assemble the generators properly
If it's a steel box then it will shield EMI regardless of grounding. If you want RFI shielding also, then the box has to be grounded. That's my simple minded understanding anyway.

I suppose we could put poor Victor's oscillator into all kinds of tins, like they do with headphone amps... coffee cans, Altoids tins, Earl Grey Tea tins... etc. Maybe some exotic Estonian snuff tin.... <g>
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Old 18th September 2012, 04:34 AM   #408
Frex is offline Frex  France
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Quote:
Well if you look at my FFT plots then you see that H2 and H3 are at least 20dB over the ADC noise floor, hence when those spurs would disappear measured THD I think should already be at or below 1 ppm
What i am saying is if these spurs come from the ADC itself, they never disappear.

You seem to build an interesting project that allowing to perform spectral
analysis from DC to more than MHz. This type of tool could be very useful for many hobbyist. Is a commercial product ? Do you think to share it in this forum ? Or maybe sell it ? I would be interested to know more about it.
Regards.

Frex.
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Old 18th September 2012, 05:15 PM   #409
RNMarsh is offline RNMarsh  United States
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Quote:
Originally Posted by Frex View Post
You seem to build an interesting project that allowing to perform spectral
analysis from DC to more than MHz. This type of tool could be very useful for many hobbyist. Is a commercial product ? Do you think to share it in this forum ? Or maybe sell it ? I would be interested to know more about it.
Regards.

Frex.
DITTO !!
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Old 18th September 2012, 05:43 PM   #410
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Quote:
Originally Posted by Frex View Post
What i am saying is if these spurs come from the ADC itself, they never disappear.

You seem to build an interesting project that allowing to perform spectral
analysis from DC to more than MHz. This type of tool could be very useful for many hobbyist. Is a commercial product ? Do you think to share it in this forum ? Or maybe sell it ? I would be interested to know more about it.
Regards.

Frex.
Hi

we are working on commercial projects. What I can say that is it likely that we will have also products that we will be offering for sale.

Click the image to open in full size.

In the picture above is our current VICTIM, AD7760 SMD module, visible with part of the base board. The module is 22.5x22.5 size large, it fits PLCC68 SMD socket footprint. SMD pads are 1.27 mm pitch.

On the module are implemented AD7760 with ALL needed bypassing, ADP7102 low noise LDO's, 5V clock buffer IC, 4.096V Voltage Reference. For operation the module should be powered from 7.5V supply (analog part) and 2.5V for digital part.

AD7760 datasheet says also that it can be used with Audio standard sampling frequencies, so I assume it has been used for some audio use

We have several more analog blocks in similar form factor, and we plan more of them.

Right now we measured 0.01% THD and 79dB SNR when generating 100KHz sinus with our AD9952 based signal generator module, this without bandpass filtering, only 1MHz lowpass. I must make more homework, but I think 0.01% THD at 100KHz is already not bad at all.

I promise to stick around

Ah, and I will be assembling frex 10KHz oscillator so I will be able to compare these two oscillators with AD7760.

Ah, the AD7760 module is being tested with USB test board, that is not so good, we did not know that FTDI FT2232 fast streaming only works in port A! So we can not do continuous streaming at full 2.5MSPS rate
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