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Old 29th December 2010, 01:27 AM   #1
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Default Distortion measurements, THD, IMD-what card?

Hello,
I would like to ask you,what equipment are you using to measure distortions for your projects. I would expect that professional equipment is not affordable for most of DIY's. Are you using soundcards? What performance you can get from your card? Would like to figure out the best soundcard for the job. Looking for something affordable with good performance. Probably 24/192, to be able to measure up to 96 KHz
It also interesting, what software are you using together with your card.
Thanks.
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Old 29th December 2010, 02:40 AM   #2
Pano is offline Pano  United States
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A search of the forum will turn up a lot of good leads.

I've seen good results with the M-Audio Audiophile card and ARTA or AudioTester software.
At home I use M-Audio USB cards but am limited to 96Khz.
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Old 29th December 2010, 03:27 AM   #3
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Problem with soundcards is that the iron-man test of an amplifier is the distortion of higher frequency signals -- this is where amplifier problems really manifest themselves. Soundcards aren't going to give you a lot of information on the 2nd and 3rd harmonics of a 19kHz signal.

If you're starting out, I would suggest looking at the Heathkit THD% and IM analyzers -- further -- take a look at Bob Cordell's "Distortion Magnifier" as well.

If you have some ability to program, Texas Instruments has some great USB digital signal processing development and mother boards. They come with a bit of canned software and the results I obtained were pretty darned good. You can get a lot of utility out of this stuff.
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Old 29th December 2010, 03:42 AM   #4
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seeing higher harmonics directly can be helpful - but we really should be interested in audio frequency IMD - and we can measure that to very high resolution with prosumer soundcards and some helper circuitry

I have made indirect measurements of ~160 dB resolution with a US$130 Juli@ soundcard by looking for the difference frequency IMD products with a +30 dB gain/filter stage - part of the trick is to sum single tones from separate channels to avoid the IMD generation in the DAC and on card buffer circuitry

the Juli@ does 192K so its really as good as the "standard" audio 80KHz analyzers
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Old 29th December 2010, 02:41 PM   #5
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Juli@ looks like a good choice. I have PCM 2902 USB evaluation board from TI. It works very well for 16 bit and shows about 0.004% of THD (DAC to ADC), close to full range.
But I wanted to get like 10X better distortion reading and wider bandwidth. I think that 80-90KHz bandwidth should be good enough for most measurements. Main concern-linearity of converter and noise.

Any other than Juli@ good cards?
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Old 29th December 2010, 02:58 PM   #6
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Well there is stuff like the PicoScope. I have the 3224 model here, it does 10Mhz, but is limited to 12 bits. Still pretty useful for FFT, tho.

Not in the same price range as the soundcards.
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Old 29th December 2010, 03:37 PM   #7
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Quote:
Originally Posted by panomaniac View Post
Well there is stuff like the PicoScope. I have the 3224 model here, it does 10Mhz, but is limited to 12 bits. Still pretty useful for FFT, tho.

Not in the same price range as the soundcards.
12 bit resolution is limiting distortion measurements to 0.025% at the best. What are you getting with PicoScope?
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Old 29th December 2010, 03:55 PM   #8
MiiB is online now MiiB  Denmark
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what software to use with the juli@-card
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Old 29th December 2010, 08:07 PM   #9
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E-MU 0202, 0204 and 0404 USB2 seems to be an interesting option. Portability and good performance. Unfortunately, they are using AK AD/DA. As I understand, some of Burr-Brown products are offering better THD performance.
But 0404 has SPDIF in/out, so it could be easily expanded with external AD/DA if required.
Has anybody tried one of these products?
Have you seen any other usb cards, supporting 24/192?
Or just USB to SPDIF in/out, supporting 24/192?
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Old 29th December 2010, 08:58 PM   #10
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Quote:
Originally Posted by begemot61 View Post
What are you getting with PicoScope?
I dunno. Here is a quick look at my computer soundcard playing a 1KHz tone.
Pico set to 156Khz limit, zoomed in. 1M termination (the scope).

It does allow you to see noise and other stuff pretty low.
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