Trouble FFT and distortion mesure on Pico 2203

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Glubbein
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Joined: Mon Jul 02, 2012 2:14 pm

Trouble FFT and distortion mesure on Pico 2203

Post by Glubbein »

Hello,

I bought a PICO 2203 about one year ago, but starded to use it juste a few weeks ago. Everything works fine except distortion analysis

To show you the problem I've done two tests :
a) with a very low distortion sinus generator (Marconi TF 2005R), an HP8903 distortion analyseur (witch cane give me TRS and TDH (in %)) in one hand, and the Pico2203 (using same cable than with HP8903) one the other hand.
- the THD value given for a 200Hz signal is : HP : 0,07 PICO : 0,78
- the THD value given for a 633Hz signal is : HP : 0,038 PICO : 0,157
- the THD value given for a 6kHz signal is : HP : 0,033 PICO : 0,229
RMS and frequency values are the same with HP and PICO.
The Pico FFT analyzer gives me the folowing screens (Hanning window) :
200Hz signal :
FFT Hann Average Marconi 2.jpg
633Hz signal :
FFT Hann Average Marconi.jpg
b) with the Pico AWG (at 600Hz) with signal out shorted with channel A (using the same cable as before and Hanning window) :
Sinus 600Hz AWG.jpg

As ou can see something is wrong... Is this due to sofware or hardware???

Thanks a lot for helping.

Best regards.

Glubbein

ziko
Advanced User
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Posts: 1705
Joined: Fri Dec 01, 2006 10:03 am
Location: St Neots

Re: Trouble FFT and distortion mesure on Pico 2203

Post by ziko »

HI and thank you for your post.

From the post you sent I can tell that you have put averaging on (picture says it!), but I require more information about your setup. For example you say Hanning Window do you mean Hamming or do you mean Hann window? Also what is the amplitude of the signal that you are feeding in.

Also you seem to have a big frequency span across your signal, 5MHz, I would drop the frequency range down to say something like 9.766 kHz or lower.

Could you send a settings file?

Bear in mind this is an 8 bit oscilloscope and not a dedicated spectrum analyser.

Kind regards
Ziko

Technical Specialist

Glubbein
Newbie
Posts: 0
Joined: Mon Jul 02, 2012 2:14 pm

Re: Trouble FFT and distortion mesure on Pico 2203

Post by Glubbein »

big distortion!!) :oops:

2°) But I still have many problems :
a) mesurement of THD (in %) :
signal : 1,7kHz sinus 3V peak to peak
spectrum settings : 39,06kHz Hann Window average amplitude : +/-20V to +/-2V (see below)
+/- 2V ---> 0,07%
+/- 5V ---> 0,03%
+/- 10V ---> 0,05%
+/- 20V ---> 0,67%
With my HP8903E (setting with 30kHz filter to reduce bandwidth) : 0,01%
b) worth :cry:
I put a 250Hz, 1V peak to peak in both channel (at the same time) with my Marconi generator :
results are completely different on channel A and on channel B...
A and B +-2V 250Hz 1V.jpg
I thought it could be my cables, so I invert the two cables : still the same!
A and B (inverted cables) +-2V 250Hz 1V.jpg
Why do I don't have the same specrum view in both channels??? Why do I have so many harmonics?

Best regards

Glubb
-->
Hello Ziko,

thanks a lot for your help.

1°) I've done a mistake : the amplitude setting in spectrum analyzer was wrong (too low ---> big distortion!!) :oops:

2°) But I still have many problems :
a) mesurement of THD (in %) :
signal : 1,7kHz sinus 3V peak to peak
spectrum settings : 39,06kHz Hann Window average amplitude : +/-20V to +/-2V (see below)
+/- 2V ---> 0,07%
+/- 5V ---> 0,03%
+/- 10V ---> 0,05%
+/- 20V ---> 0,67%
With my HP8903E (setting with 30kHz filter to reduce bandwidth) : 0,01%
b) worth :cry:
I put a 250Hz, 1V peak to peak in both channel (at the same time) with my Marconi generator :
results are completely different on channel A and on channel B...
A and B +-2V 250Hz 1V.jpg
I thought it could be my cables, so I invert the two cables : still the same!
A and B (inverted cables) +-2V 250Hz 1V.jpg
Why do I don't have the same specrum view in both channels??? Why do I have so many harmonics?

Best regards

Glubb