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FRF CPS plot in Matlab

FRF CPS plot in Matlab

FRF CPS plot in Matlab

(OP)
Hallo!

I plot the FRF in Matlab with the command:
plot(f_vec,20*log10(sum(abs(frf_mat)./(size(frf_mat,1)))),'b','LineWidth',1.5);

-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.

 

Cheers

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

(OP)
Thanks...

RE: FRF CPS plot in Matlab

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

Greetz

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

(OP)
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

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

Greetings

RE: FRF CPS plot in Matlab

<soapbox>

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.

</soapbox>

- Steve

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