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Inaccurate THD measurement !!!

Forum for discussing PicoScope version 6 (non-automotive version)

Inaccurate THD measurement !!!

Postby pub2000 » Wed May 07, 2014 6:21 pm

Hello,

Is here a tutorial to measure THD (%) at output audio amplifier using PicoScope 6?

I tried various settings, for example: I edited measuring THD use "Advanced Mode", I tried various settings to "harmonic detector" but the results are very inaccurate.

I mention that I own a 2000 Series Oscilloscope, 2x10Mhz.

Someone can guide me to achieve an accurate THD measurement ? A tutorial or some advice would be helpful.

Best Regards,
pub
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Re: Inaccurate THD measurement !!!

Postby Hitesh » Fri May 09, 2014 9:35 am

Hi pub,

Could you please send an example data file to support@picotech.com and we can look into the measurements that you are getting.

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Re: Inaccurate THD measurement !!!

Postby pub2000 » Fri May 09, 2014 1:32 pm

I sent a message to that email address with some samples.

I notice the THD errors occur especially when:
1. Changes measured parameters >> Properties >> Advanced> Harmonic detector;
2. Modify value "Capture Count" (I noticed a smaller value it is better!!!).

Errors occur, I suppose, because if I increase the number of harmonics to be taken into account (eg, 16), it is possible that some signal have 8 harmonics of 16, below or at the limit noise. It should be obvious that when harmonics reach the noise limit, the program no longer taken into account, but do not do this. A solution would be to increase the "Harmonic Noise Floor" but it's difficult to assess how many dB??
It's a vicious circle, ie:
a) or use a smaller number of harmonics to be taken into account;
b) or use a larger number of harmonics to be taken into account while using a high "Harmonic Noise Floor".

I tried to apply the two solutions above, but the results are very deplorable. Maybe I didn't understand correctly, maybe, that's why I opened this topic (or I send a email to support), may succeed find a solution. I hope.

Regards,
pub
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Re: Inaccurate THD measurement !!!

Postby Hitesh » Thu May 15, 2014 2:19 pm

Hello pub,

Have you tried altering the frequency range that the spectrum view is displayed over?

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Re: Inaccurate THD measurement !!!

Postby pub2000 » Fri May 16, 2014 6:19 pm

Hi,

I tried but I don't think the result is the most appropriate .

For example , I did this:
1 ) First, I used the PicoScope signal generator (AWG output) with no load on the output and the THD is ~0.05 % at 1 kHz. In fact, varies between 0.05 % and 100 % (quite frustrating);
2 ) I connected the generator output audio input signal (it is a transistor amplifier having input like in the picture attached below):
intput amplifier.jpg
intput amplifier.jpg (12.96 KiB) Viewed 2101 times

After I increased voltage almost 0.5Vrms, distortion THD reached about 0.3 % on the input of the amplifier (amplifier isn't powered).
3 ) in this step I powered audio amplifier and gave its input signal in order to reach half of the maximum power that can be delivered (half the maximum peak to peak voltage of the output). In this case THD increases from ~0.75 % to 21 % ? ? I tried to decrease the frequency band, as you said, at 6kHz, but the distortion is a few percent to half of the nominal power. I think a lot.

The phenomenon described above is happening on three types of audio amplifiers with different schematics, including TDA7294?? Why? I mean, if tested three different audio amplifiers, all three have the same results (very close) in terms of input and output THD amplifier ?? How can this happen even more as one of the three amplifiers was fed from another voltage source?

Another weird aspect is the measurement of THD + N (dBc). In a first test, when the spectrum analyzer was set to 1 MHz, at maximum power amplifier was obtained THD+N = 48.68dBc and THD = 0.368%. Then, I reduced the bandwidth of at spectrum analyzer, in this case 6kHz, and THD+N reached ~45dBc and THD=~1%. Now, if you use this website:
http://www.sengpielaudio.com/calculator-thd.htm
you'll see 48.86dBc means 0.368% and 45dBc means 0.562%. Now, in other words, you want to tell me that: the THD is higher than THD + N? or equal ?? Is impossible.

One thing rather frustrating is refresh rate measured values ​​(very high). By the way, how I can reduce this refresh rate ?

The fact that, under certain conditions, PicoScope software can not calculate exactly a THD and take steps like the one from 0.7% to 21% shows that the software is not optimized, does not know the difference between useful signal and noise. You should mix the noise filtering method using the software - I don't know if you did this or you don't did that. So, I don't think you've managed to make it work very well. Surely something must be improved.

Best regards,
pub
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Re: Inaccurate THD measurement !!!

Postby pub2000 » Tue May 20, 2014 9:01 am

Maybe someone can give me a answer.
Anyway, I sent my message above and to technical support.
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Re: Inaccurate THD measurement !!!

Postby Hitesh » Wed May 21, 2014 1:10 pm

Hi pub,

We are currently looking through the information that you have provided to help determine expected calculations with a view to passing this to the Development team for further investigation if we can repeat the dicrepancies.

If you have any other information e.g. data files with your latest tests, please send them by reply with your Helpdesk ticket reference in the subject line, indicating your expected results.

We hope to resolve this as soon as possible.

Many thanks,
Hitesh

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Re: Inaccurate THD measurement !!!

Postby Hitesh » Thu May 22, 2014 9:53 am

Hi pub,

Just an update to let you know that we can reproduce issues with THD measurements in PicoScope 6 and have been able to compare some calculations with third party software.

A bug report has been submitted to the Development Team incorporating your test files and comments in this thread.

Thank you for the information provided so far. We hope to provide an update as soon as possible.

Many thanks,
Hitesh

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Re: Inaccurate THD measurement !!!

Postby pub2000 » Sat May 24, 2014 6:30 am

Then I wish you much success in improving the PicoScope software.

I wanted an answer today because I need to do some actual measurements but if you can not today, then in the next few days or next week.

I recommend you buy at least three kits DIY audio amplifiers and test them as I said above. If you wish, you can use for comparison and trade audio amplifiers fully assembled.

I'm wondering how did PicoScope version 6 without doing some rudimentary tests in this regard. You have looked
somehow it only at the software? Bad!

I will send other measurements today or tomorrow.

Best regards,
"pub"
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Re: Inaccurate THD measurement !!!

Postby Hitesh » Wed Jun 04, 2014 1:08 pm

Hi pub,

The THD measurement can be affected when the peak frequency of a signal is close to DC in the spectrum view.

The recommendation is to have your data acquisition controlled in Spectrum mode (set the main view as spectrum mode, then add a scope view).

Adjust the frequency range so that the peak frequency is not too close to DC, and also increase the number of spectrum bins and consider using a Blackman Harris window.

I hope this helps.

Best wishes,
Hitesh

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