harmonic distorsion

[molavec]

Hi,

   I have had problem using the harmonic distorsion module using it with a data adquisition card (but I think is not a card problem).


  I tested the module in different ways:

- Using it in simulated condition with generator
- Using it with Card

In PDF attached there are three tables that show the values have been gotten from channel 3 in Hamonic Distorsion module:

- The first is the theorical THD
- The second is the THD got using DasyLab Generator
- The third is the THD got using the card

As you see in the images, using simulated schema is the THD is (second table in pdf),
but using the card the THD is wrong although Amplitude values got from channel 3 (third table in pdf).

Could you help me with ideas that help me to detect what is worng or if the module has problem?


More details:
--------------------------
DasyLab v10.0.1


THD type 3

FFT Details:
Channel 0: amplitude spectrum (without dB)
Channel 2: Phase spectrum
Filtering Disable

Sample data : 20ks/s
Block sizes: 8192

Period Check:
search for zero ....
zero line = 0
block size = 8192
----------------------------

[fromm8]

Hello Movalec,

Could you tell me what DAQ card you are using?

Regards

Frank R.

[molavec]

DAQCard-AI-16XE-50

http://200.126.14.82/web/NationalInstruments/Instrupedia/instrupedia/test/nioc.vp-cid=1070&.htm

anyway, but I think not a Card problem. For me is critical resolve problem because if I can't resolve problem I must use other modules to get harmonic distorsion, so if you help me you will be my life better

Thanks

[molavec]

If you see the second picture FFT its OK and the parameters are the same as in simulated situation.

[fromm8]

Hi Molavec,

Are you measuring THD of other system (signal) or you are trying to verify its specifications? So what are you simulating? I would like to understand why those values could be wrong.

Regards

Frank R.

[molavec]

I taken a signal from a tone. Anyway, It doesn't care. The problem is if you see the second FFT the tone is 1Khz and harmonics are almost zero and the THD from Harmonic distorsion is almost 1. That is absolutly wrong. I just simulated a signal to comprobate if  I used the Hamonic Distorsion wrong, but is ok.

So the question is how could comprobate how the module is calculating the THD or what could producing that THD was Equals to 1? I used the FFT without decibels, could it provocate worng calculating? There some options in FFT  that I should use or some block size that could producing this error?

Thanks in advance.

[molavec]

Hi again,



    I follow developing others parts of my program and left to end this part. Probabbly, something I'm doing worng. I used only a simulated example and to discard DAQ problem.



I used a simulated generator with next amplitudes

1khz  -> 1.0000
2khz  -> 0,5000
3khz  -> 0,2500
4khz  -> 0,1250
5khz  -> 0,0625
6khz  -> 0,0313
7khz  -> 0,0156

Getting next result:

After that I used folling values:

1khz  -> 1.0000
2khz  -> 0.0050
3khz  -> 0.0020
4khz  -> 0.0009
5khz  -> 0.0008
6khz  -> 0.0007
7khz  -> 0.0006

The result:

Obviously, there is a problem in THD Value.

So, The quetions are;

1.- The result is in percentage (%) or is absolute?? (From documentation I guess that it will be absolute)

2.- What  would provocate that error in THD calculation?

Configuration Harmmonic distorsion module:

Thanks in advance for your help (thrust me I need help!!)