05-31-2012 01:04 PM
I have a simple .vi that is attempting to calculate the total PSD over a 500Hz bandwidth. My question pertains to the Windowing function and
what affect it has on the power calculation. I have much on windowing etc. in the NI's help but cannot seem to arrive at the value I am simulating to. I have including my .VI and the associated input file. (this file represents Pulse Densisty Modulated data output from an ADC)
thanks for any help
06-01-2012 01:46 PM
What exactly is your question here? In general, windows are designed to have a minimal effect on things like power spectrum density measurements. In your VI, you are feeding a digital signal into the window which will basically just characterize the window. Perhaps you need to feed in the analog signal or a quantized digital signal. Also, what value are you trying to simulate? The process in your VI seems to make sense.
06-01-2012 02:23 PM
My digital signal is actually Pulse Density Modulated data. It is the output of my ADC, and represents a DC value with inherent system noise, which in turn is analog. Because the noise is random, I am windowing the FFT function to reduce any spectral leakage that may be present. I found some info in help documents that tells me to divide my window array by the coherent gain of the window, and then to take the sum of the powers over the frequency band I am interested in and divide it by the Noise Power Bandwidth of the window I am using. I added these functions to my .vi and it appears to be working correctly now.
06-01-2012 02:43 PM
You may want to try "FFT Power Spectrum and PSD.vi". it takes care of the compensation for the window effect automatically.
Be careful that "Amplitude" compensation is used for Power Spectrum, and "Energy" compensation for PSD. The difference is a scaling factor of 1.5 for Hanning window.