FRF - Coherence, Real, and Imaginary parts not believable

[adalogue]

I'm new to Signal Express and have had to some issues getting realistic Coherence, Real, and Imaginary data output from the FRF analysis tool. Even in test situations where there should be no coherence between the output and input signals, I get values of 1, acrossed the entire spectrum.

I can get realistic magnitude and phase plots. But, Real and Imaginary parts are always near zero, with amplitudes increasing exponentially at higher frequencies

Thoughts?

[Trevor-B]

Hi,

The coherency functionality required multiple samples to be averaged: http://zone.ni.com/reference/en-XX/help/371361J-01/lvanlsconcepts/lvac_coherence_function/

You can do this in the Averaging tab of the step setup. Once Averaging is turned on, when you run the task, the coherency should no longer be unity. 

As for your questions about the real and imaginary magnitudes, It is hard to say whether or not that is expected without seeing the signals you are comparing.

[adalogue]

Excellent, thank you! I was able to get the coherence to work properly by doing as you suggested, and one other thing: I was using the output of one USB-4431 and the input of another USB-4431. When I kept my input/output to the same device, coherence cleaned up nicely. Is it possible to sync two USB-4431s so I can do FRFs across the two devices?

Thanks again for your help.

[Trevor-B]

Hi,

Unfortunately, there is no way to synchronize the internal clocks of our USB DAQ products. You can set up an external clock to trigger acquisition, and share that clock between the two 4431s. Here is an example to work from: http://zone.ni.com/devzone/cda/epd/p/id/5009

[adalogue]

Thank you. Can you suggest an NI product for use as an external clock that I can sync two USB 4431s to?

[adalogue]

Perhaps a standard function generator will work, I'm running at 1kS/S. However, I can not locate the pinout for the DB9 connector on the back to know which pin is ground and which are PF0-PF7. Also, please confirm I can sync to an external clock on the USB-4431 using Labview Signal Express (2009 S&V Toolkit) via the digital I/O connector and that PF0-PF7 aren't just for triggering.

Thanks again!

[Trevor-B]

HI,

Here is the method I use to find device pinouts: http://www.ni.com/white-paper/4053/en/. Here is the online version of that help file: http://zone.ni.com/reference/en-XX/help/370466V-01/device_pinouts/4431_pinout/

As for how to use an external clock, it is certainly possible. When you drop a DAQmx Acquire task, in the Advanced Timing tab, you can change the sample clock settings to use an external clock source, and you can also specify which PFI you would like to trigger off of.