40
Complex Spectrum Sx(f) :
This is also known as Linear Spectrum , which is
derived from the Fourier transform of x(t) Note that spectrum is a complex
function which has both real and imaginary part. Alternatively, it can be
displayed in Amplitude and Phase, too. Due to the vectorial nature of linear
spectrum, its averaged result may not be correct unless that the time data is
synchronized by an appropriate triggering.When the phase data is not required,
it is recommended to use the auto power spectrum.
Power Spectrum Sxx(f) :
This is also known as Auto Power Spectrum.
Sxx(f) is defined as :
Sxx(f) = Real(Sx(f))^2 + Imag(Sx(f))^2.
The Auto Spectrum is a real function that preserves the “amplitude” of the
Linear Spectrum and removes its “Phase”. Although its unit is the power of the
engineering unit by definition, the cursor displays its value in engineering unit
for convenience.
Cross Spectrum Sxy(f) :
This is the short form of Cross Power Spectrum.
Cross Power Spectrum is the cross product of Spectrum
Sx(f) and Sy(f). Sxy(f) = Sx(f) Sy*(f)
This function is a complex function. It is usually used for analyzing a vector
field. For example, sound intensity field or ODS measurement.
FRF Sy/x(f) :
This is the short form of Frequency Response function, and is
also known as Transfer function. It is defined as the response Spectrum
Sy(f) divided by the input Spectrum.
Sx(f). Sy/x(f) = Sy(f)/ Sx(f)
FRF is a complex function that is often used for modal analysis or operational
deflection shape analysis.
