Questions about the frequency response step

[rico1985]

Hi,

I am using Signal Express 3.0. I am not clear about the transfer functions in the frequency response step with different averaging modes. What I got from the help file is that H(f)=Sab(f)/Saa(f) which is cross spectrum over auto spectrum where a is the stimulus signal and b is the response signal. When RMS averaging is used, I am wondering whether the transfer function becomes magnitude of cross spectrum divided by magnitude of auto spectrum. When vector averaging is used, everthing is in complex numbers.Is it doing averaging of the time-domain signals, frequency domain signals or the results from transfer functions?

Thanks a lot.

[DFGray · Accepted solution]

The algorithm to calculate the spectrum is the same in both modes.  However, the averaging mode can have a huge effect on the result.  RMS averaging is done on the spectrum itself, after the calculation.  Vector averaging is done on the input signals before the calculation.  With vector averaging, the signals must be coherent (have the same phase) or the result will be wrong due to the signal being averaged away.