Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

FRF CPS plot in Matlab

FRF CPS plot in Matlab

FRF CPS plot in Matlab


I plot the FRF in Matlab with the command:

-f_vec is the Frequency in Hz
-frf_mat ist the frf matrix

The data is imported from a universal file and measured with the LMS Cada-X Software. My Question: I have also measured the Cross Power Spectrum and want to plot it in MatLAb like the FRF. The x axis with the frequency (f_vec) is correct, but the y axis is not correct displayed, the dB value is moren than the double as it should. I only want to plot the CPS and not compute, the data is already measured.

Thanks from Germany

RE: FRF CPS plot in Matlab

The CPS is probably already in power units tehefore you need to use 10*log10 instead of 20.



Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: FRF CPS plot in Matlab


RE: FRF CPS plot in Matlab

But it doesnt work, the dB value is now shifted by round about 10 dB. Is still something missing in the command line??


RE: FRF CPS plot in Matlab

The heart of the problem here is understanding exactly what Cada-X is exporting.  Don't assume it does what the manuals say either (BTDT), export something that's a known quantity and check the numbers by hand.

- Steve

RE: FRF CPS plot in Matlab

In the universal file the values are saved as crosspower spectrum (double power) with the units g^2 and Hz.

RE: FRF CPS plot in Matlab

P, P2P, RMS, other??  What about window correction factors?

- Steve

RE: FRF CPS plot in Matlab

RMS values and uniform window, i don't know the correction factors.


RE: FRF CPS plot in Matlab


You really have to create a sine wave that has exact known attributes and put it through the system.  Otherwise you'll always be wondering.  I have wasted hours and hours before realising and doing this.


- Steve

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close