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Inherent Total Harmonic Distortion w/ PS2000?

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Inherent Total Harmonic Distortion w/ PS2000?

Postby AudioRodeo » Tue Jun 02, 2015 4:21 pm

Hoping to get some clarification:

I'm using a PicoScope 2000 to perform some simple audio testing in the vocal range. Channels A and B of the PS are plugged into the headphone jack of my output device. The PS2000 generates a signal sweep in the audio range which is played through speakers, whose output is recorded through a microphone/amplifier and analyzed with the PicoScope.

In analyzing what appeared to be very high levels of Total Harmonic Distortion (presumably from the speakers themselves) I decided to get a baseline THD of the PicoScope itself to use as a reference point and was very surprised to see very high THD with a simple sine wave at 1kHz. Attachment 1 is a screenshot of the spectrum view with associated vitals.
PS2000 THD.jpg


Attachment 2 highlights the spectrum of interest for data collected during a full sweep from 300Hz to 3kHz.
PS2000 THD 2.jpg


All I did was plug the PS2000 output into the PS2000 Ch.A input, in effect using the PS2000 to analyze it's own signal. So....

What is the source of this distortion? Is real, or an artifact?
Might it influence the THD of my system of interest?
How do separate it from that THD generated by my audio system to be tested?

....Thanks in advance for your help.
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Re: Inherent Total Harmonic Distortion w/ PS2000?

Postby Hitesh » Tue Jun 09, 2015 10:34 am

Hi AudioRodeo,

Would it be possible to send in the PicoScope 6 data files corresponding to the screenshots so that we can check your settings please? You can post them here or e-mail them to support@picotech.com

Which version of PicoScope 6 are you using?

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Re: Inherent Total Harmonic Distortion w/ PS2000?

Postby AudioRodeo » Tue Jun 09, 2015 2:46 pm

OK....will do
Last edited by AudioRodeo on Tue Jun 09, 2015 4:19 pm, edited 1 time in total.
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Re: Inherent Total Harmonic Distortion w/ PS2000?

Postby AudioRodeo » Tue Jun 09, 2015 2:56 pm

(.....not clear if my post is under review or if it simply didn't go through. Here it is again.)

Here are equipment specifics, and the attached data set:

PicoScope® 6 - PC Oscilloscope software version: 6.10.16.2
Copyright © 1995-2015, Pico Technology Ltd

Model: PicoScope 2204A
Serial Number: CR522/301
USB Version: 2.0
Calibration Date: Tuesday, January 06, 2015
Hardware Version: 17
Driver Version: 2.1.4.37

Thanks, again, for your help.
Attachments
PicoScope Harmonics Question.psdata
(457.22 KiB) Downloaded 58 times
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Re: Inherent Total Harmonic Distortion w/ PS2000?

Postby Hitesh » Thu Jun 18, 2015 1:54 pm

Hi AudioRodeo,

Thank you for the information.

We are looking into this. In the meantime the article on Low-distortion measurements with high-resolution oscilloscopes might be helpful.

There is also a video in the page that will provide some useful information:



Regards,
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Re: Inherent Total Harmonic Distortion w/ PS2000?

Postby AudioRodeo » Fri Jun 19, 2015 9:43 pm

Thanks.....will read up on your info.
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Re: Inherent Total Harmonic Distortion w/ PS2000?

Postby AudioRodeo » Tue Jun 23, 2015 8:40 pm

Thanks for the YouTube seminar; it was very useful! I'm really surprised, however, that the THD of the 2205 (as I measured it) is almost three times worse than the 4206.....if that's even the correct way of interpreting my result.

I guess I should expect that the 12bit resolution of my inexpensive 2204 unit is no match for the 16bit resolution of the far more expensive 4206 unit illustrated in your example, but I'm not clear where the distortion "lives" on my unit when I measure the self-produced signal of my picoscope. Does the signal generator produce a "distorted" signal to begin with? and does the 2204 simply add to that distortion?
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Re: Inherent Total Harmonic Distortion w/ PS2000?

Postby Hitesh » Thu Jun 25, 2015 12:07 pm

Hi AudioRodeo,

Just to clarify, the PicoScope 2204A is an 8-bit resolution device. The 12-bit resolution can be obtained via the PicoScope 6 Resolution Enhancement feature.

It is likely to be a combination of the PicoScope 2204A signal generator and the inputs.

Comparing the specfications for harmonic distortion:

PicoScope 2204A: < -50 dB at 100 kHz, full–scale input

PicoScope 4262: -95 dB typical @ 10 kHz, -1 dBfs input

The PicoScope 4262 has a lower distortion value.

I also tried connecting the signal generator of the PicoScope 2204A to the input of the PicoScope 4262 and vice-versa with the following results:

2204A_Spectrum.png
PicoScope 2204A Spectrum view


4262_Spectrum.png
PicoScope 4262 Spectrum view


The PicoScope 4262 is better suited for audio related measurements compared to the PicoScope 2204A.

The data files are attached below (they were saved using PicoScope 6.11.4 Beta).

2204A_Spectrum.psdata
PicoScope 2204A data file
(73.72 KiB) Downloaded 56 times


4262_Spectrum.psdata
PicoScope 4262 data file
(380.04 KiB) Downloaded 56 times


Hope this helps.
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Re: Inherent Total Harmonic Distortion w/ PS2000?

Postby AudioRodeo » Thu Jun 25, 2015 1:38 pm

Thanks Hitesh;

I think that's exactly what I was looking for. Your help is much appreciated.

-Rich
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