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# FRF CPS plot in Matlab

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## 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.

(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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