"power" and "magnitude" in power spectrum?

[rico1985]

When I choose "power in the power spectrum step, numbers in my data were negative. I don't understand why. What's the difference between "power" and "magnitude"?

And what are the "dB" and "linear" in the magnitude scale selection mean? Does "dB" mean the following equation? Vnorm=20*log10(Vnorm)? Does "linear"mean magnitude?

Thank you so much!

[CarlaU]

Hi Ningyu Zhao,

Can you explain a bit more background about what you are looking at? Are you talking about the power spectrum function in LabVIEW? Also, where you are looking at that equation? Is that an equation you found in the LabVIEW help? Does it pertain to a specific VI? Also, you might want to look at more information just about power spectrum in general. Hopefully this helps a bit!

Carla

[rico1985]

Hi Carla,

Thank you for your reply. I am using Labview SignalExpress.

I am a graduate studnet majoring in structural engineering in Tufts University. My research area is floor vibration. We built a scale building model in our lab. A DAQ system, computers and several accelerometers are availabe in our lab. We put a shaker beneath the scale model and use SignalExpress to generate and aquire data (velocity and force)  from accelerometers. We use Matlab to process data.

I have learned how to use SignalExpress for only one month and I feel kind of confused about the data I got from it. The project in SignalExpress was built step by step and here are my questions about the power spectrum  and frequency response step. I am not clear about what transfer functions inside these steps.

When I choose "power in the power spectrum step, numbers in my data were negative. I don't understand why. What's the difference between "power" and "magnitude"? And what are the "dB" and "linear" in the magnitude scale selection mean? Does "dB" mean the following equation? Vnorm=20*log10(Vnorm)? Does "linear"mean magnitude?

I really need some help. Thank you so much.

Ningyu

[CarlaU · Accepted solution]

Ningyu,

I’m assuming that you are doing the following steps. If not, please correct me.  Did you create a step to acquire some sort of signal (such as a voltage analog signal). Then you added a second step that is performing an analysis on the data, using the frequency domain measurement power spectrum.  Basically, the power spectrum can return the spectra or the time domain signal in root-mean-square, peak and peak-to-peak units. The signal that is produced in the graph has two views. One of the views shows the time signals and the other shows the spectra.  The spectra view shows the data as frequency versus magnitude. This magnitude can be expressed in either dB (decibels) or linear units. There is an equation that shows the relation of these two units. The view in time signals shows the signal on the graph as time versus voltage.  These are all just difference ways of analysis the data. You can also take a look at the LabVIEW Analysis Concepts , which goes into great detail about power, magnitude and their relationship. In particular, take a look at Chapter 4 (pages 4-8 and 4-22). I hope this helps!

Carla