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Old 1st February 2010, 03:00 PM   #111
EUVL is offline EUVL  Europe
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> Does your Handyscope come with FFT and THD software?

Yes.

> Being able to do spectrum analysis from 10 Hz to 2 MHz is really nice.

I should correct myself. 3MHz sampling at 14 bit. Still not bad :

http://www.tiepie.com/downloads/documents/HS3_spec.pdf

Not cheap though !


Patrick
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Old 1st February 2010, 06:55 PM   #112
ghg is offline ghg  Austria
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Well, got a HS3 for test, but in spectrum mode the harmonics are way too high, even at 16bit. Got about -70dB THD, see attachment.
At 16bit, we should see about 90dB dynamic. Source was Krohn-Hite 4400A and E-MU 1616m, these are down -120dB normally.

Patrick, if you have some spare time, would you mind to check your HS3 results against Bob's analyzer and, maybe, some FFT's via ARTA/Spectralab etc.

BR

Gary from Vienna
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File Type: png HS3_E-MU-1616m.png (27.8 KB, 508 views)
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Old 1st February 2010, 07:33 PM   #113
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Quote:
Originally Posted by ghg View Post
Well, got a HS3 for test, but in spectrum mode the harmonics are way too high, even at 16bit. Got about -70dB THD, see attachment.
At 16bit, we should see about 90dB dynamic. Source was Krohn-Hite 4400A and E-MU 1616m, these are down -120dB normally.

Patrick, if you have some spare time, would you mind to check your HS3 results against Bob's analyzer and, maybe, some FFT's via ARTA/Spectralab etc.

BR

Gary from Vienna
Patrick corrected himself to 14 bits, but that should yield between 78 and 84 dB. It is also possible that the -70 dB just comes from the analog electronics in front of the ADC, since scope vertical amplifiers that go to 2 MHz or more are not often designed to do on the order of 0.01% THD.

Anyway, this points out the value in using the DM. if the DM is set to a magnification of 40 dB, then with the HSO at -70 you might get down to -110 dB, or 0.003%.

Cheers,
Bob
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Old 1st February 2010, 07:37 PM   #114
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Such devices are handy to do spectral analysis of the notch filter residual but surely entirely unsuitable for direct distortion measurements. Better converters exist (e.g. LTC2215-LTC2217) but -90 dB is still somewhat short of excellence.

With recent audio DA/AD converters and sufficient care/experience it should be possible to get somewhere near -110 dB THD+N at 1 kHz and for a 20 kHz bandwidth. Good enough for most purposes but still inferior to an all analogue analyser. Bandwidth becomes less a problem as more and more converters support 384 kHz and up. Which of course doesn't mean that the noise and distortion performance keeps up at frequencies above the audio range.

Samuel

PS: Bob, are the schematics of your DA still available?
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Old 1st February 2010, 07:40 PM   #115
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Quote:
Patrick corrected himself to 14 bits, but that should yield between 78 and 84 dB.
Actually the number of bits doesn't set any limits to distortion if proper dithering is applied. Not sure if the later is the case for such converters.

Samuel
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Old 1st February 2010, 08:17 PM   #116
AuroraB is offline AuroraB  Norway
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I was looking for the DA yesterday.. sure would be interesting if that schematic became avaialble again....
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Old 1st February 2010, 08:26 PM   #117
anatech is offline anatech  Canada
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Hi Patrick,
You did an amazing job on your build! Fantastic!

Your PC scope sounds really promising. Most DSOs are 8 bit, with more expensive models allowing a 12 bit mode. Yours at 16 bit, 2 MHz is very attractive. I did buy one with far lower specs - and it died. I'm having trouble getting it repaired. They won't release schematics and getting it there (far east) is becoming a real issue. It might be garbage.

Hi Bob,
I think that a computer connection is optional and for extended features. Things like setup recall, computer control and data storage and sampling. With a normalized DC, your ADC choices are far more relaxed. Too bad DDS technology wasn't lower distortion, because that would give you an analog sweep ability - or spot checks. Still, the basic instrument could easily be uP run with a keypad on the panel. An analog meter is something I would keep, even if you had a high resolution digital reading. Ever adjust an FM tuner discriminator with only a digital meter? Keep the analog meter!

Updating your basic idea with newer parts should give you a lower noise floor on an already excellent instrument. We do have a number of members that could do the micro-controller side. This would make the killer instrument while also providing a rare kit building experience. I have to admit to missing Heathkit a lot.

You know what Bob? You are to be heart-fully <spell checker> thanked for the creation and publishing of your first project. I'm sure it was a labor of love, and an impressive feat of engineering. The idea of a type II is extremely attractive, especially if you have some help. I don't think a PC scope can replace a low noise purpose built THD meter. Besides, with your meter, you can control the ground currents and noise. Something I found was difficult with a USB based thing.

And yes, the lucky winning bidder has an excellent deal. I wonder how much he knows just how good an instrument he now has?

-Chris
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Old 1st February 2010, 08:35 PM   #118
anatech is offline anatech  Canada
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Hi Patrick,
I see they have a 100:1 probe available @ 250 MHz. Not a bad price for something essential for a USB class "'scope". It is pricey though (HS-3)

-Chris
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Old 1st February 2010, 11:47 PM   #119
EUVL is offline EUVL  Europe
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Gary's measurements are correct, or similar to mine. I am not sure though whether it comes from the scope or the built-in func gen. But as Bob said, a DM will enhance that by another 40dB or so. I am not looking for a perfect THD analyser, or a substitute for an Audio Precision or the like. I already have the HS3 for quite a few years now, which I find excellent for what I need (scope, func gen, better than 8 bit resoultion). No other USB scope has all these functionalities.

With the DM and a HS3, I can see distortion residuals down to 0.005mV from a 5V signal, which is better than 100dB. The absolute value does not interest me, and I never try to build amplifiers with -200dB THD. The frequency spectrum of the distortion is more interesting, as it allows me to trade between 2nd and 3rd, etc.

Of course you can invest in a proper spectrum analyser, and use that in combination with a good func gen (e.g. Bob's) and a DM to better results. I am sure there are also other good combinations. I merely want to point out what simple, low-cost solution one can have to allow some not-so-absolutely-serious DIYers to do simple THD measurements (namely Soundcard and DM).

I actually asked Bob whether he was willing to let me publish his DM design. He said he would rather tidy it up to his usual high standard first before posting at his website. I of course respect that. So you will just have to wait a while till I am ready to publish my own version, inspired by Bob's but different in many details.


Patrick
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Old 2nd February 2010, 01:39 PM   #120
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Quote:
Originally Posted by AuroraB View Post
I was looking for the DA yesterday.. sure would be interesting if that schematic became avaialble again....
Were you looking for the DA as in Distortion Analyzer, or were you actually looking for the DM as in Distortion Magnifier?

Best,
Bob
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