The swept-sine algorithm tends to reject distortion products (harmonics)
caused by system nonlinearities. However, nonlinearities can also cause errors
in the measurement of the fundamental frequency (the frequency of the source
sine wave). You can examine the nonlinear behavior by doing two or more
sine sweeps at different source levels and compare the measured results. The
magnitude of the change indicates how nonlinear the system is. Another
symptom of measurement nonlinearities that can be observed using the max
order mode of the curve fitter, is a failure of the curve fitter to stop
incrementing system orders after the fit appears to be very good. This can
occur on FFT based measurements as well.
Another way to check for nonlinearities is to stimulate the system with a fixed
frequency sine wave (in FFT Analysis mode; in Swept Sine mode, you can
define a math function to compute the FFT of channel 2 time data) and observe
the linear spectrum in the response channel. If harmonic distortion is evident
in the spectrum, then the system contains some form of nonlinearity.
Agilent 35607A
Operator's Guide Curve Fit Option 1D3
16-13
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