Questions about averaging mode in power spectrum and frequency response.

[rico1985]

Hi,

I have some questions about averaging mode. When I generate a sine wave signal from an output channel to two input channels to see whether my DAQ board works well and use vector averaging in power spectrum to get DFT, the amplitudes was different from the previou one of the amplitude, which was supposed to be 1 v peak. They varied from 0.5 v to 0.6 v peak. When the averaging model is RMS, the amplitudes were close to 1. I am wondering what are the fomulas of RMS and vector averaging.Does that mean I could not get accurate data if I use vector averaging? In the frequency response step, why did I get difference coherences and magnitudes by using vector and RMS averaging?

Thanks,

 Ningyu

[G-Money]

rico1985,

         What VIs (name specifically) are you wondering about? You can look at the logic behind the VIs by double left clicking the icon. This will open up the front panel and block diagram of the VI so you can look at the math logic itself.

[G-Money]

Also, Are you using LabVIEW or SignalExpress?

[rico1985]

Hi Grant,

Thank you for your reply. I am using Labview Signal Express 3.0. Although it has some explanations for each box and step, I still feel confused about  some steps. In the DAQmx Generate Analog Output step, what are the differences among1 sample, N samples and continuous samples in the generation mode step? Does the setup of Signal Output Range have some influence on the qualisy of the data aquisition?

What does "Additional Timing Settings" mean in Advanced Timing? And I feel confused about how to set up the terminal configuration.

Thanks a lot.

[G-Money · Accepted solution]

rico1985,

           The differences in the generation modes are as they sound: 1 sample only outputs one sample per write, N Samples will output however many samples you configure it for every write, and the Continuous Samples will output samples continuously until a user specified action happens (you press the stop button or some logic that you created gets fulfulled). The Signal Output Range allows you to set a high and low level celing for your output signal and it only influences the quality by keeping it within this range. Additional Timing allows you to set a timeout for the time in between sample aquisition. If a new sample doesn't become available before the timeout setting then you will get an error. This is useful for watching a network because if the network goes down and you stop getting data from a machine then you would like to know about it. I want to point you to these videos which are short tutorials on how to do most of these actions in SignalExpress.The SignalExpress 3.0 Help File is also your first point of contact for all of your getting started questions. These two resources should get you up and running in SignalExpress in no time. (by the way all of your questions were answered using these resources) Cheers!!

[rico1985]

Grant,

Thank you for your reply and explanation. I didn't find anything about the averaging mode in the power spectrum step in those videos and Signal Express 3.0 Help File. I would like to ask you about this.

I am not sure how Signal Express calculates data using Vector averaging and RMS averaging. When I used RMS averaging, the data became smooth and clean. However, when Vector averaging was used, the data had larger peaks and easily contaminated. Some people told me that RMS averaing was averaging the magnitudes of the data while Vector averaging was averaging the real part and the imaginary part seperately. Then I got more errors by using Vector averaging.

Thanks,
Ningyu

[rico1985]

Thanks a lot. I found it in the Labview Help file.